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1. include record def s include linux s PURPOSE This function reads a record from the file descriptor 4 INPUT The file descriptor and a buffer 4 OUTPUT This function writes the data to the buffer and returns a status code STACK LOCAL VARIABLES equ ST_READ_BUFFER 8 egqu ST_FILEDES 12 section text globl read_record type read_record read_record pushl movl pushl mov mov mov function mov int NOT pop za mov pop ret Sebp Sesp ebp Sebx ST_FILEDES Sebp ebx ST_READ_BUFFER ebp ecx SRECORD_SIZE edx SSYS_READ eax SLINUX_SYSCALL give back Sebx Sebp esp Sebp Seax has t he return value which we will to our calling program It s a pretty simply function It just reads data the size of our structure into an appropriately sized buffer from the given file descriptor The writing one is similar 69 Chapter 6 Reading and Writing Simple Records include linux s include record def s PURPOSE This function writes a record to the file descriptor 4 INPUT The file descriptor and a buffer 4 OUTPUT This function produces a status code STACK LOCAL VARIABLES equ ST_WRITE_BUFFER 8 egqu ST_FILEDES 12 section text globl writ
2. L numbers equ SYS_OPEN 5 equ SYS_WRITE 4 equ SYS_READ 3 egqu SYS_CLOSE 6 equ SYS_EXIT 1l options for open look at usr include asm fcentl h for various values You can combine them by adding them equ O_RDONLY 0 egu O_CREAT_WRONLY_TRUNC 03101 Standard file descriptors equ STDIN equ STDOUT equ STDERR 0 1 2 37 Chapter 5 Dealing with Files system call interrupt equ LINUX_SYSCALL 0x80 equ END_OF_FILE 0 This is the return value of read which means we ve hit the end of the file equ NUMBER_ARGUMENTS 2 section bss Buffer this is where the data is loaded into from the data file and written from into the output file This should never exceed 16 000 for various reasons equ BUFFER_SIZE 500 lcomm BUFFER_DATA BUFFER_SIZE section text STACK POSITIONS egqu ST_SIZE_RESERVE 8 equ ST_FD_IN 4 egqu ST_FD_OUT 8 equ ST_ARGC 0 Number of arguments equ ST_ARGV_O 4 Name of program equ ST_ARGV_1 8 Input file name equ ST_ARGV_2 12 Output file name globl _start _start INITIALIZE PROGRAM Ssave the stack pointer movl esp ebp Allocate space for our file descriptors on the stack subl SST_SIZE_RESERVE esp open_files open_fd_in OPEN INPUT FILE
3. pushl Srecord2 call write_record addl 8 esp Write the third record pushl ST_FILE_DESCRIPTOR ebp pushl Srecord3 call write_record addl 8 esp Close the file descriptor movl SSYS_CLOSE eax movl ST_FILE_DESCRIPTOR ebp ebx int SLINUX_SYSCALL Exit the program movl S SYS_EXIT eax movl 0 ebx int SLINUX_SYSCALL This is a fairly simple program It merely consists of defining the data we want to write in the data section and then calling the right system calls and function calls to accomplish it For a refresher of all of the system calls used see Appendix C You may have noticed the lines include linux s include record def s 73 Chapter 6 Reading and Writing Simple Records These statements cause the given files to basically be pasted right there in the code You don t need to do this with functions because the linker can take care of combining functions exported with glob1 However constants defined in another file do need to be imported in this way Also you may have noticed the use of a new assembler directive rept This directive repeats the contents of the file between the rept and the endr directives the number of times specified after rept This is usually used the way we used it to pad values in the data section In our case we are adding null bytes to the end
4. include lt gnome h gt Program definitions define MY_APP_TITLE define MY_APP_ID gno define MY_APP_VERSION Gnome Example Program me example Ww 000 define MY_BUTTON_ TEXT I Want to Quit the GNOME define MY_QUIT_QU EST Example Program ON Are you sure you want to quit used Must declare functions before they ar int destroy_handler gpointer window int click_handler gpointer window GdkEventAny int main int argc gpointer appPtr gpointer btnQuit cha Initialize GNOME 1 gnome_init MY_APP_ID r argv application window quit button ibraries MY_APP_VERSION argc Create new application window appPtr gnome_app_new MY_APP_ID MY_APP_TITLE Create new button btnQuit gtk_button_new_with_label MY_ GdkEventAny int delete_handler gpointer window GdkEventAny BUTTON_T xe gpointer data e gpointer data e gpointer data argv EXT Make the button show up inside the application window gnome_app_set_contents appPtr btnQuit 1 179 Appendix A GUI Programming Makes the button show up gtk_widget_show btnQuit Makes the application window show up gtk_widget_show appPtr Connect the signal handlers g
5. Instead of going on the the _start section of the program go to the end where we define the convert_to_upper function This is the part that actually does the conversion Starting out we have a set of constants we are using The reason these are put here rather than at the top is that they only deal with this one function We have equ LOWERCASE _ A a The lower boundary of our search equ LOWERCASE_Z 2z The upper boundary of our search equ UPPER_CONVERSION A a Conversion between upper and lower case The first two simply define the letters that are the boundaries of what we are searching for Remember that in the computer letters are represented as numbers Therefore we can use LOWERCASE_A in comparisons additions subtractions or anything else we can use numbers in Also notice we define the constant UPPER_CONVERSION Since letters are represented as numbers we can subtract them Subtracting an upper case letter from the same lower case letter gives us how much we need to add to a lower case letter to make it upper case If that doesn t make sense look at the ASCII code tables themselves see Appendix D You ll notice that the number for the character A is 65 and the character a is 97 The conversion factor is then 32 For any lowercase letter if you add 32 you will get it s capital equivalent 5 This is fairly sta
6. Examples of suitable formats for Transparent copies include plain ASCII without markup Texinfo input format LaTeX input format SGML or XML using a publicly available DTD and standard conforming simple HTML designed for human modification Opaque formats include PostScript PDF proprietary formats that can be read and edited only by proprietary word processors SGML or XML for which the DTD and or processing tools are not generally available and the machine generated HTML produced by some word processors for output purposes only The Title Page means for a printed book the title page itself plus such following pages as are needed to hold legibly the material this License requires to appear in the title page For works in formats which do not have any title page as such Title Page means the text near the most prominent appearance of the work s title preceding the beginning of the body of the text 2 VERBATIM COPYING You may copy and distribute the Document in any medium either commercially or noncommercially provided that this License the copyright notices and the license notice saying this License applies to the Document are reproduced in all copies and that you add no other conditions whatsoever to those of this License You may not use technical measures to obstruct or control the reading or further copying of the copies you make or distribute However you may accept compensation in exchange fo
7. Memory Register to memory transfers on little endian systems Other processors are big endian processors which means that they store the big end of their words first the way we would naturally read a number Register Byte 0 Byte 1 Byte 2 Byte 3 r Y Y F Byte 0 Byte 1 Byte 2 Byte 3 Memory 144 Chapter 10 Counting Like a Computer Register to memory transfers on big endian systems This difference is not normally a problem although it has sparked many technical controversies throughout the years Because the bytes are reversed again or not if it is a big endian processor when being read back into a register the programmer usually never notices what order the bytes are in The byte switching magic happens automatically behind the scenes However the byte order can cause problems in several instances e If you try to read in several bytes at a time using mov1 but deal with them on a byte by byte basis using the least significant byte i e by using al and or shifting of the register this will be in a different order than they appear in memory e If you read or write files written for different architectures you may have to account for whatever order they write their bytes in e If you read or write to network sockets you may have to account for a different byte order in the protocol As long as you are aware of the issue it usually isn t a big deal For more in depth
8. Performs unsigned division Divides the contents of the double word contained in the combined edx eax registers by the value in the register or memory location specified The eax register contains the resulting quotient and the sedx register contains the resulting remainder If the quotient is too large to fit in eax it triggers a type 0 interrupt idiv R M O S Z A P Performs signed division Operates just like div1 above imull R M R O S Z A P C Performs signed multiplication and stores the result in the second operand If the second operand is left out it is assumed to be eax and the full result is stored in the double word edx eax incl R M O S Z A P Increments the given register or memory location Use incb to increment a byte instead of a word mull IR M T R O S Z A P C Perform unsigned multiplication Same rules as apply to imu11 negl R M O S Z A P C 187 Appendix B Common x86 Instructions Instruction Operands Affected Flags Negate the given register or memory location sbbl I R M R M O S Z A P C Performs subtraction with borrowing subl I R M R M O S Z A P C Perform subtraction Logic Instructions These instructions operate on memory as bits instead of words Table B 3 Logic Instructions Instruction Operands Affected Flags andl I R M R M O S Z P C Performs a logical and of the contents o
9. caki prantt pushl 0 call exit It s even shorter Now building programs which use shared libraries is a little different than normal You can build the first program normally by doing this as helloworld nolib s o helloworld nolib o ld helloworld nolib o o helloworld nolib However in order to build the second program you have to do this as helloworld lib s o helloworld lib o ld dynamic linker lib ld linux so 2 o helloworld lib helloworld lib o lc The backslash in the first line simply means that the command continues on the second line The option dynamic linker lib 1d linux so 2 allows our program to be linked to libraries This builds the executable so that before executing the operating system will load the program lib 1d linux so 2 to load in external libraries and link them with the program This program is known as a dynamic linker The 1c option says to link to the c library named Libc so on GNU Linux systems Given a library name c in this case usually library names are longer than a single letter the GNU Linux linker prepends the string 1ib to the beginning of the library name and appends so to the end of it to form the library s filename This library contains many functions to automate all types of tasks The two we are using are printf which prints strings and exit which exits the program Notice that the symbols printf and exit ar
10. esssseesesesesessesresseesresresressersessresresresseseresresee 86 Handling Errors Eff ctivelyssmcsnsncnaennnunnaman n i 88 Making Our Program More Robust ssseesssesesssesesssesseesserssetssseessreseessseesseesseesseesseeesseessesset 89 REVE Woese cat T E E dn sy occas A E E E E Se eena Sata aaa nae 91 8 Sharing Functions with Code Libraries e seseesessescesessossesossesossossesossesoososoossesossesossossssossso 93 Usinga Shared PAC AI ci e gatos EEE E E o sath yes oda nian ose ae ea meee ates 94 How Shared Libraries Work 5 3 2c5sia5c lt 58hs430css0alscadeansnee aastevessteeroes E E p s 96 Finding Information about Labraries yc 22c 02 ce cgaiteisees sata aces cegns atoee assoc gyno Gaeten names 97 ISM PUNCH OMS cates T es sdeuatzasaduveaeaseaytes ca TEE E E EE 101 Building a Slared Ei Dray cesses csads fics sys i a a ceed ialda E EE 102 RS Tawa oa ca tae ens a cas sala eet SON ase RUS 2a econ Sa cee 103 9 Intermediate Memory TOpiCS sessessossoesoessessossossoesoesoosoossoesoesoesoossoesoesoesoossoesoesoesoossossoesossose 107 How a Computer Views Memory 25 33 disascsecaesavasiciaties hacsvscceeasaveegeadintensicdasdhncedianpesctedesnaene 107 Phe AMS PUG HOR Pointer Neree e a aaea auc EEA L a STEE 108 The Memory Layout of a Linux Propraiits yisscss since ayes cence eee 109 Every Memory Addressisa Tiernan i aie usted a E E Ciakketae 110 Gettin Mor Memory tat 9 seis ri ce aa E E N R A AE E A E 112 A Simple Memory Mana
11. movl SSYS_OPEN eax open syscall movl ST_ARGV_1 ebp ebx input filename into ebx movl SO_RDONLY ecx read only flag movl 0666 edx this doesn t really matter for reading int SLINUX_SYSCALL call Linux store_fd_in 58 movl Chapter 5 Dealing with Files Seax ST_FD_IN ebp save the given file descriptor open_fd_out OPEN OUTPUT FILE mov mov mov mov int SSYS_OPEN eax open the file ST_ARGV_2 Sebp ebx Ooutput filename into ebx SO_CREAT_WRONLY_TRUNC ecx flags for writing to the file 0666 edx mode for new file if it s created SLINUX_SYSCALL call Linux store_fd_out movl Seax ST_FD_OUT ebp store the file descriptor here BEG N MAIN LOOP read_loop_begin READ IN A BLOCK FROM THE INPUT FILE mov mov mov mov int FHEX SSYS_READ eax ST_FD_IN ebp ebx get the input file descriptor SBUFFER_DATA ecx the location to read into SBUFFER_SIZE edx the size of the buffer SLINUX_SYSCALL Size of buffer read is returned in eax cmpl jle T IF WE VE REACHED THE END SEND_OF_FILE eax check for end of file marker end_loop if found or on error go to the end continue_read_loop CONVERT THE BLOCK TO UPPER CASE pushl SBUFFER_DATA location of the buffer pushl eax Ssize
12. struct person char firstname 40 char lastname 40 int age This doesn t do anything by itself except give you ways of intelligently using 84 bytes of data You can do basically the same thing using equ directives in assembly language Like this equ PERSON_SIZE 84 equ PERSON_FIRSTNAME_OFFSET 0 equ PERSON_LASTNAME_OFFSET 40 equ PERSON_AGE_OFFSET 80 203 Appendix E C Idioms in Assembly Language When you declare a variable of this type all you are doing is reserving 84 bytes of space So if you have this in C void foo struct person p Do stuff here In assembly language you would have foo Standard header beginning pushl Sebp movl esp sebp Reserve our local variable subl SPERSON_SIZE esp This is the variable s offset from ebp equ P_VAR 0 PERSON_SIZE Do Stuff Here Standard function ending movl sebp sesp popl sebp ret To access structure members you just have to use base pointer addressing with the offsets defined above For example in C you could set the person s age like this p age 30 In assembly language it would look like this movl 30 P_VAR PERSON_AGE_OFFSET ebp Pointers Pointers are very easy Remember pointers are simply the address that a value resides at Let s start by taking a look at global variables For exa
13. The program counter is used to tell the computer where to fetch the next instruction from We mentioned earlier that there is no difference between the way data and programs are stored they are just interpreted differently by the CPU The program counter holds the memory address of the next instruction to be executed The CPU begins by looking at the program counter and fetching Chapter 2 Computer Architecture whatever number is stored in memory at the location specified It is then passed on to the instruction decoder which figures out what the instruction means This includes what process needs to take place addition subtraction multiplication data movement etc and what memory locations are going to be involved in this process Computer instructions usually consist of both the actual instruction and the list of memory locations that are used to carry it out Now the computer uses the data bus to fetch the memory locations to be used in the calculation The data bus is the connection between the CPU and memory It is the actual wire that connects them If you look at the motherboard of the computer the wires that go out from the memory are your data bus In addition to the memory on the outside of the processor the processor itself has some special high speed memory locations called registers There are two kinds of registers general registers and special purpose registers General purpose registers are where the main action happens
14. This is your standard error It is a write only file and usually represents your screen display Most regular processing output goes to STDOUT but any error messages that come 3 As we mentioned earlier in Linux almost everything is a file Your keyboard input is considered a file and so is your screen display 55 Chapter 5 Dealing with Files up in the process go to STDERR This way if you want to you can split them up into separate places This is always file descriptor 2 Any of these files can be redirected from or to a real file rather than a screen or a keyboard This is outside the scope of this book but any good book on the UNIX command line will describe it in detail The program itself does not even need to be aware of this indirection it can just use the standard file descriptors as usual Notice that many of the files you write to aren t files at all UNIX based operating systems treat all input output systems as files Network connections are treated as files your serial port is treated like a file even your audio devices are treated as files Communication between processes is usually done through special files called pipes Some of these files have different methods of opening and creating them than regular files i e they don t use the open system call but they can all be read from and written to using the standard read and write system calls Using Files in a Program We are going to write
15. addl pushl pushl push pushl call addl call add 174 pushl pushl pushl pushl Have appPtr gtk_widget_show 4 esp GNOME call our delete_handler function whenever a delete event occurs SNULL extra data to pass to our function we don t use any Sdelete_handler function address to call Ssignal_delete_event name of the signal Have appPtr widget to listen for events on gtk_signal_connect 16 esp recover stack GNOME call our destroy_handler function whenever a destroy event occurs SNULL extra data to pass to our function we don t use any Sdestroy_handler function address to call Ssignal_destroy name of the signal appPtr widget to listen for events on gtk_signal_connect 16 esp recover stack Appendix A GUI Programming Have GNOME call our click_handler function whenever a click event occurs Note that the previous signals were listening on the fapplication window while this one is only listening on the button pushl SNULL pushl Sclick_handler pushl Ssignal_clicked pushl btnQuit call gtk_signal_connect addl 16 esp Transfer control to GNOME Everything that happens from here out is in reaction to user events which call signal handlers This main function just sets up the main window and connects Signal handlers and the signal handlers take care of the rest call gtk _mai
16. return values 50 source code 14 source file 14 source operand 30 special register 39 special purpose register 9 special purpose registers 7 17 stack 37 stack frame 39 46 stack memory 38 stack pointer 39 stack register 38 standard error 55 standard input 55 standard output 55 static variables 36 40 status code 19 status register 27 STDERR 55 STDIN 55 STDOUT 55 subl 17 superscalar processors 7 symbol 16 36 system call 18 28 system calls 18 35 tab 24 text section 16 unconditional jump 21 variables 25 Von Neumann architecture 5 6 word 8 30 write 53 229
17. Addition subtraction multiplication comparisions and other operations generally use general purpose registers for processing However computers have very few general purpose registers Most information is stored in main memory brought in to the registers for processing and then put back into memory when the processing is completed special purpose registers are registers which have very specific purposes We will discuss these as we come to them Now that the CPU has retrieved all of the data it needs it passes on the data and the decoded instruction to the arithmetic and logic unit for further processing Here the instruction is actually executed After the results of the computation have been calculated the results are then placed on the data bus and sent to the appropriate location in memory or in a register as specified by the instruction This is a very simplified explanation Processors have advanced quite a bit in recent years and are now much more complex Although the basic operation is still the same it is complicated by the use of cache hierarchies superscalar processors pipelining branch prediction out of order execution microcode translation coprocessors and other optimizations Don t worry if you don t know what those words mean you can just use them as Internet search terms if you want to learn more about the CPU Some Terms Computer memory is a numbered sequence of fixed size storage locations The number att
18. e Find one other program we have done so far and add error checking to that program e Add a recovery mechanism for add year s that allows it to read from STDIN if it cannot open the standard file Going Further e What if anything should you do if your error reporting function fails Why e Try to find bugs in at least one open source program File a bug report for it e Try to fix the bug you found in the previous exercise 92 Chapter 8 Sharing Functions with Code Libraries Need to also include scanf and other input mechanisms By now you should realize that the computer has to do a lot of work even for simple tasks Because of that you have to do a lot of work to write the code for a computer to even do simple tasks In addition programming tasks are usually not very simple Therefore we neeed a way to make this process easier on ourselves There are several ways to do this including e Write code in a high level language instead of assembly language e Have lots of pre written code that you can cut and paste into your own programs Have a set of functions on the system that are shared among any program that wishes to use it All three of these are usually used to some degree in any given project The first option will be explored further in Chapter 11 The second option is useful but it suffers from some drawbacks including e Code that is copied often has to be majorly modified to fit the surrounding code e Every p
19. eax is the biggest oe e movl data_items edi 4 start_loop Start loop cmpl 0 eax check to see if we ve hit the end je loop_exit incl edi load next value movl data_items edi 4 eax cmpl ebx eax compare values jle start_loop jump to loop beginning if the new 22 Chapter 3 Your First Programs one isn t bigger movl eax sebx move the value as the largest jmp start_loop jump to loop beginning loop_exit Sebx is the return value and it already has the number movl 1 eax 1 is the exit syscall int 0x80 Now assemble and link it with these commands as maximum s o maximum o ld maximum o o maximum Now run it and check it s status maximum echo You ll notice it returns the value 222 Let s take a look at the program and what it does If you look in the comments you ll see that the program finds the maximum of a set of numbers aren t comments wonderful You may also notice that in this program we actually have something in the data section These lines are the data section data_items These are the data items long 3 67 34 222 45 75 54 34 44 33 22 11 66 0 Lets look at this data_items is a label that refers to the location that follows it Then there is a directive that starts with Long That causes the assembler to reserve memory for the list of
20. Chapter 4 All About Functions status code to the operating system However Linux requires that the program s exit status be stored in Sebx not eax so we have to move it Then we do the standard exit system call The nice thing about function calls is that e Other programmers don t have to know anything about them except it s arguments to use them e They provide standardized building blocks from which you can form a program e They can be called multiple times and it always knows how to get back to where they were since call pushes the return address onto the stack These are the main advantages of functions Larger programs also use functions to break down complex pieces of code into smaller simpler ones In fact almost all of programming is writing and calling functions Let s now take a look at how the factorial function itself is implemented Before the function starts we have this directive type factorial function factorial The t ype directive tells the linker that factorial is a function This isn t really needed unless we were using factorial in other programs We have included it for completeness The line that says factorial gives the symbol factorial the storage location of the next instruction That s how call knew where to go when we said call factorial The first real instructions of the function are pushl Sebp movl esp ebp As shown in the previous program this creates the stack fram
21. Chapter 6 Reading and Writing Simple Records as read records s o read records o ld read record o count chars o write newline o read records o 0o read records You can run your program by doing read records As you can see this program opens the file and then runs a loop of reading checking for the end of file and writing the firstname The one construct that might be new is the line that says pushl SRECORD_FIRSTNAME record_buffer It looks like we are combining and add instruction with a push instruction but we are not You see both RECORD_FIRSTNAME and record_buffer are constants The first is a direct constant created through the use of a equ directive while the latter is defined automatically by the assemble through its use as a label it s value being the address that the data that follows it will start at Since they are both constants that the assembler knows it is able to add them together while it is assembling your program so the whole instruction is a single immediate mode push of a single constant The RECORD_FIRSTNAME constant is the number of bytes after the beginning of a record before we hit the first name record_buf fer is the name of our buffer for holding records Adding them together gets us the address of the first name member of the record stored in record_buffer Modifying the Records In this section we will write a program that Opens an input an
22. GNU Free Documentation License 226 Index ah 31 al 31 Jax 31 eax 17 28 41 48 53 64 83 90 97 195 ebp 17 39 44 212 Yecx 17 53 64 202 edi 17 edx 17 53 64 2 eflags 18 27 eip 17 39 esi 17 esp 17 38 207 212 15 ascii 24 bss 54 byte 23 54 data 54 endr 71 globl 16 24 int 24 lcomm 55 long 23 rept 71 section 16 text 16 54 type 44 49 0x80 19 function 44 0 24 n 24 t 24 _ start 16 24 ABI 41 addl 17 address 7 addressing modes 10 29 Application Binary Interface 41 Arithmetic and logic unit 6 as 14 ASCII 8 assemble 14 assembler 16 assembler directives 16 Assembly Language 4 14 base case 46 base pointer 40 base pointer addressing 40 49 base pointer addressing mode 38 base pointer register 39 base pointer addressing mode 11 30 branch prediction 7 buffers 54 byte 7 30 C language calling convention 38 C programming language 37 cache hierarchies 7 call 37 39 44 48 calling conventions 37 41 close 54 cmpl 27 comments 15 computer architecture 5 conditional jump 21 coprocessors 7 CPU 5 6 Data bus 6 data section 16 23 decl 50 destination operand 30 direct addressing 18 direct addressing mode 10 29 55 echo 15 echo 19 exit 18 exit status 18 25 exit status code 15 fetch execute cycle 6 file d
23. call write_newline Exit movl SSYS_EXIT eax movl 0 ebx int SLINUX_SYSCALL To build the program issue the following commands as integer to number s o integer to number o 149 Chapter 10 Counting Like a Computer as count chars s 0o count chars o as write newline s o write newline o as conversion program s 0o conversion program o ld integer to number o count chars o write newline o conversion program o o conversion program To run just type conversion program and the output should say 824 Review Know the Concepts e Convert the decimal number 5 294 to binary e Add the binary numbers 10111001 and 101011 e Multiply the binary numbers 1100 1010110 e Convert the results of the previous two problems into decimal e Describe how and or not and xor work e What is masking for e What number would you use for the flags of the open system call if you wanted to open the file for writing and create the file if it doesn t exist e How would you represent 55 in a thirty two bit register e Sign extend the previous quantity into a 64 bit register e Describe the difference between little endian and big endian storage of words in memory Use the Concepts e Go back to previous programs that returned numeric results through the exit status code and rewrite them to print out the results instead using our integer to string conversion function e Modify the integer2n
24. ebp esp popl ebp ret The error code just returns 0 in eax so we won t discuss it So let s look at the rest of the loop What happens if the current memory being looked at isn t past the end Well let s look movl HDR_SIZE_OFFSET Seax edx cmpl SUNAVAILABLE HDR_AVAIL_OFFSET eax je next_location First we grab the size of the memory segment and put it in sedx Then we look at the available flag to see if it is set to UNAVAILABLE If so that means that memory is in use so we ll have to skip over it So if the available flag is set to UNAVAILABLE you go to the code labeled next_location If the available flag is set to AVAILABLE then we keep on going This is known as falling through because we didn t test for this condition and jump to another location this is the default We didn t have to jump here it s just the next instruction 123 Chapter 9 Intermediate Memory Topics So let s say that the space was available and so we fall through Then we check to see if this space is big enough to hold the requested amount of memory The size of this segment is being held in edx so we do cmpl Sedx ecx jle allocate_here So if the requested size is less than or equal to the current segment size we can use this block It doesn t matter if the current segment is larger than requested because the extra space wil
25. have to learn the reason the hard way by modifying the code and watching it break or just leaving it alone 15 Chapter 3 Your First Programs After the comments the next line says section data Anything starting with a period isn t directly translated into a machine instruction Instead it s an instruction to the assembler itself These are called assembler directives or pseudo operations because they are handled by the assembler and are not actually run by the computer The section command breaks your program up into sections This command starts the data section where you list any memory storage you will need for data Our program doesn t use any so we don t need the section It s just here for completeness Almost every program you write in the future will have data Right after this you have section text which starts the text section The text section of a program is where the program instructions live The next instruction is globl _start This instructs the assembler that _st art is important to remember _start is a symbol which means that it is going to be replaced by something else either during assembly or linking Symbols are generally used to mark locations of programs or data so you can refer to them by name instead of by their location number Imagine if you had to refer to every memory location by it s address First of all it would be very confusing because you would have to memorize or look up
26. learning how to research answers and learning how to look ahead are all a major part of a programmer s work If you have problems that you just can t get past there is a mailing list for this book where readers can discuss and get help with what they are reading The address is pgubook readers nongnu org This mailing list is open for any type of question or discussion along the lines of this book Your Tools This book teaches assembly language for x86 processors and the GNU Linux operating system Therefore we will be giving all of the examples using the GNU Linux standard GCC tool set If you are not familiar with GNU Linux and the GCC tool set they will be described shortly If you are new to Linux you should check out the guide available at http rute sourceforge net What I intend to show you is more about programming in general than using a specific tool set on a specific platform but standardizing on one makes the task much easier Those new to Linux should also try to get involved in their local GNU Linux User s Group User s Group members are usually very helpful for new people and will help you from everything from installing Linux to learning to use it most efficiently A listing of GNU Linux User s Groups is available at http www linux org groups All of these programs have been tested using Red Hat Linux 8 0 and should work with any other GNU Linux distribution too They will not work with non Linux operating
27. s even better to have the function check and have a reliable exit plan if it happens Now we start our loop First it moves a byte into c1 The code for this is movb Seax edi 1 scl This says to start at seax and go edi locations forward with each location being 1 byte big Take the value found there and put it in c1 After this it checks to see if that value is in the range of lower case a to lower case z To check the range it simply checks to see if the letter is smaller than a If it is it can t be a lower case letter Likewise if it is larger than z it can t be a lower case letter So in each of these cases it simply moves on If it is in the proper range it then adds the uppercase conversion and stores it back Either way it then goes to the next value by incrementing cl Next it checks to see if we are at the end of the buffer If we are not at the end we jump back to the beginning of the loop the convert_loop label If we are at the end it simply continues on to the end of the function Because we are modifying the buffer directly we don t need to return anything to the calling program the changes are already in the buffer The label end_convert_loop is not needed but it s there so it s easy to see where the parts of the program are 63 Chapter 5 Dealing with Files Now we know how the conversion process works Now we need to figure out how to get the data in and out of the files Before reading
28. s look at the next line movl eax ebx We have the first item to look at stored in eax Since it is the first item we know it s the biggest one we ve looked at We store it in ebx since that s where we are keeping the largest number found Also even though mov1 stands for move it actually copies the value so eax and Sebx both contain the starting value Now we move into a loop A loop is a segment of your program that might run more than once We have marked the starting location of the loop in the symbol start_loop The reason we are doing a loop is because we don t know how many data items we have to process but the procedure will be the same no matter how many there are We don t want to have to rewrite our program for every list length possible In fact we don t even want to have to write out code for a comparison for every list item Therefore we have a single section of code a loop that we execute over and over again for every element in data_items In the previous section we outlined what this loop needed to do Let s review e Check to see if the current value being looked at is zero If so that means we are at the end of our data and should exit the loop 9 The instruction doesn t really use 4 for the size of the storage locations although looking at it that way works for our purposes now It s actually what s called a multiplier basically the way it works is that you start at the location s
29. with symbols labels and comments The debugger we will be looking at is GDB the GNU Debugger This application is present on almost all GNU Linux distributions It can debug programs in multiple programming languages including assembly language An Example Debugging Session The best way to explain how a debugger works is by using it The program we will be using the debugger on is the maximum program used in Chapter 3 Let s say that you entered the program perfectly except that you left out the line incl edi When you run the program it just goes in an infinite loop it never exits To determine the cause you need to run the program under GDB However to do this you need to have the assembler include debugging information in the executable All you need to do to enable this is to add the gstabs option to the as command So you would assemble it like this as gstabs maximum s 0o maximum o Linking would be the same as normal stabs is the debugging format used by GDB Now to run the program under the debugger you would type in gdb maximum Be sure that the source files are in the current directory The output should look similar to this GNU gdb Red Hat Linux 5 2 1 4 Copyright 2002 Free Software Foundation Inc GDB is free software covered by the GNU General Public License and you are welcome to change it and or distribute copies of it under certain conditions Type show copying to see th
30. you can test it out yourself with a calculator 4 3 2 1 24 I m guessing you didn t understand the whole code listing Let s go through it a line at a time to see what is happening _start pushl 4 call factorial Okay this program is intended to compute the factorial of the number 4 When programming functions you are supposed to put the parameters of the function on the top of the stack right before you call it Remember a function s parameters are the data that you want the function to work with In this case the factorial function takes 1 parameter the number you want the factorial of The push1 instruction puts the given value at the top of the stack The ca11 instruction then makes the function call Next we have these lines addl 4 esp movl eax Sebx movl 1 eax int 0x80 This takes place after factorial has finished and computed the factorial of 4 for us Now we have to clean up the stack The add1 instruction moves the stack pointer back to where it was before we pushed the 4 onto the stack Since the factorial function You should always clean up your stack parameters after a function call returns The next instruction moves eax to ebx What s in eax Itis factorial s return value In our case it is the value of the factorial function With 4 as our parameter 24 should be our return value Remember return values are always stored in eax We want to return this value as the 48
31. 55555 0 rept 209 Padding to 240 bytes byte 0 endr long 45 record2 ascii Marilyn o rept 32 Padding to 40 bytes byte 0 endr ascii Taylor o rept 33 Padding to 40 bytes 71 Chapter 6 Reading and Writing Simple Records byte 0 endr ascii 2224 S Johannan St nChicago IL 12345 0 rept 203 Padding to 240 bytes byte 0 endr long 29 record3 ascii Derrick 0 rept 32 Padding to 40 bytes byte 0 endr ascii McIntire 0 rept 31 Padding to 40 bytes byte 0 endr ascii 500 W Oakland nSan Diego CA 54321 0 rept 206 Padding to 240 bytes byte 0 endr long 36 This is the name of the file we will write to file_name ascii test dat 0 cqu ST_FILE_DESCRIPTOR 4 globl _start _start Copy the stack pointer to ebp movl esp ebp Allocate space to hold the file descriptor subl 4 esp Open the file movl SSYS_OPEN eax movl Sfile_name ebx movl 0101 ecx This says to create if it 72 Chapter 6 Reading and Writing Simple Records doesn t exist and open for writing movl 0666 edx int SLINUX_SYSCALL Store the file descriptor away movl eax ST_FILE_DESCRIPTOR ebp Write the first record pushl ST_FILE_DESCRIPTOR ebp pushl Srecordl call write_record addl 8 esp Write the second record pushl ST_FILE_DESCRIPTOR ebp
32. Also if you have a lot of programs that are all moving around too fast into memory your machine can get so bogged down moving pages on and off of disk that the system becomes unusable Programmers call this swap 111 Chapter 9 Intermediate Memory Topics death It s usually recoverable if you start terminating your programs but it s a pain Getting More Memory We know that Linux maps all of our virtual memory into real memory or swap If you try to access a piece of virtual memory that hasn t been mapped yet it triggers an error known as a segmentation fault which will terminate your program The program break point if you remember is the last valid address you can use Now this is all great if you know beforehand how much storage you will need You can just add all the memory you need to your data section and it will all be there But let s say you don t know how much memory you will need For example with a text editor you don t know how long the person s file will be You could try to find a maximum file size and just tell the user that they can t go beyond that but that s a waste if the file is small So Linux has a facility to move the break point If you need more memory you can just tell Linux where you want the new break point to be and Linux will map all the memory you need and then move the break point The way we tell Linux to move the break point is the same way we told Linux to exit our program
33. Carry flag Used in arithmetic to say whether or not the result should be carried over to an additional byte If the carry flag is set that usually means that the destination register could not hold the full result It is up to the programmer to decide on what action to take i e propogate the result to another byte signal an error or ignore it entirely Other flags exist but they are much less important Data Transfer Instructions These instructions perform little if any computation Instead they are mostly used for moving data from one place to another Table B 1 Data Transfer Instructions Instruction Operands Affected Flags movl I R M I R M O S Z A C This copies a word of data from one location to another movl eax ebx copies the contents lof eax to ebx Imovb I R M I R M O S Z A C Same as mov 1 but operates on individual bytes leal M I R M O S Z A C This takes a memory location given in the standard format and instead of loading the contents of the memory location loads the computed address For example leal 5 Sebp ecx 1 eax loads the address computed by 5 ebp 1 ecx and stores that in eax popl R M O S Z AIC Pops the top of the stack into the given location This is equivalent to performing mov1 esp R M followed by addl 4 esp popf1 is a variant which pops the top of the stack into the seflags register pushl I R M O S Z A C
34. Chapter 6 Reading and Writing Simple Records Registers used Secx character count Sal current character Sedx current character address type count_chars function globl count_chars This is where our one parameter is on the egqu ST_STRING_START_ADDRESS 8 count_chars pushl Sebp movl esp ebp Counter starts at zero movl 0 ecx Starting address of data movl ST_STRING_START_ADDRESS ebp edx count_loop_begin Grab the current character movb Sedx tal Is it null cmpb 0 al If yes we re don je count_loop_end Otherwise increment the counter and the pointer incl ecx incl edx Go back to the beginning of the loop jmp count_loop_begin count_loop_end We re done Move the count into eax and return movl ecx Seax popl ebp ret stack As you can see it s a fairly straightforward function It simply loops through the bytes counting as it goes until it hits a null byte Then it returns the count 75 Chapter 6 Reading and Writing Simple Records Our record reading program will be fairly straightforward too It will do the following e Open the file e Attempt to read a record If we are at the end of the file exit Otherwise count the characters of the first name e Write the first name to STDOUT e Write a newline to STDOUT e Go back to read another record To write this we need one more simple
35. For example if the Document is in part a textbook of mathematics a Secondary Section may not explain any mathematics The relationship could be a matter of historical connection with the subject or with related matters or of legal commercial philosophical ethical or political position regarding them The Invariant Sections are certain Secondary Sections whose titles are designated as being those of Invariant Sections in the notice that says that the Document is released under this License The Cover Texts are certain short passages of text that are listed as Front Cover Texts or Back Cover Texts in the notice that says that the Document is released under this License 219 Appendix H GNU Free Documentation License A Transparent copy of the Document means a machine readable copy represented in a format whose specification is available to the general public whose contents can be viewed and edited directly and straightforwardly with generic text editors or for images composed of pixels generic paint programs or for drawings some widely available drawing editor and that is suitable for input to text formatters or for automatic translation to a variety of formats suitable for input to text formatters A copy made in an otherwise Transparent file format whose markup has been designed to thwart or discourage subsequent modification by readers is not Transparent A copy that is not Transparent is called Opaque
36. However only in rare circumstances should the error message try to predict why the error occurred It should simply relate what happened Back in 1995 I worked for an Internet Service Provider One of the web browsers we supported tried to guess the cause for every network error rather than just reporting the error If the computer wasn t connected to the Internet and the user tried to connect to a website it would say that there was a problem with the Internet Service Provider that the server was down and that the user should contact their Internet Service Provider to correct the problem Nearly a quarter of our calls were from people who had received this message but merely needed to connect to the Internet before trying to use their browser As you can see trying to diagnose what the problem is can lead to a lot more problems than it fixes It is better to just include a troubleshooting guide which includes possible reasons and courses for action for each error message Recovery Points In order to simplify error handling it is often useful to break your program apart into distinct units where each unit fails and is recovered as a whole For example you could break your program up so that reading the configuration file was a unit If reading the configuration file failed at any point opening the file reading the file trying to decode the file etc then the amp 8 Chapter 7 Developing Robust Programs program would simply treat
37. In C a while loop looks like this while a lt b Do stuff here Finished Looping This can be rendered in assembly language like this loop_begin movl a eax movl b ebx cmpl eax ebx jge loop_end loop_body Do stuff here jmp loop_begin loop_end Finished looping The x86 assembly language has some direct support for looping as well The ecx register can be used as a counter that ends with zero The loop instruction will decrement ecx and jump to a specified address unless ecx is zero For example if you wanted to execute a statement 100 times you would do this in C 202 Appendix E C Idioms in Assembly Language for i 0 i lt 100 i Do process here In assembly language it would be written like this loop_initialize movl 100 ecx loop_begin 7 Do Process Here L Decrement ecx and loops if not zero loop loop_begin rest_of_program Continues on to here One thing to notice is that the loop instruction requires you to be counting backwards to zero If you need to count forwards or use another ending number you should use the loop form which does not include the loop instruction For really tight loops of character string operations there is also the rep instruction but we will leave learning about that as an exercise to the reader Structs Structs are simply descriptions of memory blocks For example in C you can say
38. In hexadecimal a single digit can hold 16 values instead of the normal 10 This is done by utilizing the letters a through f in addition to the regular digits a represents 10 b represents 11 and so on 0x10 represents the number 16 and so on This will be discussed more in depth later but just be aware that numbers starting with 0x are in hexadecimal Tacking on an H at the end is also sometimes used instead but we won t do that in this book For more information about this see Chapter 10 6 Actually the interrupt transfers control to whoever set up an interrupt handler for the interrupt number In the case of Linux all of them are set to be handled by the Linux kernel 7 If you don t watch Veggie Tales you should 19 Chapter 3 Your First Programs e Where will the original list of numbers be stored e What procedure will we need to follow to find the maximum number e How much storage do we need to carry out that procedure e Will all of the storage fit into registers or do we need to use some memory as well You might not think that something as simple as finding the maximum number from a list would take much planning You can usually tell people to find the maximum number and they can do so with little trouble However our minds are used to putting together complex tasks automatically Computers need to be instructed through the process In addition we can usually hold any number of things in our mind without much trouble
39. Invariant Sections of all of the original documents unmodified and list them all as Invariant Sections of your combined work in its license notice The combined work need only contain one copy of this License and multiple identical Invariant Sections may be replaced with a single copy If there are multiple Invariant Sections with the same name but different contents make the title of each such section unique by adding at the end of it in parentheses the name of the original author or publisher of that section if known or else a unique number Make the same adjustment to the section titles in the list of Invariant Sections in the license notice of the combined work In the combination you must combine any sections entitled History in the various original documents forming one section entitled History likewise combine any sections entitled Acknowledgements and any sections entitled Dedications You must delete all sections entitled Endorsements 6 COLLECTIONS OF DOCUMENTS You may make a collection consisting of the Document and other documents released under this License and replace the individual copies of this License in the various documents with a single copy that is included in the collection provided that you follow the rules of this License for verbatim copying of each of the documents in all other respects You may extract a single document from such a collection and dispbibute it i
40. Okay now let s look at the code Variables and Constants At the beginning of the program we have two locations set up heap_begin Long 0 current_break long 0 119 Chapter 9 Intermediate Memory Topics The section of memory being managed is commonly referred to as the heap Now when we assemble the program we have no idea where the beginning of the heap is nor where the current break point is Therefore we reserve space for them but just fill them with a O for the time being You ll notice that the comments call them global variables A set of terms commonly used are global and local variables A local variable is a variable that is allocated on the stack when a procedure is run A global variable is declared as above and is allocated when the program begins So global variables last for the length of the program while local variables only last for the run of the procedure It is good programming practice to use as few global variables as possible but there are some cases where its unavoidable We will look more at local variables later Next we have a section called constants A constant is a symbol that we use to represent a number For example here we have equ UNAVAILABLE 0 cqu AVAILABLE 1 This means that anywhere we use the symbol UNAVAILABLE to make it just like we re using the number 0 and any time we use the symbol AVAILABLE to make it just like we re using the number 1
41. Pushes the given value onto the stack This is the equivalent to performing subl 4 esp followed by movl I R M esp pushf1 is a variant which pushes the current contents of the seflags register onto the top of the stack xchgl IR M R M O S Z A C Exchange the values of the given operands 186 Appendix B Common x86 Instructions Integer Instructions These are basic calculating instructions that operate on signed or unsigned integers Table B 2 Integer Instructions Instruction Operands Affected Flags adcl I R M R M O S Z A P C Add with carry Adds the first argument to the second and if there is an overflow sets overflow and carry to true add I R M R M O S Z A P C Signed integer addition cdq O S Z A P C Converts the eax word into the double word consisting of sedx eax with sign extension The q signifies that it is a quad word It s actually a double word but it s called a quad word because of the terminology used in the 16 bit days This is usually used before issuing an idiv instruction cmpl I R M R M O S Z A P C Compares two integers It does this by subtracting the first operand from the second It discards the results but sets the flags accordingly Usually used before a conditional jump decl R M O S Z A P Decrements the register or memory location Use decb to decrement a byte instead of a word divl R M O S Z A P
42. So we need to print it out It will look like this gdb print d Sedi 2 0 gdb 211 Appendix F Using the GDB Debugger This indicates that edi is set to zero which is why it keeps on loading the first element of the array This should cause you to ask yourself two questions what is the purpose of sedi and how should its value be changed To answer the first question we just need to look in the comments edi is holding the current index of data_items Since our search is a sequential search through the list of numbers in data_items it would make sense that sedi should be incremented with every loop iteration Scanning the code there is no code which alters edi at all Therefore we should add a line to increment edi at the beginning of every loop iteration This happens to be exactly the line we tossed out at the beginning Assembling linking and running the program again will show that it now works correctly Hopefully this exercise provided some insight into using GDB to help you find errors in your programs Breakpoints and Other GDB Features The program we entered in the last section had an infinite loop and could be easily stopped using control c Other programs may simply abort or finish with errors In these cases control c doesn t help because by the time you press control c the program is already finished To fix this you need to set breakpoints A breakpoint is a place in the source code that you have ma
43. This makes the program much more readable We also have several others that make the program more readable like equ BRK 45 equ LINUX_SYSCALL 0x80 It is much easier to read int S LINUX_SYSCALL than int 0x80 even though their meanings are the same In general you should replace any hardcoded value in your code that has a meaning with equ statements Next we have structure definitions The memory that we will be handing out has a definite structure it has four bytes for the allocated flag four bytes for the size and the rest for the actual memory The eight bytes at the beginning are known as the header They contain descriptive information about the data but aren t actually a part of the data Anyway we have the following definitions egqu HEADER_SIZE 8 equ HDR_AVAIL_OFFSET 0 equ HDR_SIZE_OFFSET 4 So this says that the header is 8 bytes the available flag is offset 0 positions from the beginning it s the first thing and the size field is offset 4 positions from the beginning right after the available flag Since all of our structures are defined here if we needed to rearrange for some reason all we have to do is change the numbers here If we needed to add another field we would 120 Chapter 9 Intermediate Memory Topics just define it here and change the HEADER_SIZE So putting definitions like this at the top of the program is useful espec
44. We usually don t even realize we are doing it For example if you scan a list of numbers for the maximum you will probably keep in mind both the highest number you ve seen so far and where you are in the list While your mind does this automatically with computers you have to explicitly set up storage for holding the current position on the list and the current maximum number You also have other problems such as how to know when to stop When reading a piece of paper you can stop when you run out of numbers However the computer only contains numbers so it has no idea when it has reached the last of your numbers In computers you have to plan every step of the way So let s do a little planning First of all just for reference let s name the address where the list of numbers starts as data_items Let s say that the last number in the list will be a zero so we know where to stop We also need a value to hold the current position in the list a value to hold the current list element being examined and the current highest value on the list Let s assign each of these a register e edi will hold the current position in the list e ebx will hold the current highest value in the list e eax will hold the current element being examined When we begin the program and look at the first item in the list since we haven t seen any other items that item will automatically be the current largest element in the list Also we will se
45. a smattering of examples of the kinds of local optimizations possible However remember that the maintainability and readability of code is much more important except under extreme circumstances Global Optimization Global optimization has two goals The first one is to put your code in a form where it is easy to do local optimiztions For example if you have a large procedure that performs several slow complex calculations you might see if you can break parts of that procedure into their own functions where the values can be precomputed or memoized Stateless functions functions that only operate on the parameters that were passed to them i e no globals or system calls are the easiest type of functions to optimize in a computer The more stateless parts of your program you have the more opportunities you have to optimize In the e commerce situation I wrote about above the computer had to find all of the associated parts for specific inventory items This required about 12 database calls and in the worst case took about 20 seconds However the goal of this program was to be interactive and a long wait would destroy that goal However I knew that these inventory configurations do not change Therefore I converted the database calls into their own functions which were stateless I was then able to memoize the functions At the beginning of each day the function results were cleared in case anyone had changed them and several inventory
46. about handles A Simple Memory Manager Here I will show you a simple memory manager It is extremely slow at allocating memory 112 Chapter 9 Intermediate Memory Topics especially after having been called several times However it shows the principles quite well and as we learn more sophisticated programming techniques we will improve upon it As usual I will give you the program first for you to look through Afterwards will follow an in depth explanation PURPOSE Program to manage memory usage allocates and deallocates memory as requested 4 NOTES The programs using these routines will ask for a certain size of memory We actually use more than that size but we put it at the beginning before the pointer we hand back We add a size field and an AVAILABLE UNAVAILABLE marker So the memory looks like this 4 EEEE EEEE EH HE E E HE EE HE HE HE HE HE HE HE HE EE HE E E HE E E EE E E E E E E E E E E E E E E E E E HF Available Marker Size of memory Actual memory locations HEE EEE EE HH HEE HH EEE EH HE HE HE HE HEE EE EE HE EE HE E E E E E E E EE E E E E E E HE H Returned pointer points here The pointer we return only points to the actual locations requested to make it easier for the calling program It also allows us to change our
47. address and then load it wherever you want So we could just say jo amp a leal A_VAR ebp eax movl eax B_VAR ebp It s the same number of lines but a little cleaner Then to use this value you simply have to move it to a general purpose register and use indirect addressing as shown in the example above Getting GCC to Help One of the nice things about GCC is it s ability to spit out assembly language code To convert a C language file to assembly you can simply do gcc S file c The output will be in file s It s not the most readable output most of the variable names have been removed and replaced either with numeric stack locations or references to automatically generated labels To start with you probably want to turn off optimizations with 00 so that the assembly language output will follow your source code better 206 Appendix E C Idioms in Assembly Language Something else you might notice is that GCC reserves more stack space for local variables than we do and then AND s Sesp This is to increase memory and cache efficiency by double word aligning variables Finally at the end of functions we usually do the following instructions to clean up the stack before issuing a ret instruction movl sebp sesp popl sebp However GCC output will usually just include the instruction leave This instruction is simply the combination of the above two instructions We do not use leave in this
48. and is absolutely necessary if you are temporarily saving registers on the stack 41 Chapter 4 All About Functions A Function Example Let s take a look at how a function call works in a real program The function we are going to write is the power function We will give the power function two parameters the number and the power we want to raise it to For example if we gave it the paramters 2 and 3 it would raise 2 to the power of 3 or 2 2 2 giving 8 In order to make this program simple we will only allow numbers and greater The following is the code for the complete program As usual an explanation follows Name the file power s PURPOSE Program to illustrate how functions work This program will compute the value of DO3o A2 Everything in the main program is stored in registers so the data section doesn t have anything section data section text globl _start _start pushl 3 push second argument pushl 2 push first argument call power call the function addl 8 esp move the stack pointer back pushl Seax save the first answer before calling the next function pushl 2 push second argument pushl 5 push first argument call power call the function addl 8 esp move the stack pointer back popl ebx The second answer is already in eax We saved the first answer onto the stack s
49. because we don t need to initialize them anyway we just need to reserve storage In order to do this we do the following commands section bss lcomm my_buffer 500 This directive 1comm will create a symbol my_buf fer that refers to a 500 byte storage location that we can use as a buffer We can then do the following assuming we have opened a file for reading and have placed the file descriptor in sebx movl Smy_buffer ecx movl 500 edx movl 3 eax int 0x80 This will read up to 500 bytes into our buffer In this example I placed a dollar sign in front of my_buffer Remember that the reason for this is that without the dollar sign my_buf fer is treated as a memory location and is accessed in direct addressing mode The dollar sign switches it to immediate mode addressing which actually loads the number represented by my_buffer i e the address of the start of our buffer which is the address of my_buf fer itself into ecx Standard and Special Files You might think that programs start without any files open by default This is not true Linux programs usually have at least three open file descriptors when they begin They are STDIN This is the standard input It is a read only file and usually represents your keyboard This is always file descriptor 0 STDOUT This is the standard output It is a write only file and usually represents your screen display This is always file descriptor 1 STDERR
50. different instructions execute at different speeds It s true they do And these operations are pretty much the fastest For example you saw that XORing a number with itself produces 0 Well the XOR operation is faster than the loading operation so many programmers use it to load a register with zero For example the code movl 0 eax is often replaced by xorl eax Seax We ll discuss speed more in the optimization chapter but I want you to see how programmers often do tricky things especially with these binary operators to make things fast Now let s look at how we can use these operators to manipulate true false values Earlier we discussed how binary numbers can be used to represent any number of things Let s use binary numbers to represent what things my Dad and I like First let s look at the things I like 135 Chapter 10 Counting Like a Computer Food yes Heavy Metal Music yes Wearing Dressy Clothes no Football yes Now let s look at what my Dad likes Food yes Heavy Metal Music no Wearing Dressy Clothes yes Football yes Now let s use a 1 to say yes we like something and a 0 to say no we don t Now we have Me Food 1 Heavy Metal Music 1 Wearing Dressy Clothes 0 Football Dad Food 1 Heavy Metal Music 0 Wearing Dressy Clothes 1 Football Now if we just memorize which position each of these are in we have Me 1101 Dad 1011 Now let s see we want t
51. do you think that is How does the programming language s input output methods differ from that of the Linux system calls Going Further Having seen languages which have such brevity as Perl why do you think this book started you with a language as verbose as assembly language How do you think high level languages have affected the process of programming Why do you think so many languages exist Learn two new high level languages How do they differ from each other How are they similar What approach to problem solving does each take 158 Chapter 12 Optimization Optimization is the process of making your application run more effectively You can optimize for many things speed memory space usage disk space usage etc This chapter however focuses on speed optimization When to Optimize It is better to not optimize at all than to optimize too soon When you optimize your code generally becomes less clear because it becomes more complex Readers of your code will have more trouble discovering why you did what you did which will increase the cost of maintenance of your project Even when you know how and why your program runs the way it does optimized code is harder to debug and extend It slows the development process down considerably both because of the time it takes to optimize the code and the time it takes to modify your optimized code Compounding this problem is that you don t even know beforehand where the
52. every protocol or type of data on the planet Python however has a cleaner syntax and often lends itself to more straightforward solutions It s cross platform GUI tools are also excellent PHP makes writing web applications simple Common LISP has more power and features than any other environment for those willing to learn it Scheme is the model of simplicity and power combined together Each language is different and the more languages you know the better programmer you will be Knowing the concepts of different languages will help you in all programming because you can match the programming language to the problem better and you have a larger set of tools to work with Even if certain features aren t directly supported in the language you are using often they can be simulated However if you don t have a broad experience with languages you won t know of all the possibilities you have to choose from Your First C Program Here is your first C program which prints Hello world to the screen and exits Type it in and give it the name Hello World c include lt stdio h gt PURPOSE This program is mean to show a basic C program All it does is print Hello World to the y screen and exit Main Program int main int argc char argv puts Hello World n Print our string to standard output return 0 Exit with status 0 154 Chapter 11 High Level Languag
53. expensive to implement The computer s memory is used for a number of different things All of the results of any calculations are stored in memory In fact everything that is stored is stored in memory Think of your computer at home and imagine what all is stored in your computer s memory e The location of your cursor on the screen e The size of each window on the screen e The shape of each letter of each font being used e The layout of all of the controls on each window e The graphics for all of the toolbar icons e The text for each error message and dialog box e The list goes on and on In addition to all of this the Von Neumann architecture specifies that not only computer data should live in memory but the programs that control the computer s operation should live there too In fact in a computer there is no difference between a program and a program s data except how it is used by the computer They are both stored and accessed the same way The CPU So how does the computer function Obviously simply storing data doesn t do much help you need to be able to access manipulate and move it That s where the CPU comes in The CPU reads in instructions from memory one at a time and executes them This is known as the fetch execute cycle The CPU contains the following elements to accomplish this e Program Counter e Instruction Decoder e Data bus e General purpose registers e Arithmetic and logic unit
54. export LD_LIBRARY_PATH Alternatively if that gives you an error do this instead setenv LD_LIBRARY_PATH Now you can run write records normally by typing write records Setting LD_LIBRARY_PATH tells the linker to add whatever paths you give it to the library search path for dynamic libraries For further information about dynamic linking see the following sources on the Internet e The man page for 1d so contains a lot of information about how the Linux dynamic linker works e http www benyossef com presentations dlink is a great presentation on dynamic linking in Linux e http www linuxjournal com article php sid 1059 and http www linuxjournal com article php sid 1060 provide a good introduction to the ELF file format with more detail available at http www cs ucdavis edu haungs paper node10 html e http www iecc com linker linker10 html contains a great description of how dynamic linking works with ELF files which is the main executable file format used by Linux Review Know the Concepts e What are the advantages and disadvantages of shared libraries e Given a library named foo what would the library s filename be e What does the 1dd command do 103 Chapter 8 Sharing Functions with Code Libraries Let s say we had the files foo o and bar o and you wanted to link them together and dynamically link them to the library kramer What would the linking
55. function a function to write out a newline to STDOUT Put the following code into write newline s include linux s globl write_newline type write_newline function section data newline ascii n section text equ ST_FILEDES 8 write_newline pushl Sebp movl esp ebp movl S SYS_WRITE eax movl ST_FILEDES ebp Sebx movl Snewline ecx movl 1 edx int SLINUX_SYSCALL movl ebp esp popl ebp ret Now we are ready to write the main program Here is the code to read records s include linux s 76 Chapter 6 Reading and Writing Simple Records include record def s section data file name ascii test dat 0 section bss lcomm record_buffer RECORD_SIZE section text Main program globl _start 2s8tart These are the locations on the stack where we will store the input and output descriptors FYI we could have used memory addresses in a data section instead equ ST equ Si INPUT_DESCRIPTOR 4 OUTPUT_DESCRIPTOR 8 Copy the stack pointer to ebp mov A1 sub Sesp ebp Locate space to hold the file descriptors 8 esp Open the file SSYS_OPEN eax Sfile_name ebx mov mov mov mov int 0 ecx This says to open read only 0666 edx SLI Save fil movl NUX_SYSCALL le descripto
56. in the Modified Version s license notice These titles must be distinct from any other section titles You may add a section entitled Endorsements provided it contains nothing but endorsements of your Modified Version by various parties for example statements of peer review or that the text has been approved by an organization as the authoritative definition of a standard You may add a passage of up to five words as a Front Cover Text and a passage of up to 25 words as a Back Cover Text to the end of the list of Cover Texts in the Modified Version Only one passage of Front Cover Text and one of Back Cover Text may be added by or through arrangements made by any one entity If the Document already includes a cover text for the same cover previously added by you or by arrangement made by the same entity you are acting on 222 Appendix H GNU Free Documentation License behalf of you may not add another but you may replace the old one on explicit permission from the previous publisher that added the old one The author s and publisher s of the Document do not by this License give permission to use their names for publicity for or to assert or imply endorsement of any Modified Version 5 COMBINING DOCUMENTS You may combine the Document with other documents released under this License under the terms defined in section 4 above for modified versions provided that you include in the combination all of the
57. interests you From the Bottom Up This list is in the best reading order I could find It s not necessarily easiest to hardest but based on subject matter 167 Chapter 13 Moving On from Here Programming from the Ground Up by Jonathan Bartlett Introduction to Algorithms by Thomas H Cormen Charles E Leiserson and Ronald L Rivest The Art of Computer Programming by Donald Knuth 3 volume set volume 1 is the most important Programming Languages by Samuel N Kamin Modern Operating Systems by Andrew Tanenbaum Linkers and Loaders by John Levine Computer Organization and Design The Hardware Software Interface by David Patterson and John Hennessy From the Top Down These books are arranged from the simplest to the hardest However they can be read in any order you feel comfortable with How to Design Programs by Matthias Felleisen Robert Bruce Findler Matthew Flatt and Shiram Krishnamurthi available online at http www htdp org Simply Scheme An Introduction to Computer Science by Brian Harvey and Matthew Wright How to Think Like a Computer Scientist Learning with Python by Allen Downey Jeff Elkner and Chris Meyers available online at http www greenteapress com thinkpython Structure and Interpretation of Computer Programs by Harold Abelson and Gerald Jay Sussman with Julie Sussman available online at http mitpress mit edu sicp Design Patterns by Erich Gamma Richard Helm Ralph Johnson and John Vliss
58. it as a configuration file problem This way you only need one error handling mechanism for all of the possible problems that could occur with your program Note that even with recovery points your error messages need to be specific as to what the problem was Recovery points are basic units for error reporting and recovery not for error detection Error detection still needs to be extremely exact Also with recovery points you often need to include cleanup code to handle different contingencies For example in our configuration file example the recovery function would need to include code to check and see if the configuration file was open and if so to close it so the program can return to a consistent state The simplest way to handle recovery points is to wrap the whole program into a single recovery point You would just have a simple error reporting function that you can call with an error code and a message The function would print them and and simply exit the program Making Our Program More Robust This section will go through making the add year s program from Chapter 6 a little more robust Since this is a pretty simple program we will limit ourselves to a single recovery point that covers the whole program The only thing we will do to recover is to print the error and exit The code to do that is pretty simple include linux s equ ST_ERROR_CODE egu ST_ERROR_MSG 12 globl error_exit type error_exit f
59. items were automatically preloaded From then on during the day the first time someone accessed an inventory item it would take the 20 seconds it did beforehand but afterwards it would take less than a second because the database results had been memoized Global optimization usually often involves achieving the following properties in your functions Parallelization Parallelization means that your algorithm can effectively be split among multiple processes For example pregnancy is not very parallelizable because no matter how many women you have it still takes nine months However building a car is parallelizable because you can have one worker working on the engine while another one is working on the interior Usually applications have a limit to how parallelizable they are The more parallelizable 163 Chapter 12 Optimization your application is the better it can take advantage of multiprocessor and clustered computer configurations Statelessness As we ve discussed stateless functions and programs are those that rely entirely on the data explicitly passed to them for functioning Most processes are not entirely stateless but they can be within limits In my e commerce example the function wasn t entirely stateless but it was within the confines of a single day Therefore I optimized it as if it were a stateless function but made allowances for changes at night Two great benefits resulting from statelessness is that m
60. make up the entire operating system GNU Linux For the most part this book will be using the computer s low level assembly language There are essentially three kinds of languages Machine Language This is what the computer actually sees and deals with Every command the computer sees is given as a number or sequence of numbers Assembly Language This is the same as machine language except the command numbers have been replaced by letter sequences which are easier to memorize Other small things are done to make it easier as well High Level Language High level languages are there to make programming easier Assembly language requires you to work with the machine itself High level languages allow you to describe the program in a more natural language A single command in a high level language usually is equivalent to several commands in an assembly language In this book we will learn assembly language although we will cover a bit of high level languages Chapter 2 Computer Architecture Before learning how to program you need to first understand how a computer interprets programs You don t need a degree in electrical engineering but you need to understand some basics Modern computer architecture is based off of an architecture called the Von Neumann architecture named after its creator The Von Neumann architecture divides the computer up into two main parts the CPU for Central Processing Unit and the memory T
61. makes some basic use of that library 104 Chapter 8 Sharing Functions with Code Libraries e Research the use of LD_PRELOAD What is it used for Try building a shared library that contained the exit function and have it write a message to STDERR before exitting Use LD_PRELOAD and run various programs with it What are the results 105 Chapter 8 Sharing Functions with Code Libraries 106 Chapter 9 Intermediate Memory Topics Okay so the last chapter was quite a doozy This may seem overwhelming at first but if you can stick it out you will have the background you need to being a successful programmer How a Computer Views Memory Let s review how memory within a computer works You may also want to re read Chapter 2 A computer looks at memory as a long sequence of numbered storage locations A sequence of millions of numbered storage locations Everything is stored in these locations Your programs are stored there your data is stored there everything Each storage location looks like every other one The locations holding your program are just like the ones holding your data In fact the computer has no idea which are which So we ve seen how numbers are stored each value takes up four storage locations How are the instructions stored Each instruction is a different length Most instructions take up one or two storage locations for the instruction itself and then storage locations for the instruction s argume
62. may use the same title as a previous version if the original publisher of that version gives permission e B List on the Title Page as authors one or more persons or entities responsible for authorship of the modifications in the Modified Version together with at least five of the principal authors of the Document all of its principal authors if it has less than five e C State on the Title Page the name of the publisher of the Modified Version as the publisher e D Preserve all the copyright notices of the Document e E Add an appropriate copyright notice for your modifications adjacent to the other copyright notices e F Include immediately after the copyright notices a license notice giving the public permission to use the Modified Version under the terms of this License in the form shown in the Addendum below 221 Appendix H GNU Free Documentation License e G Preserve in that license notice the full lists of Invariant Sections and required Cover Texts given in the Document s license notice e H Include an unaltered copy of this License e I Preserve the section entitled History and its title and add to it an item stating at least the title year new authors and publisher of the Modified Version as given on the Title Page If there is no section entitled History in the Document create one stating the title year authors and publisher of the Document as given on its Title Page then add an item d
63. memory location 4 would be used In direct addressing we would just load the value 4 but in indirect addressing we use 4 as the address to use to find the data we want Finally there is the base pointer addressing mode This is similar to indirect addressing but you also include a number called the offset to add to the register s value before using it for lookup We will use this mode quite a bit in this book In the Section called Interpreting Memory we discussed having a structure in memory holding customer information Let s say we wanted to access the customer s age which was the eighth byte of the data and we had the address of the start of the structure in a register We could use base pointer addressing and specify the register as the base pointer and 8 as our offset This is a lot like indexed addressing with the difference that the offset is constant and the pointer is held in a register and in indexed addressing the offset is in a register and the pointer is constant There are other forms of addressing but these are the most important ones Review Know the Concepts e Describe the fetch execute cycle e What is a register How would computation be more difficult without registers How do you represent numbers larger than 255 e How big are the registers on the machines we will be using e How does a computer know how to interpret a given byte or set of bytes of memory e What are the addressing modes and what a
64. needing more memory cmpl ebx eax je move_break eax holds the current memory segment being examined and ebx holds the location past the current break Therefore if this condition occurs we need more memory to allocate this space Notice too that this is the case for the first call after allocate_init So let s skip down to move_break and see what happens there move_break add SHEADER_SIZE ebx addl ecx ebx pushl Seax pushl Secx pushl Sebx 122 Chapter 9 Intermediate Memory Topics movl S BRK eax int SLINUX_SYSCALL So when we reach this point in the code sebx holds where we want the next segment of memory to be The size should be the size requested plus the size of our headers So we add those numbers to ebx and that s where we want the program break to be We then push all the registers we want to save on the stack and call the break system call After that we check for errors cmpl 0 eax je error Afterwards we pop the registers back off the stack mark the memory as unavailable record the size of the memory and make sure eax points to the start of usable memory after the headers popl ebx popl ecx popl eax movl SUNAVAILABLE HDR_AVAIL_OFFSET eax movl ecx HDR_SIZE_OFFSET eax add SHEADER_SIZE eax Then we store the new program break and return movl ebx current_break movl
65. number is d 0 section text pushl 88 pushl Stext_string carl printf popl eax popl eax seax is just a dummy variable nothing is actually being done with the value You can also directly re adjust Sesp to the proper location Variables and Assignment Global and static variables are declared using data or bss entries Local variables are declared by reserving space on the stack at the beginning of the function This space is given back at the end of the function 200 Appendix E C Idioms in Assembly Language Interestingly global variables are accessed differently than local variables in assembly language Global variables are accessed using direct addressing while local variables are accessed using base pointer addressing For example consider the following C code int my_global_var int foo int my_local_var my_local_var 1 my_global_var 2 return 0 This would be rendered in assembly language as section data lcomm my_global_var 4 type foo function foo pushl Sebp Save old base pointer movl esp Sebp make stack pointer base pointer subl 4 esp Make room for my_local_var equ my_local_var 4 Can now use my_local_var to find the local variable movl 1 my_local_var ebp movl 2 my_global_var movl ebp esp Clean up function and return popl ebp ret What may not be obvious is that accessing the glob
66. of ecx S CASE_A a The lower boundary of our search CASE_Z 2 The upper boundary of our search cqu UPPER_CONVERSION A a Conversion between upper and lower case ESTACK ST equ ST_BU equ ST_BU 60 UFF HHT FFER_LEN 8 Length of buffer FFER 12 factual buffer Chapter 5 Dealing with Files convert_to_upper pushl Sebp movl esp ebp SET UP VARIABLES movl ST_BUFFER ebp eax mov ST_BUFFER_LEN ebp ebx movl 0 edi if a buffer with zero length was given us just leave cmpl 0 ebx je end_convert_loop convert_loop get the current byte movb Seax edi 1 Scl go to the next byte unless it is between a and 2z cmpb SLOWERCASE_A cl JL next_byte cmpb SLOWERCASE_Z c jg next_byte otherwise convert the byte to uppercase addb SUPPER_CONVERSION cl fand store it back movb cl eax sedi 1 next_byte incl edi next byte cmpl edi ebx continue unless we ve reached th jne convert_loop end_convert_loop no return value just leave movl ebp esp popl ebp ret Type in this program as toupper s and then enter in the following commands as toupper s o toupper o ld toupper o o toupper nd 61 Chapter 5 Dealing with Files This builds a program called toupper which converts all of the lowercase characters in a file to upperc
67. of any later version that has been published not as a draft by the Free Software Foundation If the Document does not specify a version number of this License you may choose any version ever published not as a draft by the Free Software Foundation Addendum To use this License in a document you have written include a copy of the License in the document and put the following copyright and license notices just after the title page Copyright YEAR YOUR NAME Permission is granted to copy distribute and or modify this document under the terms of the GNU Free Documentation License Version 1 1 or any later version published by the Free Software Foundation with the Invariant Sections being LIST THEIR TITLES with the Front Cover Texts being LIST and with the Back Cover Texts being LIST A copy of the license is included in the section entitled GNU Free Documentation License 224 Appendix H GNU Free Documentation License If you have no Invariant Sections write with no Invariant Sections instead of saying which ones are invariant If you have no Front Cover Texts write no Front Cover Texts instead of Front Cover Texts being LIST likewise for Back Cover Texts If your document contains nontrivial examples of program code we recommend releasing these examples in parallel under your choice of free software license such as the GNU General Public License to permit their use in free software 225 Appendix H
68. of each field until they are their defined lengths To build the application run the commands as write records s o write record o as write record s o write record o ld write record o write records o o write records Here we are assembling two files separately and then combining them together using the linker To run the program just type the following write records This will cause a file called test dat to be created containing the records However since they contain non printable characters the null character specifically they may not be viewable by a text editor Therefore we need the next program to read them for us Reading Records Now we will consider the process of reading records In this program we will read each record and display the first name listed with each record Since each person s name is a different length we will need a function to count the number of characters we want to write Since we pad each field with null characters we can simply count characters until we reach a null byte Note that this means our records must contain at least one null byte each Here is the code Put it in a file called count chars s PURPOSE Count the characters until a null byte is reached INPUT The address of the character string OUTPUT Returns the count in eax PROCESS 2 Ifyou have used C this is what the strlen function does 74
69. of the buffer call convert_to_upper popl eax get the size of the read back addl 4 esp move the stack the rest of the way back WRITE THE BLOCK OUT TO THE OUTPUT FILE movl eax edx Ssize of the buffer movl S SYS_WRITE eax movl ST_FD_OUT ebp ebx file to use mov SBUFFER_DATA ecx location of the buffer int SLINUX_SYSCALL 59 Chapter 5 Dealing with Files e Sho 4k 4h E th CONTINUE THE LOOP jmp read_loop_begin nd_loop CLOSE TH E FILES NOTE we err don t need to do error checking on these becaus or conditions don t signify anything special here movl S SYS_CLOSE eax movl ST_FD int SLINU OUT Sebp ebx X_SYSCALL movl S SYS_CLOSE eax movl ST_FD _IN Sebp ebx X_SYSCALL movl S SYS_EXIT eax int SLINU H H EXITHHH movl 0 int SLINU PURPOSE INPUT OUTPUT VARIABLES CONSTANT equ LOWER equ LOWER ebx X_SYSCALL This function actually does the conversion to upper case for a block The first parameter is the location of the block of memory to convert The second parameter is the length of that buffer This function overwrites the current buffer with the upper casified version seax beginning of buffer Sebx length of buffer Sedi current buffer offset Scl current byte being examined first part
70. owner of the file can read write and execute the file the group members can read and execute the file and everyone else can only execute the file Anyway as you can see octal is used to group bits binary digits into threes The way the assembler knows that a number is octal is because octal numbers are prefixed with a zero For example 010 means 10 in octal which is 8 in decimal If you just write 10 that means 10 in decimal The beginning zero is what differentiates the two So be careful not to put any leading zeroes in front of decimal numbers or they will be interepreted as octal numbers Hexadecimal numbers also called just hex use the numbers 1 15 for each digit however since 10 15 don t have their own numbers hexadecimal uses the letters a through f to represent them For example the letter a represents 10 the letter b represents 11 and so on 10 in hexadecimal is 16 in decimal In octal each digit represented three bits In hexadecimal each digit represents 142 Chapter 10 Counting Like a Computer four bits Every two digits is a full byte and eight digits is a 32 bit word So you see it is considerably easier to write a hexadecimal number than it is to write a binary number The most important number to remember in hexadecimal is which means that all bits are set So if I want to set all of the bits of a register to 1 I can just do movl SOxFFFFFFFF eax Which is considerably easier and less error prone
71. prints out an error message to STDOUT when it occurs 66 Chapter 6 Reading and Writing Simple Records As mentioned in Chapter 5 many applications deal with data that is persistent meaning that the data lives longer than the program by being stored on disk inf files You can shut down the program and open it back up and you are back where you started Now there are two basic kinds of persistent data structured and unstructured Unstructured data is like what we dealt with in the toupper program It just dealt with text files that were entered by a person The contents of the files weren t usable by a program because a program can t interpret what the user is trying to say in random text Structured data on the other hand is what computers excel at handling This is data that is divided up into fields and records For the most part the fields and records are fixed length Because the data is divided into fixed length records and fixed format fields the computer can interpret the data Structured data can contain variable length fields but at that point you are usually better off with a database This section deals with reading and writing simple fixed length records Let s say we wanted to store some basic information about people we know We could imagine the following example fixed length record about people e Firstname 40 bytes e Lastname 40 bytes e Address 240 bytes Age 4 bytes In this everything is char
72. program In the case of the maximum program there would only be one value maximum Other programs take more arguments When we run as for example we give it several arguments as maximum s o and maximum o After these we have the stack This is where all of our data goes when we do pushes pops and calls Since the stack is at the top of the memory it grows down esp 2 You may be thinking what if they grow toward each other and overlap Although this is possible it is extremely unlikely because the amount of space in between is huge 3 The NULL character is actually the number 0 not to be confused with the character 0 whose numeric value is not zero Every possible letter symbol or number you can type with your keyboard has a number associated with it These numbers are called ASCII codes We ll deal more with these later 109 Chapter 9 Intermediate Memory Topics always holds the current address of where the next value will be put on the stack It then gets decreased whenever there is a push and increased whenever there is a pop So pushl Seax is equivalent to movl Seax esp subl 4 esp sub1 does subtraction Since eax is four bytes big we have to subtract 4 from esp In the same way popl eax is the same as movl Sesp eax addl 4 esp Now notice on the mov1 we had esp in parenthesis That s because we wanted the value that esp pointed to not the actual address If we
73. program does is start with the base number and store it both as the multiplier stored in ebx and the current value stored in 4 ebp It also has the power stored in ecx It then continually multiplies the current value by the multiplier decreases the power and leaves the loop if the power in secx gets down to 1 By now you should be able to go through the program without help The only things you should need to know is that imul does integer multiplication and stores the result in the second operand and decl decreases the given register by 1 For more information on these and other instructions see Appendix B A good project to try now is to extend the program so it will return the value of a number if the power is 0 hint anything raised to the zero power is 1 Keep trying If it doesn t work at first try going through your program by hand with a scrap of paper keeping track of where sebp and esp are pointing what is on the stack and what the values are in each register Recursive Functions The next program will stretch your brains even more The program will compute the factorial of a number A factorial is the product of a number and all the numbers between it and one For example the factorial of 7 is 7 6 5 4 3 2 1 and the factorial of 4 is 4 3 2 1 Now one thing you might notice is that the factorial of a number is the same as the product of a number and the factorial just below it For example the factorial of 4 is 4
74. repository of reference material on x86 x86 64 and compatible processors http www x86 org Dr Dobb s Journal Microprocessor Resources http www drpaulcarter com pcasm Dr Paul Carter s PC Assembly Language Page http webster cs ucr edu The Art of Assembly Home Page http www intel com design pentium manuals Intel s manuals for their processors http www amd com us en Processors DevelopWithAMD 0 30_2252_739_7203 00 html AMD s processor documentation http www janw easynet be Jan Wagemaker s Linux assembly language examples http www azillionmonkeys com qed asm html Paul Hsieh s x86 Assembly Page 170 Appendix A GUI Programming Introduction to GUI Programming The purpose of this appendix is not to teach you how to do Graphical User Interfaces It is simply meant to show how writing graphical applications is the same as writing other applications just using an additional library to handle the graphical parts As a programmer you need to get used to learning new libraries Most of your time will be spent passing data from one library to another The GNOME Libraries The GNOME projects is one of several projects to provide a complete desktop to Linux users The GNOME project includes a panel to hold application launchers and mini applications called applets several standard applications to do things such as file management session management and configuration and an API for creating application
75. returns It s kind of like a scratch pad of paper You get a new piece of paper every time the function is activated and you have to throw it away when you are finished processing Local variables of a function are not accessible to any other function within a program static variables Static variables are data storage that a function uses while processing that is not thrown away afterwards but is reused for every time the function s code is activated This data is not accessible to any other part of the program Static variables are generally not used unless absolutely necessary as they can cause problems later on global variables Global variables are data storage that a function uses for processing which are managed outside the function For example a simple text editor may put the entire contents of the file it is working on in a global variable so it doesn t have to be passed to every function that operates on it Configuration values are also often stored in global variables 1 This is generally considered bad practice Imagine if a program is written this way and in the next version they decided to allow a single instance of the program edit multiple files Each function would then have to be modified so that the file that was being manipulated would be passed as a parameter If you had simply passed it as a parameter to begin with most of your functions could have survived your upgrade unchanged 36 Chapter 4 All Ab
76. somewhere or the loop will continue forever This condition is called an infinite loop If we accidentally left out step 1 2 or 3 the loop and our program would never end In the next section we will implement this program that we have planned Program planning sounds complicated and it is to some degree When you first start programming it s often hard to convert our normal thought process into a procedure that the computer can understand We often forget the number of temporary storage locations that our minds are using to process problems As you read and write programs however this will eventually become very natural to 21 Chapter 3 Your First Programs you Just have patience Finding a Maximum Value Enter the following program as maximum s PURPOSE This program finds the maximum number of a set of data items 4 VARIABLES The registers have the following uses 4 edi Holds the index of the data item being examined Sebx Largest data item found Seax Current data item The following memory locations are used data_items contains the item data A 0 is used to terminate the data section data data_items These are the data items long 3 67 34 222 45 75 54 34 44 33 22 11 66 0 section text globl _start astart movl 0 edi move 0 into the index register ax load the first byte of data movl eax ebx Since this is the first item
77. speed issues in your program will be Even experienced programmers have trouble predicting which parts of the program will be the bottlenecks which need optimization so you will probably end up wasting your time optimizing the wrong parts the Section called Where to Optimize will discuss how to find the parts of your program that need optimization While you develop your program you need to have the following priorities e Everything is documented e Everything works as documented e The code is written in an modular easily modifiable form Documentation is essential especially when working in groups The proper functioning of the program is essential You ll notice application speed was not anywhere on that list Optimization is not necessary during early development for the following reasons e Minor speed problems can be usually solved through hardware which is often much cheaper than a programmer s time e Your application will change dramatically as you revise it therefore wasting most of your efforts to optimize it e Speed problems are usually localized in a few places in your code finding these is difficult before you have most of the program finished 1 Many new projects often have a first code base which is completely rewritten as developers learn more about the problem they are trying to solve Any optimization done on the first codebase is completely wasted 159 Chapter 12 Optimization Therefore the tim
78. systems such as 1 This is quite a large document You certainly don t need to know everything to get started with this book You simply need to know how to navigate from the command line and how to use an editor like pico emacs or vi or others 2 By GNU Linux distribution I mean an x86 GNU Linux distribution GNU Linux distributions for the Power Macintosh the Alpha processor or other processors will not work with this book Chapter 1 Introduction BSD or other systems However all of the skills learned in this book should be easily transferable to any other system If you do not have access to a GNU Linux machine you can look for a hosting provider who offers a Linux shell account which is a command line only interface to a Linux machine There are many low cost shell account providers but you have to make sure that they match the requirements above i e Linux on x86 Someone at your local GNU Linux User s Group may be able to give you one as well Shell accounts only require that you already have an Internet connection and a telnet program If you use Windows you already have a telnet client just click on start then run then type in telnet However it is usually better to download PuTTY from http www chiart greenend co uk sgtatham putty because Windows telnet has some weird problems There are a lot of options for the Macintosh too NiftyTelnet is my favorite If you don t have GNU Linux and can t fin
79. t give them a name In this program we have several variables a variable for the current maximum number found e a variable for which number of the list we are currently examining called the index e a variable holding the current number being examined In this case we have few enough variables that we can hold them all in registers In larger programs you have to put them in memory and then move them to registers when you are ready to use them We will discuss how to do that later When people start out programming they usually underestimate the number of variables they will need People are not used to having to think through every detail of a process and therefore leave out needed variables in their first programming attempts In this program we are using ebx as the location of the largest item we ve found sedi is used as the index to the current data item we re looking at Now let s talk about what an index is When we read the information from data_items we will start with the first one data item number 0 then go to the second one data item number 1 then the third data item number 2 and so on The data item number is the index of data_items You ll notice that the first instruction we give to the computer is movl 0 edi Since we are using edi as our index and we want to start looking at the first item we load edi with 0 Now the next instruction is tricky but crucial to what we re doing It says mo
80. that it is created with a write only create if doesn t exist truncate if does exist mode It s file descriptor is stored as well Now we get to the main part the read write loop Basically we will read fixed size chunks of data from the input file call our conversion function on it and write it back to the output file Although we are reading fixed size chunks the size of the chunks don t matter for this program we are just operating on straight sequences of characters We could read it in with as little or as large of chunks as we want and it still would work properly The first part of the loop is to read the data This uses the read system call This call just takes a file descriptor to read from a buffer to write into and the size of the buffer i e the maximum number of bytes that could be written The system call returns the number of bytes actually read or end of file the number 0 After reading a block we check eax for an end of file marker If found it exits the loop Otherwise we keep on going After the data is read the convert_to_upper function is called with the buffer we just read in and the number of characters read in the previous system call After this function executes the buffer should be capitalized and ready to write out The registers are then restored with what they had before Finally we issue a write system call which is exactly like the read system call except that it moves the data from
81. that storage onto the stack before calling the function As you can see 155 Chapter 11 High Level Languages it s a lot less work Finally our function returns the number 0 In assembly language we stored our return value in eax but in C we just use the return command and it takes care of that for us The return value of the main function is what is used as the exit code for the program As you can see using compilers and interpreters makes life much easier It also allows our programs to run on multiple platforms more easily In assembly language your program is tied to both the operating system and the hardware platform while in compiled and interpreted languages the same code can usually run on multiple operating systems and hardware platforms For example this program can be built and executed on x86 hardware running Linux Windows UNIX or most other operating systems In addition it can also run on Macintosh hardware running a number of operating systems For more information about C you should also see Appendix E Perl Perl is an interpreted language existing mostly on Linux and UNIX based platforms It actually runs on almost all platforms but you find it most often on Linux and UNIX based ones Anyway here is the Perl version of the program which should be typed into a file named Hello World pl usr bin perl print Hello world n Since Perl is interpreted you don t need to compile or li
82. the next digit is 0 Finally we divide 1 by 2 and get 0 with 1 left over so the next digit to the left is 1 Now there s nothing left so we re done So the number we wound up with is 10001 Previously we converted to binary 11101011001001 to decimal 15049 Let s do the reverse to make sure that we did it right 15049 2 7524 Remaining 1 7524 2 3762 Remaining 0 3762 2 1881 Remaining 0 1881 2 940 Remaining 1 940 2 470 Remaining 0 470 2 235 Remaining 0 235 2 117 Remaining 1 117 2 58 Remaining 1 58 2 29 Remaining 0 29 2 14 Remaining 1 14 2 7 Remaining 0 7 2 3 Remaining 1 3 2 1 Remaining 1 1 2 0 Remaining 1 Then we put the remaining numbers back together and we have the original number Remember the first division remainder goes to the far right so from the bottom up you have 11101011001001 Each digit in a binary number is called a bit which stands for binary digit Computers divide up their memory into storage locations called bytes Each storage location on an x86 processor and most others is 8 bits long Earlier we said that a byte can hold any number between 0 and 255 The reason for this is that the largest number you can fit into 8 bits is 255 You can see this for yourself if you convert binary 11111111 into decimal 132 Chapter 10 Counting Like a Computer 11111111 CA AE 2A o Fr CE 26 A Bn eee O Ree Ae aN e DAB RIE AR ADDY oe EDS ROD Te oe 128 64 32
83. the numeric memory address of every piece of code or data In addition every time you had to insert a piece of data or code you would have to change all the addresses in your program Symbols are used so that the assembler and linker can take care of keeping track of addresses and you can concentrate on writing your program glob1 means that the assembler shouldn t discard this symbol after assembly because the linker will need it start is a special symbol that always needs to be marked with glob1l because it marks the location of the start of the program Without marking this location in this way when the computer loads your program it won t know where to begin running your program The next line _start defines the value of the _start label A label is a symbol followed by a colon Labels define a symbol s value When the assembler is assembling the program it has to assign each data value whether it is still needed or not You should always document any strange behavior your program performs Unfortunately figuring out what is strange and what is straightforward comes mostly with experience 16 Chapter 3 Your First Programs and instruction an address Labels tell the assembler to make the symbol s value be wherever the next instruction or data element will be This way if the actual physical location of the data or instruction changes you don t have to rewrite any references to it the symbol automatically gets th
84. then only move the small part al into the character string popl eax movb al edx Decreasing ecx so we know when we are finished decl ecx Increasing edx so that it will be pointing to the next byte incl edx Check to see if we are finished cmpl 0 ecx If so jump to the end of the function je end_copy_reversing_loop Otherwise repeat the loop jmp copy_reversing_loop end_copy_reversing_loop Done copying Now write a null byte and return movb 0 edx movl ebp esp popl ebp ret To show this used in a full program use the following code along with the count_chars and write_newline functions written about in previous chapters The code should be in a file called conversion program s include linux s section data This is where it will be stored 148 Chapter 10 Counting Like a Computer tmp_buffer ascii O 0 0 0 0 0 0 0 0 0 0 section text globl _start Starti movl esp ebp Storage for the result pushl tmp_buffer Number to convert pushl 824 call integer2number addl 8 esp Get the character count for our system call pushl Stmp_buffer call count_chars addl 4 esp ti The count goes in edx for SYS_WRIT movl S eax Sedx Make the system call movl S SYS_WRITE eax movl S STDOUT ebx movl Stmp_buffer ecx int SLINUX_SYSCALL Write a carriage return pushl SSTDOUT
85. we compare our current value stored in eax to our biggest value so far stored in sebx If the current value is less or equal to our biggest value so far we don t care about it so we just jump back to the beginning of the loop Otherwise we need to record that value as the largest one movl eax ebx jmp start_loop which moves the current value into sebx which we are using to store the current largest value and starts the loop over again Okay so the loop executes until it reaches a 0 when it jumps to loop_exit This part of the program calls the Linux kernel to exit If you remember from the last program when you call the operating system remember it s like signaling Batman you store the system call number in eax 1 for the exit call and store the other values in the other registers The exit call requires that we put our exit status in ebx We already have the exit status there since we are using ebx as our largest number so all we have to do is load eax with the number one and call the kernel to exit Like this movl 1 eax int 0x80 12 The names of these symbols can be anything you want them to be as long as they only contain letters and the underscore character _ The only one that is forced is _start and possibly others that you declare with glob1 However if its a symbol you define and only you use feel free to call it anything you want that is adequately descriptive remember that others will have to modify
86. with try to store them all in registers In high level languages you do not always have this option the compiler decides what goes in registers and what doesn t Inline Functions Functions are great from the point of view of program management they make it easy to break up your program into independent understandable and reuseable parts However function calls do involve the overhead of pushing arguments onto the stack and doing the jumps remember locality of reference your code may be swapped out on disk instead of in memory For high level languages it s often impossible for compilers to do optimizations across function call boundaries However some languages support inline functions or function macros These functions look smell taste and act like real functions except the compiler has the option to simply plug the code in exactly where it was called This makes the program faster but it also increases the size of the code There are also many functions like recursive functions which cannot be inlined because they call themselves either directly or indirectly Optimized Instructions Often times there are multiple assembly language instructions which accomplish the same purpose A skilled assembly language programmer knows which instructions are the fastest However this can change from processor to processor For more information on this topic you need to see the user s manual that is provided for the specific chip
87. 0000000001 This will make the value of the register 1 if my Dad likes dressy clothes and 0 if he doesn t Then we can do a comparison to 1 and print the results The code would look like this NOTE assume that the register ebx holds my Dad s preferences movl ebx eax This copies the information into eax so we don t lose the original data shrl 1 eax This is the shift operator It stands for Shift Right Long This first number is the number of positions to shift and the second is the register to shift andl SObO00000000000000000000000000000001 eax This does the masking cmpl SOb00000000000000000000000000000001 Seax Check to see if the re sult is 1 or 0 je yes_he_likes_dressy_clothes jmp no_he_doesnt_like_dressy_clothes And then we would have two labels which printed something about whether or not he likes dressy clothes and then exits The 0b notation means that what follows is a binary number In this case it wasn t needed because is the same in any numbering system We also didn t need the 31 zeroes but I put them in to make a point that the number you are using is 32 bits When a number represents a set of options for a function or system call the individual true false elements are called flags Many system calls have numerous options that are all set in the same register using a mechanism like we ve described The open
88. 0001 would represent the number 1 and 10000000000000000000000000000001 would represent 1 This makes a lot of sense and in fact some old processors work this way However it has some problems First of all it takes a lot more circuitry to add and subtracted signed numbers Even more so this representation has a problem with the number 0 In this system you could have both a negative and a positive 0 This leads to a lot of questions like should negative zero be equal to positive zero What should the sign of zero be in various circumstances These problems were overcome by using a representation of negative numbers called two s complement representation To get the negative representation of a number in two s complement form you must perform the following steps 1 Perform a NOT operation on the number 2 Add one to the resulting number So to get the negative of 00000000000000000000000000000001 you would first do a NOT operation which gives 11111111111111111111111111111110 and then add one giving 11111111111111111111111111111111 To get negative two first take 00000000000000000000000000000010 The NOT of that number is 11111111111111111111111111111101 Adding one gives 11111111111111111111111111111110 With this representation you can add numbers just as if they were positive and come out with the right answers For example if you add one plus negative one in binary you will notice that all of the numbers flip to zero Als
89. 16 8 4 4 2 4 t15 255 The largest number that you can hold in 16 bits is 65535 The largest number you can hold in 32 bits is 4294967295 4 billion The largest number you can hold in 64 bits is 18 446 744 073 709 551 615 The largest number you can hold in 128 bits is 340 282 366 920 938 463 463 374 607 43 1 768 211 456 Anyway you see the picture For x86 processors most of the time you will deal with 4 byte numbers 32 bits because that s the size of the registers Truth Falsehood and Binary Numbers Now we ve seen that the computer stores everything as sequences of 1 s and 0 s Let s look at some other uses of this What if instead of looking at a sequence of bits as a number we instead looked at it as a set of switches For example let s say there are four switches that control lighting in the house We have a switch for outside lights a switch for the hallway lights a switch for the living room lights and a switch for the bedroom lights We could make a little table showing which of these were on and off like so Outside Hallway Living Room Bedroom On OLE On On It s obvious from looking at this that all of the lights are on except the hallway ones Now instead of using the words On and Off let s use the numbers 1 and 0 1 will represent on and 0 will represent off So we could represent the same information as Outside Hallway Living Room Bedroom 1 0 1 1 Now instead of having labels on the li
90. 2 is the least significant half You can t quite divide it like that for registers since they operate on base 2 rather than base 10 numbers but that s the basic idea For more information on this topic see Chapter 10 31 Chapter 3 Your First Programs What does the linker do How do you check the result status code of the last program you ran What is the difference between movl 1 eaxandmovl 1 eax Which register holds the system call number What are indexes used for Why do indexes usually start at 0 If I issued the command movl data_items edi 4 eax and data_items was address 3634 and edi held the value 13 what address would you be using to move into eax List the general purpose registers What is the difference between mov1 and movb What is flow control What does a conditional jump do What things do you have to plan for when writing a program Go through every instruction and list what addressing mode is being used for each operand Use the Concepts 32 Modify the first program to return the value 3 Modify the maximum program to find the minimum instead Modify the maximum program to use the number 255 to end the list rather than the number 0 Modify the maximum program to use an ending address rather than the number 0 to know when to stop Modify the maximum program to use a length count rather than the number 0 to know when to stop What would the instruction movl _start eax d
91. C Depending on the system call other registers may have to have values in them as well Note that system calls is not the only use or even the main use of registers It is just the one we are dealing with in this first program Later programs will use registers for regular computation The operating system however usually needs more information than just which call to make For example when dealing with files the operating system needs to know which file you are dealing with what data you want to write and other details The extra details called parameters are stored in other registers In the case of the exit system call the operating system requires a status code be loaded in ebx This value is then returned to the system This is the value you retrieved when you typed echo So we load ebx with 0 by typing the following movl 0 ebx Now loading registers with these numbers doesn t do anything itself Registers are used for all sorts of things besides system calls They are where all program logic such as addition subtraction and comparisons take place Linux simply requires that certain registers be loaded with certain parameter values before making a system call eax is always required to be loaded with the system call number For the other registers however each system call has different requirements In the exit system call ebx is required to be loaded with the exit status We will 4 You may be wondering why do al
92. Programming from the Ground Up Jonathan Bartlett Edited by Dominick Bruno Jr Programming from the Ground Up by Jonathan Bartlett Edited by Dominick Bruno Jr Copyright 2003 by Jonathan Bartlett Permission is granted to copy distribute and or modify this document under the terms of the GNU Free Documentation License Version 1 1 or any later version published by the Free Software Foundation with no Invariant Sections with no Front Cover Texts and with no Back Cover Texts A copy of the license is included in Appendix H In addition you are granted full rights to use the code examples for any purpose without even having to credit the authors This book can be purchased in paperback at http www bartlettpublishing com This book is not a reference book it is an introductory book It is therefore not suitable by itself to learn how to professionally program in x86 assembly language as some details have been left out to make the learning process smoother The point of the book is to help the student understand how assembly language and computer programming works not to be a definitive reference to the subject To receive a copy of this book in electronic form please visit the website http savannah nongnu org projects pgubook This site contains the instructions for downloading a transparent copy as defined by the GNU FDL of this book Table of Contents 1 Introduction sicouies eeisascetegcs Sadvisats Satei ss tac
93. SE This program is to demonstrate how to call printf section data This string is called the format string It s the first parameter and printf uses it to find out how many parameters it was given and what kind they are firststring ascii Hello s is a s who loves the number d n 0 name ascii Jonathan 0 personstring ascii person o This could also have been an equ but we decided to give it a real memory location just for kicks numberloved long 3 section text globl _start start note that the parameters are passed in the reverse order that they are listed in the function s prototype pushl numberloved This is the Sd pushl Spersonstring This is the second s This is the first s pushl Sname pushl S firststring This is the format string in the prototype call printf pushl 0 call exit Type it in with the filename print f example s and then do the following commands 98 Chapter 8 Sharing Functions with Code Libraries as printf example s 0o printf example o ld printf example o o printf example lc dynamic linker lib 1ld linux so 2 Then run the program with printf example and it should say this Hello Jonathan is a person who loves the number 3 Now if you look at the code you ll see that we actually push the format string last even though it s the first argument You
94. We load eax with the system call number 45 in this case and load ebx with the new breakpoint Then you call int 0x80 to signal Linux to do its work Linux will do its thing and then return either 0 if there is no memory left or the new break point in eax The new break point might actually be larger than what you asked for because Linux rounds up to the nearest page The problem with this method is keeping track of the memory Let s say I need to move the break to have room to load a file and then need to move a break again to load another file Later let s say you get rid of the first file You now have a giant gap in memory that s mapped but you aren t using If you continue to move the break for each file you load you can easily run out of memory So what you need is a memory manager A memory manager consists of two basic functions allocate and deallocate A memory manager usually also has an initialization function Note that the function names might not be allocate and deallocate but that the functionality will be the same Whenever you need a certain amount of memory you can simply tell allocate how much you need and it will give you back an address to the memory When you re done with it you tell deallocate that you are through with it Then allocate will be able to reuse the memory This minimizes the number of holes in your memory making sure that you are making the best use of it you can FIXME what about talking
95. a extending memory mapping to disk For example let s say you only have 16 Megabytes of physical memory Let s also say that 8 Megabytes are being used by Linux and some basic applications and you want to run a program that requires 20 Megabytes of memory Can you The answer is yes if you have set up a swap file or partition What happens is that after all of your remaining 8 Megabytes of physical memory have been mapped into virtual memory Linux starts mapping parts of your disk into memory So if you access a memory location in your program that location may not actually be in memory at all but on disk When you access the memory Linux notices that the memory is on disk and moves that portion of the disk back into physical memory and moves another part of physical memory back onto the disk So not only can Linux have a virtual address map to a different physical address it can also move those mappings around as needed Memory is separated out into groups called pages When running Linux on x86 processors a page is four thousand ninety six bytes of memory All of the memory mappings are done a page at a time What this means to you is that whenever you are programming try to keep most memory accesses within the same basic range of memory so you will only need a page or two of memory Otherwise Linux will have to keep moving pages on and off of disk to keep up with you Disk access is slow so this can really slow down your program
96. a simple program to illustrate these concepts The program will take two files and read from one convert all of its lower case letters to upper case and write to the other file Before we do so let s think about what we need to do to get the job done e Have a function that takes a block of memory and converts it to upper case This function would need an address of a block of memory and its size as parameters e Have a section of code that repeatedly reads in to a buffer calls our conversion function on the buffer and then writes the buffer back out to the other file e Begin the program by opening the necessary files Notice that I ve specified things in reverse order that they will be done That s a useful trick in writing complex programs first decide the meat of what is being done In this case it s converting blocks of characters to upper case Then you think about what all needs to happen to get that done In this case you have to open files and continually read and write blocks to disk One of the keys of programming is continually breaking down problems into smaller and smaller chunks until it s small enough that you can easily solve the problem Then you can build these chunks back up until you have a working program You may have been thinking that you will never remember all of these numbers being thrown at you the system call numbers the interrupt number etc In this program we will also introduce a new dire
97. ached to each storage location is called it s address The size of a single storage location is called a byte On x86 processors a byte is a number between 0 and 255 You may be wondering how computers can display and use text graphics and even large numbers when all they can do is store numbers between 0 and 255 First of all specialized hardware like graphics cards have special interpretations of each number When displaying to the Chapter 2 Computer Architecture screen the computer uses ASCII code tables to translate the numbers you are sending it into letters to display on the screen with each number translating to exactly one letter or numeral For example the capital letter A is represented by the number 65 The numeral is represented by the number 49 So to print out HELLO you would actually give the computer the sequence of numbers 72 69 76 76 79 To print out the number 100 you would give the computer the sequence of numbers 49 48 48 A list of ASCII characters and their numeric codes is found in Appendix D In addition to using numbers to represent ASCII characters you as the programmer get to make the numbers mean anything you want them to as well For example if I am running a store I would use a number to represent each item I was selling Each number would be linked to a series of other numbers which would be the ASCII codes for what I wanted to display when the items were scanned in I would have more number
98. acter data except for the age which is simply a numeric field using a standard 4 byte word we could just use a single byte for this but keeping it at a word makes it easier to process In programming you often have certain definitions that you will use over and over again within the program or perhaps within several programs It is good to separate these out into files that are simply included into the assembly language files as needed For example in our next programs we will need to access the different parts of the record above This means we need to know the offsets of each field from the beginning of the record in order to access them using base pointer addressing The following constants describe the offsets to the above structure Put them in a file named record def s 1 A database is a program which handles persistent structured data for you You don t have to write the programs to read and write the data to disk to do lookups or even to do basic processing It is a very high level interface to structured data which although it adds some overhead and additional complexity is very useful for complex data processing tasks References for learning how databases work are listed in Chapter 13 67 Chapter 6 Reading and Writing Simple Records equ RECORD_FIRSTNAME 0 equ RECORD_LASTNAME 40 equ RECORD_ADDRESS 80 equ RECORD_AGE 320 equ RECORD_SIZE 324 In addition there are several c
99. actorial One thing to remember is that after a function call we can never know what the registers are except esp and ebp So even though we had the value we were called with in seax it s not there any more Therefore we need pull it off the stack from the same place we got it the first time at 8 Sebp So we do this movl 8 ebp ebx Now we want to multiply that number with the result of the factorial function If you remember our previous discussion the result of functions are left in seax So we need to multiply sebx with eax This is done with this instruction imul ebx eax This also stores the result in eax which is exactly where we want the return value for the function to be Since the return value is in place we just need to leave the function If you remember at the start of the function we pushed ebp and moved esp into ebp to create the current stack frame Now we reverse the operation to destroy the current stack frame and reactivate the last one end_factorial movl sebp sesp popl sebp Now we re already to return so we issue the following command 50 Chapter 4 All About Functions ret This pops the top value off of the stack and then jumps to it If you remember our discussion about call we said that ca11 first pushed the address of the next instruction onto the stack before it jumped to the beginning of the function So here we pop it back off so we can return there The function is
100. actually being phased out in favor of an international standard known as Unicode which allows you to display any character from any known writing system in the world As you may have noticed ASCII only has support for English characters Unicode is much more complicated however because it requires more than one byte to encode a single character There are several different methods for encoding Unicode characters The most common is UTF 8 and UTF 32 UTF 8 is somewhat backwards compatible with ASCII it is stored the same for English characters but expands into multiple byte for international characters UTF 32 simply requires four bytes for each character rather than one Windows uses UTF 16 which is a variable length encoding which requires at least 2 bytes per character so it is not backwards compatible with ASCII A good tutorial on internationalization issues fonts and Unicode is available in a great Article by Joe Spolsky called The Absolute Minimum Every Software Developer Absolutely Positively Must Know About Unicode and Character Sets No Excuses available online at 197 Appendix D Table of ASCII Codes http www joelonsoftware com articles Unicode html 198 Appendix E C Idioms in Assembly Language This appendix is for C programmers learning assembly language It is meant to give a general idea about how C constructs can be implemented in assembly language lf Statement In C an if statement consists of three pa
101. ake exit This program is much more dynamic in that it is directed by data Depending on what data it receives it will follow different instruction paths In this program this will be accomplished by two different instructions the conditional jump and the unconditional jump The conditional jump changes paths based on the results of a previous comparison or calculation The unconditional jump just goes directly to a different path no matter what The unconditional jump may seem useless but it is very necessary since all of the instructions will be laid out on a line If a path needs to converge back to the main path it will have to do this by an unconditional jump We will see more of both of these jumps in the next section Another use of flow control is in implementing loops A loop is a piece of program code that is meant to be repeated In our example the first part of the program setting the current position to 0 and loading the current highest value with the current value was only done once so it wasn t a loop However the next part is repeated over and over again for every number in the list It is only left when we have come to the last element indicated by a zero This is called a loop because it occurs over and over again It is implemented by doing unconditional jumps to the beginning of the loop at the end of the loop which causes it to start over However you have to always remember to have a conditional jump to exit the loop
102. al variable takes fewer machine cycles than accessing the global variable However that may not matter because the stack is more likely to be in physical RAM instead of swap than the global variable is Also note that after loading a value into a register that value will likely stay in that register until that register is needed for something else It may also move registers For example if you have a variable foo it may start on the stack but the compiler will eventually move it into registers for 201 Appendix E C Idioms in Assembly Language processing If there aren t many variables in use the value may simply stay in the register until it is needed again Otherwise when that register is needed for something else the value if it s changed is copied back to its corresponding memory location In C you can use the keyword volatile to make sure all modifications and references to the variable are done to the memory location itself rather than a register copy of it in case other processes threads or hardware may be modifying the value while your function is running Loops Loops work a lot like if statements in assembly language the blocks are formed by jumping around In C a while loop consists of a loop body and a test to determine whether or not it is time to exit the loop A for loop is exactly the same with optional initialization and counter increment sections These can simply be moved around to make a while loop
103. always push the arguments in reverse order You may be wondering how the printf function knows how many arguments there are Well it searches through your string and counts how many ds and ss it finds and then grabs that number of arguments from the stack If the argument matches a a it treats it as a number and if it matches a s it treats it as a pointer to a null terminated string printf has many more features than this but these are the most used ones So as you can see printf can make output a lot easier but it also has a lot of overhead because it has to count the number of characters in the string look through it for all of the control characters it needs to replace pull them off the stack convert them to a suitable representation numbers have to be converted to strings etc and stick them all together appropriately We ve seen how to use the C prototypes to call library functions To use them effectively however you need to know several more of the possible data types for reading functions Here are the main ones int An int is an integer number 4 bytes on x86 processor long A long is also an integer number 4 bytes on an x86 processor long long A long long is an integer number that s larger than a long 8 bytes on an x86 processor short A short is an integer number that s shorter than an int 2 bytes on an x86 processor 1 The reason that arguments are pushed in the reverse order is bec
104. amed string the name isn t important except to use for talking about it which has a type char char means that it wants a character The after it means that it doesn t actually want a character as an argument but instead it wants the address of a character or sequence of characters If you look back at our helloworld program you will notice that the function call looked like this pushl Shello call printf So we pushed the address of the hello string rather than the actual characters You might notice that we didn t push the length of the string The way that print found the end of the string was because we ended it with a null character 0 Many functions work that way especially C language functions The int before the function definition tell what type of value the function will return in eax when it returns printf will return an int when it s through Now after the char string we have a series of periods This means that it can take an 97 Chapter 8 Sharing Functions with Code Libraries indefinite number of additional arguments after the string Most functions can only take a specified number of arguments print f however can take many It will look into the string parameter and everywhere it sees the characters s it will look for another string from the stack to insert and everywhere it sees d it will look for a number from the stack to insert This is best described using an example T PURPO
105. ams that run multiple files Chapter 8 has additional information on what this is used for Since this program is only in one file it would work just the same with this left out however it is good practice After that we define the value of the power label power As mentioned previously this defines the symbol power to be the address where the instructions following the label begin This is how call power works It transfers control to this spot of the program The difference between call and jmp is that call also pushes the return address onto the stack so that the function can return while the jmp does not Next we have our instructions to set up our function 44 Chapter 4 All About Functions pushl Sebp movl esp ebp subl 4 esp At this point our stack looks like this Base Number lt 12 ebp Power lt 8 S ebp Return Address lt 4 ebp Old ebp lt Sebp Current result lt 4 ebp and esp Although we could use a register for temporary storage this program uses a local variable in order to show how to set it up Often times there just aren t enough registers to store everything so you have to offload them into local variables Other times your function will need to call another function and send it a pointer to some of your data You can t have a pointer to a register so you have to store it in a local variable in order to send a pointer to it Basically what the
106. and writing the files we must open them The UNIX open system call is what handles this It takes the following parameters e eax contains the system call number as usual 5 in this case e ebx contains a pointer to a string that is the name of the file to open The string must be terminated with the null character e ecx contains the options used for opening the file These tell Linux how to open the file They can indicate things such as open for reading open for writing open for reading and writing create if it doesn t exist delete the file if it already exists etc We will not go into how to create the numbers for the options until the Section called Truth Falsehood and Binary Numbers in Chapter 10 For now just trust the numbers we come up with e edx contains the permissions that are used to open the file This is used in case the file has to be created first so Linux knows what permissions to create the file with These are expressed in octal just like regular UNIX permissions After making the system call the file descriptor of the newly opened file is stored in eax So what files are we opening In this example we will be opening the files specified on the command line Fortunately command line parameters are already stored by Linux in an easy to access location and are already null terminated When a Linux program begins all pointers to command line arguments are stored on the stack The number of arguments is st
107. app_new Creates a new application window and returns a pointer to it Takes the application id and the window title as arguments gtk_button_new_with_label Creates a new button and returns a pointer to it Takes one argument the text that is in the button 177 Appendix A GUI Programming gnome_app_set_contents This takes a pointer to the gnome application window and whatever widget you want a button in this case and makes the widget be the contents of the application window gtk_widget_show This must be called on every widget created application window buttons text entry boxes etc in order for them to be visible However in order for a given widget to be visible all of it s parents must be visible as well gtk_signal_connect This is the function that connects widgets and their signal handling callback functions This function takes the widget pointer the name of the signal the callback function and an extra data pointer After this function is called any time the given event is triggered the callback will be called with the widget that produced the signal and the extra data pointer In this application we don t use the extra data pointer so we just set it to NULL which is 0 gtk_main This function causes GNOME to enter into it s main loop To make application programming easier GNOME handles the main loop of the program for us GNOME will check for events and call the appropriate callback functions w
108. are what lead to what is known as DLL hell However it is generally assumed that the advantages outweigh the disadvantages 93 Chapter 8 Sharing Functions with Code Libraries In programming these shared code files are referred to as shared libraries shared objects dynamic link libraries DLLs or so files We will refer to them as shared libraries Using a Shared Library The program we will examine here is simple it writes the characters hello world to the screen and exits The regular program hel loworld nolib s looks like this T PURPOSE This program writes the message hello world and exits include linux s section data helloworld ascii hello world n helloworld_end cqu helloworld_len helloworld_end helloworld section text globl _start stare movl S STDOUT ebx movl Shelloworld ecx movl Shelloworld_len edx movl S SYS_WRITE eax int SLINUX_SYSCALL movl 0 ebx movl S SYS_EXIT eax int SLINUX_SYSCALL That s not too long However take a look at how short he lloworld 1ib is which uses a library T PURPOSE This program writes the message hello world and exits 94 Chapter 8 Sharing Functions with Code Libraries section data helloworld ascii hello world n 0 section text globl _start Stark pushl Shelloworld
109. as binary The nice thing about base two is that the basic math tables are very short In base ten the multiplication tables are ten columns wide and ten columns tall In base two it is very simple Table of binary addition 0 i o i eO 1 1 10 Table of binary multiplication OI O 0 1 So let s add the numbers 10010101 with 1100101 10010101 1100101 11111010 Now let s multiply them 10010101 1100101 1001010 00000000 00000000 10010101 10010101 130 Chapter 10 Counting Like a Computer 11101011001001 Conversions Between Binary and Decimal Let s learn how to convert numbers from binary base two to decimal base ten This is actually a rather simple process If you remember each digit stands for some grouping of two So we just need to add up what each digit represents and we will have a decimal number Take the binary number 10010101 To find out what it is in decimal we take it apart like this Individual units 2 0 0 groups of 2 2 1 1 group of 4 2 2 0 groups of 8 2 3 1 group of 16 2 4 0 groups of 32 2 5 0 groups of 64 2 6 1 group of 128 2 7 1 0 0 1 0 1 0 1 and then we add all of the pieces together like this EAT 2Z 8 te OO A OS 2h ts PAG OPEB eh DEA ee ORS ae Ld 128 16 4 1 149 So 10010101 in binary is 149 in decimal Le
110. ase For example to convert the file toupper s to uppercase type in the command toupper toupper s toupper uppercase and you will find in the file toupper uppercase an uppercase version of your original file Let s examine how the program works The first section of the program is marked CONSTANTS In programming a constant is a value that is assigned when a program assembles or compiles and is never changed I make a habit of placing all of my constants together at the beginning of the program It s only necessary to declare them before you use them but putting them all at the beginning makes them easy to find Making them all upper case makes it obvious in your program which values are constants and where to find them In assembly language we declare constants with the equ directive as mentioned before Here we simply give names to all of the standard numbers we ve used so far like system call numbers the syscall interrupt number and file open options The next section is marked BUFFERS We only use one buffer in this program which we call BUFFER_DATA We also define a constant BUFFER_SIZE which holds the size of the buffer If we always refer to this constant rather than typing out the number 500 whenever we need to use the size of the buffer if it later changes we only need to modify this value rather than having to go through the entire program and changing all of the values individually
111. ause of functions which take a variable number of arguments like printf The arguments pushed in last will be in a known position relative to the top of the stack The program can then use these arguments to determine where on the stack the additional arguments are and what type they are If we pushed the known arguments first you wouldn t be able to tell where they were on the stack 99 Chapter 8 Sharing Functions with Code Libraries char A char is a single byte integer number This is mostly used for storing character data since ASCII strings usually are represented with one byte per character float A float is a floating point number 4 bytes on an x86 processor Floating point numbers will be explained in more depth in the Section called Floating point Numbers in Chapter 10 double A double is a floating point number that is larger than a float 8 bytes on an x86 processor Like floats it only represent approximate values unsigned unsigned is a modifier used for any of the above types which keeps them from being used as signed quantities The difference between signed and unsigned numbers will be discussed in Chapter 10 An asterisk is used to denote that the data isn t an actual value but instead is a pointer to a location holding the given value 4 bytes on an x86 processor So let s say in memory location my_location you have the number 20 stored If the prototype said to pass an int you would use direc
112. be free in the same sense It complements the GNU General Public License which is a copyleft license designed for free software We have designed this License in order to use it for manuals for free software because free software needs free documentation a free program should come with manuals providing the same freedoms that the software does But this License is not limited to software manuals it can be used for any textual work regardless of subject matter or whether it is published as a printed book We recommend this License principally for works whose purpose is instruction or reference 1 APPLICABILITY AND DEFINITIONS This License applies to any manual or other work that contains a notice placed by the copyright holder saying it can be distributed under the terms of this License The Document below refers to any such manual or work Any member of the public is a licensee and is addressed as you A Modified Version of the Document means any work containing the Document or a portion of it either copied verbatim or with modifications and or translated into another language A Secondary Section is a named appendix or a front matter section of the Document that deals exclusively with the relationship of the publishers or authors of the Document to the Document s overall subject or to related matters and contains nothing that could fall directly within that overall subject
113. buffer call read_record addl 8 esp 80 Chapter 6 Reading and Writing Simple Records Returns the number of bytes read If it isn t the same number we requested then it s either an end of file or an error so we re quitting cmpl SRECORD_SIZE eax jne loop_end Increment the age incl record_buffer RECORD_AGI Write the record out pushl ST_OUTPUT_DESCRIPTOR ebp pushl Srecord_buffer call write_record addl 8 esp jmp loop_begin loop_end movl S SYS_EXIT eax movl 0 ebx int SLINUX_SYSCALL You can type it in as add year s To build it type the following as add year s o add year o ld add year o read record o write record o o add year To run the program just type in the following add year This will add a year to every record listed in test dat and write the new records to the file testout dat As you can see writing fixed length records is pretty simple You only have to read in blocks of data to a buffer process them and write them back out Unfortunately this program doesn t write the new ages out so you can verify your program s effectiveness This is because we won t get to 4 This assumes that you have already built the object files read record o and write record o in the previous examples If not you will have to do so 5 This is assuming yo
114. but these are the important ones The Program Status Register We ve seen how bits on a register can be used to give the answers of yes no and true false statements On your computer there is a register called the program status register This register holds a lot of information about what happens in a computation For example have you ever wondered what would happen if you added two numbers and the result was larger than would fit in a register The program status register has a flag called the overflow flag You can test it to see if the last computation overflowed the register There are flags for a number of different statuses 139 Chapter 10 Counting Like a Computer In fact when you do a compare cmp 1 instruction the result is stored in this register The jump instructions 4ge jne etc use these results to tell whether or not they should jump jmp the unconditional jump doesn t care what is in the status register since it is unconditional Let s say you needed to store a number larger than 32 bits So let s say the number is 2 registers wide or 64 bits How could you handle this If you wanted to add two 64 bit numbers you would add the least significant registers first Then if you detected an overflow you could add 1 to the most significant register before adding them In fact this is probably the way you learned to do decimal addition If the result in one column is more than 9 you simply carried the number to t
115. bytes long long VALUES Takes a sequence of numbers separated by commas and inserts those numbers as 4 byte words right where they are in the program rept COUNT Repeats everything between this directive and the endr directives the number of times Specified section SECTION NAME Switches the section that is being worked on Common sections include t ext for code data for data embedded in the program itself and bss for uninitialized global data type SYMBOL function Tells the linker that the given symbol is a function 191 Appendix B Common x86 Instructions Differences in Other Syntaxes and Terminology The syntax for assembly language used in this book is known at the AT amp T syntax It is the one supported by the GNU tool chain that comes standard with every Linux distribution However the official syntax for x86 assembly language known as the Intel syntax is different It is the same assembly language for the same platform but it looks different Some of the differences include e In Intel syntax the operands of instructions are often reversed The destination operand is listed before the source operand e In Intel syntax registers are not prefixed with the percent sign e In Intel syntax a dollar sign is not required to do immediate mode addressing Instead non immediate addressing is accomplished by surrounding the address with b
116. command be to generate the final executable What is typedef for What are structs for What is the difference between a data element of type int and int How would you access them differently in your program If you had a object file called foo o what would be the command to create a shared library called bar What is the purpose of LD_LIBRARY_PATH Use the Concepts Rewrite one or more of the programs from the previous chapters to print their results to the screen using printf rather than returning the result as the exit status code Also make the exit status code be 0 Use the factorial function you developed in the Section called Recursive Functions in Chapter 4 to make a shared library Then re write the main program so that it links with the library dynamically Rewrite the program above so that it also links with the c library Use the c library s printf function to display the result of the factorial call Rewrite the toupper program so that it uses the c library functions for files rather than system calls Going Further Make a list of all the environment variables used by the GNU Linux dynamic linker Research the different types of executable file formats in use today and in the history of computing Tell the strengths and weaknesses of each What kinds of programming are you interested in graphics databbases science etc Find a library for working in that area and write a program that
117. ctive equ which should help out equ allows you to assign names to numbers For 4 Maureen Sprankle s Problem Solving and Programming Concepts is an excellent book on the problem solving process applied to computer programming 56 Chapter 5 Dealing with Files example if you did equ LINUX_SYSCALL 0x80 any time after that you wrote LINUX_SYSCALL the assembler would substitue 0x80 for that So now you can write i nt SLINUX_SYSCALL which is much easier to read and much easier to remember Coding is complex but there are a lot of things we can do like this to make it easier Here is the program Note that we have more labels than we actually use for jumps because some of them are just there for clarity Try to trace through the program and see what happens in various cases An in depth explanation of the program will follow 4 4 4 4 se SE PURPOSE Gd PROCESSING This program converts an input file to an output file with all letters converted to uppercase Open the input file 2 Open the output file 4 While we re not at the end of the input file a read part of the file into our piece of memory b go through each byte of memory if the byte is a lower case letter convert it to uppercase c write the piece of memory to the output file section data system call CONSTANTS
118. d and return the number of the button that the user clicked on The button number is based on the order the buttons were pushed on in the gnome_message_box_new function pushl Seax call gnome_dialog_run_and_close addl 4 esp 176 Appendix A GUI Programming Button 0 is the Yes button If this is the button they clicked on tell GNOME to quit it s event loop Otherwise do nothing cmpl 0 eax jne click_handler_end call gtk_main_quit click_handler_end leave ret To build this application execute the following commands as gnome example s 0o gnome example o gcc gnome example o gnome config libs gnomeui o gnome example Then type in gnome example to run it This program like most GUI programs makes heavy use of passing pointers to functions as parameters In this program you create widgets with the GNOME functions and then you set up functions to be called when certain events happen These functions are called callback functions All of the event processing is handled by the function gtk_main so you don t have to worry about how the events are being processed All you have to do is have callbacks set up to wait for them Here is a short description of all of the GNOME functions that were used in this program gnome_init Takes the command line arguments argument count application id and application version and initializes the GNOME libraries gnome_
119. d a shell account service then you can download Knoppix from http www knoppix org Knoppix is a GNU Linux distribution that boots from CD so that you don t have to actually install it Once you are done using it you just reboot and remove the CD and you are back to your regular operating system So what is GNU Linux GNU Linux is an operating system modeled after UNIX The GNU part comes from the GNU Project http www gnu org which includes most of the programs you will run including the GCC tool set that we will use to program with The GCC tool set contains all of the programs necessary to create programs in various computer languages Linux is the name of the kernel The kernel is the core part of an operating system that keeps track of everything The kernel is both an fence and a gate As a gate it allows programs to access hardware in a uniform way Without the kernel you would have to write programs to deal with every device model ever made The kernel handles all device specific interactions so you don t have to It also handles file access and interaction between processes For example when you type your typing goes through several programs before it hits your editor First the kernel is what handles your hardware so it is the first to receive notice about the keypress The keyboard sends in scancodes to the kernel which then converts them to the actual letters numbers and symbols they represent If you are using a windowi
120. d output file e Reads records from the input e Increments the age e Writes the new record to the output file Like most programs we ve encountered recently this program is pretty straightforward include linux s include record def s section data input_file_name 3 You will find that after learning the mechanics of programming most programs are pretty straightforward once you know exactly what it is you want to do Most of them initialize data do some processing in a loop and then clean everything up 79 Chapter 6 Reading and Writing Simple Records ascii test dat 0 output_file_name testout dat 0 ascil secti lcomm record_buffer Stack offsets of local equ ST_INPUT_DESCRIPTOR 4 on bss RECORD_SIZE variables cequ ST_OUTPUT_DESCRIPTOR secti on text globl _start start 8 variables Copy stack pointer and make room for local PTOR ebp movl esp ebp subl 8 esp Open file for reading movl S SYS_OPEN eax movl Sinput_file_name ebx movl 0 ecx movl 0666 edx int SLINUX_SYSCALL movl eax ST_INPUT_DESCRI Open file for writing movl S SYS_OPEN eax movl Soutput_file_name ebx mov 0101 ecx movl 0666 edx int SLINUX_SYSCALL movl eax ST_OUTPUT_DESCRIPTOR ebp loop_begin pushl ST_INPUT_DESCRIPTOR Sebp pushl Srecord_
121. device files It s actual usage varies based on the type of file or device your descriptor references A more complete listing of system calls along with additional information is available at http www xhp in berlin de Ihpsyscal html You can also get more information about a system call by typing inman 2 SYSCALLNAME which will return you the information about the system call from section 2 of the UNIX manual However this refers to the usage of the system call from the C programming language and may or may not be directly helpful For information on how System Calls are implemented on Linux see the Linux Kernel 2 4 Internals section on how system calls are implemented at http www faqs org docs kernel_2_4 lki 2 html ss2 11 196 Appendix D Table of ASCII Codes To use this table simply find the character or escape that you want the code for and add the number on the left and the top Table D 1 Table of ASCII codes in decimal 0 1 2 3 4 5 6 7 NUL SOH STX ETX EOT ENQ ACK BEL 8 BS HT LF VT FF CR SO SI 16 DLE DCI DC2 DC3 DC4 NAK SYN ETB 24 CAN EM SUB ESC FS GS RS US B2 o amp 40 f 48 0 1 2 3 4 5 6 j 56 8 9 lt gt 64 A B C D E F G 72 H I J K L M N O 80 P Q R S T U V W 88 X Y Z A us 96 a b c d e f g 104 h i j k m n o 112 p q r S t u v w 120 X y Z l DEL ASCII is
122. done and we have our answer Like our previous program you should look over the program again and make sure you know what everything does Looking back through this section and the previous sections for the explanation of anything you don t understand Then take a piece of paper and go through the program step by step keeping track of what the values of the registers are at each step and what values are on the stack Doing this should deepen your understanding of what is going on Review Know the Concepts e What are primitives e What are calling conventions e What is the stack e How do push1 and pop affect the stack What special purpose register do they affect e What are local variables and what are they used for e Why are local variables so necessary in recursive functions e What are ebp and esp used for e What is a stack frame Use the Concepts e Write a function called square which receives argument and returns the square of that argument e Write a program to test your square function e Convert the maximum program given in the Section called Finding a Maximum Value in Chapter 3 so that it is a function which takes a pointer to several values and returns their 51 Chapter 4 All About Functions maximum Write a program that calls maximum with 3 different lists and returns the result of the last one as the program s exit status code Explain the problems that would arise without a standard cal
123. e are many jump statements that you can use je Jump if the values were equal jg Jump if the second value was greater than the first value jge Jump if the second value was greater than or equal to the first value jl Jump if the second value was less than the first value jle Jump if the second value was less than or equal to the first value 11 notice that the comparison is to see if the second value is greater than the first I would have thought it the other way around You will find a lot of things like this when learning programming It occurs because different things make sense to different people Anyway yov ll just have to memorize such things and go on 27 Chapter 3 Your First Programs jmp Jump no matter what This does not need to be preceeded by a comparison The complete list is documented in Appendix B In this case we are jumping if eax holds the value of zero If so we are done and we go to loop_exit If the last loaded element was not zero we go on to the next instructions incl edi movl data_items edi 4 eax If you remember from our previous discussion edi contains the index to our list of values in data_items incl increments the value of sedi by one Then the mov is just like the one we did beforehand However since we already incremented edi eax is getting the next value from the list Now eax has the next value to be tested So let s test it cmpl ebx eax jle start_loop Here
124. e conditions 209 Appendix F Using the GDB Debugger There is absolutely no warranty for GDB Type show warranty for details This GDB was configured as i386 redhat linux gdb Depending on which version of GDB you are running this output may vary slightly At this point the program is loaded but is not running yet The debugger is waiting your command To run your program just type in run This will not return because the program is running in an infinite loop To stop the program hit control c The screen will then say this Starting program home johnnyb BartlettPublishing Books PGU Move maximum Program received signal SIGINT Interrupt start_loop at maximum s 34 34 movl data_items edi 4 eax Current language auto currently asm gdb This tells you that the program was interrupted by the SIGINT signal from your control c and was within the section labelled start_loop and was executing on line 34 when it stopped It gives you the code that it is about to execute Depending on exactly when you hit control c it may have stopped on a different line or a different instruction than the example One of the best ways to find bugs in a program is to follow the flow of the program to see where it is branching incorrectly To follow the flow of this program keep on entering stepi for step instruction which will cause the computer to execute one instruction at a time If you do t
125. e different approaches but a good programmer takes all of them into account The bottom up approaches help you understand the machine aspects the top down approaches help you understand the problem area aspects and the middle approaches help you with practical questions and answers To leave any of these aspects out would be a mistake Computer Programming is a vast subject As a programmer you will need to be prepared to be constantly learning and pushing your limits These books will help you do that They not only teach their subjects but also teach various ways and methods of thinking As Alan Perlis said A language that doesn t affect the way you think about programming is not worth knowing http www cs yale edu homes perlis alan quotes html If you are constantly looking for new and better ways of doing and thinking you will make a successful programmer If you do not seek to enhance yourself A little sleep a little slumber a little folding of the hands to rest and poverty will come on you like a bandit and scarcity like an armed man Proverbs 24 33 34 NIV Perhaps not quite that severe but still it s best to always be learning These books were selected because of their content and the amount of respect they have in the computer science world Each of them brings something unique There are many books here The best way to start would be to look through online reviews of several of the books and find a starting point that
126. e for this function These two lines will be the way you should start every function The next instruction is this movl 8 ebp eax This uses base pointer addressing to move the first parameter of the function into eax Remember ebp has the old ebp 4 ebp has the return address and 8 ebp is the location of the first parameter to the function If you think back this will be the value 4 on the first call since that was what we pushed on the stack before calling the function with push1 4 parameter into seax As this function calls itself it will have other values too 49 Chapter 4 All About Functions Next we check to see if we ve hit our base case a parameter of 1 If so we jump to the instruction at the label end_factorial where it will be returned it s already in eax which we mentioned earlier is where you put return values That is accomplished by these lines cmpl 1 eax je end_factorial If it s not our base case what did we say we would do We would call the factorial function again with our parameter minus one So first we decrease eax by one decl eax dec1 stands for decrement It subtracts 1 from the given register or memory location Seax in our case incl is the inverse it adds 1 After decrementing eax we push it onto the stack since it s going to be the parameter of the next function call And then we call factorial again pushl Seax call factorial Okay now we ve called f
127. e memory segment addl edx eax is the sum of the size requested Currently stored in edx plus another 8 storage locations for the header 4 for the AVAILABLE UNAVAILABLE flag fand 4 for the size of the segment So fadding edx and 8 to eax will get the address of the next memory segment jmp alloc_loop_begin go look at the next location allocate_here if we ve made it here that means that the segment header of the segment to allocate is in eax movl SUNAVAILABLE HDR_AVAIL_OFFSET eax mark space as unavailable 116 add SHEADER_SIZE eax movl ebp esp popl ebp ret move_break addl SHEADER SIZE ebx addl ecx ebx pushl Seax pushl Secx pushl Sebx movl SBRK eax int SLINUX_SYSCALL cmpl 0 eax je error popl ebx Chapter 9 Intermediate Memory Topics move eax past the header to the usable memory since that s what we return return from the function that means that we have exhausted all if we ve made it here memory that we can address Sebx holds the current endpoint of the data we need to ask for more and ecx holds its size now we need to increase ebx to where we _want_ memory to end so we add space for the headers structure add space to the break for the data requested now its time to ask Linux for more memory save n ded registers reset the break break point Seb
128. e new value Now we get into actual computer instructions The first such instruction is this movl 1 eax When the program runs this instruction transfers the number 1 into the eax register In assembly language many instructions have operands mov1 has two operands the source and the destination In this case the source is the literal number 1 and the destination is the eax register Operands can be numbers memory location references or registers Different instructions allow different types of operands See Appendix B for more information on which instructions take which kinds of operands On most instructions which have two operands the first one is the source operand and the second one is the destination Note that in these cases the source operand is not modified at all Other instructions of this type are for example addl sub1 and imul These add subtract multiply the source operand from to by the destination operand and and save the result in the destination operand Other instructions may have an operand hardcoded in idiv for example requires that the dividend be in eax and edx be zero and the quotient is then transferred to seax and the remainder to edx However the divisor can be any register or memory location On x86 processors there are several general purpose registers which can be used with mov1 e Seax e Sebx e SeCx e Sedx e S edi e S esi In addition to these general purpose registers there are a
129. e or is contained in the c1 register sarl I c1 R M C Arithmetic shift right The least significant bit is shifted out to the carry flag The sign bit is shifted in and kept as the sign bit Other bits are simply shifted to the right The number of bits to shift is either specified in immediate mode or is contained in the c1 register shll I c1 R M C Logical shift left This shifts all bits to the left sign bit is not treated specially The leftmost bit is pushed to the carry flag The number of bits to shift is either specified in immediate mode or is contained in the c1 register shrl I c1 R M C Logical shift right This shifts all bits in the register to the right sign bit is not treated specially The rightmost bit is pushed to the carry flag The number of bits to shift is either specified in immediate mode or is contained in the c1 register testl I R M R M O S Z A P C Does a logical and of both operands and discards the results but sets the flags accordingly xorl I R M R M O S Z A P C Does an exclusive or on the two operands and stores the result in the second operand Sets the overflow and carry flags to false Flow Control Instructions These instructions may alter the flow of the program Table B 4 Flow Control Instructions Instruction Operands Affected Flags call destination address O S Z A C This pushes what would be t
130. e others likely are If there are registers you want to save before calling a function you need to save them by pushing them on the stack before pushing the function s paramters You can then pop them back off in reverse order after popping off the parameters Even if you know a function does not overwrite a register you should save it because future versions of that function may Other languages calling conventions may be different For example other calling conventions may place the burden on the function to save any registers it uses Be sure to check to make sure the calling conventions of your languages are compatible before trying to mix languages Or in the case of assembly language be sure you know how to call the other language s functions them Extended Specification Details of the C language calling convention also known as the ABI or Application Binary Interface is available online We have oversimplified and left out several important pieces to make this simpler for new programmers For full details you should check out the documents available at http www linuxbase org spec refspecs Specifically you should look for the System V Application Binary Interface Intel386 Architecture Processor Supplement 4 This is not always strictly needed unless you are saving registers on the stack before a function call The base pointer keeps the stack frame in a reasonably consistent state However it is still a good idea
131. e simply referred to by name within the program In previous chapters the linker would resolve all of the names to physical memory addresses and the names would be thrown away When using dynamic linking the name itself resides within the 95 Chapter 8 Sharing Functions with Code Libraries executable and is resolved by the dynamic linker when it is run When the program is run by the user the dynamic linker loads the libraries listed in our link statement and then finds all of the function and variable names that were named by our program but not found at link time and matches them up with corresponding entries in the shared libraries it loads It then replaces all of the names with the addresses which they are loaded at This sounds time consuming It is to a small degree but it only happens once at program startup time How Shared Libraries Work In our first programs all of the code was contained within the source file Such programs are called statically linked executables because they contained all of the necessary functionality for the program that wasn t handled by the kernel In the programs we wrote in Chapter 6 we used both our main program file and files containing routines used by multiple programs In these cases we combined all of the code together using the linker at link time so it was still statically linked However in the he lloworld 1lib program we started using shared libraries When you use shared libraries
132. e to optimize is toward the end of development when you have determined that your correct code actually has performance problems In a web based e commerce project I was involved in I focused entirely on correctness This was much to the dismay of my colleagues who were worried about the fact that each page took twelve seconds to process before it ever started loading most web pages process in under a second However I was determined to make it the right way first and put optimization as a last priority When the code was finally correct after 3 months of work it took only three days to find and eliminate the bottlenecks bringing the average processing time under a quarter of a second By focusing on the correct order I was able to finish a project that was both correct and efficient Where to Optimize Once you have determined that you have a performance issue you need to determine where in the code the problems occur You can do this by running a profiler A profiler is a program that will let you run your program and it will tell you how much time is spent in each function and how many times they are run gprof is the standard GNU Linux profiling tool but a discussion of using profilers is outside the scope of this text After running a profiler you can determine which functions are called the most or have the most time spent in them These are the ones you should focus your optimization efforts on If a program only spends 1 of its t
133. e will do this by storing the characters on the stack as we compute them This way as we pop them back off to fill in the buffer it will be in the reverse order that we pushed them on The code for the function should be put in a file called integer to number s and should be entered as follows T PURPOSE Convert an integer number to a decimal string for display INPUT A buffer large enough to hold the largest possible number An integer to convert OUTPUT The buffer will be overwritten with the decimal string Variables Secx will hold the count of characters processed eax will hold the current value edi will hold the base 10 equ ST_VALUE 8 equ ST_BUFFE globl integer2number type integer2number function integer2number Normal function beginning pushl Sebp movl esp ebp Current character count movl 0 ecx Move the value into position movl ST_VALUE ebp eax When we divide by 10 the 10 must be in a register or memory location movl 10 edi conversion_loop Division is actually performed on the 146 Chapter 10 Counting Like a Computer o o combined edx eax register so first clear out edx movl 0 edx Divide edx eax which are implied by 10 Store the quotient in eax and the remainder in edx both of which are implied divl edi Quoti
134. e_record type write_record function write_record pushl Sebp movl esp ebp pushl movl movl movl mov ebx SSYS_WRITE eax ST_FILEDES ebp ebx ST_WRITE_BUFFER ebp ecx SRECORD_SIZE edx int NOT popl ti movl popl ret Now that SLINUX_SYSCALL eax has the return value which we will give back to our calling program Sebx Sebp esp Sebp we have our basic definitions down we are ready to write our programs Writing Records This program will simply write some hardcoded records to disk It will e Open the file e Write three records 70 Chapter 6 Reading and Writing Simple Records e Close the file Type the following code into a file called write records s include linux s include record def s section data Constant data of the records we want to write Each text data item is padded to the proper length with null i e 0 bytes rept is used to pad each item rept tells the assembler to repeat the section between rept and endr the number of times specified This is used in this program to add extra null characters at the end of each field to fill it up recordl ascii Fredrick 0 rept 31 Padding to 40 bytes byte 0 endr ascii Bartlett 0 rept 31 Padding to 40 bytes byte 0 endr ascii 4242 S Prairie nTulsa OK
135. eason for it is that when we think of a stack of anything dishes papers etc we think of adding and removing to the top of it However in memory the stack starts at the top of memory and grows downward due to other architectural considerations Therefore when we refer to the top of the stack remember it s at the bottom of the stack s memory When we are referring to the top or bottom of memory we will specifically say so You can also pop values off the top using an instruction called popl When we push a value onto the stack the top of the stack moves to accomodate the addition value We can actually continually push values onto the stack and it will keep growing further and further down in memory until we hit our code or data So how do we know where the current top of the stack is The stack register esp always contains a pointer to the current top of the stack wherever it is Every time we push something onto the stack with pushl esp gets subtracted by 4 so that it points to the new top of the stack remember each word is four bytes long and the stack grows downward If we want to remove something from the stack we simply use the pop1 instruction which adds 4 to esp and puts the previous top value in whatever register you specified push1 and pop each take one operand the register to push onto the stack for push1 or receive the data that is popped off the stack for pop1 If we simply want to access the value on the t
136. ed by the instruction movb 30 Chapter 3 Your First Programs However since the registers we have discussed are word sized and not byte sized you cannot use the full register Instead you have to use a portion of the register Take for instance eax If you only wanted to work with two bytes at a time you could just use ax Sax is the least significant half i e the last part of the number of the eax register and is useful when dealing with two byte quantities sax is further divided up into sal and ah al is the least significant byte of sax and Sah is the most significant byte Loading a value into eax Will wipe out whatever was in al and ah and also ax since ax is made up of them Similarly loading a value into either al or ah will corrupt any value that was formerly in eax Basically it s wise to only use a register for either a byte or a word but never both at the same time eax PS S ah al S se ax Layout of the eax register For a more comprehensive list of instructions see Appendix B Review Know the Concepts e What does if mean if a line in the program starts with the character e What is the difference between an assembly language file and an object code file 13 When we talk about the most or least significant byte it may be a little confusing Let s take the number 5432 In that number 54 is the most significant half of that number and 3
137. ed into memory each sect ion is loaded into its own spot The actual code the text section is loaded at the address 0x08048000 The data section is loaded immediately after that followed by the bss see the Section called Buffers and bss in Chapter 5 section Remember the bss section has all of the memory locations that we reserve that we don t put values in until run time In the data section we put actual values into the storage spaces we reserved with the long directive This information is embedded in the program file and loaded when the program starts The bss section is not initialized until after the program is run Therefore the data doesn t have to be stored in the program file itself it just notes that it needs a certain number of storage spaces Anyway we ll talk more about that later The last storage location that can be addressed is location Oxbfffffff The text data and bss sections all start at 0x08048000 and grow larger The next sections start at the end and grow back downward First at the very end of memory there are two words that just contain zeroes After that comes the name of the program Each letter takes up one byte and it is ended by the NULL character the 0 we talked about earlier After the program name comes the program environment values These are not important to us now Then come the program arguments These are the values that the user typed in on the command line to run this
138. embly Language ccccsscccsssssccssscsccccssssccssssccccssesecssssssssscssssssscssssessecees 199 F Using the GDB Debugger vesicisietks sessrccceceessocsactesdacdastsceuensstsveussacsareusdcsduecadiesectdescosuesadscdsdivacs 209 G Docum nt HIStOry ssississiossssisssesoisisoteceisessossnisssossssisosssssssspoa sisis oss sses sss sonsasaatucavacssseaveussabnaden 217 H GNU Free Documentation License e ssoesssessesssocssossssesssessoossoossosesseessoossossssesssossoossosssssse 219 DETO C EE E A E TTA E EE 227 vi Chapter 1 Introduction Welcome to Programming I love programming I enjoy the challenge to not only make a working program but to do so with style Programming is like poetry It conveys a message not only to the computer but to those who modify and use your program With a program you build your own world with your own rules You create your world according to your conception of both the problem and the solution Masterful programmers create worlds with programs that are clear and succinct much like a poem or essay One of the greatest programmers Donald Knuth describes programming not as telling a computer how to do something but telling a person how they would instruct a computer to do something The point is that programs are meant to be read by people not just computers Your programs will be modified and updated by others long after you move on to other projects Thus programming is not as much about c
139. emory address addr using repeat count r size s and format f Repeat count defaults to 1 if not specified Size can be b byte h halfword w word or g double word Size defaults to word if not Specified Format is the same as for print with the additions of s string and i instruction info display Shows a numbered list of expressions set up to display automatically at each break 214 Appendix F Using the GDB Debugger Examining Registers and Memory display f reg At each break print the contents of register reg using format f display si addr At each break print the contents of memory address addr using size s same options as for the x command display ss addr At each break print the string of size s that begins in memory address addr undisplay displaynum IRemove displaynum from the display list Examining the Call Stack where Print the call stack backtrace Print the call stack frame Print the top of the call stack up Move the context toward the bottom of the call stack down Move the context toward the top of the call stack 215 Appendix F Using the GDB Debugger 216 Appendix G Document History e 12 17 2002 Version 0 5 Initial posting of book under GNU FDL e 07 18 2003 Version 0 6 Added ASCII appendix finished the discussion of the CPU in the Memory chapter reworked exercises into a new format corrected seve
140. emory managers the deallocate function is one of the fastest Later we will see how to improve allocate considerably without slowing down deallocate too much Another performance problem is the number of times we re calling the break system call System calls take a long time They aren t like functions because the processor has to switch modes Your program isn t allowed to map itself memory but the Linux kernel is So the processor has to switch into kernel mode then it maps the memory and then switches back to user mode This is also called a context switch This takes a long time because although your program looks at its virtual memory Linux looks at the physical memory Therefore the processor has to forget all of its page mappings All of this takes a lot of time So you should avoid calling the kernel unless you really need to The problem that we have is that we aren t recording where Linux actually sets the break In our previous discussion we mentioned that Linux might actually set the break past where we requested it If we wanted to save time we should record that location in move_break and the next time we ask for memory look to see if the break is already where we need it Along the same lines it might be wise to always ask for a lot more memory than we really need in order to reduce the number of times we have to call the break system call We just have to remember that Linux has to map everything even if we don t
141. ent is in the right place edx has the remainder which now needs to be converted into a number So edx has a number that is 0 through 9 You could also interpret this as fan index on the ASC table starting from the character 0 The ascii code for 0 plus zero is still the ascii code for 0 The ascii code for 0 plus 1 is the ascii code for the character 71 Therefore the following instruction will give us the character for the number stored in edx addl 0 edx Now we will take this value and push it on the stack This way when we are done we can just pop off the characters one by one and they will be in the right order Note that we are pushing the whole register but we only need the byte in dl the last byte of the Sedx register for the character pushl Sedx Increment the digit count incl ecx Check to see if eax is zero yet go to next step if so cmpl 0 eax je end_conversion_loop Seax already has its new value jmp conversion_loop end_conversion_loop The string is now on the stack if we pop it off a character at a time we can copy it into 147 Chapter 10 Counting Like a Computer the buffer and be done Get the pointer to the buffer in edx movl ST_BUFFER ebp edx copy_reversing_loop We pushed a whole register but we only need the last byte So we are going to pop off to the o entire eax register but
142. es Compatibility Note The listed sizes are for intel compatible x86 machines Other machines will have different sizes Also even when shorter sized parameters are passed to functions they are passed as longs That s how to read function documentation Now let s get back to the question of how to find out about libraries Most of your system libraries are in usr 1ib or 1ib If you want to just see what symbols they define just run objdump R FILENAME where FILENAME is the full path to the library The output of that isn t too helpful though for finding an interface that you might need Usually you have to know what library you want at the beginning and then just read the documentation Most libraries have manuals or man pages for their functions The web is the best source of documentation for libraries Most libraries from the GNU project also have info pages on them which are a little more thorough than man pages Useful Functions Several useful functions you will want to be aware of from the c library include e size_t strlen const char s calculates the size of null terminated strings e int strcmp const char sl const char s2 compares two strings alphabetically e char strdup const char s takes the pointer to a string and creates a new copy in a new location and returns the new location e FILE fopen const char filename const char opentype opens a managed buf
143. es As you can see it s a pretty simple program To compile it run the command gcc o HelloWorld Hello World c To run the program do HelloWorld Let s look at how this program was put together Comments in C are started with and ended with Comments can span multiple lines but many people prefer to start and end comments on the same line so they don t get confused include lt stdio h gt is the first part of the program This is a preprocessor directive C compiling is split into two stages the preprocessor and the main compiler This directive tells the preprocessor to look for the file stdio h and paste it into your program The preprocessor is responsible for putting together the text of the program This includes sticking different files together running macros on your program text etc After the text is put together the preprocessor is done and the main compiler goes to work Now everything in stdio h is now in your program just as if you typed it there yourself The angle brackets around the filename tell the compiler to look in it s standard paths for the file usr include and usr local include usually If it was in quotes like include stdio h it would look in the current directory for the file Anyway stdio h contains the declarations for the standard input and output functions and variables These declarations tell the compiler what functions are available for input and output The next few li
144. escribing the Modified Version as stated in the previous sentence e J Preserve the network location if any given in the Document for public access to a Transparent copy of the Document and likewise the network locations given in the Document for previous versions it was based on These may be placed in the History section You may omit a network location for a work that was published at least four years before the Document itself or if the original publisher of the version it refers to gives permission e K In any section entitled Acknowledgements or Dedications preserve the section s title and preserve in the section all the substance and tone of each of the contributor acknowledgements and or dedications given therein e L Preserve all the Invariant Sections of the Document unaltered in their text and in their titles Section numbers or the equivalent are not considered part of the section titles e M Delete any section entitled Endorsements Such a section may not be included in the Modified Version e N Do not retitle any existing section as Endorsements or to conflict in title with any Invariant Section If the Modified Version includes new front matter sections or appendices that qualify as Secondary Sections and contain no material copied from the Document you may at your option designate some or all of these sections as invariant To do this add their titles to the list of Invariant Sections
145. escriptor 53 227 file descriptors 55 files 53 flow control 21 27 function 39 45 function call 42 function parameters 39 functions 35 GCC 3 General purpose registers 6 17 global variables 36 40 GNU Linux 2 hexadecimal 19 High Level Language 4 idiv 17 immediate mode 10 30 immediate mode addressing 18 40 imul 17 incl 28 50 index 25 index register 10 26 29 indexed addressing mode 10 26 29 indirect addressing mode 11 30 38 infinite loop 21 Instruction Decoder 6 instruction pointer 9 39 int 19 interrupt 19 jmp 44 kernel 3 Knoppix 3 label 23 labels 36 Larry Boy 19 Id 15 link 14 linker 14 Linux 3 19 local variable 41 45 local variables 36 39 46 loop 26 228 loops 21 Machine Language 4 memory 5 memory address 29 microcode translation 7 movb 30 movl 17 25 multiplier 10 26 29 newline 24 null 71 object file 14 offset 11 open 53 operands 17 out of order execution 7 padding 71 parameter 49 parameters 18 35 36 39 48 permissions 53 persistance 53 pipelining 7 pointer 38 45 pointers 9 10 popl 38 primitive functions 35 primitives 35 profiler 160 Program Counter 6 programming 1 pseudo operations 16 pushl 37 48 read 53 recursive 45 register 45 register addressing mode 10 30 registers 7 8 18 25 41 ret 40 return address 37 39 return value 37 41 48
146. ess all of the data we need for this function by using base pointer addressing using different offsets from ebp ebp was made specifically for this purpose which is why it is called the base pointer You can use other registers in base pointer addressing mode but the x86 architecture makes using the ebp register a lot faster Global variables and static variables are accessed just like we have been accessing memory in previous chapters The only difference between the global and static variables is that static variables are only used by one function while global variables are used by many functions Assembly language treats them exactly the same although most other languages distinguish them When a function is done executing it does three things First it stores it s return value in eax Second it resets the stack to what it was before being called it gets rid of the current stack frame and puts the stack frame of the calling code back into effect Third it returns control back to wherever it was called from Returning control is done using the ret instruction which pops whatever value is at the top of the stack and sets the instruction pointer eip to that value So before a function returns control to the code that called it it must restore the previous stack frame Note also that without doing this ret wouldn t work because in our current stack frame the return address is not at the top of the stack Therefore before we retur
147. etaieits catussudecedsshenntadedacsedededs casts daveonsec sudusasecnstdeesusasteipocsats 1 Welcome to Prosranimins icc ceccisasssgecciss stancoteesusccdivacage convuaneecdacuesescdecneneuadssavecetandeatcdessaeeieas gt 1 Your ToO Sis gie i a E eased seca esata A EA aaa 2 2 Computer Architecture essseseessesoesoossoeseesoesoossossecsoesoosoosseesoesossoessessoseossoosseesesoossoesossoessossossosso 5 Struct re of Computer Memory essay tes ch cus toeseahostuen i a E a eae aaa ear asss 5 TNS SP E E T E A E E EE tee Re 6 Some Terms m aie e agp en AE ea E da ela E E E EE 7 Interpreting Memory soneinioniunraie a E E EE E aa E a 9 Mata Accessing Meth dS incre ennn n R A E NE E 10 REVIEW ea s A E E E E EE E E dente r T 11 3 Your First PROGSTAMs wesseescidecansadadevaviadadlsatndaconapsedaceuascacsaacauseoscuedustecneaueel saunbilentealesedaduiuadessiaes 13 Ent n n the PRO ean a oa eE cade ie EE E R ea gee E EE E RR 13 Outline of an Assembly Language Program esssseeeesereseeresressessreeresressesseserssressessesereseesee 15 Planning the Progra esnie p aE E RAEE EE AS ls aed tat avec le ed 19 Findings a Maxim m Valie tere conden n a aa a a a alge 22 POGOe SSNS Mode Ssnin e access e e ee E E aE 29 Sea s CoA ae ree ceo m a e a a eR a a A A 31 A AML ADOUt FUNCHHOMNS sescccescssscsdeaseeccisssseescceeseseccssodesscoccssevedeocasseeceasssbtbesecasoesdscsesdeccessssessececeocsse 35 Dealing with Complexity wa icciss ascsstasnaccesuslesescsislegeca
148. eter 2 lt 12 ebp Parameter 1 lt 8 S ebp Return Address lt 4 ebp Old Sebp lt esp and ebp As you can see each parameter can be accessed using base pointer addressing mode using the ebp register Next the function reserves space on the stack for any local variables it needs This is done by simply moving the stack pointer out of the way Let s say that we are going to need 2 words of memory to run a function We can simply move the stack pointer down 2 words to reserve the space This is done like this 39 Chapter 4 All About Functions subl 8 esp This subtracts 8 from zesp remember a word is four bytes long This way we can use the stack for variable storage without worring about clobbering them with pushes that we may make for function calls Also since it is allocated on the stack frame for this function call the variable will only be alive during this function When we return the stack frame will go away and so will these variables That s why they are called local they only exist while this function is being called So now we have 2 words for local storage Our stack now looks like this Parameter N lt N 4 4 Sebp Parameter 2 lt 12 ebp Parameter 1 lt 8 ebp Return Address lt 4 ebp Old Sebp lt Sebp Local Variabl i ee 4 ebp Local Variabl 2 lt 8 ebp and esp So we can now acc
149. f the two operands and stores the result in the second operand Sets the overflow and carry flags to false notl R M Performs a logical not on each bit in the operand Also known as a one s complement orl I R M R M O S Z A P C Performs a logical or between the two operands and stores the result in the second operand Sets the overflow and carry flags to false rell I Scl1 R M O C Rotates the given location s bits to the left the number of times in the first operand which is either an immediate mode value or the register c1 The carry flag is included in the rotation making it use 33 bits instead of 32 Also sets the overflow flag rerl I c1 R M O C Same as above but rotates right roll I Scl R M O C Rotate bits to the left It sets the overflow and carry flags but does not count the carry flag as part of the rotation The number of bits to roll is either specified in immediate mode or is contained in the c1 register rorl I c1 R M O C Same as above but rotates right sall I c1 R M C 188 Appendix B Common x86 Instructions Instruction Operands Affected Flags Arithmetic shift left The sign bit is shifted out to the carry flag and a zero bit is placed in the least significant bit Other bits are simply shifted to the left This is the same as the regular shift left The number of bits to shift is either specified in immediate mod
150. fer size Writes the buffer to the file descriptor int descriptor 5 open null option list permission Opens the given file Returns the terminated mode file descriptor or an error number file name 6 close file Closes the give file descriptor descriptor 12 chdir null Changes the current directory of terminated your program directory name 19 Iseek file offset mode Repositions where you are in the descriptor given file The mode called the whence should be 0 for absolute positioning and 1 for relative positioning 20 getpid Returns the process ID of the current process 195 Appendix C Important System Calls Seax Name ebx ecx edx Notes 39 mkdir null permission Creates the given directory terminated mode Assumes all directories leading up directory to it already exist name 40 rmdir null Removes the given directory terminated directory name 41 dup file Returns a new file descriptor that descriptor works just like the existing file descriptor 42 pipe pipe array Creates two file descriptors where writing on one produces data to read on the other and vice versa ebx is a pointer to two words of storage to hold the file descriptors 45 brk new Sets the system break i e the end system of the data section If the system break break is 0 it simply returns the current system break 54 ioctl file request arguments This is used to set parameters on descriptor
151. fered file allows easier reading and writing than using file descriptors directly 2 stdin stdout and stderr all lower case can be used in these programs to refer to the files of their corresponding file descriptors 3 FILE is a struct You don t need to know it s contents to use it You only have to store the pointer and pass it to the relevant other functions 101 Chapter 8 Sharing Functions with Code Libraries e int fclose FILE stream closes a file opened with fopen char fgets char s int count FILE stream fetches a line of characters into string s e int fputs const char s FILE stream writes a string to the given open file e int fprintf FILE stream const char template is just like printf but it uses an open file rather than defaulting to using standard output You can find the complete manual on this library by going to http www gnu org software libc manual Building a Shared Library Let s say that we wanted to take all of our shared code from Chapter 6 and build it into a shared library to use in our programs The first thing we would do is assemble them like normal as write record s o write record o as read record s 0o read record o Now instead of linking them into a program we want to link them into a shared library This changes our linker command to this ld shared write record o read record o o librecord so T
152. from the first bit to the 32nd and run the AND operation on that bit in both registers In this case 10100010101010010101101100101010 AND 10001000010101010101010101111010 10000000000000010101000100101010 You ll see that the resulting set of bits only has a one where both numbers had a one and in every other position it has a zero Let s look at what an OR looks like 10100010101010010101101100101010 OR 10001000010101010101010101111010 10101010111111010101111101111010 In this case the resulting number has a where either number has a in the given position Let s look at the NOT operation 134 Chapter 10 Counting Like a Computer NOT 10100010101010010101101100101010 01011101010101101010010011010101 This just reverses each digit Finally we have XOR which is like an OR except if both digits are 1 it returns 0 10100010101010010101101100101010 XOR 10001000010101010101010101111010 00101010111111000000111001010000 This is the same two numbers used in the OR operation so you can compare how they work Also if you XOR a number with itself you will always get 0 like this 10100010101010010101101100101010 XOR 10100010101010010101101100101010 ODD00000000000000000000000000000 These operations are useful for two reasons e The computer can do them extremely fast e You can use them to compare many truth values at the same time You may not have known that
153. g was okay it returns 0 UNIX programs return numbers other than zero to indicate failure or other errors warnings or statuses The programmer determines what each number means You can view this code by typing in echo In the following section we will look at what each part of the code does Outline of an Assembly Language Program Take a look at the program we just entered At the beginning there are lots of lines that begin with hashes These are comments Comments are not translated by the assembler They are used only for the programmer to talk to anyone who looks at the code in the future Most programs you write will be modified by others Get into the habit of writing comments in your code that will help them understand both why the program exists and how it works Always include the following in your comments e The purpose of the code e An overview of the processing involved e Anything strange your program does and why it does it 1 If you are new to Linux and UNIX you may not be aware that files don t have to have extensions In fact while Windows uses the exe extension to signify an executable program UNIX executables usually have no extension 2 refers to the current directory in Linux and UNIX systems 3 You ll find that many programs end up doing things strange ways Usually there is a reason for that but unfortunately programmers never document such things in their comments So future programmers either
154. ger ss sesssesssssesseessessesesseeessesseessetesstesseesseesseesstessesseeesseeeseesseeeseet 112 TELENIO EEE TE E AE E EE EE EA E E E A S 127 10 Counting Like a Computer sessssooesssoosssooecesoccessscoessoosessooeesssecesssoossscossssoosesosesesssessssoosseso 129 Co nun ges iea a OR a dus wad a A E A E E tuaht aii bared 129 Truth Falsehood and Binary NUMDEIS lt 6cacs ttle aeusanis Cela eaten auien 133 The Program Stats Register 30 c 2sca6so5ecceaste3elaceasags laseavavec cau vias edacveuncucauaneusacdiubansas iseiusaceusagece 139 Other Number SV stem Se Gh ccd Goce i wes oa AE AAE AE A EE uaa 140 Octal and Hexadecimal Numbers o aare ieat AE LAE E EEEE aE E eae E Eart 142 Order of Bytesima Worde mernans e e e e eosa eded geese eeit 143 Converting Numbers for Display recerca n a a eee 145 RE VICW heponi no unn ae alate nes case Rado Naso eRe AO EE i 150 11 High Level Languages is ctecsseccudectissentisuaversdessasediasuutscusaceudscesditesseusiverceeuedssusevestssuduacstesedunass 153 Compiled and Interpreted Languages coos 5 5 vacances aconcaecvieushinsael aconazs wwacsuu te seonaaze seen ctarecqousta ts 153 Your First C Prostam dys cos atcun secs tae cyaban tales e eo a a E epee 154 PPE a cis T E wala E ln DA peatess Dts agua shad sad see coe aaa do ooo cea Bee sede 156 PHC OT 2 ease aiden tel T dante ara eae Ede ee ele eed eee 157 RO VIGW eeren an e Ra E E E E ueene 157 12 OUI Z AION secinasssccecairinditnsastadevandetadececd
155. ght switches let s say we just memorized which position went with which switch Then the same information could be represented as 1 0 1 il or as 1011 133 Chapter 10 Counting Like a Computer This is just one of many ways you can use the computers storage locations to represent more than just numbers The computers memory just sees numbers but programmers can use these numbers to represent anything their imaginations can come up with Not only can you do regular arithmetic with binary numbers they also have a few operations of their own The standard binary operations are AND e OR NOT e XOR Before we look at examples I ll describe them for you AND takes two bits and returns one bit AND will return a 1 only if both bits are 1 and a 0 otherwise For example 1 AND 1 is 1 but 1 AND 0 is 0 0 AND 1 is 0 and 0 AND 0 is 0 OR takes two bits and returns one bit It will return 1 if either of the original bits is 1 For example 1 OR 1 is 1 1 OR 0 is one 0 OR 1 is 1 but 0 OR O is 0 NOT only takes one bit and returns it s opposite NOT 1 is 0 and NOT Ois 1 Finally XOR is like OR except it returns 0 if both bits are 1 Computers can do these operations on whole registers at a time For example if a register has 10100010101010010101101100101010 and another one has 10001000010101010101010101111010 you can run any of these operations on the whole registers For example if we were to AND them the computer will run
156. he next most significant column If you added 65 and 37 first you add 7 and 4 to get 12 You keep the 2 in the right column and carry the one to the next column There you add 6 3 and the you carried This results in 10 So you keep the zero in that column and carry the one to the next most significant column which is empty so you just put the one there As you can see most computer operations are exactly like their human counterparts except you have to describe them in excruciating detail Additional program status register flags are examined in Appendix B Other Numbering Systems What we have studied so far only applies to positive integers However real world numbers are not always positive integers Negative numbers and numbers with decimals are also used Floating point Numbers So far the only numbers we ve dealt with are integers numbers with no decimal point Computers have a general problem with numbers with decimal points because computers can only store fixed size finite values Decimal numbers can be any length including infinite length think of a repeating decimal like the result of 1 3 The way a computer handles this is by storing decimals at a fixed precision A computer stores decimal numbers in two parts the exponent and the mantissa The mantissa contains the actual digits that will be used and the exponent is what magnitude the number is For example 12345 2 is stored as 1 23452 1044 The mantissa
157. he address in Seax ret O S Z A C Pops a value off of the stack and then sets eip to that value Used to return from function calls 190 Appendix B Common x86 Instructions Assembler Directives These are instructions to the assembler and linker instead of instructions to the processor These are used to help the assembler put your code together properly and make it easier to use Table B 5 Assembler Directives Directive Operands aSCli QUOTED STRING Takes the given quoted string and converts it into byte data byte VALUES Takes a comma separated list of values and inserts them right there in the program as data endr Ends a repeating section defined with rept equ LABEL VALUE Sets the given label equivalent to the given value The value can be a number a character or an constant expression that evaluates to a a number or character From that point on use of the label will be substituted for the given value globl LABEL Sets the given label as global meaning that it can be used from separately compiled object files include FILE Includes the given file just as if it were typed in right there lcomm SYMBOL SIZE This is used in the bss section to specify storage that should be allocated when the program is executed Defines the symbol with the address where the storage will be located and makes sure that it is the given number of
158. he next value for eip onto the stack and jumps to the destination address Used for function calls Alternatively the destination address can be an asterisk followed by a register for an indirect function call For example call eax will call the function at the address in Seax int I O S Z A C 189 Appendix B Common x86 Instructions Instruction Operands Affected Flags Causes an interrupt of the given number This is usually used for system calls and other kernel interfaces Joc destination address O S Z A C Conditional branch cc is the condition code Jumps to the given address if the condition code is true set from the previous instruction probably a comparison Otherwise goes to the next instruction The condition codes are e n a e above unsigned greater than An n can be added for not and an e can be added for or equal to n b e below unsigned less than e n e equal to e n z zero e n g e greater than signed comparison e n 1 e less than signed comparison e n c carry flag set e n o overflow flag set e p p parity flag set e n s sign flag set e ecxz ecx is zero jmp destination address O S Z A C An unconditional jump This simply sets eip to the destination address Alternatively the destination address can be an asterisk followed by a register for an indirect jump For example jmp eax will jump to t
159. hen they occur This function will continue to process events until gtk_main_quit is called by a signal handler gtk_main_quit This function causes GNOME to exit it s main loop at the earliest opportunity gnome_message_box_new This function creates a dialog window containing a question and response buttons It takes as parameters the message to display the type of message it is warning question etc and a list of buttons to display The final parameter should be NULL to indicate that there are no more buttons to display gtk_window_set_modal This function makes the given window a modal window In GUI programming a modal window is one that prevents event processing in other windows until that window is closed This is often used with Dialog windows gnome_dialog_run_and_close This function takes a dialog pointer the pointer returned by gnome_message_box_new 178 Appendix A GUI Programming can be used here and will set up all of the appropriate signal handlers so that it will run until a button is pressed At that time it will close the dialog and return to you which button was pressed The button number refers to the order in which the buttons were set up in gnome_mes sage_box_new The following is the same program written in the C language Type it in as gnome example c c PURPOSE This program is meant to be an example of what GUI programs look like written with the GNOME libraries
160. his several times your output will look something like this gdb stepi 35 cmpl ebx eax compare values gdb stepi 36 jle start_loop jump to loop beginning if the new gdb stepi 32 cmpl 0 eax check to see if we ve hit the end gdb stepi 33 je loop_exit gdb stepi 34 movl data_items edi 4 eax gdb stepi 35 cmpl ebx eax compare values gdb stepi 36 jle start_loop jump to loop beginning if the new gdb step 32 cmpl 0 eax check to see if we ve hit the end 210 Appendix F Using the GDB Debugger As you can tell it has looped In general this is good since we wrote it to loop However the problem is that it is never stopping Therefore to find out what the problem is let s look at the point in our code where we should be exitting the loop cmpl 0 eax je loop_exit Basically it is checking to see if eax hits zero If so it should exit the loop There are several things to check here First of all you may have left this piece out altogether It is not uncommon for a programmer to forget to include a way to exit a loop However this is not the case here Second you should make sure that loop_exit actually is outside the loop If we put the label in the wrong place strange things would happen However again this is not the case Neither of those potential problems are the culprit So the next option is that perhaps seax has the wro
161. his architecture is used in all modern computers including personal computers supercomputers mainframes and even cell phones Structure of Computer Memory To understand how the computer views memory imagine your local post office They usually have a room filled with PO Boxes These boxes are similar to computer memory in that each are numbered sequences of fixed size storage locations For example if you have 256 megabytes of computer memory that means that your computer contains roughly 256 million fixed size storage locations Or to use our analogy 256 million PO Boxes Each location has a number and each location has the same fixed length size The difference between a PO Box and computer memory is that you can store all different kinds of things in a PO Box but you can only store a single number in a computer memory storage location 2204 2205 l 2206 2207 2208 2209 2210 2211 l i l 2212 2213 9014 4 2215 l l I I 2216 2217 2218 2219 Memory locations are like PO Boxes You may wonder why a computer is organized this way It is because it is simple to implement If Chapter 2 Computer Architecture the computer were composed of a lot of differently sized locations or if you could store different kinds of data in them it would be difficult and
162. his links both of these files together into a shared library called Librecord so This file can now be used for multiple programs If we need to update the functions contained within it we can just update this one file and not have to worry about which programs use it Let s look at how we would link against this library To link the write records program we would do the following as write records s 0o write records ld L dynamic linker lib ld linux so 2 o write records lrecord write records o In this command L told the linker to look for libraries in the current directory it usually only searches 1ib directory usr 1ib directory and a few others As we ve seen the option dynamic linker lib 1ld linux so 2 specified the dynamic linker The option lrecord tells the linker to search for functions in the file named librecord so Now the write records program is built but it will not run If we try it we will get an error like the following 102 Chapter 8 Sharing Functions with Code Libraries write records error while loading shared libraries librecord so can not open shared object file No such file or directory This is because by default the dynamic linker only searches 1ib usr 1ib and whatever directories are listed in etc 1d so conf for libraries In order to run the program you either need to move the library to one of these directories or execute the following command LD_LIBRARY_PATH
163. his works PURPOSE Given a number this program computes the factorial For example the factorial of 3 is 3 2 1 or 6 The factorial of As avs fe Bok Do Ne Jor 24 and s0 Ons This program shows how to call a function You call a function by first pushing all the arguments then you call the function and the resulting value is in eax The program can also change the passed parameters if it wants to section data This program has no global data section text globl _start globl factorial this is unneeded unless we want to share 5 By running at the same time I am talking about the fact that one will not have finished before a new one is activated I am not implying that their instructions are running at the same time 46 Chapter 4 All About Functions this function among other programs The factorial takes one argument the number we want a factorial of So it gets pushed run the factorial function Scrubs the parameter that was pushed on the stack factorial returns the answer in eax but we want it in tebx to send it as our exit status call the kernel s exit function is the actual function definition Sstandard function stuff we have to restore ebp to its prior state before returning so we have to push it This is because we don t want to modify the stack pointer so we
164. how a Window that has a button to quit the application When that button is clicked it will ask you if you are sure and if you click yes it will close the application To run this program type in the following as gnome example s PURPOSE This program is meant to be an exampl 171 Appendix A GUI Programming of what GUI programs look like written with the GNOME libraries INPUT The user can only click on the Quit button or close the window OUTPUT The application will close PROCESS If the user clicks on the Quit button the program will display a dialog asking if they are sure If they click Yes it will close the application Otherwise it will continue running section data T GNOME definitions These were found in the GNOME header files for the C language and converted into their assembly equivalents GNOME Button Names GNOME _STOCK_BUTTON_YES ascii Button_Yes 0 GNOME_STOCK_BUTTON_NO ascii Button_No 0 E Gnome MessageBox Types GNOME_MESSAGE_BOX_QUESTION ascii question 0 Standard definition of NULL equ NULL 0 GNOME signal definitions signal_destroy ascii destroy o0 signal_delete_event ascii delete_event 0 signal_clicked ascii clicked 0 Application specific definitions 172 Appendix A GUI Programming Application i
165. ially for long term maintenance Just remember that these are only valid for the current file The allocate init function Okay this is a simple function All it does is set up the heap_begin and current_break variables we discussed earlier So if you remember the discussion earlier the current break can be found using the break system call So the function looks like this pushl Sebp movl esp ebp movl S BRK eax movl 0 ebx int SLINUX_SYSCALL incl eax Anyway after int S LINUX_SYSCALL eax holds the last valid address We actually want the first invalid address so we just increment eax Then we move that value to the heap_begin and current_break locations Then we leave the function Like this movl eax current_break movl eax heap_begin movl ebp esp popl ebp So why do we want to put an invalid address as the beginning of our heap Because we don t control any memory yet Our allocate function will notice this and reset the break so that we actually have memory The allocate function This is the doozy function Let s start by looking at an outline of the function 1 Start at the beginning of the heap 2 Check to see if we re at the end of the heap 3 If we are at the end of the heap grab the memory we need from the kernel mark it as unavailable and return it If the kernel won t give us any more return a 0 121 Chapter 9 Intermediate Memory Topics 4 If
166. ides What not How The Rules Approach to Application Development by Chris Date The Algorithm Design Manual by Steve Skiena Programming Language Pragmatics by Michael Scott Essentials of Programming Languages by Daniel P Friedman Mitchell Wand and Christopher T Haynes From the Middle Out Each of these is the best book on its subject If you need to know these languages these will tell you all you need to know 168 Chapter 13 Moving On from Here e Programming Perl by Larry Wall Tom Christiansen and Jon Orwant e Common LISP The Language by Guy R Steele e ANSI Common LISP by Paul Graham The C Programming Language by Brian W Kernighan and Dennis M Ritchie The Waite Group s C Primer Plus by Stephen Prata The C Programming Language by Bjarne Stroustrup Thinking in Java by Bruce Eckel available online at http www mindview net Books TIJ The Scheme Programming Language by Kent Dybvig e Linux Assembly Language Programming by Bob Neveln Specialized Topics These books are the best books that cover their topic They are thorough and authoritative To get a broad base of knowledge you should read several outside of the areas you normally program in e Practical Programming Programming Pearls and More Programming Pearls by Jon Louis Bentley e Databases Understanding Relational Databases by Fabian Pascal e Project Management The Mythical Man Month by Fred P Brooks UNIX Programming The Art of UNIX Pr
167. ike byte This was so you could have a better grasp on what was being done From here on we will be using the above terms instead so be sure you know what they mean The Instruction Pointer Previously we have concentrated on general registers and how they work The only special register we ve dealt with is the status register and we really didn t say much about it The next special register we will deal with is the instruction pointer or se ip We mentioned earlier that the computer sees every byte on the computer in the same way If we have a number that is an entire word the computer doesn t know what address that word starts or ends at The computer doesn t know the difference between instructions and data either Any value in memory could be instructions data or the middle of an instruction or piece of data So how does the computer know what to execute The answer is the instruction pointer The instruction pointer always has the value of the next instruction When the computer is ready to execute an instruction it looks at the instruction pointer to see where to go next It then increments the instruction pointer to point to the next instruction After it finishes executing the current instruction it looks at the instruction pointer again That s all well and good but what about jumps the jmp family of instructions At the end of those instructions the computer does _not_ look at the next instruction it goes to an instructio
168. ike this the top of the stack is at the bottom on this example Parameter N Parameter 2 Parameter 1 Return Address lt Sesp Each of the parameters of the function have been pushed onto the stack and finally the return address is there Now the function itself has some work to do The first thing it does is save the current base pointer register sebp by doing pushl ebp The base pointer is a special register used for accessing function parameters and local variables Next it copies the stack pointer to ebp by doing movl esp ebp This allows you to be able to access the function parameters as fixed indexes from the base pointer You may think that you can use the stack pointer for this However during your program you may do other things with the stack such as pushing arguments to other functions Copying the stack pointer into the base pointer at the beginning of a function allows you to always know where your parameters are and as we will see local variables too even while you may be pushing things on and off the stack sebp will always be where the stack pointer was at the beginning of the function so it is more or less a constant reference to the stack frame the stack frame consists of all of the stack variables used within a function including parameters local variables and the return address At this point the stack looks like this Parameter N lt N 4 4 ebp Param
169. ikely to enter data that makes sense to the computer User s enter data that makes sense to them Allowing non programmers to use your program usually gives you much more accurate results as to how robust your program truly is Data Testing When designing programs each of your functions needs to be very specific about the type and range of data that it will or won t accept You then need to test these functions to make sure that they perform to specification Most important is testing corner cases or edge cases Corner cases are the inputs that are most likely to cause problems or behave unexpectedly When testing numeric data there are several corner cases you always need to test The number 0 86 Chapter 7 Developing Robust Programs The number 1 e A number within the expected range A number outside the expected range The first number in the expected range e The last number in the expected range The first number below the expected range The first number above the expected range For example if I have a program that is supposed to accept values between 5 and 200 I should test 0 1 4 5 153 200 201 and 255 at a minimum 153 and 255 were randomly chosen inside and outside the range respectively The same goes for any lists of data you have You need to test that your program behaves as expected for lists of 0 items 1 item and so on In addition you should also test any turning points you have For example if yo
170. ime in a given function then no matter how much you speed it up you will only achieve a maximum of a 1 overall speed improvement However if a program spends 20 of its time in a given function then even minor improvements to that functions speed will be noticeable Therefore profiling gives you the information you need to make good choices about where to spend your programming time In order to optimize functions you need to understand in what ways they are being called and used The more you know about how and when a function is called the better position you will be in to optimize it appropriately There are two main categories of optimization local optimizations and global optimizations Local optimizations consist of optimizations that are either hardware specific such as the fastest way to perform a given computation or program specific such as making a specific piece of code perform the best for the most often occuring case Global optimization consist of optimizations which are structural For example if you were trying to find the best way for three people in different cities to meet in St Louis a local optimization would be finding a better road to get there while a global optimization would be to decide to teleconference instead of meeting in person Global optimization often involves restructuring code to avoid performance problems rather than trying to find the best way through them 160 Chapter 12 Optimization Loca
171. inux will then return to you a file descriptor in eax Remember this is a number that you use to refer to this file throughout your program 3 Next you will operate on the file doing reads and or writes each time giving Linux the file descriptor you want to use read is system call 3 and to call it you need to have the file descriptor in ebx the address of a buffer for storing the data that is read in ecx and the size of the buffer in sedx Buffers will be explained in the Section called Buffers and bss read will return with either the number of characters read from the file or an error code Error codes can be distinguished because they are always negative numbers more information on negative numbers can be found in Chapter 10 write is system call 4 and it 1 This will be explained in more detail in the Section called Truth Falsehood and Binary Numbers in Chapter 10 53 Chapter 5 Dealing with Files requires the same parameters as the read system call except that the buffer should already be filled with the data to write out The write system call will give back the number of bytes written in eax or an error code 4 When you are through with your files you can then tell Linux to close them Afterwards your file descriptor is no longer valid This is done using close system call 6 The only parameter to close is the file descriptor which is placed in sebx Buffers and bss In the previous section we mentioned buffe
172. is 1 23452 and the exponent is 4 All numbers are stored as X XXXXX 10 XXXX The number 1 is stored as 1 00000 10 0 Now the mantissa and the exponent are only so long which leads to some interesting problems For example when a computer stores an integer if you add 1 to it the resulting number is one larger This does not necessarily happen with floating point numbers If the number is sufficiently big like 5 234 10 5000 adding 1 to it might not even register in the mantissa remember both parts are only so long This affects several things especially order of operations Let s say that I add 1 to 5 234 10 5000 a few billion or trillion times Guess what the number won t change at all However if I add one to itself a few trillion or billion times and then add it to the original number it might make a dent 140 Chapter 10 Counting Like a Computer Note that I haven t actually computed this nor do I know the details of the representation I m just trying to let you in on how this works in the computer so it doesn t surprise you later on You should note that it takes most computers a lot longer to do floating point arithmetic than it does integer arithmetic So for programs that really need speed integers are mostly used Negative Numbers How would you think that negative numbers on a computer might be represented One thought might be to use the first digit of a number as the sign so 0000000000000000000000000000
173. ize for the customer s name and address say 50 ASCII characters for each which would be 50 bytes for each Then after that have a number for the customer s age and their customer id In this case you would have a block of memory that would look like this Start of Record Customer s name 50 bytes start of record Customer s address 50 bytes start of record 50 bytes Customer s age 1 word 4 bytes start of record 100 bytes 2 Note that here we are talking about general computer theory Some processors and operating systems actually mark the regions of memory that can be executed with a special marker that indicates this Chapter 2 Computer Architecture Customer s id number 1 word 4 bytes start of record 104 bytes This way given the address of a customer record you know where the rest of the data lies However it does limit the customer s name and address to only 50 ASCII characters each What if we didn t want to specify a limit Another way to do this would be to have in our record pointers to this information For example instead of the customer s name we would have a pointer to their name In this case the memory would look like this Start of Record Customer s name pointer 1 word start of record Customer s address pointer 1 word start of record 4 Customer s age 1 word start of record 8 Customer s id number 1 word start of record 12 The ac
174. just did movl esp eax eax would just have the pointer to the end of the stack So the stack grows downward while the bss section grows upward This middle part is called the break and you are not allowed to access it until you tell the kernel that you want to If you try you will get an error the error message is usually segmentation fault The same will happen if you try to access data before the beginning of your program 0x08048000 In general it s best not to access any location unless you have reserved storage for it in the stack data or bss sections Every Memory Address is a Lie So why does the computer not allow you to access memory in the break area To answer this question we will have to delve into the depths of how your computer really handles memory Be warned reading this section is like taking the blue pill You may have wondered since every program gets loaded into the same place in memory don t they step on each other or overwrite each other It would seem so However as a program writer you only access virtual memory Physical memory refers to the actual RAM chips inside your 4 asin the movie The Matrix 110 Chapter 9 Intermediate Memory Topics computer and what they contain It s usually between 16 and 512 Megabytes If we talk about a physical memory address we are talking about where exactly on these chips a piece of memory is located So what s virtual memory Virtual memor
175. l Optimizations The following are some well known methods of optimizing pieces of code When using high level languages some of these may be done automatically by your compiler s optimizer Precomputing Calculations Sometimes a function has a limitted number of possible inputs and outputs In fact it may be so few that you can actually precompute all of the possible answers beforehand and simply look up the answer when the function is called This takes up some space since you have to store all of the answers but for small sets of data this works out really well especially if the computation normally takes a long time Remembering Calculation Results This is similar to the previous method but instead of computing results beforehand the result of each calculation requested is stored This way when the function starts if the result has been computed before it will simply return the previous answer otherwise it will do the full computation and store the result for later lookup This has the advantage of requiring less storage space because you aren t precomputing all results This is sometimes termed caching or memoizing Locality of Reference Locality of reference is a term for where in memory the data items you are accessing are With virtual memory you may access pages of memory which are stored on disk In such a case the operating system has to load that memory page from disk and unload others to disk Let s say for insta
176. l just be unused So let s jump down to allocate_here and see what happens there movl UNAVAILABLE HDR_AVAIL_OFFSET eax add l SHEADER_SIZE eax movl ebp esp popl ebp ret So we mark the memory as being unavailable move eax past the header and use it as the return value for the function Okay so let s say the segment wasn t big enough What then Well we fall through again to the code labeled next_location This section of code is used both for falling through and for jumping to any time that we figure out that the current memory segment won t work for allocating memory All it does is advance eax to the next possible memory segment and go back to the beginning of the loop Remember that edx is holding the size of the memory segment and HEADER_SIZE is the symbol for the size of the memory segment s header So we have add SHEADER_SIZE eax addl edx eax jmp alloc_loop_begin And the function runs another loop Now whenever you have a loop you must make sure that it will always end In our case we have the following possibilities e We will reach the end of the heap e We will find a memory segment that s available and large enough e We will go to the next location The first two items are conditions that will cause the loop to end The third one will keep it going This loop will always end Even if we never find an
177. l of these registers begin with the letter e The reason is that early generations of x86 processors were 16 bits rather than 32 bits Therefore the registers were only half the length they are now In later generations of x86 processors the size of the registers doubled They kept the old names to refer to the first half of the register and added an e to refer to the extended versions of the register Usually you will only use the extended versions Newer models also offer a 64 bit mode which doubles the size of these registers yet again and uses an r prefix to indicate the larger registers i e rax is the 64 bit version of eax However these processors are not widely used and are not covered in this book 18 Chapter 3 Your First Programs discuss different system calls as they are needed For a list of common system calls and what is required to be in each register see Appendix C The next instruction is the magic one It looks like this int 0x80 The int stands for interrupt The 0x80 is the interrupt number to use An interrupt interrupts the normal program flow and transfers control from our program to Linux so that it will do a system call You can think of it as like signaling Batman or Larry Boy if you prefer You need something done you send the signal and then he comes to the rescue You don t care how he does his work it s more or less magic and when he s done you re back in control In this case all
178. lding Linux assembly language programs In addition you will learn the structure of assembly language programs and a few assembly language commands As you go through this chapter you may want to refer also to Appendix B and Appendix F These programs may overwhelm you at first However go through them with diligence read them and their explanations as many times as necessary and you will have a solid foundation of knowledge to build on Please tinker around with the programs as much as you can Even if your tinkering does not work every failure will help you learn Entering in the Program Okay this first program is simple In fact it s not going to do anything but exit It s short but it shows some basics about assembly language and Linux programming You need to enter the program in an editor exactly as written with the filename exit s The program follows Don t worry about not understanding it This section only deals with typing it in and running it In the Section called Outline of an Assembly Language Program we will describe how it works PURPOSE Simple program that exits and returns a status code back to the Linux kernel INPUT none OUTPUT returns a status code This can be viewed by typing echo after running the program VARIABLES Seax holds the system call number this is always the case Sebx holds the re
179. le c c gnome config cflags libs gnomeui o gnome example c Run it by typing gnome example c Finally we have a version in Python Type it in as gnome example py PURPOSE This program is meant to be an exampl of what GUI programs look like written with the GNOME libraries Import GNOME libraries import gtk import gnome ui DEFINE CALLBACK FUNCTIONS FIRST In Python functions have to be defined before they are used so we have to define our callback functions first def destroy_handler event 181 Appendix A GUI Programming gtk mainquit return 0 def delete_handler window event return 0 def click_handler event Create the Are you sure dialog msgbox gnome ui GnomeMessageBox Are you sure you want to quit gnome ui MESSAGE_BOX_QUESTION gnome ui STOCK_BUTTON_YES gnome ui STOCK_BUTTON_NO msgbox set_modal 1 msgbox show result msgbox run_and_close Button 0 is the Yes button If this is the button they clicked on tell GNOME to quit it s event loop Otherwise do nothing if result 0 gtk mainquit return 0 MAIN PROGRAM Create new application window myapp gnome ui GnomeApp gnome example Gnome Example Program Create new button mybutton gtk GtkButton I Want to Quit the GNOME Example program myapp set_contents mybut
180. ling convention Going Further 52 Do you think it s better for a system to have a large set of primitives or a small one assuming that the larger set can be written in terms of the smaller one The factorial function can be written non recursively Do so Find an application on the computer you use regularly Try to locate a specific feature and practice breaking that feature out into functions Define the function interfaces between that feature and the rest of the program Come up with your own calling convention Rewrite the programs in this chapter using it An example of a different calling convention would be to pass paramters in registers rather than the stack to pass them in a different order to return values in other registers or memory locations Whatever you pick be consistent and apply it throughout the whole program Can you build a calling convention without using the stack What limitations might it have What test cases should we use in our example program to check to see if it is working properly Chapter 5 Dealing with Files A lot of computer programming deals with files After all when we reboot our computers the only thing that remains from previous sessions are the things that have been put on disk Data which is stored in files is called persistent data because it persists in files that remain on the disk even when the program isn t running The UNIX File Concept Each operating system has it s
181. look at byte order issues you should read DAV s Endian FAQ at http www rdrop com cary html endian_faq html especially the article On Holy Wars and a Plea for Peace by Daniel Cohen Converting Numbers for Display So far we have been unable to display any number stored to the user except by the extremely limitted means of passing it through exit codes In this section we will discuss converting positive numbers into strings for display The function will be called integer2string and it will take two parameters an integer to convert and a string buffer filled with null characters zeroes The buffer will be assumed to be big enough to store the entire number as a string at least 11 characters long to include a trailing null character Remember that the way that we see numbers is in base 10 Therefore to access the individual decimal digits of a number we need to be dividing by 10 and displaying the remainder for each digit Therefore the process will look like this e Divide the number by ten e The remainder is the current digit Convert it to a character and store it e We are finished if the quotient is zero 145 Chapter 10 Counting Like a Computer e Otherwise take the quotient and the next location in the buffer and repeat the process The only problem is that since this process deals with the one s place first it will leave the number backwards Therefore we will have to finish by reversing the characters W
182. lso several special purpose registers including e Sebp e Sesp e Seip 17 Chapter 3 Your First Programs We ll discuss these later just be aware that they exist Some of these registers like te ip and ef lags can only be accessed through special instructions The others can be accessed using the same instructions as general purpose registers but they have special meanings special uses or are simply faster when used in a specific way So the mov1 instruction moves the number 1 into eax The dollar sign in front of the one indicates that we want to use immediate mode addressing refer back to the Section called Data Accessing Methods in Chapter 2 Without the dollar sign it would do direct addressing loading whatever number is at address 1 We want the actual number 1 loaded in so we have to use immediate mode The reason we are moving the number 1 into eax is because we are preparing to call the Linux Kernel The number 1 is the number of the exit system call We will discuss system calls in more depth soon but basically they are requests for the operating system s help Normal programs can t do everything Many operations such as calling other programs dealing with files and exiting have to be handled by the operating system through system calls When you make a system call which we will do shortly the system call number has to be loaded into eax for a complete listing of system calls and their numbers see Appendix
183. ly Before you read the next paragraph try to think about what the problems with it might be Okay the biggest problem here is speed Now if there are only a few allocations made then speed won t be a big issue But think about what happens if you make a thousand allocations On allocation number 1000 you have to search through 999 memory segments to find that you have to request more memory As you can see that s getting pretty slow In addition remember that Linux can keep pages of memory on disk instead of in memory So since you have to go through every piece of memory that means that Linux has to load every part of memory from disk to check to see if its available You can see how this could get really really slow This method is said to run in linear time which means that every element you have to manage makes your program take longer A program that runs in constant time takes the same amount of time no matter how many elements you are managing Take the deallocate function for instance It only runs 4 instructions no matter how many elements we are managing or where they are in 6 This is why adding more memory to your computer makes it run faster The more memory your computer has the less it puts on disk so it doesn t have to always be interrupting your programs to retreive pages off the disk 125 Chapter 9 Intermediate Memory Topics memory In fact although our allocate function is one of the slowest of all m
184. mple int global_data 30 204 Appendix E C Idioms in Assembly Language In assembly language this would be section data global_data long 30 Taking the address of this data in C a amp global_data Taking the address of this data in assembly language movl Sglobal_data eax You see with assembly language you are almost always accessing memory through pointers That s what direct addressing is To get the pointer itself you just have to go with immediate mode addressing Local variables are a little more difficult but not much Here is how you take the address of a local variable in C void foo int a int b a 30 b amp a b 44 The same code in assembly language foo Standard opening pushl Sebp movl esp ebp Reserve two words of memory subl 8 Sesp equ A_VAR 4 equ B_VAR 4 205 Appendix E C Idioms in Assembly Language ja 30 movl 30 A_VAR ebp jo amp a movl SA_VAR B_VAR ebp addl tebp B_VAR ebp j b 30 movl B_VAR ebp eax movl 30 eax Standard closing movl sebp sesp popl sebp ret As you can see to take the address of a local variable the address has to be computed the same way the computer computes the addresses in base pointer addressing There is an easier way the processor provides the instruction lea which stands for load effective address This lets the computer compute the
185. n After the program is finished leave movl 0 eax leave ret A destroy event happens when the widget is being removed In this case when the application window is being removed we simply want the event loop to quit destroy_handler pushl Sebp movl esp ebp This causes gtk to exit it s event loop as soon as it can call gtk_main_quit movl 0 eax leave ret A delete event happens when the application window gets clicked in the x that you normally use to 175 Appendix A GUI Programming close a window delete_handler movl 1 eax ret A click event happens when the widget gets clicked click_handler pushl Sebp movl esp ebp Create the Are you sure dialog pushl SNULL End of buttons pushl SGNOME_STOCK_BUTTON_NO Button 1 pushl SGNOME_STOCK_BUTTON_YES Button 0 pushl SGNOME_MESSAGE_BOX_QUESTION Dialog type pushl Squit_question Dialog mesasge call gnome_message_box_new addl 16 esp recover stack eax now holds the pointer to the dialog window Setting Modal to 1 prevents any other user interaction while the dialog is being shown pushl 1 pushl Seax call gtk_window_set_modal popl eax addl 4 esp Now we show the dialog pushl Seax call gtk_widget_show popl eax This sets up all the necessary signal handlers in order to just show the dialog close it when one of the buttons is clicke
186. n we have to reset the stack pointer esp and base pointer ebp to what they were before the function was called Therefore to return from the function you have to do the following movl sebp sesp 3 Just a reminder the dollar sign in front of the eight indicates immediate mode addressing meaning that we load the number 8 into esp rather than the value at address 8 40 Chapter 4 All About Functions popl sebp ret At this point you should consider all local variables to be disposed of The reason is that after you move the stack pointer back future stack pushes will likely overwrite everything you put there Therefore you should never save the address of a local variable past the life of the function it was created in or else it will be overwritten after the life of it s stack frame ends Control has now beenhanded back to the calling code which can now examine eax for the return value The calling code also needs to pop off all of the parameters it pushed onto the stack in order to get the stack pointer back where it was you can also simply add 4 number of paramters to esp using the addl instruction if you don t need the values of the parameters anymore Destruction of Registers When you call a function you should assume that everything currently in your registers will be wiped out The only register that is guaranteed to be left with the value it started with is sebp eax is guaranteed to be overwritten and th
187. n groups of ten 7 groups of ten and 9 sheep left ungrouped So is there anything significant about grouping things by ten No It s just that grouping by ten is how we ve always done it because we have ten fingers We could have grouped at nine or at eleven in which case we would have had to make up a new symbol The only difference between the different groupings of numbers is that we have to re learn our multiplication addition subtraction and division tables The rules haven t changed just the way we represent them Also some of our tricks that we learned don t always apply either For example let s say we grouped by nine instead of ten Moving the decimal point one digit to the right no longer multiplies by ten it now multiplies by nine In base nine 500 is only nine times as large as 50 Counting Like a Computer The question is how many fingers does the computer have to count with The computer only has two fingers So that means all of the groups are groups of two So let s count in binary 0 zero 1 one 10 two one group of two 11 three one group of two and one left over 100 four two groups of two 101 five two groups of two and one left over 110 six two groups of two 129 Chapter 10 Counting Like a Computer and one group of two and so on In base two moving the decimal one digit to the right multiplies by two and moving it to the left divides by two Base two is also referred to
188. n in a totally different place How does this work Because jmp somewhere is exactly the same as movl Ssomewhere eip Where somewhere is a symbol referring to a program section Now you can t actually do this because you are not allowed to refer directly to eip but if you could this would be how Also note that we put a dollar sign in front of somewhere How do we know when to put a dollar sign and when not to The dollar sign says to use immediate mode addressing which means to treat somewhere as a value If the dollar sign weren t there it would switch to direct addressing mode 108 Chapter 9 Intermediate Memory Topics moving the value in the somewhere s address into Seip which is not what we want In our previous programs we often will load registers like this movl 0 ebx The dollar sign in front of the zero indicates that this is an immediate mode instruction meaning that we load the value zero itself If we accidentally left out the dollar sign instead of putting the number zero in ebx we would be using direct addressing mode putting whatever was at address zero on our computer into ebx To refresh your memory of addressing modes see the Section called Data Accessing Methods in Chapter 2 The Memory Layout of a Linux Program This section is based off of Konstantin Boldyshev s document Startup state of a Linux i386 ELF binary available at http inuxassembly org startup html When you program is load
189. n index register to offset that address For example we could specify address 2002 and an index register If the index register contains the number 4 the actual address the data is loaded from would be 2006 This way if you have a set of numbers starting at location 2002 you can cycle between each of them using an index register On x86 processors you can also specify a multiplier for the index This allows you to access memory a byte at a time or a word at a time 4 bytes If you are accessing an entire word your index will need to be multiplied by 4 to get the 10 Chapter 2 Computer Architecture exact location of the fourth element from your address For example if you wanted to access the fourth byte from location 2002 you would load your index register with 3 remember we start counting at 0 and set the multiplier to 1 since you are going a byte at a time This would get you location 2005 However if you wanted to access the fourth word from location 2002 you would load your index register with 3 and set the multiplier to 4 This would load from location 2014 the fourth word Take the time to calculate these yourself to make sure you understand how it works In the indirect addressing mode the instruction contains a register that contains a pointer to where the data should be accessed For example if we used indirect addressing mode and specified the seax register and the eax register contained the value 4 whatever value was at
190. n of the C programming language because it is the most widely used for our examples and then show you some other possibilities Assembly Language Functions using the C Calling Convention You cannot write assembly language functions without understanding how the computer s stack works Each computer program that runs uses a region of memory called the stack to enable functions to work properly Think of a stack as a pile of papers on your desk which can be added to indefinitely You generally keep the things that you are working on toward the top and you take things off as you are finished working with them Your computer has a stack too The computer s stack lives at the very top addresses of memory You can push values onto the top of the stack through an instruction called push1 which pushes 2 A convention is a way of doing things that is standardized but not forcibly so For example it is a convention for people to shake hands when they meet If I refuse to shake hands with you you may think I don t like you Following conventions is important because it makes it easier for others to understand what you are doing and makes it easier for programs written by multiple independent authors to work together 37 Chapter 4 All About Functions either a register or value onto the top of the stack Well we say it s the top but the top of the stack is actually the bottom of the stack s memory Although this is confusing the r
191. n the ones spot For example Shift left 10010111 00101110 Rotate left 10010111 00101111 Notice that if you rotate a number for every digit it has i e rotating a 32 bit number 32 times you wind up with the same number However if you shift a number for every digit you have you wind up with 0 So what are these shifts useful for Well if you have binary numbers representing things you use shifts to peek at each individual value Let s say for instance that we had my Dad s likes stored in a register 32 bits It would look like this 00000000000000000000000000001011 Now as we said previously this doesn t work as program output So in order to do output we would need to do shifting and masking Masking is the process of eliminating everything you don t want In this case for every value we are looking for we will shift the number so that value is in the ones place and then mask that digit so that it is all we see For example let s say we 137 Chapter 10 Counting Like a Computer wanted to print out whether my Dad likes dressy clothes or not That data is the second value from the right So we have to shift the number right digit so it looks like this 00000000000000000000000000000101 and then we just want to look at that digit so we mask it by ANDing it with 00000000000000000000000000000001 00000000000000000000000000000101 AND 00000000000000000000000000000001 0000000000000000000000
192. nce that the operating system will allow you to have 20k of memory in physical memory and forces the rest of it to be on disk and your application uses 60k of memory Let s say your program has to do 5 operations on each piece of data If it does one operation on every piece of data and then goes through and does the next operation on each piece of data eventually every page of data will be loaded and unloaded from the disk 5 times Instead if you did all 5 operations on a given data item you only have to load each page from disk once When you bundle as many operations on data that is physically close to each other in memory then you are taking advantage of locality of reference In addition processors usually store some data on chip in a cache If you keep all of your operations within a small area of physical memory you can bypass even main memory and only use the chips ultra fast cache memory This is all done for you all you have to do is to try to operate on small sections of memory at a time rather than bouncing all over the place in memory Register Usage Registers are the fastest memory locations on the computer When you access memory the 161 Chapter 12 Optimization processor has to wait while it is loaded from the memory bus However registers are located on the processor itself so access is extremely fast Therefore making wise usage of registers is extremely important If you have few enough data items you are working
193. ndard practice among programmers in all languages 62 Chapter 5 Dealing with Files After this we have some constants labelled STACK POSITIONS Remember that function parameters are pushed onto the stack before function calls These constants prefixed with ST for clarity define where in the stack we should expect to find each piece of data The return address is at position 4 the length of the buffer is at position 8 and the address of the buffer is at position 12 This way when I use these stack addresses in the program it s easier to see what is happening Next comes the label convert_to_upper This is the entry point of the function The first two lines are our standard function lines to save the stack pointer The next two lines movl ST_BUFFER ebp eax movl ST_BUFFER_LEN ebp ebx move the function parameters into the appropriate registers for use Then we load zero into edi What we are going to do is iterate through each byte of the buffer by loading from the location eax edi incrementing edi and repeating until sedi is equal to the buffer length in sebx The lines cmpl 0 ebx je end_convert_loop are just a sanity check to make sure that noone gave us a buffer of zero size If they did we just clean up and leave Guarding against potential user and programming errors is an important task of a programmer You can always specify that your function should not take a buffer of zero size but it
194. ndividually under this License provided you insert a copy of this License into the extracted document and follow this License in all other respects regarding verbatim copying of that document 7 AGGREGATION WITH INDEPENDENT WORKS A compilation of the Document or its derivatives with other separate and independent documents or works in or on a volume of a storage or distribution medium does not as a whole count as a Modified Version of the Document provided no compilation copyright is claimed for the compilation Such a compilation is called an aggregate and this License does not apply to the other self contained works thus compiled with the Document on account of their being thus compiled if they are not themselves derivative works of the Document If the Cover Text requirement of section 3 is applicable to these copies of the Document then if the Document is less than one quarter of the entire aggregate the Document s Cover Texts may be placed on 223 Appendix H GNU Free Documentation License covers that surround only the Document within the aggregate Otherwise they must appear on covers around the whole aggregate 8 TRANSLATION Translation is considered a kind of modification so you may distribute translations of the Document under the terms of section 4 Replacing Invariant Sections with translations requires special permission from their copyright holders but you may include translation
195. nes are simply comments about the program Then there is the line int main int argc char argv This is the start of a function C Functions are declared with their name arguments and return type This declaration says that the function s name is main it returns an int integer 4 bytes long on the x86 platform and has two arguments an int called argc andachar called argv You don t have to worry about where the arguments are positioned on the stack the C compiler takes care of that for you You also don t have to worry about loading values into and out of registers because the compiler takes care of that too The main function is a special function in the C language it is the start of all C programs much like _start in our assembly language programs It always takes two parameters The first parameter is the number of arguments given to this command and the second parameter is a list of the arguments that were given The next line is a function call In assembly language you had to push the arguments of a function onto the stack and then call the function C takes care of this complexity for you You simply have to call the function with the parameters in parenthesis In this case we call the function puts with a single parameter This parameter is the character string we want to print We just have to type in the string in quotations and the compiler takes care of defining storage and moving the pointers to
196. nformation app_id ascii gnome example 0 app_version ascii 1 000 0 app_title ascii Gnome Example Program 0 Text for Buttons and windows button_quit_text ascii I Want to Quit the GNOME Example Program 0 gquit_question ascii Are you sure you want to quit 0 section bss Variables to save the created widgets in equ WORD_SIZE 4 lcomm appPtr WORD_SIZE lcomm btnQuit WORD_SIZE section text globl main type main function main pushl Sebp movl esp ebp Initialize GNOME libraries pushl 12 ebp argv pushl 8 ebp argc pushl app_version pushl app_id call gnome_init addl 16 esp recover the stack Create new application window pushl Sapp_title Window title pushl Sapp_id Application ID call gnome_app_new addl 8 esp recover the stack 173 Appendix A GUI Programming movl eax appPtr save the window pointer Create new button pushl button_quit_text button text call addl movl Make pushl pushl call addl gtk_button_new_with_label 4 Sesp recover the stack seax btnQuit save the button pointer the button show up inside the application window btnQuit appPtr gnome_app_set_contents 8 esp Makes the button show up only after it s window shows up though pushl btnQuit call addl gtk_widget_show 4 esp Makes the application window show up pushl cal
197. ng 192 Appendix B Common x86 Instructions Where to Go for More Information Intel has a set of comprehensive guides to their processors These are available at http www intel com design pentium manuals Note that all of these use the Intel syntax not the AT amp T syntax In addition you can find a lot of information in the manual for the GNU assembler available online at http www gnu org software binutils manual gas 2 9 1 as html Similarly the manual for the GNU linker is available online at http www gnu org software binutils manual ld 2 9 1 ld html 193 Appendix B Common x86 Instructions 194 Appendix C Important System Calls These are some of the more important system calls to use when dealing with Linux For most cases however it is best to use library functions rather than direct system calls because the system calls were designed to be minimalistic while the library functions were designed to be easy to program with For information about the Linux C library see the manual at http www gnu org software libc manual Remember that eax holds the system call numbers and that the return values and error codes are also stored in Seax Table C 1 Important Linux System Calls Seax Name ebx ecx edx Notes 1 exit return Exits the program value int 3 read file buffer start buffer size Reads into the given buffer descriptor int 4 write file buffer start buf
198. ng system like Microsoft Windows or the X Window System then the windowing system reads the keypress from the kernel and delivers it to whatever program is currently in focus on the user s display Example 1 1 How the computer processes keyboard sigals Keyboard gt Kernel gt Windowing system gt Application program The kernel also controls the flow of information between programs The kernel is a program s gate to the world around it Every time that data moves between processes the kernel controls the 3 The GNU Project is a project by the Free Software Foundation to produce a complete free operating system Chapter 1 Introduction messaging In our keyboard example above the kernel would have to be involved for the windowing system to communicate the keypress to the application program As a fence the kernel prevents programs from accidentally overwriting each other s data and from accessing files and devices that they don t have permission to It limits the amount of damage a poorly written program can do to other running programs In our case the kernel is Linux Now the kernel all by itself won t do anything You can t even boot up a computer with just a kernel Think of the kernel as the water pipes for a house Without the pipes the faucets won t work but the pipes are pretty useless if there are no faucets Together the user applications from the GNU project and other places and the kernel Linux
199. ng the majority of the time and try to optimize it Was the reason for the slowness different than you imagined e Has the compiler eliminated the need for local optimizations Why or why not e What kind of problems might a compiler run in to if it tried to optimize code across function call boundaries 2 Since these programs are usually short enough not to have noticeable performance problems looping through the program thousands of times will exaggerate the time it takes to run enough to make calculations 165 Chapter 12 Optimization 166 Chapter 13 Moving On from Here Congratulations on getting this far You should now have a basis for understanding the issues involved in many areas of programming Even if you never use assembly language again you have gained a valuable perspective and mental framework for understanding the rest of computer science There are essentially three methods to learn to program e From the Bottom Up This is how this book teaches It starts with low level programming and works toward more generalized teaching e From the Top Down This is the opposite direction This focuses on what you want to do with the computer and teaches you how to break it down more and more until you get to the low levels e From the Middle This is characterized by books which teach a specific programming language or API These are not as concerned with concepts as they are with specifics Different people lik
200. ng value There are two ways to check the contents of register in GDB The first one is the command info register This will display the contents of all registers in hexadecimal However we are only interested in eax at this point To just display seax we can do print S eax to print it in hexadecimal or do print d eax to print it in decimal Notice that in GDB registers are prefixed with dollar signs rather than percent signs Your screen should have this on it gdb print d Seax This means that the result of your first inquiry is 3 Every inquiry you make will be assigned a number prefixed with a dollar sign Now if you look back into the code you will find that 3 is the first number in the list of numbers to search through If you step through the loop a few more times you will find that in every loop iteration eax has the number 3 This is not what should be happening eax should go to the next value in the list in every iteration Okay now we know that eax is being loaded with the same value over and over again Let s search to see where eax is being loaded from The line of code is this movl data_items edi 4 eax So step until this line of code is ready to execute Now this code depends on two values data_items and edi data_items is a symbol and therefore constant It s a good idea to check your source code to make sure the label is in front of the right data but in our case it is Therefore we need to look at edi
201. nk it Just run in with the following command perl Hello World pl As you can see the Perl version is even shorter than the C version With Perl you don t have to declare any functions or program entry points You can just start typing commands and the interpreter will run them as it comes to them In fact this program only has two lines of code one of which is optional The first optional line is used for UNIX machines to tell which interpreter to use to run the program The tells the computer that this is an interpreted program and the usr bin perl tells the computer to use the program usr bin per1 to interpret the program However since we ran the program by typing in perl Hello World pl we had already specified that we were using the perl interpreter 156 Chapter 11 High Level Languages The next line calls a Perl builtin function print This has one parameter the string to print The program doesn t have an explicit return statement it knows to return simply because it runs off the end of the file It also knows to return 0 because there were no errors while it ran You can see that interpreted languages are often focused on letting you get working code as quickly as possible without having to do a lot of program setup One thing about Perl that isn t so evident from this example is that Perl treats strings as a single value In assembly language we had to program according to the computer s memory a
202. nt bpnum only if expression expr is non zero 213 Appendix F Using the GDB Debugger Using Breakpoints command bpnum cmd1 cmd2 Execute commands cmd cmd2 etc whenever breakpoint bpnum or the current breakpoint is hit continue Continue executing the program kill Stop executing the program delete bpnum1 bpnum2 Delete breakpoints bpnum1 bpnum2 etc or all breakpoints if none specified clear addr Clear the breakpoint at memory address addr clear fn Clear the breakpoint at function fn or the current breakpoint clear linenum Clear the breakpoint at line number linenum disable bpnum1 bpnum2 Disable breakpoints bpnum1 bpnum2 etc or all breakpoints if none specified enable bpnum 1 bpnum2 Stepping through the Program Enable breakpoints bpnum1 bpnum2 etc or all breakpoints if none specified Examining Registers and Memory nexti Step over the next instruction doesn t follow function calls stepi Step into the next instruction follows function calls finish Step out of the current function info registers Print the contents of all registers print f reg Print the contents of register reg using format f The format can be x hexadecimal u unsigned decimal o octal a address c character or f floating point x rsf addr Print the contents of m
203. nts For example movl data_items edi 4 ebx takes up 7 storage locations The first two hold the instruction the third one tells which registers to use and the next four hold the storage location of data_items In memory these look just like all the other numbers and the instructions themselves can be moved into and out of registers just like numbers because that s what they are Now let s define a few terms Address An address is the number of a storage location For example the first storage location on a computer has an address of 0 the second has an address of 1 and so on Every piece of data on the computer not in a register has an address Normally we don t ever type the exact address of something but we use symbols instead like using data_items in our second program Pointer A pointer is a register or memory storage location whose value is an address In our second example ebp was a pointer to the current stack position Programming uses a lot of pointers which we will see eventually 1 You actually never use addresses this low but it works for discussion 107 Chapter 9 Intermediate Memory Topics Byte This is the size of a storage location On x86 processors a byte can hold numbers between 0 and 255 Word This is the size of a normal register On x86 processors a word is four storage locations bytes long We have been using terms like storage location instead of their proper terms l
204. numbers that follow it data_items refers to the location of the first one Because data_items is a label any time in our program where we need to refer to this address we can use the data_items symbol and the assembler will substitute it with the address where the numbers start during assembly For example the instruction movl data_items eax would move the value 3 into eax There are several different types of memory locations other than long that can be reserved The main ones are as follows byte Bytes take up one storage location for each number They are limited to numbers between 0 and 255 23 Chapter 3 Your First Programs Lint Ints which differ from the int instruction take up two storage locations for each number These are limitted to numbers between 0 and 65535 long Longs take up four storage locations This is the same amount of space the registers use which is why they are used in this program They can hold numbers between 0 and 4294967295 ascii The ascii directive is to enter in characters into memory Characters each take up one storage location they are converted into bytes internally So if you gave the directive ascii Hello there 0 the assembler would reserve 12 storage locations bytes The first byte contains the numeric code for H the second byte contains the numeric code for e and so forth The last character is represented by 0 and it is the terminating character it will never dis
205. o Be specific based on your knowledge of both addressing modes and the meaning of _start How would this differ from the instruction movl _start eax Chapter 3 Your First Programs Going Further e Modify the first program to leave off the int instruction line Assemble link and execute the new program What error message do you get Why do you think this might be e So far we have discussed three approaches to finding the end of the list using a special number using the ending address and using the length count Which approach do you think is best Why Which approach would you use if you knew that the list was sorted Why 33 Chapter 3 Your First Programs 34 Chapter 4 All About Functions Dealing with Complexity In Chapter 3 the programs we wrote only consisted of one section of code However if we wrote real programs like that it would be impossible to maintain them It would be really difficult to get multiple people working on the project as any change in one part might adversely affect another part that another developer is working on To assist programmers in working together in groups it is necessary to break programs apart into separate pieces which communicate with each other through well defined interfaces This way each piece can be developed and tested independently of the others making it easier for multiple programmers to work on the project Programmers use functions to break their programs int
206. o the first digit still carries the sign bit making it simple to determine whether or not the number is positive or negative Negative numbers will always have a 1 in the leftmost bit One thing to note about the two s complement representation of signed numbers is that unlike unsigned quantities if you increase the number of bits you can t just add zeroes to the left of the number For example let s say we are dealing with four bit quantities and we had the number 3 1101 If we were to extend this into an eight bit register we could not represent it as 00001101 as this would represent 13 not 3 When you increase the size of a signed quantity in two s complement representation you have to perform sign extension Sign extension means that you 141 Chapter 10 Counting Like a Computer have to pad the left hand side of the quantity with whatever digit is in the sign digit So if we extend a negative number by 4 digits we should fill the new digits with a 1 If we extend a positive number by 4 digits we should fill the new digits with a 0 So the extension of 3 from four to eight bits will yield 11111101 The x86 processor has different forms of several instructions depending on whether they expect the quantities they operate on to be signed or unsigned These are listed in Appendix B For example the x86 processor has both a sign preserving shift right sar1 and a shift right which does not preserve the sign bit
207. o get a list of things both my Dad and I like You would use the AND operation So 1101 AND 1011 Which translates to Things we both like 136 Chapter 10 Counting Like a Computer Food yes Heavy Metal Music no Wearing Dressy Clothes no Football yes Remember the computer has no idea what the ones and zeroes represent That s your job Obviously later down the road you would examine each bit and tell the user what it s for If you asked a computer what two people agreed on and it answered 1001 it wouldn t be very useful Anyway let s say we want to know the things that we disagree on For that we would use XOR because it will return 1 only if one or the other is 1 but not both So 1101 XOR 1011 And ll let you translate that back out So you see how these work The previous operations AND OR NOT and XOR are called boolean operator because they were first studied by a guy with the last name of Boole So if someone mentiones boolean operators or boolean algebra you now know what they are talking about Anyway there are also two binary operators that aren t boolean shift and rotate Shifts and rotates each do what their name implies and can do so to the right or the left A left shift moves each digit of a binary number one space to the left puts a zero in the ones spot and chops off the furthest digit to the left A left rotate does the same thing but takes the furthest digit to the left and puts it i
208. o if another function moved the break between allocates and used the memory allocate would have no idea and just write over it That would suck Finally we have a problem that we have unrestricted access to global variables namely heap_begin and current_break Now heap_begin isn t a problem because it is set once and then only read However current_break changes quite often Later we will see cases 126 Chapter 9 Intermediate Memory Topics where you might be in allocate when you need to call allocate again because of an external event If the two allocates are both trying to modify current_break it could be disasterous If you are totally confused by this that s okay m just warning you about later chapters Just be aware that you should avoid using global variables as much as possible In this book we will use them a decent amount because the generally give shorter simpler programs which is good for a book but not so good for real life Review Know the Concepts e Describe the Linux stack e What are the initial contents of the Linux stack e What happens when you access unmapped memory e How does the operating system prevent processes from writing over each other s memory e What happens if a piece of memory you are using is currently residing on disk e What is the current break e Why do you need an allocator Use the Concepts Going Further 127 Chapter 9 Intermediate Memory Topics 128 Cha
209. o now we can just pop it out into ebx addl eax sebx fadd them together result in ebx 42 Chapter 4 All About Functions movl 1 eax exit ebx is returned int 0x80 PURPOSE This function is used to compute the value of a number raised to a power INPUT First argument the base number Second argument the power to raise it to OUTPUT Will give the result as a return value NOTES The power must be 1 or greater VARIABLES Sho sk Sk SE Sk Sk ER type power power pushl Sebp Sebx holds the base number secx holds the power 4 ebp holds the current result Seax is used for temporary storage function save old base pointer movl esp ebp make stack pointer the base pointer subl 4 esp get room for our local storage movl 8 ebp ebx put first argument in eax movl 12 ebp secx put second argument in ecx movl ebx 4 ebp Sstore current result power_loop_start cmp1 je movl imul movl decl 1 ecx if the power is 1 we are done end_power 4 ebp eax move the current result into eax Sebx eax multiply the current result by the base number Seax 4 Sebp store the current result SECX decrease the power 43 Chapter 4 All About Functions jmp power_loop_start run for the next power end_power movl 4 ebp eax re
210. o pieces which can be independently developed and tested Functions are units of code that do a defined piece of work on specified types of data For example in a word processor program I may have a function called handle_typed_character which is activated whenever a user types in a key The data the function uses would probably be the keypress itself and the document the user currently has open The function would then modify the document according to the keypress it was told about The data items a function is given to process are called it s parameters In the word processing example the key which was pressed and the document would be considered parameters to the handle_typed_characters function Much care goes into determining what parameters a function takes because if it is called from many places within a project it is difficult to change if necessary A typical program is composed of thousands of functions each with a small well defined task to perform However ultimately there are things that you cannot write functions for which must be provided by the system Those are called primitive functions or just primitives they are the basics which everything else is built off of For example imagine a program that draws a graphical user interface There has to be a function to create the menus That function probably calls other functions to write text to write icons to paint the background calculate where the mouse pointer is etc Ho
211. o wherever you want afterwards There is somewhat of a chicken and egg problem in teaching programming especially assembly language There is a lot to learn it s almost too much to learn almost at once but each piece Chapter 1 Introduction depends on all the others Therefore you must be patient with yourself and the computer while learning to program If you don t understand something the first time reread it If you still don t understand it it is sometimes best to take it by faith and come back to it later Often after more exposure to programming the ideas will make more sense Don t get discouraged It s a long climb but very worthwhile At the end of each chapter are three sets of review exercises The first set is more or less regurgitation they check to see if can you give back what you learned in the chapter The second set contains application questions they check to see if you can apply what you learned to solve problems The final set is to see if you are capable of broadening your horizons Some of these questions may not be answerable until later in the book but they give you some things to think about Other questions require some research into outside sources to discover the answer Still others require you to simply analyze your options and explain a best solution Many of the questions don t have right or wrong answers but that doesn t mean they are unimportant Learning the issues involved in programming
212. ocations after the actual start of the memory segment All we have to do is go back 8 locations and mark that memory as available Sho Sh E SE Sk HE so that the allocate function knows it can use it globl deallocate 118 Chapter 9 Intermediate Memory Topics type deallocate function cqu ST_MEMORY_SEG 4 stack position of the memory segment to free deallocate Since the function is so simple we don t need any of the fancy function SUf movl ST_MEMORY_SEG esp eax get the address of the memory to free normally this is 8 ebp but since we didn t push ebp or move esp to Sebp we can just do 4 esp subl SHEADER_SIZE eax get the pointer to the real beginning of the memory movl SAVAILABLE HDR_AVAIL_OFFSET eax mark it as available ret return END OF FUNCTION The first thing to notice is that there is no __start symbol The reason is that this is just a section of a program A memory manager by itself is not a full program it doesn t do anything It has to be linked with another program to work Will will see that happen later So you can assemble it but you can t link it So type in the program as alloc s and then assemble it with as alloc s o alloc o
213. ogramming by Eric S Raymond available online at http www catb org esr writings taoup e UNIX Programming Advanced Programming in the UNIX Environment by W Richard Stevens e Network Programming UNIX Network Programming 2 volumes by W Richard Stevens e Generic Programming Modern C Design by Andrei Alexandrescu e Compilers The Art of Compiler Design Theory and Practice by Thomas Pittman and James Peters e Compilers Advanced Compiler Design and Implementation by Steven Muchnick Development Process Refactoring Improving the Design of Existing Code by Martin Fowler Kent Beck John Brant William Opdyke and Don Roberts e Typesetting Computers and Typesetting 5 volumes by Donald Knuth e Cryptography Applied Cryptography by Bruce Schneier 169 Chapter 13 Moving On from Here Linux Professional Linux Programming by Neil Matthew Richard Stones and 14 other people Linux Kernel Linux Device Drivers by Alessandro Rubini and Jonathan Corbet Open Source Programming The Cathedral and the Bazaar Musings on Linux and Open Source by an Accidental Revolutionary by Eric S Raymond Computer Architecture Computer Architecture A Quantitative Approach by David Patterson and John Hennessy Further Resources on Assembly Language In assembly language your best resources are on the web http www linuxassembly org a great resource for Linux assembly language programmers http www sandpile org a
214. ommunicating to a computer as it is communicating to those who come after you A programmer is a problem solver a poet and an instructor all at once Your goal is to solve the problem at hand doing so with balance and taste and teach your solution to future programmers I hope that this book can teach at least some of the poetry and magic that makes computing exciting Most introductory books on programming frustrate me to no end At the end of them you can still ask how does the computer really work and not have a good answer They tend to pass over topics that are difficult even though they are important I will take you through the difficult issues because that is the only way to move on to masterful programming My goal is to take you from knowing nothing about programming to understanding how to think write and learn like a programmer You won t know everything but you will have a background for how everything fits together At the end of this book you should be able to do the following e Understand how a program works and interacts with other programs e Read other people s programs and learn how they work Learn new programming languages quickly e Learn advanced concepts in computer science quickly I will not teach you everything Computer science is a massive field especially when you combine the theory with the practice of computer programming However I will attempt to get you started on the foundations so you can easily g
215. on allocate_init pushl Sebp standard function stuff movl esp ebp If the brk system call is called with 0 in ebx it returns the last valid usable address movl S BRK eax find out where the break is movl 0 ebx int SLINUX_SYSCALL incl eax Seax now has the last valid address and we want the memory location after that movl S eax current_break store the current break movl eax heap_begin store the current break as our first address This will cause 114 movl ebp popl S ebp ret Chapter 9 Intermediate Memory Topics the allocate function to get more memory from Linux the first time it is run esp exit the function END OF FUNCTION 4444 allocate PURPOSE PARAMETERS 4 This function is used to grab a section of memory It checks to see if there are any free blocks and if not it asks Linux for a new one This function has one parameter the size of the memory block we want to allocate RETURN VALUE This function returns the address of the allocated memory in Seax If there is no memory available it will return 0 in eax H HHHH PROCESSINGHHHHHHHH Variables used ecx hold the size of the requested memory first only parameter eax current memory segment being examined ebx current break position sedx size of current memory segment We scan
216. onstants that we have been defining over and over in our programs and it is useful to put them in a file so that we don t have to keep entering them over and over again Put the following constants in a file called Linux s Common Linux Definitions System Call Numbers equ SYS_EXIT 1 egu SYS_READ 3 equ SYS_WRITE 4 equ SYS_OPEN u u on equ SYS_CLOSE equ SYS_BRK 45 System Call Interrupt Number equ LINUX_SYSCALL 0x80 Standard File Descriptors equ STDIN 0 equ STDOUT 1 egqu STDERR 2 Common Status Codes equ END_OF_FILE 0 We will write three programs in this chapter using the structure defined above The first program will build a file containing several records as defined above The second program will display the records in the file The third program will add 1 year to the age of every record In addition to the standard constants we will be using throughout the programs there are also two functions that we will be using in several of the programs one which reads a record and one which writes a record What parameters do these functions need in order to operate We basically need 68 Chapter 6 Reading and Writing Simple Records The location of a buffer that we can read a record into The file descriptor that we want to read from or write to Let s look at our reading function first
217. op of the stack we can simply use the sesp register in indirect addressing mode For example the following code moves whatever is at the top of the stack into eax movl esp eax If we were to just do this movl esp eax then eax would just hold the pointer to the top of the stack rather than the value at the top Putting zesp in parenthesis causes the computer to go to indirect addressing mode and therefore we get the value pointed to by esp If we want to access the value right below the top of the stack we can simply issue this instruction movl 4 esp eax This instruction uses the base pointer addressing mode see the Section called Data Accessing Methods in Chapter 2 which simply adds 4 to esp before looking up the value being pointed to In the C language calling convention the stack is the key element for implementing a function s local variables parameters and return address 38 Chapter 4 All About Functions Before executing a function a program pushes all of the parameters for the function onto the stack in the reverse order that they are documented Then the program issues a ca11 instruction indicating which function it wishes to start The ca11 instruction does two things First it pushes the address of the next instruction which is the return address onto the stack Then it modifies the instruction pointer Seip to point to the start of the function So at the time the function starts the stack looks l
218. open segment we will eventually reach the end of the heap Whenever you write a loop you must always make sure it ends or else the 124 Chapter 9 Intermediate Memory Topics computer will waste all of its time there and yov ll have to terminate your program This is called an infinite loop because it could go on forever without stopping The deallocate function The deallocate function is much easier than the allocate one That s because it doesn t have to do any searching at all It can just mark the current memory segment as AVAILABLE and allocate will find it next time it is run So we have movl ST_MEMORY_SEG esp eax sub SHEADER_SIZE eax movl SAVAILABLE HDR_AVAIL_OFFSET eax ret In this function we don t have to save Sebp or zesp since we re not changing them nor do we have to restore them at the end All we re doing is reading the address of the memory segment from the stack backing up to the beginning of the header and marking the segment as available This function has no return value so we don t care what we leave in eax Performance Issues and Other Problems Okay so we have our nice little memory manager It s a very simplistic one Most memory managers are much more complex Ours was simple so you could see the basics of what a memory manager has to deal with Now our memory manager does work it just doesn t do so optimal
219. ored at 8 Sesp the name of the program is stored at 12 sesp and the arguments are stored from 16 esp on In the C Programming language this is referred to as the argv array so we will refer to it that way in our program The first thing our program does is save the current stack position in ebp and then reserve some space on the stack to store the file descriptors After this it starts opening files The first file the program opens is the input file which is the first command line argument We do this by setting up the system call We put the file name into ebx a readonly mode into ecx the default mode of 0666 into Sedx and the system call number into eax After the system call the file is open and the file descriptor is stored in eax The file descriptor is then transferred to it s appropriate place on the stack 6 If you aren t familiar with UNIX permissions just put 0666 here Don t forget the leading zero as it means that the number is an octal number 7 Notice that we don t do any error checking on this That is done just to keep the program simple In normal programs every system call should normally be checked for success or failure In failure cases eax will hold an error code instead of a return value Error codes are negative so they can be detected by comparing eax to zero and jumping if it is less than zero 64 Chapter 5 Dealing with Files The same is then done for the output file except
220. ost stateless functions are parallelizable and often benefit from memoization Global optimization takes quite a bit of practice to know what works and what doesn t Deciding how to tackle optimization problems in code involves looking at all the issues and knowing that fixing some issues may cause others Review Know the Concepts e At what level of importance is optimization compared to the other priorities in programming e What is the difference between local and global optimizations e Name some types of local optimizations e How do you determine what parts of your program need optimization e At what level of importance is optimization compared to the other priorities in programming Why do you think I repeated that question Use the Concepts e Go back through each program in this book and try to make optimizations according to the procedures outlined in this chapter 164 Chapter 12 Optimization e Pick a program from the previous exercise and try to calculate the performance impact on your code under specific inputs Going Further e Find an open source program that you find particularly fast Contact one of the developers and ask about what kinds of optimizations they performed to improve the speed e Find an open source program that you find particularly slow and try to imagine the reasons for the slowness Then download the code and try to profile it using gprof or similar tool Find where the code is spendi
221. out Functions return address The return address is an invisible parameter in that it isn t directly used during the function The return address is a parameter which tells the function where to resume executing after the function is completed This is needed because functions can be called to do processing from many different parts of your program and the function needs to be able to get back to wherever it was called from In most programming languages this parameter is passed automatically when the function is called In assembly language the call instruction handles it for you return value The return value is the main method of transferring data back to the main program Most programming languages only allow a single return value for a function although some allow multiple These pieces are present in most programming languages How you specify each piece is different in each one however The way that the variables are stored and the parameters and return values are transferred by the computer varies from language to language as well This variance is known as a language s calling convention because it describes how functions expect to get and receive data when they are called Assembly language can use any calling convention it wants to You can even make one up yourself However if you want to interoperate with functions written in other languages you have to obey their calling conventions We will use the calling conventio
222. own way of dealing with files However the UNIX method which is used on Linux is the simplest and most universal UNIX files no matter what program created them can all be accessed as a sequential stream of bytes When you access a file you start by opening it by name The operating system then gives you a number called a file descriptor which you use to refer to the file until you are through with it You can then read and write to the file using its file descriptor When you are done reading and writing you then close the file which then makes the file descriptor useless In our programs we will deal with files in the following ways 1 Tell Linux the name of the file to open and in what mode you want it opened read write both read and write create it if it doesn t exist etc This is handled with the open system call which takes a filename a number representing the mode and a permission set as its parameters eax will hold the system call number which is 5 The address of the first character of the filename should be stored in sebx The read write intentions represented as a number should be stored in ecx For now use 0 for files you want to read from and 03101 for files you want to write to you must include the leading zero Finally the permission set should be stored as a number in edx If you are unfamiliar with UNIX permissions just use 0666 for the permissions again you must include the leading zero N L
223. pecified by data_items then you add edi 4 storage locations and retrieve the number there Usually you use the size of the numbers as your multiplier but in some circumstances you ll want to do other things 10 Also the 1 in movi stands for move long since we are moving a value that takes up four storage locations 26 Chapter 3 Your First Programs e We have to load the next value of our list e We have to see if the next value is bigger than our current biggest value e Ifitis we have to copy it to the location we are holding the largest value in e Now we need to go back to the beginning of the loop Okay so now lets go to the code We have the beginning of the loop marked with start_loop That is so we know where to go back to at the end of our loop Then we have these instructions cmpl 0 eax je end_loop The cmp1 instruction compares the two values Here we are comparing the number 0 to the number stored in seax This compare instruction also affects a register not mentioned here the eflags register This is also known as the status register and has many uses which we will discuss later Just be aware that the result of the comparison is stored in the status register The next line is a flow control instruction which says to jump to the end_loop location if the values that were just compared are equal that s what the e of je means It uses the status register to hold the value of the last comparison We used je but ther
224. play it just tells other parts of the program that that s the end of the characters Letters and numbers that start with a backslash represent characters that are not typeable on the keyboard or easily viewable on the screen For example n refers to the newline character which causes the computer to start output on the next line and t refers to the tab character All of the letters in an ascii directive should be in quotes In our example the assembler reserves 14 Longs one right after another Since each long takes up 4 bytes that means that the whole list takes up 56 bytes These are the numbers we will be searching through to find the maximum data_items is used by the assembler to refer to the address of the first of these values Take note that the last data item in the list is a zero I decided to use a zero to tell my program that it has hit the end of the list I could have done this other ways I could have had the size of the list hard coded into the program Also I could have put the length of the list as the first item or in a separate location I also could have made a symbol which marked the last location of the list items No matter how I do it I must have some method of determining the end of the list The computer knows nothing it can only do what its told It s not going to stop processing unless I give it some sort of signal Otherwise it would continue processing past the end of the list into the data that follow
225. pter 10 Counting Like a Computer Counting Counting Like a Human In many ways computers count just like humans So before we start learning how computers count let s take a deeper look at how we count How many fingers do you have No it s not a trick question Humans normally have ten fingers Why is that significant Look at our numbering system At what point does a one digit number become a two digit number That s right at ten Humans count and do math using a base ten numbering system Base ten means that we group everything in tens Let s say we re counting sheep 1 2 3 4 5 6 7 8 9 10 Why did we all of a sudden now have two digits and re use the 1 That s because we re grouping our numbers by ten and we have 1 group of ten sheep Okay let s go to the next number 11 That means we have 1 group of ten sheep and 1 sheep left ungrouped So we continue 12 13 14 15 16 17 18 19 20 Now we have 2 groups of ten 21 2 groups of ten and 1 sheep ungrouped 22 2 groups of ten and 2 sheep ungrouped So let s say we keep counting and get to 97 98 99 and 100 Look it happened again What happens at 100 We now have ten groups of ten At 101 we have ten groups of ten and 1 ungrouped sheep So we can look at any number like this If we counted 60879 sheep that would mean that we had 6 groups of ten groups of ten groups of ten groups of ten 0 groups of ten groups of ten groups of ten 8 groups of te
226. r eax ST_INPUT_DESCRIPTOR ebp a local Even though it s a constant we are saving the output file descriptor in variable so that if we later decide that it isn t always going to be STDOUT we can change it easily movl SSTDOUT ST_OUTPUT_DESCRIPTOR ebp 77 Chapter 6 Reading and Writing Simple Records record_read_loop pushl ST_INPUT_DESCRIPTOR Sebp pushl Srecord_buffer call read_record addl 8 esp Returns the number of bytes read If it isn t the same number we requested then it s either an end of file quitting or an error so we re cmpl SRECORD_SIZE eax jne finished_reading Otherwise print out the first name but first we must know it s size pushl SRECORD_FIRSTNAME record_buffer call count_chars addl 4 esp movl Seax Sedx movl ST_OUTPUT_DESCRIPTOR ebp ebx movl SSYS_WRITE eax movl SRECORD_FIRSTNAME record_buffer int SLINUX_SYSCALL pushl ST_OUTPUT_DESCRIPTOR ebp call write_newline addl 4 esp jmp record_read_loop finished_reading movl SSYS_EXIT eax movl 0 ebx int SLINUX_SYSCALL ecx To build this program we need to assemble all of the parts and link them together as read record s 0o read record o as count chars s 0o count chars o as write newline s o write newline o 78
227. r e Rewrite the programs in this chapter to use command line arguments to specify the filesnames 82 Chapter 6 Reading and Writing Simple Records Research the 1seek system call Rewrite the add year program to open the source file for both reading and writing use 2 for the read write mode and write the modified records back to the same file they were read from Research the various error codes that can be returned by the system calls made in these programs Pick one to rewrite and add code that checks eax for error conditions and if one is found writes a message about it to STDERR and exit Write a program that will add a single record to the file by reading the data from the keyboard Remember you will have to make sure that the data has at least one null character at the end and you need to have a way for the user to indicate they are done typing Because we have not gotten into characters to numbers conversion you will not be able to read the age in from the keyboard so you ll have to have a default age Write a function called compare strings that will compare two strings up to 5 characters Then write a program that allows the user to enter 5 characters and have the program return all records whose first name starts with those 5 characters 83 Chapter 6 Reading and Writing Simple Records 84 Chapter 7 Developing Robust Programs This chapter deals with developing programs that are robust Robust programs a
228. r copies If you distribute a large enough number of copies you must also follow the conditions in section 3 You may also lend copies under the same conditions stated above and you may publicly display copies 3 COPYING IN QUANTITY If you publish printed copies of the Document numbering more than 100 and the Document s license notice requires Cover Texts you must enclose the copies in covers that carry clearly and legibly all these Cover Texts Front Cover Texts on the front cover and Back Cover Texts on the back cover Both covers must also clearly and legibly identify you as the publisher of these copies The front cover must present the full title with all words of the title equally prominent and visible You may add other material on the covers in addition Copying with changes limited to the covers as long as they preserve the title of the Document and satisfy these conditions can be 220 Appendix H GNU Free Documentation License treated as verbatim copying in other respects If the required texts for either cover are too voluminous to fit legibly you should put the first ones listed as many as fit reasonably on the actual cover and continue the rest onto adjacent pages If you publish or distribute Opaque copies of the Document numbering more than 100 you must either include a machine readable Transparent copy along with each Opaque copy or state in or with each Opaque copy a publicly accessible computer ne
229. rackets e In Intel syntax the instruction name does not include the size of data being moved If that is ambiguous it is explicitly stated as BYTE WORD or DWORD immediately after the instruction name e The way that memory addresses are represented in Intel assembly language is much different shown below e Because the x86 processor line originally started out as a 16 bit processor most literature about x86 processors refer to words as 16 bit values and call 32 bit values double words However we use the term word to refer to the standard register size on a processor which is 32 bits on an x86 processor The syntax also keeps this naming convention DWORD stands for double word in Intel syntax and is used for standard sized registers which we would call simply a word e Intel assembly language has the ability to address memory as a segment offset pair We do not mention this because Linux does not support segmented memory and is therefore irrelevant to normal Linux programming Other differences exist but they are small in comparison To show some of the differences consider the following instruction movl eax 8 ebx sedi 4 In Intel syntax this would be written as mov 8 ebx 1 edi eax The memory reference is a bit easier to read than it s AT amp T counterpart because it spells out exactly how the address will be computed However but the order of operands in Intel syntax can be confusi
230. ral errors Thanks to Harald Korneliussen for the many suggestions and the ASCII table e 01 11 2004 Version 0 7 Added C translation appendix added the beginnings of an appendix of x86 instructions added the beginnings of a GDB appendix finished out the files chapter finished out the counting chapter added a records chapter created a source file of common linux definitions corrected several errors and lots of other fixes e 01 22 2004 Version 0 8 Finished GDB appendix mostly finished w appendix of x86 instructions added section on planning programs added lots of review questions and got everything to a completed initial draft state e 01 29 2004 Version 0 9 Lots of editting of all chapters Made code more consistent and made explanations clearer Added some illustrations 217 Appendix G Document History 218 Appendix H GNU Free Documentation License 0 PREAMBLE The purpose of this License is to make a manual textbook or other written document free in the sense of freedom to assure everyone the effective freedom to copy and redistribute it with or without modifying it either commercially or noncommercially Secondarily this License preserves for the author and publisher a way to get credit for their work while not being considered responsible for modifications made by others This License is a kind of copyleft which means that derivative works of the document must themselves
231. rce files and you will convert each one into an object file The linker is the program that is responsible for putting the object files together and adding information to it so that the kernel knows how to load and run it In our case we only have one object file so the linker is only adding the information to enable it to run To link the file enter the command ld exit o o exit 14 Chapter 3 Your First Programs 1d is the command to run the linker exit o is the object file we want to link and o exit instructs the linker to output the new program into a file called exit If any of these commands reported errors you have either mistyped your program or the command After correcting the program you have to re run all the commands You must always re assemble and re link programs after you modify the source file for the changes to occur in the program You can run exit by typing in the command exit The is used to tell the computer that the program isn t in one of the normal program directories but is the current directory instead You ll notice when you type this command the only thing that happens is that you ll go to the next line That s because this program does nothing but exit However immediately after you run the program if you type in echo It will say 0 What is happening is that every program when it exits gives Linux an exit status code which tells it if everything went all right If everythin
232. rchitecture which meant that strings had to be treated as a sequence of multiple values with a pointer to the first letter Perl pretends that strings can be stored directly as values and thus hides the complication of manipulating them for you In fact one of Perl s main strengths is it s ability and speed at manipulating text However that is outside the scope of this book Python The python version of the program looks almost exactly like the Perl one However Python is really a very different language than Perl even if it doesn t seem so from this trivial example Type the program into a file named Hello World py The program follows usr bin python print Hello World You should be able to tell what the different lines of the program do Review Know the Concepts e What is the difference between an intepretted language and a compiled language e What reasons might cause you to need to learn a new programming language Use the Concepts e Learn the basic syntax of a new programming language Recode one of the programs in this book in that language 157 Chapter 11 High Level Languages In the program you wrote in the question above what specific things were automated in the programming language you chose Modify your program so that it runs 10 000 times in a row both in assembly language and in your new language Then run the time command to see which is faster Which does come out ahead Why
233. rd it is loaded as an instruction Other than that the computer has no way of knowing the difference between programs and other types of data Interpreting Memory Computers are very exact Because they are exact programmers have to be equally exact A computer has no idea what your program is supposed to do Therefore it will only do exactly what you tell it to do If you accidentally print out a regular number instead of the ASCII codes that make up the number s digits the computer will let you and you will wind up with jibberish on your screen it will try to look up what your number represents in ASCII and print that If you tell the computer to start executing instructions at a location containing data instead of program instructions who knows how the computer will interpret that but it will certainly try The computer will execute your instructions in the exact order you specify even if it doesn t make sense The point is the computer will do exactly what you tell it no matter how little sense it makes Therefore as a programmer you need to know exactly how you have your data arranged in memory Remember computers can only store numbers so letters pictures music web pages documents and anything else are just long sequences of numbers in the computer which particular programs know how to interpret For example say that you wanted to store customer information in memory One way to do so would be to set a maximum s
234. re able to handle error conditions gracefully They are programs that do not crash no matter what the user does Building robust programs is essential to the practice of programming Writing robust programs takes discipline and work it is usually finding every possible problem that can occur and coming up with an action plan for your program to take Where Does the Time Go Programmers schedule poorly In almost every programming project programmers will take two four or even eight times as long to develop a program or function than they originally estimated There are many reasons for this problem including e Programmers don t always schedule time for meetings or other non coding activities that make up every day e Programmers often underestimate feedback times how long it takes to pass change requests and approvals back and forth for projects e Programmers don t always understand the full scope of what they are producing e Programmers often have to estimate a schedule on a totally different kind of project than they are used to and thus are unable to schedule accurately e Programmers often underestimate the amount of time it takes to get a program fully robust The last item is the one we are interested in here t takes a lot of time and effort to develop robust programs More so than people usually guess including experienced programmers Programmers get so focused on simply solving the problem at hand that they fail
235. re they used for e What does the instruction pointer do ll Chapter 2 Computer Architecture Use the Concepts What data would you use in an employee record How would you lay it out in memory If I had the pointer the the beginning of the employee record above and wanted to access a particular piece of data inside of it what addressing mode would I use In base pointer addressing mode if you have a register holding the value 3122 and an offset of 20 what address would you be trying to access In indexed addressing mode if the base address is 6512 the index register has a 5 and the multiplier is 4 what address would you be trying to access In indexed addressing mode if the base address is 123472 the index register has a 0 and the multiplier is 4 what address would you be trying to access In indexed addressing mode if the base address is 9123478 the index register has a 20 and the multiplier is 1 what address would you be trying to access Going Further 12 What are the minimum number of addressing modes needed for computation Why include addressing modes that aren t strictly needed Research and then describe how pipelining or one of the other complicating factors affects the fetch execute cycle Research and then describe the tradeoffs between fixed length instructions and variable length instructions Chapter 3 Your First Programs In this chapter you will learn the process for writing and bui
236. resents testl 1 eax jns continue_processing 90 Chapter 7 Developing Robust Programs Send the error section data no_open_file_cod ascii 0001 O no_open_file_msg ascii Can t Open Input File o section text pushl Sno_open_file_msg pushl Sno_open_file_code call error_exit continue_processing Rest of program So after calling the system call we check and see if we have an error If so we call our error reporting and exit routine After every system call or other location which can have erroneous results you should add error checking and handling code To assemble and link the files do as add year s o add year o as error exit s 0o error exit o ld add year o write newline o error exit o read record o write record o count chars o o add year Now try to run it without the necessary files It now exits cleanly and gracefully Review Know the Concepts e What are the reasons programmer s have trouble with scheduling e What are corner cases Can you list examples of numeric corner cases e Why is user testing so important e What are stubs and drivers used for What s the difference between the two e What are recovery points used for 91 Chapter 7 Developing Robust Programs e How many different error codes should a program have Use the Concepts e Go through the add year s program and add error checking code after every system call
237. rked to indicate to the debugger that it should stop the program when it hits that point To set breakpoints you have to set them up before you run the program Before issuing the run command you can set up breakpoints using the break command For example to break on line 27 issue the command break 27 Then when the program crosses line 27 it will stop running and print out the current line and instruction You can then step through the program from that point and examine registers and memory To look at the lines and line numbers of your program you can simply use the command 1 This will print out your program with line numbers a screen at a time When dealing with functions you can also break on the function names For example in the factorial program in Chapter 4 we could set a breakpoint for the factorial function by typing in break factorial This will cause the debugger to break immediately after the function call and the function setup it skips the pushing of ebp and the copying of zesp When stepping through code you often don t want to have to step through every instruction of every function Well tested functions are usually a waste of time to step through except on rare occasion Therefore if you use the next i command instead of the stepi command GDB will wait until completion of the function before going on Otherwise with stepi GDB would step you through every instruction within every called function 212 Appendi
238. rogram containing the copied code has the same code in it thus wasting a lot of space Ifa bug is found in any of the copied code it has to be fixed in every application program Therefore the second option is usually used sparingly It is usually only used in cases where you copy and paste skeleton code for a specific type of task and add in your program specific details The third option is the one that is used the most often The third option includes having a central repository of shared code Then instead of each program wasting space storing the same copies of functions they can simply point to the shared files which contain the functions they need If a bug is found in one of these functions it only has to be fixed within the shared file and all applications which use it are automatically updated The main drawback with this approach is that it creates some dependency problems including e If multiple applications are all using the shared file how do we know when it is safe to delete the file For example if three applications are sharing a file of functions and 2 of the programs are deleted how does the system know that there still exists an application that uses that code and therefore it shouldn t be deleted e Some programs inadvertantly rely on bugs within shared functions Therefore if upgrading the shared program fixes a bug that a program depended on it could cause that application to cease functioning These problems
239. rpreted languages These languages require that the user run a program called an interpreter that in turn runs the given program These are usually slower than compiled programs since the translator has to read and interpret the code as it goes along However in well made translators this time can be fairly negligible There is also a class of hybrid languages which partially compile a program before execution into byte codes which are only machine readable This is done because the translator can read the byte codes much faster than it can read the regular language 153 Chapter 11 High Level Languages There are many reasons to choose one or the other Compiled programs are nice because you don t have to already have a translator installed in the user s machine You have to have a compiler for the language but the users of your program don t In a translated language you have to be sure that the user has a translator for your program and that the computer knows which translator runs your program Also translated languages tend to be more flexible while compiled languages are more rigid Language choice is usually driven by available tools and support for programming methods rather than by whether a language is compiled or interpretted In fact many languages have options for either one So why does one choose one language over another For example many choose Perl because it has a vast library of functions for handling just about
240. rs without explaining what they were A buffer is a continuous block of bytes used for bulk data transfer When you request to read a file the operating system needs to have a place to store the data it reads That place is called a buffer Usually buffers are only used to store data temporarily and it is then read from the buffers and converted to a form that is easier for the programs to handle Our programs won t be complicated enough to need that done For an example let s say that you want to read in a single line of text from a file but you do not know how long that line is You would then simply read a large number of bytes characters from the file into a buffer look for the end of line character and copy all of the characters to that end of line character to another location If you didn t find and end of line character you would allocate another buffer and continue reading You would probably wind up with some characters left over in your buffer in this case which you would use as the starting point when you next need data from the file Another thing to note is that buffers are a fixed size set by the programmer So if you want to read in data 500 bytes at a time you send the read system call the address of a 500 byte unused location and send it the number 500 so it knows how big it is You can make it smaller or bigger depending on your application s needs To create a buffer you need to either reserve static or dynamic s
241. rts the condition the true branch and the false branch However since assembly language is not a block structured language you have to work a little to implement the block like nature of C For example look at the following C code True Branch Code Here False Branch Code Here At This Point Reconverge In assembly language this can be rendered as Move a and b into registers for comparison movl a Seax movl b Sebx Compare cmpl eax ebx If True go to true branch je true_branch false_branch This label is unnecessary only here for documentation False Branch Code Here Jump to recovergence point jmp reconverge true_branch True Branch Code Here 199 Appendix E C Idioms in Assembly Language reconverge Both branches recoverge to this point As you can see since assembly language is linear the blocks have to jump around each other Recovergence is handled by the programmer not the system A case statement is written just like a sequence of if statements Function Call A function call in assembly language simply requires pushing the arguments to the function onto the stack in reverse order and issuing a ca11 instruction After calling the arguments are then popped back off of the stack For example consider the C code printf The number is d 88 In assembly language this would be rendered as section data text_string ascii The
242. s which will help you to learn different and better programming methods and ideas e Different languages are good for different types of projects e Different companies have different standard languages so knowing more languages makes your skills more marketable e The more languages you know the easier it is to pick up new ones AS a programmer you will often have to pick up new languages Professional programmers can usually pick up a new language with about a weeks worth of study and practice Languages are simply tools and learning to use a new tool should not be something a programmer flinches at In fact if you do computer consulting you will often have to learn new languages on the spot in order to keep yourself employed It will often be your customer not you who decides what language is used This chapter will introduce you to a few of the languages available to you I encourage you to explore as many languages as you are interested in Compiled and Interpreted Languages Many languages are compiled languages When you write assembly language each instruction you write is translated into exactly one machine instruction for processing With compilers a statement can translate into one or hundreds of machine instructions In fact depending on how advanced your compiler is it might even restructure parts of your code to make it faster In assembly language what you write is what you get There are also languages that are inte
243. s a constant value to the address in the register For example if you have a record where the age value is 4 bytes into the record and you have the address of the record in eax you can retrieve the age into Sebx by issuing the following instruction movl 4 eax Sebx immediate mode Immediate mode is very simple It does not follow the general form we have been using Immediate mode is used to load direct values into registers or memory locations For example if you wanted to load the number 12 into eax you would simply do the following movl 12 eax Notice that to indicate immediate mode we used a dollar sign in front of the number If we did not it would be direct addressing mode in which case the value located at memory location 12 would be loaded into eax rather than the number 12 itself register addressing mode Register mode simply moves data in or out of a register In all of our examples register addressing mode was used for the other operand These addressing modes are very important as every memory access will use one of these Every mode except immediate mode can be used as either the source or destination operand Immediate mode can only be a source operand In addition to these modes there are also different instructions for different sizes of values to move For example we have been using mov 1 to move data a word at a time in many cases you will only want to move data a byte at a time This is accomplish
244. s for the price how many I have in inventory and so on So what about if we need numbers larger than 255 We can simply use a combination of bytes to represent larger numbers Two bytes can be used to represent any number between 0 and 65536 Four bytes can be used to represent any number between 0 and 4294967295 Now it is quite difficult to write programs to stick bytes together to increase the size of your numbers and requires a bit of math Luckily the computer will do it for us for numbers up to 4 bytes long In fact four byte numbers are what we will work with by default We mentioned earlier that in addition to the regular memory that the computer has it also has special purpose storage locations called registers Registers are what the computer uses for computation Think of a register as a place on your desk it holds things you are currently working on You may have lots of information tucked away in folders and drawers but the stuff you are working on right now is on the desk Registers keep the contents of numbers that you are currently manipulating On the computers we are using registers are each four bytes long The size of a typical register is called a computer s word size x86 processors have four byte words This means that it is most natural on these computers to do computations four bytes at a time This gives us roughly 4 billion values Addresses are also four bytes 1 word long and therefore also fit into a regis
245. s it and even to locations where we haven t put any data Notice that we don t have a glob1 declaration for data_items This is because we only refer to these locations within the program No other file or program needs to know where they are located This is in contrast to the _start symbol which Linux needs to know where it is so that it knows where to begin the program s execution It s not an error to write globl 8 Note that no numbers in assembly language or any other computer language I ve seen have commas embedded in them So always write numbers like 65535 and never like 65 535 24 Chapter 3 Your First Programs data_items it s just not necessary Anyway play around with this line and add your own numbers Even though they are long the program will produce strange results if any number is greater than 255 because that s the largest allowed exit status Also notice that if you move the 0 to earlier in the list the rest get ignored Remember that any time you change the source file you have to re assemble and re link your program Do this now and see the results All right we ve played with the data a little bit Now let s look at the code In the comments you will notice that we ve marked some variables that we plan to use A variable is a dedicated storage location used for a specific purpose usually given a distinct name by the programmer We talked about these in the previous section but didn
246. s of some or all Invariant Sections in addition to the original versions of these Invariant Sections You may include a translation of this License provided that you also include the original English version of this License In case of a disagreement between the translation and the original English version of this License the original English version will prevail 9 TERMINATION You may not copy modify sublicense or distribute the Document except as expressly provided for under this License Any other attempt to copy modify sublicense or distribute the Document is void and will automatically terminate your rights under this License However parties who have received copies or rights from you under this License will not have their licenses terminated so long as such parties remain in full compliance 10 FUTURE REVISIONS OF THIS LICENSE The Free Software Foundation may publish new revised versions of the GNU Free Documentation License from time to time Such new versions will be similar in spirit to the present version but may differ in detail to address new problems or concerns See http www gnu org copyleft Each version of the License is given a distinguishing version number If the Document specifies that a particular numbered version of this License or any later version applies to it you have the option of following the terms and conditions either of that specified version or
247. s which fit in with the way the rest of the system works One thing to notice about the GNOME libraries is that they constantly create and give you pointers to large data structures but you never need to know how they are laid out in memory All manipulation of the GUI data structures are done entirely through function calls This is a characteristic of good library design Libraries change from version to version and so does the data that each data structure holds If you had to access and manipulate that data yourself then when the library is updated you would have to modify your programs to work with the new library or at least recompile them When you access the data through functions the functions take care of knowing where in the structure each piece of data is The pointers you receive from the library are opaque you don t need to know specifically what the structure they are pointing to looks like you only need to know the functions that will properly manipulate it When designing libraries even for use within only one program this is a good practice to keep in mind This chapter will not go into details about how GNOME works If you would like to know more visit the GNOME developer web site at http developer gnome org This site contains tutorials mailing lists API documentation and everything else you need to start programming in the GNOME environment A Simple GNOME Program in Several Languages This program will simply s
248. shrl Octal and Hexadecimal Numbers The numbering systems discussed so far have been decimal and binary However two others are used common in computing octal and hexadecimal In fact they are probably written more often than binary Octal is a representation that only uses the numbers 0 through 7 So the octal number 10 is actually 8 in decimal because it is one group of eight Octal 121 is decimal 81 one group of 64 8 2 two groups of 8 and one left over What makes octal nice is that every 3 binary digits make one octal digit there is no such grouping of binary digits into decimal So 0 is 000 1 is 001 2 is 010 3 is O11 4 is 100 5 is 101 6 is 110 and 7 is 111 Permissions in Linux are done using octal This is because Linux permissions are based on the ability to read write and execute The first bit is the read permission the second bit is the write permission and the third bit is the execute permission So 0 000 gives no permissions 6 110 gives read and write permission and 5 101 gives read and execute permissions These numbers are then used for the three different sets of permissions the owner the group and everyone else The number 0644 means read and write for the first permission set and read only for the second and third set The first permission set is for the owner of the file The third permission set is for the group owner of the file The last permission set is for everyone else So 0751 means that the
249. siads sed a E S aaa 35 HOW Unc tOMs WOK 1 22 c cGetnes dosceni vassals seccantesns acaudyidecsantu cutee aneedeaeetevatectonsatesare e 35 Assembly Language Functions using the C Calling Convention ee eeeeeeeeeeesteceneeeeees 37 A PUNCHON SE AMID e a a a a a R Satie Jaa tere tak Meee 41 Recursive TF UNCHOUS sssri E E E A A A E 45 ReyieW erisera eee e aa e AEEA VEE AES AA N peas AE OE AE e ER SLES 51 S D alina with Piles sasciscesacactncewsnastccaessssiccs unis sabes snes cecsunda suecs saan susasssaasnes e E6 SEC b SSESD OSE SC essi 53 MMe EINE IG CONCEDES 5 0 a E E Ea 53 Buffers ando DSS erai e E E A E E E E R 54 Standard and Special FileSesosennsat inei oe se ts eee E cee ade 55 Using Files in Pro eras sieneen EAE EEEE EEE EEA E ACSEE R VER Eaa Srodas 56 Reyi y e saved ar ds asus hae a A Senn sp Fuca tae daa sv can a E AE AENEA E AEE O TEE Ea 65 6 Reading and Writing Simple Records ssosssscssecssocesocssscessocscoesoocssscesocesooesocessccesocssosessessseee 67 Winne Records 8 tn 8 eG e E E E E E E E E E a 70 Re dins Recordsen a E a a N E aR V A RE 74 WiGdityinie the Records riase seasea dec datoe scolar a Salts ace Seiten T Saeed 79 REVIEW o ars turse eruen eE E E E A AE I E E E E aas 82 iii 7 Developing Robust Program ssccscssssccssssssscsscsscsssssccccssscsscsssssccssessscccsssecscsssssecesesees 85 Where Do s th TIMEGO ar a E E ER E O E E E EE R 85 Some Tips for Developing Robust ProgramSs
250. so 2 gt lib ld linux so 2 0x400000000 The numbers in parenthesis may be different on your system This means that the program helloworld is linked to libc so 6 the 6 is the version number which is found at lib libc so 6 and lib 1d linux so 2 is found at lib 1d linux so 2 These libraries have to be loaded before the program can be run If you are interested run the 1dd program on various programs that are on your Linux distribution and see what libraries they rely on Finding Information about Libraries Okay so now that you know about libraries the question is how do you find out what libraries you have on your system and what they do Well let s skip that question for a minute and ask another question How do programmers describe functions to each other in their documentation Let s take a look at the function printf It s calling interface usually referred to as a prototype looks like this int printi char string In Linux functions are described in the C programming language In fact most Linux programs are written in C That is why most documentation and binary compatibility is defined using the C language The interface to the printf function above is described using the C programming language This definition means that there is a function print f The things inside the parenthesis are the functions parameters or arguments The first argument here is char string This means there is an argument n
251. structure without the calling program having to change at all section data HH H H H H GLOBAL VARIABLES HH H HHHHH This points to the beginning of the memory we are managing heap_begin long 0 This points to one location past the memory we are managing current_break long 0 Ht tt HH HHH HEECONSTANTS HH EEEE HH HH equ UNAVAILABLE 0 This is the number we will use to mark 5 use the word slow but it will not be noticeably slow for any example used in this book 113 Chapter 9 Intermediate Memory Topics space that has been given out equ AVAILABLE 1 This is the number we will use to mark Space that has been returned and is 5 available for giving equ BRK 45 system call number for the break system call equ LINUX_SYSCALL 0x80 make system calls easier to read Fttti STRUCTURE INFORMATION equ HEADER_SIZE 8 size of space for memory segment header equ HDR_AVAIL_OFFSET 0 Location of the available flag in the header equ HDR_SIZE_OFFSET 4 Location of the size field in the header section text Hts tt HtPHE UNCTIONSH Hat Hat Ht HH HHH allocate_init PURPOSE call this function to initialize the functions specifically this sets heap_begin and current_break This has no parameters and no return value globl allocate_init type allocate_init functi
252. system call for example has as its 138 Chapter 10 Counting Like a Computer second parameter a list of flags to tell the operating system how to open the file Some of the flags include O_WRONLY This flag is 0600000000000000000000000000000001 in binary or 01 in octal or any number system for that matter This says to open the file in write only mode O_RDWR This flag is 0600000000000000000000000000000010 in binary or 02 in octal This says to open the file for both reading and writing O_CREAT This flag is 0600000000000000000000000001000000 in binary or 0100 in octal It means to create the file if it doesn t already exist O_TRUNC This flag is 0600000000000000000000001000000000 in binary or 01000 in octal It means to erase the contents of the file if the file already exists O_APPEND This flag is 0600000000000000000000010000000000 in binary or 02000 in octal It means to start writing at the end of the file rather than at the beginning To use these flags you simply OR them together in the combination that you want For example to open a file in write only mode and have it create the file if it doesn t exist I would use O_WRONLY 01 and 0_CREAT 0100 Or d together I would have 0101 Note that if you don t set either O_WRONLY or O_RDWR then the file is automatically opened in read only mode O_RDONLY except that it isn t really a flag since it s zero There are many other flags
253. t s look at 1100101 It can be written as 1 64 1 32 O 16 0 8 1 4 0 2 1 1 64 32 4 1 101 So we see that 1100101 in binary is 101 in decimal Let s look at one more number 11101011001001 You can convert it to decimal by doing 1 8192 1 4096 1 2048 0 1024 1 512 0 256 1 128 1 64 0 32 0 16 1 8 0 4 0 2 1 1 8192 4096 2048 512 128 644 8 1 5 15049 131 Chapter 10 Counting Like a Computer Now if you ve been paying attention you have noticed that the numbers we just converted are the same ones we used to multiply with earlier So let s check our results 101 149 15049 It worked Now let s look at going from decimal back to binary In order to do the conversion you have to divide the number into groups of two So let s say you had the number 17 If you divide it by two you get 8 with 1 left over So that means there are 8 groups of two and 1 ungrouped That means that the rightmost digit will be 1 Now we have the rigtmost digit figured out and 8 groups of 2 left over Now let s see how many groups of two groups of two we have by dividing 8 by 2 We get 4 with nothing left over That means that all groups two can be further divided into more groups of two So we have 0 groups of only two So the next digit to the left is 0 So we divide 4 by 2 and get two with 0 left over so the next digit is 0 Then we divide 2 by 2 and get 1 with 0 left over So
254. t addressing mode and do pushl my_location However if the prototype said to pass an int you would do pushl my_location an immediate mode push of the address that the value resides in In addition to indicating the address of a single value pointers can also be used to pass a sequence of consecutive locations starting with the one pointed to by the given value This is called an array struct 100 A struct 1s a set of data items that have been put together under a name For example you could declare struct teststruct int a char b and any time you ran into struct teststruct you would know that it is actually two words right next to each other the first being an integer and the second a pointer to a character or group of characters You never see structs passed as arguments to functions Chapter 8 Sharing Functions with Code Libraries Instead you usually see pointers to structs passed as arguments This is because passing structs to functions is fairly complicated since they can take up so many storage locations typedefs A typedef basically allows you to rename a type For example I can do typedef int myowntype inaC program and any time I typed myownt ype it would be just as if I typed int This can get kind of annoying because you have to look up what all of the typedefs and structs in a function prototype really mean However t ypedefs are useful for giving types more meaningful and descriptive nam
255. t the current position in the list to be zero the first element From then we will follow the following steps 1 Check the current list element eax to see if it s zero the terminating element 2 If it is zero exit 3 Increase the current position Sedi 4 Load the next value in the list into the current value register seax What addressing mode might we use here Why 20 Chapter 3 Your First Programs 5 Compare the current value eax with the current highest value ebx 6 If the current value is greater than the current highest value replace the current highest value with the current value 7 Repeat That is the procedure Many times in that procedure I made use of the word if These places are where decisions are to be made You see the computer doesn t follow the exact same sequence of instructions every time Depending on which if s are correct the computer may follow a different set of instructions The second time through it might not have the highest value In that case it will skip step 6 but come back to step 7 In every case except the last one it will skip step 2 In more complicated programs the skipping around increases dramatically These if s are a class of instructions called flow control instructions because they tell the compute which steps to follow and which paths to take In the previous program we did not have any flow control instructions as there was only one possible path to t
256. ter x86 processors can access up to 4294967296 bytes if enough memory is installed Notice that this means that we can store addresses the same way we store any other number In fact the computer can t tell the difference between a value that is an address a value that is a number a value that is an ASCII code or a value that you have decided to use for another purpose A number becomes an ASCII code when you attempt to display it A number becomes an address when you try to look up the 1 With the advent of international character sets and Unicode this is not entirely true anymore However for the purposes of keeping this simple for beginners we will use the assumption that one number translates directly to one character For more information see Appendix D Chapter 2 Computer Architecture byte it points to Take a moment to think about this because it is crucial to understanding how computer programs work Addresses which are stored in memory are also called pointers because instead of having a regular value in them they point you to a different location in memory As we ve mentioned computer instructions are also stored in memory In fact they are stored exactly the same way that other data is stored The only way the computer knows that a memory location is an instruction is that a special purpose register called the instruction pointer points to them at one point or another If the instruction pointer points to a memory wo
257. text because we want to be clear about exactly what is happening at the processor level I encourage you to take a C program you have written and compile it to assembly language and trace the logic Then add in optimizations and try again See how the compiler chose to rearrange your program to be more optimized and try to figure out why it chose the arrangement and instructions it did 1 Note that different versions of GCC do this differently 207 Appendix E C Idioms in Assembly Language 208 Appendix F Using the GDB Debugger By the time you read this appendix you will likely have written at least one program with an error in it In assembly language even minor errors usually have results such as the whole program crashing with a segmentation fault error In most programming languages you can simply print out the values in your variables as you go along and use that output to find out where you went wrong In assembly language calling output functions is not so easy Therefore to aid in determining the source of errors you must use a source debugger A debugger is a program that helps you find bugs by stepping through the program one step at a time letting you examine memory and register contents along the way A source debugger is a debugger that allows you to tie the debugging operation directly to the source code of a program This means that the debugger allows you to look at the source code as you typed it in complete
258. than writing movl 0b611111111111111111111111111111111 eax Note also that hexadecimal numbers are prefixed with 0x So when we do int S0x80 We are calling interrupt number 128 8 groups of 16 or interrupt number 0b000000000000000000000000 10000000 Hexadecimal and octal numbers take some getting used to but they are heavily used in computer programming It might be worthwhile to make up some numbers in hex and try to convert them back and forth to binary decimal and octal Order of Bytes in a Word One thing that confuses many people when dealing with bits and bytes on a low level is that when bytes are written from registers to memory their bytes are written out least significant portion first What most people expect is that if they have a word in a register say 0x5d 23 ef ee the spacing is so you can see where the bytes are the bytes will be written in that order However on x86 processors the bytes are actually written in reverse order In memory the bytes would be Oxee ef 23 5d on x86 processors The bytes are written in reverse order as they would appear conceptually but the bits within the bytes are ordered normally Not all processors behave this way The x86 processor is a little endian processor which means that it stores the little end of its words first 143 Chapter 10 Counting Like a Computer Register Byte 0 Byte 1 Byte 2 Byte 3 Byte 0 Byte 1 Byte 2 Byte 3
259. the buffer out to the file Now we just go back to the beginning of the loop After the loop exits remember it exits if after a read it detects the end of the file it simply closes its file descriptors and exits The close system call just takes the file descriptor to close in Sebx The program is then finished Review Know the Concepts e Describe the lifecycle of a file descriptor e What are the standard file descriptors and what are they used for e What is a buffer e What is the difference between the data section and the bss section e What are the system calls related to reading and writing files 65 Chapter 5 Dealing with Files Use the Concepts e Modify the toupper program so that it reads from STDIN and writes to STDOUT instead of using the files on the command line e Change the size of the buffer e Rewrite the program so that it uses storage in the bss section rather than the stack to store the file descriptors e Write a program that will create a file called heynow txt and write the words Hey diddle diddle into it Going Further e What difference does the size of the buffer make What error results can be returned by each of these system calls e Make the program able to either operate on command line arguments or use STDIN or STDOUT based on the number of command line arguments specified by ARGC e Modify the program so that it checks the results of each system call and
260. the current memory segment is marked unavailable go to the next one and go back to 2 5 If the current memory segment is large enough to hold the requested amount of space mark it as unavailable and return it 6 Go back to 2 Now look through the code with this in mind Be sure to read the comments so you ll know which register holds which value Now that you ve looked through the code let s examine it one line at a time We start off like this pushl Sebp movl esp ebp movl ST_MEM_SIZE ebp ecx movl heap_begin eax movl current_break Sebx This section initializes all of our registers The first two lines are standard function stuff The next move pulls the size of the memory to allocate off of the stack This is our function parameter Notice that we used ST_MEM_SIZE instead of the number 8 This is to make our code more readable I used the prefix ST because it is a stack offset You don t have to do this I do this just so I know which symbols refer to stack offsets After that I move the beginning heap address and the end of the heap current break into registers I am now ready to do processing The next section is marked alloca_loop_begin A loop is a section of code repeated many times in a row until certain conditions occur In this case we are going to loop until we either find an open memory segment or determine that we need more memory Our first statements check for
261. through each memory segment starting with heap_begin We look at the size of each one and if it has been allocated If it s big enough for the requested size and its available it grabs that one If it does not find a segment large enough it asks Linux for more memory In that case it moves current_break up globl allocate type allocate function equ ST_MEM_SIZE 8 stack position of the memory size allocate pushl Sebp movl esp to allocate standard function stuff Sebp 115 Chapter 9 Intermediate Memory Topics movl ST_MEM_SIZE ebp ecx ecx will hold the size we ar looking for which is the first fand only parameter movl heap_begin eax seax will hold the current search location movl current_break ebx Sebx will hold the current break point alloc_loop_begin here we iterate through each memory segment cmpl ebx Seax need more memory if these are equal je move_break movl HDR_SIZE_OFFSET eax tedx grab the size of this memory cmpl SUNAVAILABLE HDR_AVAIL_OFFSET eax If the space is unavailable go to the je next_location next one cmpl edx ecx If the space is available compare jle allocate_here the size to the needed size If its big enough go to allocate_here may want to add code here to combine allocations next_location add SHEADER_SIZE eax The total size of th
262. times the factorial of 3 The factorial of 3 is 3 times the factorial of 2 2 is 2 times the factorial of 1 The factorial of 1 is 1 This type of definition is called a recursive definition That means the definition of the factorial function 45 Chapter 4 All About Functions includes the factorial funtion itself However since all functions need to end a recursive definition must include a base case The base case is the point where recursion will stop Without a base case the function would go on forever calling itself until it eventually ran out of stack space In the case of the factorial the base case is the number 1 When we hit the number 1 we don t run the factorial again we just say that the factorial of 1 is 1 So let s run through what we want the code to look like for our factorial function 1 Examine the number 2 Is the number 1 3 If so the answer is one 4 Otherwise the answer is the number times the factorial of the number minus one This would be problematic if we didn t have local variables In other programs storing values in global variables worked fine However global variables only provide one copy of each variable In this program we will have multiple copies of the function running at the same time all of them needing their own copies of the data Since local variables exist on the stack frame and each function call gets its own stack frame we are okay Let s look at the code to see how t
263. tk_signal_connect appPtr delete_event GTK_SIGNAL_FUNC delete_handler NULL gtk_signal_connect appPtr destroy GTK_SIGNAL_FUNC destroy_handler NULL gtk_signal_connect btnQuit clicked GTK_SIGNAL_FUNC click_handler NULL Transfer control to GNOME gtk_main return 0 Function to receive the destroy signal int destroy_handler gpointer window GdkEventAny e gpointer data Leave GNOME event loop gtk_main_quit return 0 Function to receive the delete_event signal int delete_handler gpointer window GdkEventAny e gpointer data return 0 Function to receive the clicked signal int click_handler gpointer window GdkEventAny e gpointer data gpointer msgbox int buttonClicked Create the Are you sure dialog msgbox gnome_message_box_new MY_QUIT_QUESTION GNOME_MESSAGE_BOX_QUESTION GNOME_STOCK_BUTTON_YES 180 Appendix A GUI Programming GNOME_STOCK_BUTTON_NO NULL gtk_window_set_modal msgbox 1 gtk_widget_show msgbox Run dialog box buttonClicked gnome_dialog_run_and_close msgbox Button 0 is the Yes button If this is the button they clicked on tell GNOME to quit it s event loop Otherwise do nothing if buttonClicked 0 gtk_main_quit return 0 To compile it type gcc gnome examp
264. to look at the possible side issues In the toupper program we do not have any course of action if the file the user selects does not exist The program will go ahead and try to work anyway It doesn t report any error message so the user won t even know that they typed in the name wrong Let s say that the destination file is on a network drive and the network temporarily fails The operating system is returning a status code to us in Seax but we aren t checking it Therefore if a failure occurs the user is totally unaware This program is definitely not robust As you can see even in a simple program there are a lot of things that can go wrong In a large program it gets much more problematic There are usually many more possible error conditions than possible successful conditions Therefore you should always expect to spend the majority of your time checking status codes writing error handlers and performing similar tasks to make your program robust If it takes two weeks to develop a program it will likely take at 85 Chapter 7 Developing Robust Programs least two more to make it robust Remember that every error message that pops up on your screen had to be programmed in by someone Some Tips for Developing Robust Programs User Testing Testing is one of the most essential things a programmer does If you haven t tested something you should assume it doesn t work However testing isn t just about making sure
265. ton Makes the button show up mybutton show Makes the application window show up myapp show Connect signal handlers myapp connect delete_event delete_handler myapp connect destroy destroy_handler 182 Appendix A GUI Programming mybutton connect Clicked click_handler Transfer control to GNOME gtk mainloop To run it type python gnome example py GUI Builders In the previous example you have created the user interface for the application by calling the create functions for each widget and placing it where you wanted it However this can be quite burdensome for more complex applications Many programming environments including GNOME have programs called GUI builders that can be used to automatically create your GUI for you You just have to write the code for the signal handlers and for initializing your program The main GUI builder for GNOME applications is called GLADE GLADE ships with most Linux distributions There are GUI builders for most programming environments Borland has a range of tools that will build GUIs quickly and easily on Linux and Win32 systems The KDE environment has a tool called QT Designer which helps you automatically develop the GUI for their system There is a broad range of choices for developing graphical applications but hopefully this appendix gave you a taste of what GUI programming is like 183 Appendix A GUI Programming 184 Appendi
266. torage Static storage is what we have talked about so far storage locations declared using long or byte directives Dynamic storage will be discussed in the Section called Getting More Memory in Chapter 9 There are problems though with declaring buffers using byte First it is tedious to type You would have to type 500 numbers after the byte declaration and they wouldn t be used for anything but to take up space Second it uses up space in the executable In the examples we ve used so far it doesn t use up too much but that can change in larger programs If you want 500 bytes you have to type in 500 numbers and it wastes 500 bytes in the executable There is a solution to both of these So far we have discussed two program sections the text and the data sections There is another section called the bss This section is like the data section except that it doesn t take up space in the executable This section can reserve storage but it can t initialize it In the data 2 While this sounds complicated most of the time in programming you will not need to deal directly with buffers and file descriptors In Chapter 8 you will learn how to use existing code present in Linux to handle most of the complications of file input output for you 54 Chapter 5 Dealing with Files section you could reserve storage and set it to an initial value In the bss section you can t set an initial value This is useful for buffers
267. tual name and address would be stored elsewhere in memory This way it is easy to tell where each part of the data is from the start of the record without explicitly limitting the size of the name and address If the length of the fields within our records could change we would have no idea where the next field started Because records would be different sizes it would also be hard to find where the next record began Therefore almost all records are of fixed lengths Variable length data is usually store separately from the rest of the record Data Accessing Methods Processors have a number of different ways of accessing data known as addressing modes The simplest mode is immediate mode in which the data to access is embedded in the instruction itself For example if we want to initialize a register to 0 instead of giving the computer an address to read the 0 from we would specify immediate mode and give it the number 0 In the register addressing mode the instruction contains a register to access rather than a memory location The rest of the modes will deal with addresses In the direct addressing mode the instruction contains the memory address to access For example I could say please load this register with the data at address 2002 The computer would go directly to byte number 2002 and copy the contents into our register In the indexed addressing mode the instruction contains a memory address to access and also specifies a
268. turn status section data 13 Chapter 3 Your First Programs section text globl _start start movl 1 eax this is the linux kernel command number system call for exiting a program movl 0 ebx this is the status number we will return to the operating system Change this around and it will return different things to cho int 0x80 this wakes up the kernel to run the exit command What you have typed in is called the source code Source code is the human readable form of a program In order to transform it into a program that a computer can run we need to assemble and link it The first step is to assemble it Assembling is the process that transforms what you typed into instructions for the machine The machine itself only reads sets of numbers but humans prefer words An assembly language is a more human readable form of the instructions a computer understands Assembling transforms the human readable file into a machine readable one To assembly the program type in the command as exit s o exit o as is the command which runs the assembler exit s is the source file and o exit o tells the assemble to put it s output in the file exit o exit o is an object file An object file is code that is in the machine s language but has not been completely put together In most large programs you will have several sou
269. turn value goes in eax movl sebp esp restore the stack pointer popl ebp restore the base pointer ret Type in the program assemble it and run it Try calling power for different values but remember that the result has to be less than 256 when it is passed back to the operating system Also try subtracting the results of the two computations Try adding a third call to the power function and add it s result back in The main program code is pretty simple You push the arguments onto the stack call the function and then move the stack pointer back The result is stored in seax Note that between the two calls to power we save the first value onto the stack This is because the only register that is guaranteed to be saved is sebp Therefore we push the value onto the stack and pop the value back off after the second function call is complete Let s look at how the function itself is written Notice that before the function there is documentation as to what the function does what it s arguments are and what it gives as a return value This is useful for programmers who use this function This is the function s interface This lets the programmer know what values are needed on the stack and what will be in eax at the end We then have the following line type power function This tells the linker that the symbol power should be treated as a function This isn t useful now but it will be when you start building larger progr
270. twork location containing a complete Transparent copy of the Document free of added material which the general network using public has access to download anonymously at no charge using public standard network protocols If you use the latter option you must take reasonably prudent steps when you begin distribution of Opaque copies in quantity to ensure that this Transparent copy will remain thus accessible at the stated location until at least one year after the last time you distribute an Opaque copy directly or through your agents or retailers of that edition to the public It is requested but not required that you contact the authors of the Document well before redistributing any large number of copies to give them a chance to provide you with an updated version of the Document 4 MODIFICATIONS You may copy and distribute a Modified Version of the Document under the conditions of sections 2 and 3 above provided that you release the Modified Version under precisely this License with the Modified Version filling the role of the Document thus licensing distribution and modification of the Modified Version to whoever possesses a copy of it In addition you must do these things in the Modified Version e A Use in the Title Page and on the covers if any a title distinct from that of the Document and from those of previous versions which should if there were any be listed in the History section of the Document You
271. u created the file in a previous run of write records If not you need to run write records first before running this program 8l Chapter 6 Reading and Writing Simple Records displaying numbers until Chapter 8 and Chapter 10 After reading that you may want to come back and rewrite this program to display the numeric data that we are modifying Review Know the Concepts e What is a record e What is the advantage of fixed length records over variable length records e How do you include constants in multiple assembly source files e Why might you want to split up a project into multiple source files e What does the instruction incl record_buffer RECORD_AGE do What addressing mode is it using How many operands does the inc1 instructions have in this case Which parts are being handled by the assembler and which parts are being handled when the program is run Use the Concepts e Add another data member to the person structure defined in this chapter and rewrite the reading and writing functions and programs to take them into account Remember to reassemble and relink your files before running your programs e Create a program that uses a loop to write 30 identical records to a file e Create a program to find the largest age in the file and return that age as the status code of the program e Create a program to find the smallest age in the file and return that age as the status code of the program Going Furthe
272. u have different code to handle people under and over age 30 for example you would need to test it on people of ages 29 30 and 31 at least There will be some internal functions that you assume get good data because you have checked for errors before this point However while in development you often need to check for errors anyway as your other code may have errors in it To verify the consistency and validity of data during development most languages have a facility to easily check assumptions about data correctness In the C language there is the assert macro You can simply put in your code assert a gt b and it will give an error if it reaches that code when the condition is not true In addition since such a check is a waste of time after your code is stable the assert macro allows you to turn off asserts at compile time This makes sure that your functions are receiving good data without causing unnecessary slowdowns for code released to the public Module Testing Not only should you test your program as a whole you need to test the individual pieces of your program As you develop your program you should test individual functions by providing it with data you create to make sure it responds appropriately In order to do this effectively you have to develop functions whose sole purpose is to call functions for testing These are called drivers not to be confused with hardware drivers They simply loads your function suppl
273. ubadagensuasdacecduei Vacus ta suedsusty seedunsadedecusaevdsesanbandeetuaadoers 159 When to PNA ZES ie E E e ca oat Dee ache la ee Roce eo and eect E 159 Where to Optimizes fa gh a ace Se cae EE a aT Bl td a as EER 160 Local Optimizations cic ecrini kan sdiesests esi davaes A ee GLa dee ERA 160 Global Optimization eeta Syoncasuctuna say eas una E E A a aE E 163 PRG VIG W e ee E tee EE A A A e E ones E E eats 164 TS Mo yi ng On from Mere asissetiasctticcteesdlceitansi di cateveddsannuasaticadeasdenahay sachin cisasedsstanedeanstnncoebaeadbens 167 From the Botton phic goes a ec ceeek destinies eG eet eee Slee sek 167 From the Top Down os satesigsccaesysgsiastivnascciii sg sedetealistetenesees EE E S 168 From thie Middle Outremer a R R AA e teat cameeusivates 168 Speedlied Op E S A A A a E S 169 Further Resources on Assembly Language sseesssssesssesesseessersseesseeesseessesseeesseeeseesseessees 170 A GUI Programming isiaiiss cacnscicisess saceddatancdnidiiesadvisaconsodsnonaddssaconinsedpnsnsasstssonseseatsoneanastsivecsaataiens 171 B Common x86 Instructions eesseosssesssessoossoossssessesssocssossssesssessoosssosssssssosssoossossssesssosssosssssssee 185 C Important System Calls caiscssivscactssevevcavenvncvesnnccovasvovsenoesseveevacsveuesesevasvensessvenssescssatescsvceeneedes 195 D Table OF ASCH CO eS iasssscsctscssicsccssssvasctesdetaandeisecasadsssonssatersseoawsesasspacsaisuovesseoudevaacsataassatasteaes 197 E C Idioms in Ass
274. umber code to return results in octal rather than decimal e Modify the integer2number code so that the conversion base is a parameter rather than hardcoded e Write a function called is_negative that takes a single integer as a parameter and returns if the parameter is negative and 0 if the parameter is positive 150 Chapter 10 Counting Like a Computer Going Further e Modify the integer2number code so that the conversion base can be greater than 10 this requires you to use letters for numbers past 9 e Create a function that does the reverse of integer2number called number2integer which takes a character string and converts it to a register sized integer Test it by running that integer back through the integer2number function and displaying the results e Write a program that stores likes and dislikes into a single machine word and then compares two sets of likes and dislikes for commonalities 151 Chapter 10 Counting Like a Computer 152 Chapter 11 High Level Languages In this chapter we will begin to look at our first real world programming language Assembly language is the language used at the machine s level but most people including me find coding in assembly language too cumbersome for normal use Many computer languages have been invented to make the programming task easier Knowing a wide variety of languages is useful for many reasons including Different languages are based on different concept
275. unction error_exit pushl Sebp movl esp ebp Write out error code movl ST_ERROR_CODE ebp ecx pushl ecx call count_chars popl ecx movl eax edx movl SSTDERR ebx movl S SYS_WRITE eax int SLINUX_SYSCALL 89 Chapter 7 Developing Robust Programs Write out error message movl ST_ERROR_MSG ebp ecx pushl Secx call count_chars popl ecx movl eax edx movl SSTDERR ebx movl S SYS_WRITE eax int SLINUX_SYSCALL pushl S STDERR call write_newline EXit with status 1 movl S SYS_EXIT eax movl 1 ebx int SLINUX_SYSCALL Enter it in a file called error exit s To call it you just need to push the address of an error message and then an error code onto the stack and call the function Now let s look for potential error spots First of all we don t check to see if either of our open system calls actually complete properly Linux returns its status code in eax so we need to check and see if there is an error Open file for reading movl S SYS_OPEN eax movl Sinput_file_name ebx movl 0 ecx movl 0666 edx int SLINUX_SYSCALL movl eax INPUT_DESCRIPTOR ebp This will test and see if eax is negative If it is not negative it will jump to continue_processing Otherwise it will handle the error condition that the negative number rep
276. use ebp instead This is also because ebp is more flexible _start pushl 4 call factorial addl 4 esp movl S eax Sebx movl 1 eax int S0x80 This type factorial function factorial pushl Sebp movl esp ebp movl 8 Sebp eax cmpl 1 eax je end_factorial decl pushl call movl imul eax eax factorial 8 ebp ebx Sebx Seax end_factorial movl popl ret ebp ebp Sesp This moves the first argument into eax 4 ebp holds the return address and 8 Sebp holds the address of the first parameter If the number is 1 that is our base case and we simply return 1 is already in eax as the return value otherwise decrease the value push it for our next call to factorial call factorial Seax has the return value so we have to reload our parameter into ebx multiply that by the result of the last call to factorial stored in eax the answer is stored in eax which is good since that s where return values gO Standard function return stuff we have to restore tebp and esp to where they were before the function started return to the function this pops the return value too 47 Chapter 4 All About Functions Assemble link and run it with these commands as factorial s o factorial o ld factorial o o factorial factorial echo This should give you the value 24 24 is the factorial of 4
277. use it so we don t want to waste too many resources You will find that most things in programming are about balances Do we want it to go faster or use less memory Do we want an exact answer in a few hours or an approximate one in a few minutes Do we need allocate or deallocate to be faster For example let s say that our program has three times as many allocations as deallocations and then at the end it deallocates everything it hasn t used In that case we need allocate to be as fast as possible because it s used three times as often Decisions like this characterize programming Another problem we have is that if we are looking for a 5 byte segment of memory and the first open one we come to is 1000 bytes we will simply mark the whole thing as allocated and return it This leaves 995 bytes of unused but allocated memory It would be nice in such situations to break it apart so the other 995 bytes can be used later It would also be nice to combine consecutive free spaces when looking for large allocations A potentially bigger problem that we have is that we assume that we are the only program that can set the break In many programs there is more than one memory manager Also there are other reasons to map memory that we will see in a later chapter Both of these things will break using this memory manager because it assumes that it has all of free memory Trace through the program and see what kind of problems you might run int
278. vl data_items edi 4 eax Now to understand this line you need to keep several things in mind e data_items is the location number of the start of our number list e Each number is stored across 4 storage locations because we declared it using Long 25 Chapter 3 Your First Programs e edi is holding 0 at this point So basically what this line does is say start at the beginning of data_items and take the first item number because edi is 0 and remember that each number takes up four storage locations Then it stores that number in Seax This is how you write indexed addressing mode instructions in assembly language The instruction in a general form is this mov1 BEGINNINGADDRESS INDEXREGISTER WORDSIZE In our case data_items was our beginning address sedi was our index register and 4 was our word size This topic is discussed further in the Section called Addressing Modes If you look at the numbers in data_items you will see that the number 3 is now in eax If edi was set to 1 the number 67 would be in eax and if it was set to 2 the number 34 would be in eax and so forth Very strange things would happen if we used a number other than 4 as the size of our storage locations The way you write this is very awkward but if you know what each piece does it s not too difficult For more information about this see the Section called Addressing Modes Let
279. we re doing is asking Linux to terminate the program in which case we won t be back in control If we didn t signal the interrupt then no system call would have been performed Quick System Call Review To recap Operating System features are accessed through system calls These are invoked by setting up the registers in a special way and issuing the instruction int 0x80 Linux knows which system call we want to access by what we stored in the seax register Each system call has other requirements as to what needs to be stored in the other registers System call number 1 is the exit system call which requires the status code to be placed in sebx Now that you ve assembled linked run and examined the program you should make some basic edits Do things like change the number that is loaded into ebx and watch it come out at the end with echo Don t forget to assemble and link it again before running it Add some comments Don t worry the worse thing that would happen is that the program won t assemble or link or will freeze your screen That s just part of learning Planning the Program In our next program we will try to find the maximum of a list of numbers Computers are very detail oriented so in order to write the program we will have to have planned out a number of details These details include 5 You may be wondering why it s 0x80 instead of just 80 The reason is that the number is written in hexadecimal
280. wever ultimately they will reach a set of primitives provided by the operating system to do basic line or point drawing Programming can either be viewed as breaking a large program down into smaller pieces until you get to the primitive functions or building functions on top of primitives until you get the large picture in focus In assembly language the primitives are usually the same thing as the system calls even though they aren t true functions as we will talk about 35 Chapter 4 All About Functions How Functions Work Functions are composed of several different pieces function name A function s name is a symbol that represents the address where the function s code starts In assembly language the symbol is defined by typing the the function s name followed by a colon immediately before the function s code This is just like labels you have used for jumping function parameters A function s parameters are the data items that are explicitly given to the function for processing For example in mathematics there is a sine function If you were to ask a computer to find the sine of 2 sine would be the function s name and 2 would be the parameter Some functions have many parameters others have none Function parameters can also be used to hold data that the function wants to send back to the program local variables Local variables are data storage that a function uses while processing that is thrown away it
281. with zero in the equation All of the addressing modes mentioned in the Section called Data Accessing Methods in Chapter 2 except immediate mode can be represented in this fashion direct addressing mode This is done by only using the ADDRESS_OR_OFFSET portion Example movl ADDRESS eax This loads eax with the value at memory address ADDRESS indexed addressing mode This is done by using the ADDRESS_OR_OFFSET and the INDEX portion You can use any general purpose register as the index register You can also have a constant multiplier of 1 2 or 4 for the index register to make it easier to index by bytes double bytes and words For example let s say that we had a string of bytes as string_start and wanted to access the third one an index of 2 since we start counting the index at zero and ecx held the value 2 If you wanted to load it into seax you could do the following movl string_start ecx 1 eax 29 Chapter 3 Your First Programs This starts at string_start andadds1 ecx to that address and loads the value into Seax indirect addressing mode Indirect addressing mode loads a value from the address indicated by a register For example if eax held an address we could move the value at that address to sebx by doing the following movl Seax ebx base pointer addressing mode Base pointer addressing is similar to indirect addressing except that it add
282. x B Common x86 Instructions Reading the Tables The tables of instructions presented in this appendix include e The instruction code e The operands used e The flags used e A brief description of what the instruction does In the operands section it will list the type of operands it takes If it takes more than one operand each operand will be separated by a comma Each operand will have a list of codes which tell whether the operand can be an immediate mode value I a register R or a memory address M For example the mov1 instruction is listed as I R M R M This means that the first operand can be any kind of value while the second operand must be a register or memory location Note however that in x86 assembly language you cannot have more than one operand be a memory location In the flags section it lists the flags affected by the instruction The following flags are mentioned O Overflow flag This is set to true if the destination operand was not large enough to hold the result of the instruction S Sign flag This is set to the sign of the last result Z Zero flag This flag is set to true if the result of the instruction is zero A Auxillary carry flag This flag is set for carries and borrows between the third and fourth bit It is not often used P Parity flag This flag is set to true if the low byte of the last result had an even number of 1 bits 185 Appendix B Common x86 Instructions C
283. x F Using the GDB Debugger Warning One problem that GDB has is with handling interrupts Often times GDB will miss the instruction that immediately follows an interrupt The instruction is actually executed but GDB doesn t step through it This should not be a problem just be aware that it may happen GDB Quick Reference This quick reference table is copyright 2002 Robert M Dondero Jr and is used by permission in this book Parameters listed in brackets are optional Table F 1 Common GDB Debugging Commands Miscellaneous quit IExit GDB help cmd Print description of debugger command cmd Without cmd prints a list of topics directory dirl dir2 Running the Program Add directories dir1 dir2 etc to the list of directories searched for source files run arg1 arg2 Run the program with command line arguments argl arg2 etc set args arg arg2 Set the program s command line arguments to argl arg2 etc show args Using Breakpoints Print the program s command line arguments info breakpoints Print a list of all breakpoints and their numbers breakpoint numbers are used for other breakpoint commands break linenum Set a breakpoint at line number linenum break addr Set a breakpoint at memory address addr break fn Set a breakpoint at the beginning of function fn condition bpnum expr Break at breakpoi
284. x has the requested this should return the new break in eax which will be either 0 if it fails or it will be equal to or larger than under normal conditions we asked for We don t care in this program where it actually sets the break f fisn t 0 so as long as eax we don t care what it is check for error conditions restore saved registers 117 Chapter 9 Intermediate Memory Topics popl ecx popl eax movl SUNAVAILABLE HDR_AVAIL_OFFSET eax set this memory as funavailable since we re about to give it away movl ecx HDR_SIZE_OFFSET Seax set the size of the memory add SHEADER_SIZE eax move eax to the actual start of fusable memory eax now holds the return value movl S ebx current_break save the new break movl ebp esp return the function popl ebp ret error movl 0 eax On error we just return a zero movl ebp esp popl ebp ret HtHtHEFFEND OF FUNCTION HH HH HHT deallocate t PURPOSE The purpose of this function is to give back a segment of memory to the pool after we re don using it PARAMETERS The only parameter is the address of the memory we want to return to the memory pool 4 RETURN VALUE There is no return value PROCESSING If you remember we actually hand the program the start of the memory that they can use which is 8 storage l
285. y is the way your program thinks about memory Before loading your program Linux finds empty physical memory and then tells the processor to pretend that this memory is actually at the address 0x0804800 Confused yet Let me explain further Each program gets its own sandbox to play in Every program running on your computer thinks that it was loaded at memory address 0x0804800 and that it s stack starts at Oxbffffff When Linux loads a program it finds a section of memory and then tells the processor to use that section of memory as the address 0x0804800 for this program The address that a program believes it uses is called the virtual address while the actual address on the chips that it refers to is called the physical address The process of assigning virtual addresses to physical addresses is called mapping Earlier we talked about the break in memory between the bss and the stack but we didn t talk about why it was there The reason is that this segment of virtual memory addresses hasn t been mapped onto physical memory addresses The mapping process takes up considerable time and space so if every possible virtual address of every possible program were mapped you probably couldn t even run one program So the break is the area that contains unmapped memory Virtual memory can be mapped to more than just physical memory it can be mapped to disk as well Swap files swap partitions and paging files all refer to the same basic ide
286. y it with data and check the results This is especially useful if you are working on pieces of an unfinished program Since you can t test all of the pieces together you can create a driver program that will test each function individually Also the code you are testing may make calls to functions not developed yet In order to overcome this problem you can write a small function called a stub which simply returns the 87 Chapter 7 Developing Robust Programs values that function needs to proceed For example in an e commerce application I had a function called is_ready_to_checkout Before I had time to actually write the function I just set it to return true on every call so that the functions which relied on it would have an answer This allowed me to test functions which relied on is_ready_to_checkout without the function being fully implemented Handling Errors Effectively Not only is it important to know how to test but it is also important to know what to do when one is detected Have an Error Code for Everything Truly robust software has a unique error code for every possible contingency By simply knowing the error code you should be able to find the location in your code where that error was signalled This is important because the error code is usually all the user has to go on when reporting errors Therefore it needs to be as useful as possible Error codes should also be accompanied by descriptive error messages
287. you are using As an example let s look at the process of loading the number 0 into a register On most processors doing a movl 0 eax is not the quickest way The quickest way is to exclusive or the register with itself xorl eax eax This is because it only has to access the register and doesn t have to transfer any data For users of high level languages the compiler handles this kind of optimizations for you For assembly language programmers you need to know your processor well Addressing Modes 162 Different addressing modes work at different speeds The fastest are the immediate and register addressing modes Direct is the next fastest indirect is next and base pointer and indexed indirect are the slowest Try to use the faster addressing modes when possible One interesting consequence of this is that when you have a structured piece of memory that you are accessing using base pointer addressing the first element can be accessed the quickest Since it s offset is 0 you can access it using indirect addressing instead of base pointer addressing which makes it faster Chapter 12 Optimization Data Alignment Some processors can access data on word aligned memory boundaries i e addresses divisible by the word size faster than non aligned data So when setting up structures in memory it is best to keep it word aligned Some non x86 processors in fact cannot access non aligned data in some modes These are just
288. your code later and will have to figure out what your symbols mean 28 Chapter 3 Your First Programs Okay that was a lot of work and explanation especially for such a small program But hey you re learning a lot Now read through the whole program again paying special attention to the comments Make sure that you understand what is going on at each line If you don t understand a line go back through this section and figure out what the line means You might also grab a piece of paper and go through the program step by step recording every change to every register so you can see more clearly what is going on Addressing Modes In the Section called Data Accessing Methods in Chapter 2 we learned the different types of addressing modes available for use in assembly language This section will deal with how those addressing modes are represented in assembly language instructions The general form of memory address references is this ADDRESS_OR_OFFSET SBASE_OR_OFF SET s INDEX MULTIPLIER All of the fields are optional To calculate the address simply perform the following calculation ol FINAL ADDRESS ADDRESS_OR_OFFSET BASE_OR_OFFSET MULTIPLIER SINDEX ADDRESS_OR_OFFSET and MULTIPLIER must both be constants while the other two must be registers If any of the pieces is left out it is just substituted
289. your program works it s about making sure your program doesn t break For example if I have a program that is only supposed to deal with positive numbers you need to test what happens if the user enters a negative number Or a letter Or the number zero You must test what happens if they put spaces before their numbers spaces after their numbers and other little possibilities You need to make sure that you handle the user s data in a way that makes sense to the user and that you pass on that data in a way that makes sense to the rest of your program When your program finds input that doesn t make sense it needs to perform appropriate actions Depending on your program this may include ending the program prompting the user to re enter values notifying a central error log rolling back an operation or ignoring it and continuing Not only should you test your programs you need to have others test it as well You should enlist other programmers and users of your program to help you test your program If something is a problem for your users even if it seems okay to you it needs to be fixed If the user doesn t know how to use your program correctly that should be treated as a bug that needs to be fixed You will find that user s find a lot more bugs in your program than you ever could The reason is that user s don t know what the computer expects You know what kinds of data the computer expects and therefore are much more l
290. your program is then dynamically linked which means that not all of the code needed to run the program is actually contained within the program file itself but in external libraries When we put the 1c on the command to link the helloworld program it told the linker to use the c library libc so to look up any symbols that weren t already defined in he lloworld o However it doesn t actually add any code to our program it just notes in the program where to look When the helloworld program begins the file lib 1d linux so 2 is loaded first This is the dynamic linker This looks at our helloworld program and sees that it needs the c library to run So it searches for a file called libc so in the standard places listed in etc 1d so conf and in the contents of the LD_LIBRARY_PATH environment variable then looks in it for all the needed symbols printf and exit in this case and then loads the library into the program s virtual memory Finally it replaces all instances of printf in the program with the actual location of print f in the library Run the following command ldd helloworld nolib It should report back not a dynamic executable This is just like we said helloworld nolib is a statically linked executable However try this ldd helloworld lib It will report back something like libc so 6 gt lib libc so 6 0x4001d000 96 Chapter 8 Sharing Functions with Code Libraries lib ld linux

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