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CPU Sim 3.4 User`s Manual

1. lt single quoted character gt Symbol gt lt letter gt lt letter or digit or or or _ gt Here is asummary of the parts of an assembly language program The basic building blocks consist of the following items A literal is one of the following a a decimal integer b a hexadecimal integer denoted by the prefix Ox or 0x followed by one or more of the characters 0 9 a f A F c a binary integer denoted by the prefix 0b or Ob followed by one or more 0 s or 1 s d a single character surrounded by single quotes Literals other than single quoted characters can have an optional plus or minus sign in front No commas or decimal points are allowed in literals In the case of the single quoted character the value of the literal is the ASCII value of the character A string is any sequence of characters surrounded by double quotes such as abcde Note that the sequence of characters inside the quotes cannot include the double quote character and that the characters must all be on the same line A symbol consists of a sequence of characters the first character of which must be an upper or lower case letter The remaining characters must be letters or digits or or or_ CPU Sim distinguishes between upper and lower case letters hence Data and data are considered different symbols A label is a symbol followed immediately by a colon The colon is just a separator and is not consider
2. Condition Bit from the popup menu at the top of the dialog box you can edit the condition bits of the current machine A condition bit is a specific bit of a register The bit can be set to 0 or 1 by a SetCondBit microinstruction or it can be set 23 to 1 if an overflow or carry out occurs in an Arithmetic or Increment microinstruction Also you can specify whether you want the machine to halt execution when the bit gets setto 1 The meanings of all the parameters of the hardware components are described in more detail in the section entitled SPECIFICATION OF THE SIMULATED HARDWARE UNITS Modifying the microinstructions If you choose Microinstructions in the Modify menu a dialog box will appear displaying a table containing the parameters associated with the Set microinstructions Edit their values in ways similar to the ways you edit the parameters of the registers register arrays RAMs and condition bits as described above To edit the parameters of other types of microinstructions select the desired type from the popup menu at the top of the dialog box See Figure 6 above for a picture of this dialog when TransferRtoR microinstructions are displayed for the Wombat1 machine For more details on the meanings of the various parameters of each type of microinstruction see the section entitled SPECIFICATION OF THE MICROINSTRUCTIONS below Note that CPU Sim creates only one copy of each microinstruction regardless
3. described above Register width a positive integer that specifies the number of bits in the register Register Array width a positive integer that specifies the number of bits in each register in the array All registers in an array must have the same width length a positive integer specifying the number of registers in the register array Condition Bit register the register that contains the bit bit the bit of the register that is the condition bit This is a number from 0 to one 41 less than the width of the specified register Bit 0 is on the left most bit of the register halt a boolean value If this value is true then the machine will halt and display a message after the execution of any microinstruction that causes the bit s value to be set to 1 RAM length a positive integer that specifies the number of bytes in the RAM The first byte has index address 0 SPECIFICATION OF THE FETCH SEQUENCE The fetch sequence is a sequence of microinstructions that begins the execution of every machine cycle A typical fetch sequence loads into an instruction register the contents of the memory location whose address is in a program counter register Then it increments the program counter and decodes the instruction in the instruction register It is typically the case that the last microinstruction in the fetch sequence is a Decode microinstruction because the execution of a Decode microinstruction causes the
4. dialog box indicating that the program has ceased execution because a condition bit was set to 1 When execution is complete you can see the final state of the registers and RAM in the registers and RAM windows If you wish to rerun the program with different input you can proceed in two ways You can either perform a two step process of selecting Clear everything from the Execute menu and then choose Assemble load amp run from the Execute menu or you can select Clear assemble load amp run from the Execute menu While both of these methods perform the same function Clear everything and Assemble load amp run are divided into two separate operations in case you want to debug your program as explained below When you are debugging assembly code it is useful to be able to step through the execution one instruction or microinstruction at a time and to set break points in the code To practice debugging using the Wombat and the program W1 0 a first select Clear everything from the Execute menu and then choose Assemble amp load from the Execute menu Then select Debug Mode from the Execute menu You will see a toolbar appear at the top of the display see Figure 3 You will also see check boxes appear in a new column on the left side in the Main RAM window When you are in debug mode you cannot edit the machine s parameters and so the Modify menu is disabled However you can still edit
5. 12 bits of the instruction form the only operand All Wombat1 machine instructions have 4 bit and 12 bit fields but for some of the instructions the 12 bit field is ignored such fields are denoted by putting parentheses around them in the fieldLengths column As in other such dialog boxes if you wish to create a new machine instruction click New and then edit the parameters of that instruction see Figure 8 Once all machine instructions have been created click OK The detailed descriptions of all the parameters displayed in the machine instruction dialog box can be found in the section of this manual entitled SPECIFICATION OF MACHINE INSTRUCTIONS 16 BB Edit machine instructions x name opcode _ fleldLengths add s implementation Existing micros stop 0 16 ir 4 15 gt mar gt E TransferRtoR lt load 1412 store 412 Main mar gt mdr gt C TransferAtoR read al 4 12 acc mdr gt acc J TransferRtoA write 4 4 12 End gt ci Set 5 412 F gt E Test subtract 6 412 _ lt lt insert lt lt _ gt J Increment multiply 71412 o Arithmetic a k gt gt delete gt c3 shit o jmpz A412 y oia jmpn B 412 CI Decode Cf MemoryAccess gt Elo kad New Delete Duplicate o v Help OK Cancel ree DERGE ae Se ee EES e098 Edit machine instructio
6. 5 data 6 2 4 2 7 fills 6 bytes with the thr 2 byt values 4 2 7 Ascii pseudoinstruction The word ascii signifies another pseudo instruction in assembly language It provides a way for the user to insert specific ascii values in specific locations in the assembled code The pseudoinstruction has the following parts label ascii operand comment The word ascii and the operand must be included but the comment and the label are optional The label can be used as an operand in assembly language statements to refer to the first address of the data that is being specified The pseudoinstruction is terminated by the end of line character The operand must be a string of 0 or more characters surrounded by double quotes The string cannot include the end of line or return or double quote characters ascii abcde legal ascii abc de illegal contains a double quote character The effect of including an Ascii pseudoinstruction in your program is that the assembler inserts the ASCII numeric value of each of the characters in the string into the program at the point where the Ascii pseudoinstruction appears So the first example pseudoinstruction above will be assembled into 5 bytes containing the 5 ASCII numeric 40 values for the characters a b c d and e NAMES OF HARDWARE COMPONENTS MICROINSTRUCTIONS AND MACHINE INSTRUCTIONS All hardware components microinstructions and machine ins
7. EQU declarations in files that are then included using the include statements in all files in which you wish to use the declared names You can also make global EQU declarations as described above in the section detailing the Modify menu If a global EQU declaration and a local EQU declaration both use the same symbol which is legal the local EQU declaration has precedence and so hides the global EQU declaration Regular Instructions Regular instructions have the following form some of the parts are optional label operator operands comments Space and tab characters are only required between two parts if the assembler might otherwise treat the parts as one unit Statements are terminated by the newline character The label and comments are optional An operator is a symbol consisting of the name of a machine instruction of the current machine or the name of a macro Every regular instruction must have an operator An operand is a symbol or literal If it is a symbol the same symbol must appear somewhere in the program as a label or it must be an EQU constant For each instruction there must be one operand for each field specified for that instruction not counting the first field which corresponds to the opcode and not counting the don t care fields That is if a machine instruction has fields consisting of 4 2 10 then the opcode for the instruction must be 4 bits long and the instruction must have 2 operands the first one havin
8. JMPN if the value in the acc is less than 0 jump to a new location to obtain the next instruction to be executed JUMP jump to a new location unconditionally input output main memory Figure 4 Diagram of data movement in the Wombat1 We will now outline how one can create the Wombat1 machine by adding components and instructions to the default empty machine To begin this process it is always best to start at the lowest level namely the basic hardware components So choose Hardware Modules from the Modify menu The dialog box that appears allows you to edit the registers register arrays condition bits and memory of a machine You can add new components and modify or delete existing components To modify a component first select the type of the component in the popup menu at the top of the dialog This causes the parameters associated with all existing components of that type to be displayed in the table in the center of the dialog box where they can be edited The Wombat1 computer needs the six registers mentioned above so select Register as the type of module and then click the New button six times Finally edit the name and width of each of the six registers so that they match the description of the Wombat1 given above see Figure 5 The next piece of hardware to edit is the machine s main memory To do so select RAM from the popup menu at the top of the Edit Modules dialog click New and set th
9. This block is called the macro body A macro call is the use of a macro name as an instruction The assembler textually inserts the macro body wherever a macro call occurs in an assembly language program For example consider the following macro declaration MACRO foo add x y load z ENDM Assume that the declaration appears in a file that includes the following 3 instruction statements add a b foo macro call store b In this case the text that is assembled is the following add a b add x y load z 37 store b Macros can also have parameters which get textually replaced by the arguments in the macro call For example consider the following variation which has one parameter n MACRO foo n add x n load z ENDM Assume that this declaration appears in a file that includes the following 3 instruction statements add a b foo a macro call with argument a store b In this case the text that is assembled is the following add a b add x a load z store b You can include arbitrary instructions inside a macro body including labeled statements data pseudoinstructions and calls to other macros Note that macro declarations are always local which means that these macros can only be called in the file in which they are defined Therefore it is often useful to put the macro definitions in files that are then included using the include statements in all files in which you wish to make macro calls It is worth
10. arrays condition bits RAMs the microinstructions the fetch sequence and the machine instructions The file also saves the global EQUs the position and size of the register and RAM display windows the highlighting loading and IO connection options chosen by the user from the Options submenu under the Execute menu However the state of the machine is not saved in particular the current contents of the registers and RAMs are not saved 2 CPU Sim machines are saved in a text file in XML format which means that you can edit the saved machine from any text editor including CPU Sim s built in text editor But you must be careful that the edited file still fulfills the data type definition DTD of a CPU Sim machine file or CPU Sim will not be able to load your edited machine Note the DTD is included as part of the XML file The New text menu item causes CPU Sim to create and open an empty text window in which the user can type a new assembly language program The text window is like any basic text editor window including the usual cutting and pasting and an arbitrary number of undo s and redo s 19 The Open text menu item displays a dialog box asking you to choose a file You normally select a text file in which you had previously saved an assembly language program The Reopen text menu contains a list of the 10 most recently opened text files This list is saved between sessions in the properties file specified on t
11. assembly programs and display windows for viewing and editing the contents of registers and memory If you do not specify a machine on the command line when starting CPU Sim then the title of the main display window is New since if no machine is specified a new virtual machine is created that has no registers and no memory wombat ojx Wombat _ lolx File Edit Modify Execute View Text Help E Registers name width value 12 0000 0000 0000 ic 16 0000 0000 0000 0000 16 0000 0000 0000 0000 12 0000 0000 0000 16 0000 0000 0000 0000 000 i Data Binary v Cell size 2 w Address Data Comments ol 0000 0000 0000 0000 a 2 0000 0000 0000 0000 4 6 0000 0000 0000 0000 0000 0000 0000 0000 8 0000 0000 0000 0000 10 0000 0000 0000 0000 12 0000 0000 0000 0000 14 0000 0000 0000 0000 16 0000 0000 0000 0000 18 0000 0000 0000 0000 20 0000 0000 0000 0000 22 0000 0000 0000 0000 0000 0000 0000 0000 0000 0000 4 q D gt Console Figure 1 The main display after the Wombat has been loaded To load a previously saved machine choose Open machine from the File menu You will be presented with a dialog box in which you are supposed to select a file To continue with this tutorial
12. during execution are outlined in green After each step you can inspect and change the values stored in any register or RAM e You can back up one machine instruction at a time which resets the values of the registers and RAMs to their state at the start of the last machine cycle e You can also back up one microinstruction at a time which resets the values of the registers and RAMs to their state at the start of the last microinstruction e You can specify cells of RAMs to be highlighted based on the address stored in specified registers as you step through the code using the Options Highlighting menu item as discussed above For example you can cause the top cell of the stack to be highlighted after each step e You can specify break points at any cell in any RAM by checking the box on the left side of that cell in the RAM window When such a cell is accessed via a MemoryAccess microinstruction for either reading or writing the Memory Access microinstruction is executed and then execution halts When the program halts the address in RAM that caused the break is highlighted in red Additionally if the RAM window is not open at the time that the break is reached it will be opened and if the address in the RAM window where the break is located is outside of the view window CPU Sim will scroll to the specified location At that point you can inspect and or edit the contents of all registers and RAMs and back up or resume execution
13. grouped in four digit units You can also view the addresses as positive integers in base 2 10 or 16 You can view the contents of RAM in units of 1 8 bytes instead of just 1 byte Since the Wombat1 uses 16 bit instructions it is helpful to view RAM in 16 bit units so we have selected a cell size of 2 in the popup menu in the RAM window The values of all cells are editable except the cells in the address column Leave the registers and RAM windows open for the remainder of this tour Note The table columns in any of these windows can be reordered by dragging the column headers left or right to a new position The column widths can also be adjusted by dragging the line dividing the columns In all windows except the RAM windows the rows can be resorted by column by clicking in any column header The bottom panel of the main display is labeled Console and is used for console input and output That is the user can type data into the Console panel when prompted for input and CPU Sim can type data in the panel for output Now let s open a Wombat1 assembly language program To do so choose Open text from the File menu and choose the file W1 0 a This file may be found in the same folder as the Wombat1 cpu file After you have selected it a window will appear containing the text of the file see Figure 2 The first three lines are comments comments begin with a semicolon and are shown in italics font in Figure 2 The remainin
14. has already been executed by means of the Step by Micro button or if the execution of a whole cycle was halted because of an error then clicking this button causes the rest of that machine cycle to be executed The name of the machine instruction currently being executed is displayed in the debug toolbar near the right end The Step by Micro button causes execution of one microinstruction in the current fetch or execute sequence The name of the microinstruction to be executed is highlighted in the scroll box on the right end of the debug toolbar The Backup one Instr button causes the machine to back up to the state it was in at the beginning of the last machine cycle You can continue backing up one machine instruction at a time all the way to the initial state of the machine when you began execution of the current program using the current machine in debug mode The Backup one Micro button causes the machine to back up to the state it was in at the beginning of the previous microinstruction You can continue backing up one microinstruction at a time all the way to the initial state of the machine If you have been backing up by microinstruction and then use the Backup one Instr button the machine will back up to the state it was in at the beginning of the last machine cycle From there you can back up by machine instruction or microinstruction or you can step forward by machine instruction or microinstruction The R
15. list of microinstructions The default sequence is empty The current fetch sequence list is displayed in the left scroll box entitled Implementation To add microinstructions to it first display the microinstructions of the type you want in the tree of microinstructions in the right side of the dialog by clicking on the dial to the right of the folder corresponding to the desired type Then highlight the microinstruction you want by clicking on it Note If you realize you need to create or modify a microinstruction before inserting it in the fetch sequence you don t need to close this dialog Instead you can just double click on the tree of microinstructions on the right and the dialog for editing and creating microinstructions will appear Next to specify where the new microinstruction is to be inserted into the fetch sequence click on the microinstruction in the left scroll box before which you want to insert the new microinstruction Finally click lt lt insert lt lt see Figure 7 B Edit the machine s fetch sequence x Implementation All micros Ef TransferRtoR D C mar pe CY ir 4 15 gt m n Ci mar ir G mdr gt act D acc gt mdr gt gt delete D ir 4 15 gt pe gt CI TransferAtoR gt C TransferRtoA gt cI Set gt E Test gt Increment E hie Arithmetic 4 gt pc gt mar Main mar gt mdr Help OK Cancel 14 15 009e Edi
16. machine to a cease executing the fetch sequence b decode the instruction register and c begin executing the execute sequence of the machine instruction that was just decoded In other words no microinstructions of the fetch sequence get executed after a Decode microinstruction has been executed SPECIFICATION OF MACHINE INSTRUCTIONS A machine instruction consists of one or more bytes The user specifies how the bits are to be divided into one or more fields The first field stores the opcode of the instruction The remaining fields form the operands of the instruction For example consider a 2 byte instruction named load with fields of length 4 2 and 10 This instruction has a 4 bit opcode field a 2 bit operand field and a 10 bit operand field When the user wants to execute this instruction in an assembly language program the user must give the name of the instruction and 2 operands each consisting of a symbol or number The operands can optionally be separated by a comma For the load instruction mentioned above a legal assembly language instruction is load 1 96 You may optionally designate some of the fields as don t care fields To do so you surround the field lengths by parentheses in the Machine Instruction dialog By doing so you are telling CPU Sim s assembler that those fields are to be ignored In this case no operand needs to be specified in assembly language for that field when the instruction is
17. make to microinstructions are applied to the current machine even if you cancel any changes to the fetch sequence Modifying the machine instructions To modify machine instructions choose Machine Instructions from the Modify menu The dialog box that appears see Figure 8 above for a picture of this dialog box when the Wombat is loaded is similar to the fetch sequence dialog box described above except for the table on the left that displays all existing machine instructions names their opcodes and their field lengths To edit an instruction s name opcode or field lengths double click on the entry in the table on the left and then type in the new value For a discussion of what names are legal see the section above regarding naming components microinstructions and machine instructions The opcode value is displayed in hexadecimal notation and a new value for the opcode must be entered as a non negative hexadecimal value If you type in characters that are not hex characters the hex characters are 0 9 and a f and A F the opcode box you are editing will become highlighted in red and a bell will sound when you attempt to save your changes The field lengths are entered as one or more positive decimal integers separated by one or more spaces The sum of all field lengths must be a multiple of 8 You may optionally surround some of the field lengths by parentheses In doing so you are telling CPU Sim that those fields ar
18. noting that because of the textual substitution labels in macro bodies must be treated specially To understand the problem consider a macro body in which one line has a label L and suppose the macro is called twice in one file If straight text substitution were performed the label L would appear on two lines of that file violating the necessary uniqueness of all labels in a program Therefore before the textual substitution of every macro call the label and all references to the label in the macro body are replaced with a unique label For this reason labels in macro bodies cannot be referenced outside the macro body EQU declarations EQU declarations in programs define names for integer constants Those names can be used anywhere in the program where a number is expected It is legal to declare an EQU name whose value is a previously defined EQU name For example it is legal to make the following declarations A EQU 4 BEQUA In this example both A and B are now names for the constant 4 Note that the names of all EQUs must be valid symbols and the values of all EQUs must be previously defined EQU names or valid literals in the assembly language syntax The literal value must be in the range 26 to 263 1 9223372036854775808 to 9223372036854775807 Note that EQU declarations are always local which means that the declared names can only be used in the file in which they are declared Therefore it is often 38 useful to put the
19. occurs that is if the result is too big or too small to fit in the register using two s complement notation then the overflow condition bit will be set to 1 if a condition bit is used Otherwise the value of the overflow condition bit is left unchanged In the case of overflow the register will contain the rightmost n bits of the result in two s complement notation where n is the width of the register Typical uses of the increment microinstruction include incrementing the program counter during the fetch sequence or incrementing or decrementing the stack top pointer during pushes and pops Parameters Register the register whose contents are to be incremented Value the integer value that will be added to the register contents Overflow an overflow condition bit If not needed then none should be specified An IO microinstruction simulates an IO port on a CPU The microinstruction either a inputs into a buffer register a value from the user or from a text file or b outputs the value of a register to a text file or to the user An IO microinstruction that outputs a value to the user causes a dialog box to appear with the value displayed An IO microinstruction that inputs a value from the user causes a dialog box to appear in which the user is asked to type in the value To specify the source or destination of the data either to from the user or to from a text file use the Options IO connections menu item from the Exec
20. select and open the text file named Wombat1 cpu It should be found in the SampleAssignments folder included with CPU Sim Note The Wombat1 cpu file is a text file in XML format that contains the information for the Wombat machine If the machine loads without error then the title of the main display should become Wombat1 and two windows should appear inside the desktop see Figure 1 The window labeled Registers displays all the registers of the Wombat1 including their widths and current values To see the values in every register displayed in binary base 2 signed 2 s complement decimal base 10 unsigned decimal base 10 hexadecimal base 16 or as characters ASCII or Unicode select the appropriate item from the popup menu at the top of the window Values in binary and hexadecimal format are displayed in four digit groups for readability The values of the registers can be edited in any of these display formats by clicking in the appropriate cell of the table typing in a new value and then pressing the enter or return key The edited values will be automatically reformatted into groups of four characters if the user is working in binary or hexadecimal The window labeled RAM Main shows the contents of the RAM named Main As with the registers you can display the values in base 2 10 signed or unsigned 16 or in ASCII or Unicode and as with the registers values in binary and hexadecimal format are
21. step through the execution note that the next machine instruction to be executed is highlighted in the RAM window If you wish you can edit the contents of any of the registers or RAM between steps Both the fetch sequence and the execute sequence of the instructions that are fetched are comprised of a series of smaller more basic steps called microinstructions The list on right side of the debug toolbar displays the microinstructions making up the current fetch sequence or instruction If you click Step by Micro from the toolbar then only one microinstruction will execute namely the microinstruction highlighted in the scrolling list on the right end of the debug toolbar If the microinstruction that is executed changes a value in the RAM or Registers then the data in the RAM or Registers that is changed during execution is outlined in green When this microinstruction finishes execution you will see that the next microinstruction that is to be executed becomes highlighted in the scrolling list on the right end of the debug toolbar while the current machine instruction remains highlighted in the RAM window If the microinstruction that you execute is the final end microinstruction of a particular machine instruction then the current machine cycle is ended A new machine cycle will begin execution next with the fetch sequence and the scrolling list of microinstructions will be updated to display the microinstructions in the fet
22. the contents of registers or RAMs pc gt mar i 1 1 r 1 if 1 Go Step by Instr Step by Micro Backup one Instr Backup one Micro Reset all I Flush amp Reset IO Reset Control unit Fetch sequence Main mar gt mdr a mdr gt ir v pc gt mar Go Step by Instr Step by Micro Backup one Instr Backup one Micro Reset all Flush amp Reset lO Reset Control unit Fetch sequence Main mar gt mdr mdr gt ir Le Ti gt Figure 3 The debugging toolbar Let us now use debugging mode to step through the execution of the W1 0 a program Notice that on its right end the debugging toolbar says that the next instruction is the fetch sequence If you click Step by Instr from the toolbar then a complete machine cycle will be executed Do so now The fetch sequence will be executed which will fetch and decode the read instruction and then the read instruction s execute sequence will be executed causing the machine to ask you for input in the console panel at the bottom of the desktop window After typing the input in the console and pressing return you will see that the value that you typed is now in the acc register You can also see that the next instruction is again the fetch sequence In summary each click of Step by Instr causes a full machine cycle to be executed consisting of the fetch sequence followed by the execute sequence of the instruction that was fetched As you
23. using the same data stream That is if one IO microinstruction reads a value from the file then the other IO microinstruction cannot read that value from the file and instead will read the next value When CPU Sim attempts to read an integer from a text file it reads past any white space space new line carriage return and tab characters reads an optional or character and then reads and appends digits 0 9 until a non digit character is encountered When CPU Sim attempts to read a character ASCII or Unicode froma file it just reads the next character of the file If you want to store binary data in a text file to be read by an IO microinstruction precede the binary value with a 0b For example if you want to store the binary value of 1101 then the text file should have the value 0b1101 If you want to store hexademical data in a file for later reading by an IO microinstruction precede the hexadecimal value with the prefix 0x When CPU Sim attempts to write an integer to a text file it first writes a space character so that successively written integers can be distinguished from each other It then writes the integer in decimal format When CPU Sim attempts to write a character ASCII or Unicode to a file it writes the character immediately with no additional spacing When a program is run using the Assemble load amp run Run or Clear assemble load amp run menu items in the Execute me
24. value to which the bits are to be set This value must fit in the designated bits as either an unsigned or signed two s complement decimal Therefore the value must be from 2 1 to 2 1 where w is the width of the register SetCondBit The SetCondBit microinstruction sets the contents of a specified condition bit to 0 or 1 Note that this is just a convenience method since you can also set any bit of any register using the Set microinstruction Parameters bit the condition bit to be set value the value 0 or 1 to which the condition bit is to be set Branch The branch microinstruction is identical to the Test microinstruction except that it is an unconditional jump The microinstruction specifies how far to jump relative to the current branch microinstruction Parameter 49 Distance an integer positive negative or zero indicating the size of the relative jump For example if distance 0 then no jump is performed That is the microinstruction following the branch microinstruction is the next microinstruction to be executed as normally happens with other microinstructions If distance 1 then the branch microinstruction is repeatedly executed in an infinite loop Decode End The decode microinstruction is usually the last microinstruction in the fetch sequence It is used to simulate a decoder An instruction register ir must be specified CPU Sim will decode the instruction in the ir i e determine wh
25. CPU Sim USER S MANUAL Version 3 4 January 2007 by Dale Skrien assisted by Raymond Mazza III Joshua Ladieu Jonathan Weinberg Andreea Olea Thomas Cook Patrick Rodjito Tim Monahan Mike Liedtke Peter Landwehr Taylor Snook Charlie White Brian Putnam Tom Goth Mike Wolk and Cadran Cowansage Preface This manual is describes how to use version 3 4 of the CPU Sim simulation system This simulator is based on the ideas in the STARTLE simulator developed by J M Kerridge of the Department of Computer Science University of Sheffield Sheffield England Report any problems or suggestions to Dale Skrien Department of Computer Science 5851 Mayflower Hill Colby College Waterville Maine 04901 phone 207 859 5851 Email djskrien colby edu Web page www cs colby edu djskrien CPUSim Acknowledgments The author and programmers would like to acknowledge the following people whose code was used in this package e Mark Fisher for his JavaHelp demo code 1 JavaHelp for the Java Programmer by Mark Fisher JavaReport June 1999 e David Flanagan for his PageableText class e Mark Johnson for his HtmlEncoder and LAX classes e Philip Milne for his TableSorter and TableMap classes e Matthew Robinson and Pavel Vorobiev for their FindDialog DialogLayout and PrintPreview classes e Robb Shecter for his OOTableModel class e Klaus Berg for his EFileChooser package e Tom Tessier for his JScrollableDesktopPan
26. CPU Sim creates only one copy of that microinstruction and so if you edit and change its parameters then the changed microinstruction will be used in the fetch sequence and in all the machine instructions execute sequences in which it appears Editing Microinstructions from the Fetch Sequence or Machine Instruction Dialogs It is often the case that when you are modifying the fetch sequence or the machine instructions you will find that you need to modify or create a new microinstruction The easiest way to do so is to double click on the list of current microinstructions on the right side of the dialog box which will bring up the dialog for editing microinstructions When you are finished editing the microinstructions you can close that dialog box and can resume editing the fetch sequence or the machine instructions Important note Any changes that you make to microinstructions are applied to the current machine even if you cancel any changes to the fetch sequence or machine instructions Modifying the EQUs Each machine can have some equates EQUs associated with them These EQUs are global constants available for use in any assembly language program written for that machine For example if an EQU named A0 with value 0 is available then the user can type in AO wherever a numerical value is expected in assembly language programs and during assembly the symbol A0 will be replaced with the numeric value of 0 To create or edi
27. Once execution is resumed the RAM address where the break occurred will no longer be highlighted To enter debug mode select the Debug mode menu item in the Execute menu This action will cause a a debug toolbar to appear below the main menu see Figure 3 and b a new column of checkboxes to appear on the left side of all RAM windows The debug toolbar contains 8 buttons and a text component and a scroll box The text component near the right end of the toolbar always displays the machine instruction that is about to be or is currently being executed At the start of a machine cycle it displays the fetch sequence first There is also a scroll box to the right of the text component that lists the microinstructions comprising the current machine instruction or fetch sequence The Go button causes execution to continue without interruption until one of 30 the following happens e acondition bit that is designated as halt bit has its value set to 1 e abreak point is reached e the user stops the current execution by choosing Stop from the Execute menu e the user chooses Cancel when an input dialog appears e the user presses enter or return before entering a value when the Console panel asks for input e an error occurs The Step by Instr button causes the execution of one full machine cycle a fetch sequence followed by the execute sequence of the instruction decoded by the fetch sequence If part of a machine cycle
28. and ignored by the others If not needed then none should be specified Shift The shift microinstruction performs a bit wise shift of the contents of the specified source register to either the left or the right and places the result in the destination register The user specifies one of the following modes Logical shift The newly emptied bits are given the value 0 Arithmetic shift If shifting right then the leftmost bit is copied into the newly emptied bits If shifting left then the emptied bits on the right are given the value 0 Cyclic shift The bits dropped off one end are placed in the emptied bits on the other end Parameters Type either LOGICAL ARITHMETIC or CYCLIC Direction either LEFT or RIGHT Distance a non negative integer indicating the number of bit locations to the left or right the values are to be shifted Source and Destination the register whose contents are to be shifted and the register into which the shifted bits are placed These two registers must have the same width and could be the same register Logical The logical microinstructions perform the bit operations of AND OR inclusive or NOT NAND negated AND NOR negated OR or XOR exclusive or on the specified registers In all operations but NOT three registers are specified two of which are source registers and the third of which is the destination register NOT is monadic and so only the first source register is used and the s
29. ardware modules from the Modify menu You will see a dialog box with a popup menu at the top with Register currently selected See Figure 5 above for an example In the table in the dialog box you will see a list of all existing registers You can edit the parameters associated with any register or create new or delete old registers The width of a register refers to the number of bits in the register You cannot edit the contents of the registers that is the value stored in the registers using this dialog box To edit a register s contents use the window that appears when you choose Registers from the View menu You can create edit and delete register arrays by selecting Register Array from the popup menu at the top of the dialog box A register arrayis an indexed list of any number of registers If the array has name A and if the array contains 4 registers then those registers are denoted by A 0 A 1 ARI and ABT By choosing RAM from the popup menu at the top of the dialog box you can create edit or delete any number of RAMs of any length The length refers to the number of bytes of data in the RAM If you increase the length of a RAM new memory locations containing the value 0 will be added at the end of the RAM If you decrease the length of a RAM some of the bytes at the end will be deleted and their contents will be lost Note that all RAMs are byte addressable By choosing
30. arguments at the end of the command line in any order m lt machine file name gt p lt properties file name gt t lt text assembly program file name gt C If you specify a machine file using the m flag then that machine is loaded into CPU Sim during startup If you specify a text file using the t flag then that file is opened during startup If you specify the c flag which can only be used together with the t and n flags then the corresponding text file and machine file will be loaded and run from the command line If you specify a properties file using the p flag then the properties in that file are loaded into CPU Sim during startup A properties file contains information regarding the preferred fonts for text register and RAM windows and a list of the recently opened machines and text files For example to start up the Wombat1 machine and load the W1 0 a assembly language program that appear in the SampleAssignments folder you could type in the following command all on one line on Windows java cp CPUSim3 4 jar jhall jar CPUSimHelp3 4 jar cpusim Main m SampleAssignments Wombat1l cpu t SampleAssignments W1 0 a Notes o Ifthe user does not specify a machine file a new empty machine is opened o CPU Sim searches its directory for any files you specify on the command line so you must include the directories relative to that location o Ifyou use the c flag you must also specify a machine and te
31. called and instead the assembler just fills the field with 0 s For example if the 2 bit field of the load instruction mentioned above is a don t care field then in an assembly language program the programmer would type in the name of the instruction and one operand such as load 96 In that case the assembler will put the opcode in the 4 bit first field the operand 96 in the 10 bit field and will fill 42 the 2 bit field with 0 s Note that the operands can have any value that fits in the field in either 2 s complement or unsigned integer format Therefore the legal values for a 2 bit field are 2 to 3 and the legal values for a 4 bit field are 8 to 15 Each machine instruction also specifies a list of microinstructions that form the implementation of the machine instruction and that are executed when the machine instruction is to be executed This microinstruction list is called the execute sequence of the instruction Every execute sequence should contain an End microinstruction which causes the machine to stop executing the current execution sequence and start executing the fetch sequence and so causes the start of anew machine cycle Parameters Opcode a non negative integer that fits in the first field of the instruction Field Lengths a list of 1 or more positive integers whose sum is a multiple of 8 The integers corresponding to lengths of don t care fields are surrounded by parentheses Implementation a lis
32. ch sequence The first microinstruction in the fetch sequence will be highlighted and the next machine instruction will be highlighted in the RAM window You can also return to any previous state by backing up one machine or microinstruction at a time To do so repeatedly choose Backup one Instr or back up one Micro from the toolbar No matter what microinstruction you are currently executing if you click Backup one Instr the state of the CPU will revert back to the state at the beginning of the machine cycle If you click Back up one Micro the cells outlined in green in the Registers and RAM windows are updated to reflect those values that were changed in the previous microinstruction To set break points in your code check the box on the left of any line of code in the Main RAM window When the CPU accesses that line of code for example when that line is loaded into the CPU for execution the program will halt At that point you can inspect or change any of the values in the register or RAM windows and resume execution or step forward or backward To learn more about debug mode see the section of this manual entitled EXECUTE MENU RUNNING A PROGRAM ON THE SIMULATED MACHINE At this point you have many options You can continue stepping through the program one instruction or one microinstruction at a time by clicking the appropriate button or you can continue execution without stopping by clicking the Go butto
33. d Flanagan copyright c 1999 by O Reilly amp Associates 3 Programming XML in Java by Mark Johnson JavaWorld April 2000 4 The Java Tutorial at http java sun com docs books tutorial 5 Swing by Matthew Robinson and Pavel Vorobiev Manning Publications Company 1999 6 Object Oriented Widgets by Robb Shecter JavaReport December 1999 7http www javaworld com javaworld javatips jw javatip100 html 8 http www javaworld com javaworld jw 11 2001 jw 1130 jscroll_p html 9 http www jedit org 10 http www usixml org javadocs michotte be michotte util Text html 1 http jtf acm org 13 www cs colby edu djskrien CPUSim new CPU in either machine language or assembly language which can be run in the CPU Sim environment CPU Sim executes a program through repeated execution of machine cycles One cycle has two parts the fetch sequence which is a sequence of microinstructions that typically loads the next instruction to be executed into an instruction register and decodes it and the execute sequence which is the sequence of microinstructions associated with the newly decoded instruction The fetch sequence is the same for all cycles but the execute sequence can and usually does vary from cycle to cycle Execution of machine cycles halts when a halt bit is set to 1 CPU Sim has many useful features for running programs and debugging them For example CPU Sim has a built in text editor and assembler for crea
34. e don t care fields and are to be ignored In this case no operand needs to be specified in assembly language for that field when the instruction is executed Instead the assembler just fills that field with O s To modify the execute sequence of an instruction first select the instruction you wish to edit by clicking on it in the table on the left Its execute sequence or implementation that is the list of microinstructions that are executed when the machine instruction is executed will appear in the center of the dialog box Modify them as described above for the fetch sequence To delete a machine instruction first 25 select it in the table on the left Then click the Delete button To create a new machine instruction click the New or Duplicate buttons The Duplicate button allows you to create easily new machine instructions that are variations of other machine instructions To use this feature select in the table on the left the machine instruction you wish to duplicate before you click the Duplicate button A new machine instruction will be created with the same opcode field lengths and microinstructions as the instruction you had selected You can then edit the name of the new instruction its opcode field lengths and microinstructions as you would any other machine instruction Note that one microinstruction may appear in several machine instruction s execute sequences As mentioned above
35. e a machine that is a modification of an existing machine it is recommended that you create a copy of the existing machine by choosing Save machine as from the File menu and typing in a new name for the file and then modify the copy that is stored in the new file CPU Sim does not allow you to create a new machine by cutting and pasting components from an existing machine While you are in the process of creating a new machine from an existing machine you may find it frustrating in that you will not be allowed to change some parameters because of the uses to which they are currently being put For example you are not allowed to delete a register if one of its bits is being used as a condition bit For another example you cannot reduce the width of a register to less than 12 if that register is the source of a transfer microinstruction that transfers 12 of the register s bits to another register To get around these problems start by adding any new components registers register arrays condition bits and RAMs you want That is first create new registers and other components without deleting any of the old ones After this is done adjust the microinstructions to use these new components instead of the old ones Only after all this has been done should you delete any of the old components Once this task has been completed you can change the names of the new components or instructions back to the names used by the old components if yo
36. e name to Main and length to 128 for the new memory component Finally the Wombat1 needs to specify a halting condition bit and so select ConditionBit from the popup menu at the top of the dialog Click New and then set the name of the condition bit to halt bit the register to the status register the bit to 0 and check the halt box 11 12 Type of Module Register hA name width pe 12 ace 16 ir 16 mar 12 mdr 16 status 3 e608 Edit Modules Type of Module Register He g name width pc 12 acc 16 ir 16 mar 12 mdr 16 Status 3 New Delete Duplicate Help OK Y Cancel Cu ent a Figure 5 The dialog for editing the hardware modules The meanings of all the parameters of the hardware components are described in more detail at the end of this manual in the section entitled SPECIFICATION OF THE SIMULATED HARDWARE UNITS You can also find out more information from the online help that appears when you click the Help button in this dialog box and most other dialog boxes Close the Edit Modules dialog box by clicking the OK button Now we need to construct the necessary microinstructions that will be used to implement the machine instructions There are 7 transfer 4 arithmetic 2 test 1 increment 1 decode 2 io 2 memory access and 1 set condition bit microinstructions that
37. e packages e Slava Pestov for his JEditTextArea package e Benjamin Michotte for his Text utility class e The ACM Java Task Force group for the IOConsole class INTRODUCTION CPU Sim is an interactive computer simulation package in which the user specifies the details of the CPU to be simulated including the register set memory the microinstruction set machine instruction set and assembly language instructions Users can write machine or assembly language programs and run them on the CPU they ve created CPU Sim simulates computer architectures at the register transfer level That is the basic hardware units from which a hypothetical CPU is constructed consist of registers condition bits and memory RAM The user does not need to deal with individual transistors or gates on the digital logic level of a machine The basic units used to define machine instructions consist of microinstructions of a variety of types The details of how the microinstructions get executed by the hardware are not important at this level A user of CPU Sim who wants to simulate a new hypothetical CPU must first specify the hardware units and microinstructions for the CPU and then create the machine instruction set Each machine instruction is implemented via an associated sequence of microinstructions specified by the user Once the instruction set has been specified the user can write programs for the 2 Java Foundation Classes in a Nutshell by Davi
38. econd is ignored Test 46 Parameters Type which logical function AND OR NAND NOR XOR NOT is to be carried out Sourcel Source2 and Destination the three registers to be used Sourcel and Source2 contain the values used in the specified operation and the result is placed in Destination All registers must have the same width and all bits in the registers are affected by the operation The three registers need not be distinct i e the two source registers can be the same and a source register can be the same as the destination register Source2 is ignored if the type of operation is NOT The test microinstruction allows jumping to other microinstructions within the current fetch or execute sequence The test microinstruction compares the value in a part of a register with a specific value If the comparison succeeds then a specified number of successive microinstructions in the current fetch or execute sequence are skipped over Parameters Comparison the type of comparison be done LT means less than LE means less than or equal to EQ means equal to NE means not equal to GE means greater than or equal to GT means greater than Comparison value the integer to be compared with the part of the register The Comparison value must be given in decimal notation Omission an integer positive negative or zero indicating the size of the relative jump For example if omission 0 then
39. ed The Assemble amp load menu item assembles your code and if it is legal loads 27 the assembled machine code into the simulated machine s memory The RAM into which the code is loaded is not first cleared set to 0 and so the only memory locations that are changed are those into which the new code is loaded If you want to clear the rest of RAM you need to do so first by choosing the appropriate menu item from the Execute menu Note You can use the Options submenu of the Execute menu discussed below to specify which RAM is to be loaded with the machine code and what starting address is to be used The Assemble load amp run menu item is a convenience item that is equivalent to choosing the Assemble amp load menu item followed by the Run menu item Similarly the Clear assemble load amp run menu item is a convenience item that is equivalent to choosing the Clear everything menu item followed by Assemble load amp run The Run menu item causes the machine to begin execution The program will always begin execution with the first microinstruction of the fetch sequence The program will stop when one of the following events occurs a one or more condition bits that are designated as halt bits have their values sets to 1 b an error occurs c the user chooses Stop from the Execute menu d the user selects Cancel when an input dialog is displayed e the user presses
40. ed part of the label The label and colon pair is an optional feature on every line of assembly language programs including those lines that are otherwise blank or contain only comments In the latter two cases the assembler will treat the label as if it referred to the next regular instruction or data pseudoinstruction Labels can be used as operands in statements A comment is any sequence of characters preceded by a semicolon and ending at the end of the line Any line of a program can contain a comment Blank lines and lines containing only comments are also allowed in assembly language programs and are ignored by the assembler When the program is assembled regular instructions and data pseudoinstructions including the comments on the ends of the lines are saved and appear after each line of the assembled program in the Comments column of the RAM window into which the assembled program is loaded However remember that blank lines or lines with only comments are discarded when the program gets assembled A program consists of two parts The first part contains any number of EQU declarations include directives and macro definitions in any order The second part 36 contains any number of regular instructions and data pseudoinstructions one per line Include directives An Include directive is a pseudoinstruction used for temporarily inserting the text from one file into the text of another file just before assembly The pseudoins
41. edo will redo the last change that was undone An arbitrary number of undo s and redo s can be done to the changes in any of the text windows Cut Copy Paste Delete Select All Find and Replace work in the usual fashion The Preferences menu item brings up a dialog box in which you can change the font font size and font style for the text windows containing assembly code and for the display of the contents of Register and RAM windows You can also change the I 21 default syntax coloring used in text windows for the labels keywords and comments The preferences you choose are saved between sessions in the properties file specified on the command line when CPU Sim is started and so for example when you quit CPU Sim and restart it later the Register and RAM windows will use the same font as when you quit CPU Sim If no properties file is specified when CPU Sim is started then the user s font choices will not be saved between sessions and the default font size and style will be used MODIFY MENU CHANGING THE COMPONENTS There are several dialog boxes that can be called up for the purpose of changing the components of the current machine To call up these dialog boxes just select the appropriate item from the Modify menu see Figure 11 Machine Instructions Machine Instructions hfe Microinstructions Hardware modules Hardware modules Fetch Sequence Fetch Sequ
42. em creates anew HTML document containing a user friendly description of the current machine including all its details The HTML document can be opened and viewed using any web browser The Print preview menu item displays a thumbnail sketch of the pages as they would be printed for the currently selected text window The Page Setup menu item brings up the usual page setup dialog from which you can set the page size reduction or enlargement and other printing options The Print menu item brings up a dialog for printing the contents of the selected window The Exit CPU Sim menu item causes CPU Sim to quit If changes have been made to the currently loaded machine since the last time you saved it CPU Sim asks you whether you want to save the current machine before quitting Similarly if changes have been made to any text window since the last time you saved it CPU Sim asks you whether you want to save the contents before quitting EDIT MENU The Edit menu items see Figure 10 work with any text window The user must first select bring to the front the text window to be edited Undo ab L Reric SP y KCU gd l Cut a5 X Copy aC Paste gV isa H Delete Paste Ctrl SelectAll SA Delete Select All tri A Find F Re lace H Find Ctrl F ep ian Replace trl H Preferences Preferences Figure 10 The Edit menu Undo can undo the last change made in the selected text window R
43. ence EQU s EQU s Figure 11 The Modify menu Note that such modifications are not allowed when CPU Sim is in debug mode that is when the Debug Mode menu item is checked in the Execute menu Almost all of the dialog boxes display a table in which you can inspect and edit the parameters of existing components and in which you can add new components or delete components as demonstrated in the tutorial in an earlier section of this manual To delete a component displayed in the dialog box select it in the table and then click the Delete button To create a new component click in the New button at which point a new component will be added to the end of the table To edit a cell just click or double click on it Some cells are edited by typing new text in them some cells are edited by selecting items from popup menus and some cells are edited just by clicking in the check box in the cell If you try to enter an illegal value in a cell either the cell will become outlined in red for example if you type non numeric values in a cell expecting a numeric value or an error message will appear when you try to save the changes you made for example if you type in a numeric value that is out of range for that particular cell The columns in any of these tables can be reordered by dragging the column headers left or right to a new position The column widths can also be adjusted by dragging the line dividing the co
44. enter or return instead of a value when input is requested in the console panel The Stop menu item causes the currently executing program to halt This menu item is particularly useful if the program gets into an infinite loop The Clear lt X gt menu items are convenience items for initializing registers or other components to 0 before beginning execution of a program The Clear everything menu item causes all registers register arrays and RAMs to be cleared You can also clear any individual register or cell of RAM by editing its contents in the register or RAM windows The Options Highlighting menu item brings up a dialog box in which the user can choose which lines of code are to be highlighted when the code is being executed Note that this highlighting occurs only when in debug mode The highlighting is specified via pairs consisting of a register and an RAM The row of the RAM whose address corresponds to the value of the register is highlighted For example the user may wish to highlight the row of the stack corresponding to the address in a stack top pointer register The rows of RAM are highlighted after each execution step If the user wishes the highlighting to be updated after each microinstruction step then the dynamic checkbox should be checked Otherwise the highlighting is updated only at the beginning of each machine cycle The Options Loading menu item brings up a dialog box in which the user can specif
45. eset All button causes the machine to back up all at once to the initial state it was in when the user starting executing the current program in debug mode using the current machine This action also causes the IO to be flushed and reset and the control unit to be reset This buttons functions similarly to Clear everything in the Execute menu The column of checkboxes in the RAM windows that appears when CPU Sim is in debug mode is used for setting break points CPU Sim will halt execution whenever a MemoryAccess microinstruction attempts to read or write to a cell of memory with a checked box More precisely the Memory Access microinstruction will be executed the row of the RAM window with the checked box will be highlighted in red if the RAM window is closed it is opened automatically and then execution will halt At that point you can inspect and or change the contents of any register or RAM and continue execution or back up 31 By checking the box in front of a memory cell containing a machine instruction you can halt execution when that instruction is reached since that instruction needs to be loaded into a register via a MemoryAccess microinstruction before it can be executed By checking the box in front of a memory cell containing data for example data on the system stack you can halt execution when that data is read or written Note e Between every step forward or backward you can inspect and edit any values i
46. g a 2 bit value and the second having a 10 bit value If a machine instruction has fields consisting of 4 2 10 where the 10 indicates a don t care field then the opcode must be 4 bits long and the instruction must have one 2 bit operand Operands must be separated from each other by a comma and or one or more spaces and tabs Note that there are three types of sources of values for operands that are symbols local EQUs defined in the file global EQUs defined in the machine and labels defined in the file A symbol might appear in all three types of sources In that case local EQUs have priority followed by global EQUs and finally labels Data pseudoinstruction The word data signifies a pseudo instruction in assembly language It provides a way for the user to insert specific numerical values in specific locations in the assembled code The pseudoinstruction has the following parts label data operands comment The word data and the operands must be included but the comment and the label are optional The label can be used as an operand in assembly language statements to refer to the first address of the data that is being specified The pseudoinstruction is terminated by the end of line character The first operand must have a positive numerical value n that indicates the number of bytes of data that are being given specific values 39 This operand is followed by one of the following e one long value that
47. g lines consist of instructions followed by comments The instructions consist of the name of a Wombat1 machine instruction load store etc written in a bold blue font Some of the instructions are followed by an argument and some of them have a label in front labels end with a colon and are shown in a bold burgundy font The data instruction is actually a pseudo instruction used by the assembler to allocate memory for the variable sum The assembler syntax is described in more detail below in the section entitled SPECIFICATIONS OF ASSEMBLY LANGUAGE FOR CPU SIM MACHINES 9090 W1 0 a z This program reads in integers and adds them together il azr tive Then it outputs jmpn Done add sum store sum jump Start number Done load sum write stop sum data 2 0 2 byte stored Figure 2 A Wombat1 assembly language program Before you can run this program you must assemble it into machine language instructions that the Wombat1 can understand and then load those instructions into the Wombatl s memory To do so choose Assemble amp Load from the Execute menu Note If this menu item is disabled the assembly language text window W1 0 a is not the currently selected i e highlighted window In that case click on the text window before choosing Assembly amp Load You should see numbers and comments appear in the first few rows of the table in the RAM window The
48. he command line when CPU Sim is started so that when you quit CPU Sim and restart it later the Reopen text menu will be populated with the same list as it had when you quit CPU Sim If no properties file is specified when CPU Sim is started then the Reopen text menu items will not be saved between sessions The Close text menu item closes the selected topmost text window If the contents of the window have been changed since the window was last saved the user is presented with a dialog box asking whether you wish to save the contents first The Save text menu item saves the contents of the selected topmost text window to a text file If the user has not already saved the contents to a file a dialog box is displayed asking the user to select a name and location for the file The Save text as menu item displays a dialog box asking you to choose a file into which the contents of the selected topmost text window will be saved The Load RAM menu item displays a dialog box asking you to choose a file You should select a text file in which you had previously saved the contents of a RAM window using the Store RAM menu item The contents of the file will be loaded into the currently selected RAM window The Store RAM menu item displays a dialog box asking you to choose a text file into which the contents of the selected topmost RAM window will be saved The data will be saved as text in binary format that i
49. ich machine instruction is to be executed using the following algorithm CPU Sim initially inspects the leftmost bit of the ir If there is a machine instruction with an opcode of one bit that matches the leftmost bit of the ir then that machine instruction is chosen as the decoded instruction and the rest of the bits in the ir form its operands If there is no such machine instruction then the leftmost two bits of the ir are inspected and CPU Sim looks for a machine instruction with a 2 bit opcode that matches those two bits CPU Sim continues inspecting more and more bits of the ir until it finds a machine instruction whose opcode matches those bits Therefore if you have one instruction with a one bit opcode of 0 and another instruction with a two bit opcode of 01 then the second instruction will never be decoded and executed Once a machine instruction has been decoded execution of that instruction begins starting with the first microinstruction in its execute sequence Parameters ir the register that contains the current instruction to be executed The end microinstruction is used to designate the end of an execute sequence and therefore the end of a machine cycle It causes the current execute sequence to cease executing and the fetch sequence to begin executing starting with the first microinstruction in that sequence
50. iew the contents in binary hexadecimal decimal using 2 s complement notation unsigned decimal ASCII or Unicode and you can independently view the 32 addresses in binary hexadecimal or unsigned decimal You can also adjust the cell size and so view the contents of the RAM by individual bytes or by groups of bytes For example if you choose a cell size of 2 the RAM window will treat each pair of bytes as a 16 bit value and display it as such The Comments column of a RAM window can be used to store comments regarding the use of each memory cell When an assembly program is assembled and loaded into the RAM the rows in the Comments column next to each machine instruction contain the corresponding assembly language instruction from which that machine instruction was generated Note that every cell in the RAM window s table is editable except for the cells in the address column TEXT MENU DISPLAYING TEXT EDITING WINDOWS The Text menu provides a way for the user to open and or bring to the front windows for displaying and editing text usually consisting of assembly language programs see Figure 14 Tile Tile Cascade Cascade 1W1 0 a 1W1 0 a 2W1 1 a e2 Wl 1 a Figure 14 The Text menu The Tile menu item causes all text windows to expand to fill the desktop with the space divided as evenly as possible among the windows The Cascade menu item causes all the text windows to overlap in a cascading manner f
51. irst character of the range and the last character separated by a dash as in a z All characters in an assembly language program must be ASCII characters Program gt CommentsAndEOLs EquMacroIncludePart InstructionPart EOF Comment sAndEOLs gt Comment EOL Comment gt lt any sequence of characters not including EOF or EOL gt EquMacroIncludePart gt EquDeclaration MacroDeclaration Include CommentsAndEOLs Include gt inelude lt quoted sequence of characters not including EOL or EOF surrounded gt EquDeclaration gt Symbol EQU Operand acroDeclaration gt MACRO Symbol Symbol Symbol CommentsAndEOLs InstructionPart ENDM InstructionPart gt RegularInstructionCall DataPseudoinstructionCall AsciiPseudoinstructionCall Comment sAndEOLs RegulariInstructionCall gt Label CommentsAndEOLs Label Symbol Operand Operand DataPseudoinstructionCall gt Label CommentsAndEOLs Label data Operand Operand Operand Operand I Operand AsciiPseudoinstructionCall gt Label CommentsAndEOLs Label 35 ascii String Label gt Symbol Operand gt Symbol Literal Literal gt 0 9 Ox O 9a A F Ob O 1
52. itor Keyboard Shortcuts Shortcut Action Ctrl Backspace Backspace a word Ctrl Delete Delete a word Ctrl Toggle selection rect Shift Home Select Home Shift End Select End Ctrl Home Document Home Ctrl End Document End 33 34 Ctrl Shift Home Select Document Home Ctrl Shift End Select Document End Shift Left Select previous char Shift Right Select next char Ctrl Left Previous word Ctrl Right Next word Ctrl Shift Left Select previous word Ctrl Shift Right Select next word Shift Up Previous line Shift Down Next line Ctrl Enter Repeat SPECIFICATION OF ASSEMBLY LANGUAGE FOR CPU SIM MACHINES Syntax The following context free grammar CFG gives the syntax of legal assembly language programs The start symbol for the CFG is Program EOL is an end of line token and EOF indicates the end of the file Square brackets indicates an optional item Items in parentheses followed by indicate 0 or more copies of those items Items in parentheses followed by indicate 1 or more copies of those items To separate tokens that the assembler would otherwise treat as one token use one or more spaces or tab characters Terminal symbols are displayed in bold and are surrounded by quotes Terminal symbols are case sensitive A range of characters such as all small case letters of the alphabet are indicated by the f
53. lumns The rows can be resorted by column by clicking in any column header 22 Names of Components Microinstructions and Machine Instructions All hardware components microinstructions and machine instructions must be given names You can name them anything you wish with the following conditions e You must use at least one non space character in each name and you must have different names for each unit of the same type It is strongly recommended that all components microinstructions and machine instructions have unique names regardless of the type e Machine instruction names must be valid symbols in the assembly language syntax which means they must consist of one or more letters digits underscores _ plus signs and minus signs and they must start with a letter Only ASCII characters are allowed Note that if you give a register array of length 4 the name A then the 4 registers in the array will automatically be given the names A 0 A 1 A 2 A 3 Help buttons Almost every dialog box in which parts of the machine can be modified contain a button labeled Help Clicking in the Help button causes the appropriate window from the online help to appear giving you some extra hopefully helpful information about the current dialog box Modifying registers register arrays condition bits and RAMs To change the registers register arrays condition bits or RAMs of the current machine choose H
54. n Alternatively you can a backup up all the way and run the same program again with different input b create or load a new assembly language program in a text window 10 assemble it and then run it c create or load a new machine instead of the Wombat1 or d quit To quit CPU Sim just choose Quit from the File menu You may be asked whether you want to save the changes to the Wombat or an assembly language program Click No for each of these and then CPU Sim will quit running CREATING NEW MACHINES IN CPU SIM A Tour using the Wombatl To demonstrate how to use CPU Sim to create a new hypothetical machine or CPU we will outline the construction of the Wombat1 machine used in the first tour We will only demonstrate some of the principles of constructing a new machine since completing the whole machine here especially all the microinstruction and machine instruction specifications would be somewhat time consuming Start up CPU Sim or if you left it running after the previous tour choose New machine from the File menu The default new machine has no registers no memory and no machine instructions The machine we will now construct the Wombat1 is a single accumulator machine using six registers acc the accumulator pc the program counter ir the instruction register the mar memory address register mdr the memory data register and the status the status register The basic structure of
55. n the registers or RAMs e One way to find errors in your programs is to step by machine instruction until you reach a point where an error has already occurred Then back up to the instruction just before the error occurred Finally step forward one microinstruction at a time until you reach the precise microinstruction in which the problem occurred e If your program seems to be in an infinite loop you can to choose Stop from the Execute menu to halt the execution VIEW MENU DISPLAYING COMPONENTS OF THE CURRENT MACHINE The View menu provides a way for the user to open and or bring to the front the windows displaying the components registers or RAMs of the current machine see Figure 13 Registers Register array A RAM Main RAM Stack Figure 13 The View menu The Registers menu item will open a window displaying all the registers of the current machine including their widths and values You can display the registers values in either binary hexadecimal decimal using 2 s complement notation unsigned decimal ASCII or Unicode Below the Registers menu item is a list of all the register arrays in the current machine By selecting one of these menu items you can display the registers of the selected register array and their values Below the register arrays are listed all the RAMs of the current machine When you select one of these menu items a window opens displaying the contents of the RAM You can v
56. need to be created The 7 transfer microinstructions are displayed by arrows between the registers in Figure 4 above To see how to create the transfer microinstructions choose the menu item Microinstructions from the Modify menu and then choose TransferRtoR in the popup menu at the top of the dialog that appears see Figure 6 TransferRtoR type 13 microinstructions are for transferring a contiguous set of bits from one register to another register To create the first transfer microinstruction click the New button We will edit this first microinstruction so that it transfers the contents of the pc to the mar Click in the table cell with the in it in the column headed name and type in pe gt mar you can give the microinstructions any name you want but it helps to choose something descriptive Select the pc as the source register and the mar as the destination register with bit 0 as the start bit for both registers and give numBits the value 12 This means that we want to transfer all 12 bits between the registers Now this microinstruction is complete If you want to create the remaining 6 transfer microinstructions in the Wombat1 continue adding new microinstructions in the same manner see Figure 6 The microinstructions of the other types needed for the Wombat can be created similarly by first selecting the appropriate type of microinstruction in the popup menu at the to
57. no jump is performed That is the microinstruction following the test microinstruction is the next microinstruction to be executed If omission 1 and the comparison succeeds then the test microinstruction is repeatedly executed in an infinite loop Register the register whose value is to be tested Start an integer indicating the leftmost bit to be tested in the register The value must be an integer between 0 and one less than the width of the register Number of bits a non negative integer indicating the number of bits to be tested The sum of the start and the number of bits must be at most the width of the register N B When you test a register or part of a register against a value you can consider the part of the register to designate a two s complement integer value or an 47 unsigned integer value For example if you are testing to see whether two bits are 11 you can either test whether the value stored in the two bits is 1 or test whether the value is 3 Similarly to test whether one bit of a register is a 1 you can test to see whether that part of the register contains the value 1 or 1 Increment The Increment microinstruction adds an integer constant to the contents of a register The constant can be positive or negative The constant is specified in the microinstruction so if the user needs to increment by different values then different increment microinstructions need to be used If signed overflow
58. ns name opcode fieldLengths add s implementation Existing micros bai i aie ir 4 15 gt mar gt TransferRtoR store 2412 pra aay gt 3 TransferAtoR read 3 4 12 Pei a stn gt TransferRtoA write 4 4 12 EN gt J set 5412 gt 8 Test pce E lt lt insert lt lt gt Increment divide 8412 gt 3 une jump 9412 gt Shift ljmpz A412 gt gt delete gt 9 Branch jmpn B412 gt gt Logical gt Decode gt 1 MemoryAccess gt J io 3 gt SetCondBit New Delete Duplicate End Help OK Cancel Figure 8 The dialog for editing machine instructions At this point if you had created all the microinstructions machine instructions and fetch sequence the machine would be complete You can at any time go back and change any part of the machine To save the machine to a file choose Save machine or Save machine as from the File menu CPU Sim saves the details of the machine as text in an XML file To see all the details of the machine in a more user friendly format than XML choose Save machine in HTML from the File menu You will be asked to type in the name of the HTML file into which the data will be saved This file can be opened and viewed with any web browser An HTML file displaying the details of the Wombat1 machine is included with CPU Sim in a file named Wombat1 html 17 CREATING NEW MACHINES FROM EXISTING ONES If you want to creat
59. nu the data files are opened just before running and are closed when execution is halted for whatever reason If the user enters debug mode the files are opened when execution begins for the first time by clicking the Go Step by Instr or Step by Micro buttons When in debug mode the files are not closed until the user clicks the Reset All or Flush amp Reset IO buttons in the debug toolbar the user exits from debug mode or the user selects a new connection using the Options IO connections menu item for the IO microinstruction connected to the file NOTE Clicking the Backup one Instr or Backup one Micro buttons in the debug toolbar does not back up the reading or writing of text files That is the current position of reading from or writing to text files does not change when you click the Backup one Instr or Backup one Micro button 28 Debug mode Debug mode provides you with several tools for finding bugs in your code including the following e You can step through the code one machine instruction or one microinstruction at a time and as you do so the name of the current machine instruction being executed is displayed on the right end of the debug toolbar along with the microinstructions composing it which are displayed in a scroll box The specific microinstruction being executed is highlighted in the scroll box If you step by microinstruction then any register and RAM cells that are updated
60. numbers in the data column of the RAM window are the machine language instructions generated by the assembler from the assembly language program The comments column of the RAM window displays the lines of assembly code from which the corresponding machine instruction was generated Now the program in the main memory is ready to be run Make sure all the registers have been cleared set to 0 If some of them are not 0 then either edit the values to make them 0 or choose Clear all registers amp arrays from the Execute menu Then choose Run from the Execute menu The program will begin execution with the instruction whose address namely 0 is stored in the program counter pc The machine runs by repeatedly executing machine cycles consisting of the fetch sequence which loads into the ir the instruction whose address is in the pc and then decodes the instruction followed by the execute sequence which executes the machine instruction that was just decoded At this point the console panel should ask you for input Type a positive integer into the console panel and press the return key or enter key Notice that the program will halt execution until an input value has been typed into the console Repeat this process several times and then type in a negative number The program will display an output message in the console panel giving the sum of all the positive numbers you typed in This will be followed by a second output message in a new
61. of how many places it is used in the fetch sequence and execute sequences of machine instructions Therefore any changes you make to a microinstruction will affect all current uses of that microinstruction As a result it is usually a good idea to create a new microinstruction using the New or Duplicate buttons and edit that microinstruction rather than editing an existing microinstruction Modifying the fetch sequence When you choose Fetch Sequence from the Modify menu you will be presented with a dialog box in which on the left side is listed the current sequence of microinstructions forming the fetch sequence See Figure 7 above for a picture of this dialog when the Wombat1 machine has been loaded If you wish to delete one of the microinstructions from the sequence displayed on the left in the dialog select it by clicking on it Then click the gt gt delete button If you wish to insert a new microinstruction in the sequence first click on the dial to the left of the type of microinstruction you want from the list in the right part of the dialog box All existing microinstructions of that type will then be displayed Select the one you want by clicking on it and then select in the list on the left of the dialog box the microinstruction before which you wish to insert the new microinstruction Now click on the Insert button If you want to add the new microinstruction to the end of the fetch sequence then don t select an
62. p of the dialog and then creating the microinstructions of that type We will not go through the details in this tutorial When you are finished close the Edit Microinstructions dialog box by clicking OK Edit Microinstructions x Type of Microinstruction TransferRtoR w name _ source sreStantBit dest destStanBit numBits pe gt mar pc O mar 0 12 mat pec Imar Ope 0 12 ir 4 15 gt mar Jir 4 mar 0 12 mdr ir mdr Olir 0 16 mdr gt act mdr Olacc 0 16 acc gt mdr jace O madr 0 16 ir 4 15 gt pe Jir 4ipc 0 12 Duplicate Cancel eS e Edit Microinstructions Type of Microinstruction TransferRtoR E lisas name source srcStartBit dest destStartBit numBits pc gt mar pc 0 mar it 12 mar gt pc mar Opc 0 12 ir 4 15 gt ir 4 mar 0 12 mdr gt ir mdr Oir 0 16 mdr gt acc mdr O acc 0 16 acc gt mdr acc 0 mdr 0 16 ir 4 15 gt pc ir pc it 12 New Delete Help OK Cancel Figure 6 The dialog for editing microinstructions The detailed descriptions of all the parameters displayed in the microinstruction dialog boxes can be found in the section of this manual entitled SPECIFICATION OF THE MICROINSTRUCTIONS Once all the microinstructions have been created the fetch sequence can be constructed To do so choose Fetch Sequence from the Modify menu The fetch sequence is specified by a
63. reated as an unsigned integer that is they are not treated as a two s complement representation of an integer index start bit this number designates which bit is the leftmost bit of the index register used to compute the index into the array index number of bits this specifies how many consecutive bits of the index register are to be used to compute the index into the array For example if you specify 16 bit registers r1 and r2 as the source and destination registers and if you specify a source start bit of 0 a destination start bit of 12 and the number of bits as 4 then the left most 4 bits of r1 will be transferred more precisely copied into the right most 4 bits of r2 For another example suppose you specify a TransferRtoA microinstruction with source register r and destination register array A containing 8 registers If you specify an index register i and index start bit 0 and index number of bits 3 then the leftmost 3 bits of 7 are treated as an integer from 0 to 7 that is used to specify which of the 8 registers in array A is to receive the data Arithmetic The arithmetic microinstructions use three registers and optionally two condition bits The contents of the first two registers the source registers are added subtracted multiplied or divided and the result is placed in the third register the destination register If signed overflow occurs that is if the result is too big or too small to fit in the destination
64. register using two s complement notation then the overflow condition bit will be set to 1 if a condition bit is used Otherwise the value of the overflow condition bit is left unchanged In the case of overflow the destination register will contain the rightmost n bits of the result in two s complement notation where n is the width of the destination register If there is a carry in out i e an unsigned integer overflow in an addition or subtraction microinstruction then the carry condition bit is set if a condition bit is used Parameters Type the type of operation namely addition subtraction multiplication or 45 division Note that division is integer division which truncates the result i e it produces the floor of the result That is dividing 7 by 2 yields 3 whereas dividing 7 by 2 yields 4 Sourcel Source2 Destination the three registers The operation consists of combining Sourcel and Source2 by the desired method Source1 Source2 Source1 Source2 Sourcel Source2 or Sourcel Source2 and then placing the result in Destination The three registers need not all be distinct For example the Destination could be the same register as Sourcel and or Source 2 Parts of registers may not be specified The three registers need not have the same width Overflow an overflow condition bit If not needed then none should be specified Carry acarry condition bit Used only in addition and subtraction microinstructions
65. rom the upper left corner of the desktop The bottom part of the menu lists all currently opened text windows with the topmost one indicated by the radio button You can select any of these items to bring that window up front THE HELP MENU GETTING ON LINE HELP The General CPU Sim Help menu item brings up the online help window which contains virtually the same material as this manual The About CPU Sim menu item brings up a dialog box describing the CPU Sim application including the current version and contact information KEYBOARD SHORTCUTS IN CPU SIM There are many keyboard shortcuts for general actions such as menu item actions and many shortcuts specifically for editing assembly language programs in text windows Here is a list of those shortcuts Note that the Apple key is used instead of the Ctrl key on Macintosh computers General Keyboard Shortcuts Shortcut Action Ctrl N New text editor Ctrl O Open text editor Ctrl W Close text editor Ctrl S Save text editor Ctrl P Print Ctrl Z Undo Ctrl Y Redo Ctrl X Cut Ctrl C Copy Ctrl V Paste Ctrl A Select all Ctrl F Find Ctrl H Replace Ctrl G Assemble load amp run Ctrl Stop Ctrl R Clear everything Alt F Open File menu Alt E Open Edit menu Alt M Open Modify menu Alt X Open Execute menu Alt V Open View menu Alt T Open Text menu Alt H Open Help menu Text Ed
66. s the data will be saved as strings of 0 s and 1 s and the comments will be saved on the same line as the corresponding data The New machine menu item causes CPU Sim to create a new machine with no hardware registers condition bits or RAMs no machine instructions and only one microinstruction End The Open machine menu item displays a dialog box asking you to choose a file You should select a file in which you had previously saved a machine through the use of the Save machine or Save machine as menu items The Reopen machine menu contains a list of the 10 most recently opened machine files This list is saved between sessions in the properties file specified on the command line when CPU Sim is started so that when you quit CPU Sim and restart it later the Reopen machine menu will be populated with the same list as it had when you quit CPU Sim If no properties file is specified when CPU Sim is started then the Reopen machine menu items will not be saved between sessions The Save machine menu item saves the current machine in a text file in XML format If the current machine has never been saved to a file before a dialog box appears asking you to type in the name of the file into which the machine will be saved The Save machine as menu item presents you with a dialog box asking you to type in the name of the file into which the machine will be saved 20 The Save machine in HTML menu it
67. s occurs a condition bit can optionally be set to 1 The choice of which condition bit if any to set for unsigned overflow can be specified in the Arithmetic and Increment microinstructions Register Parts Several microinstructions allow the user to specify parts of registers In CPU Sim the bits of a register are indexed from left the most significant bit to right the least significant bit starting with 0 and ending with one less than the width of the register For example if a register has 8 bits the leftmost bit has index 0 and the rightmost bit has index 7 The Microinstruction set Transfer There are three types of transfers of data allowed between registers and register arrays a A register to a register denoted by TransferRtoR b A register to a register array denoted by TransferRtoA c A register array to a register denoted by TransferAtoR In all cases the user is able to transfer data to or from any set of consecutive bits of a register The old data in the part of the register receiving the data is lost and is replaced with the new data The rest of the receiving register is unchanged That is if you transfer 4 bits into the first 4 bits of a 16 bit register then the last 12 bits of the register are unchanged by the transfer In cases b and c the user must specify an index register and a sequence of consecutive bits of that register whose value will form the index of the source or des
68. t of microinstructions that form the execute sequence of the instruction SPECIFICATION OF THE MICROINSTRUCTIONS CPU Sim provides a variety of types of microinstructions that can be used in fetch sequences or execute sequences All these types are described in this section of the manual General Information Sign Convention The sign convention used in CPU Sim is 2 s complement All arithmetic calculations and tests are done using this convention Therefore whenever a sequence of bits is displayed in decimal that decimal value corresponds to the two s complement value of the bits However when the sequence of bits is displayed in binary or hexadecimal the bits are treated as an unsigned integer value and so negative signs don t appear in the display Floating point operations are not supported by CPU Sim Overflow and Carry If an arithmetic operation results in a value that it too large positive or negative as a two s complement number to fit in the destination register then the machine is in a state of signed overflow When this occurs a condition bit can optionally be set to 1 The choice of which condition bit if any to set for signed overflow can be specified in the Arithmetic and Increment microinstructions 43 Similarly if an arithmetic addition or subtraction microinstruction yields a value that results in a carry in or out of the destination register then the machine is in a state of unsigned overflow When thi
69. t the global EQUs select EQU s from the Modify menu and then edit the cells of the table as you would any of the other tables described above The values of EQUs must be specified as decimal integers Note that you can also type EQU declarations directly in your assembly language code However these EQU s are local rather than global in that they apply only to the program in which they appear Therefore they will not be saved with the current machine and will not appear in the global EQU table Note that the names of all EQUs must be valid symbols in the assembly language syntax described in the section SPECIFICATION OF ASSEMBLY LANGUAGE FOR CPU SIM MACHINES below That is they must consist of one or more letters or digits or or or _ characters and they must start with a letter Only ASCII characters are 26 allowed EXECUTE MENU RUNNING A PROGRAM ON THE SIMULATED MACHINE Execution of an assembly language program in CPU Sim involves the following steps 1 load into CPU Sim the virtual machine on which your program is to be run 2 initialize the contents of the registers to the values you desire 3 create or open a text file that contains the assembly code to be run 4 assemble and load the program into the main memory RAM of your simulated machine 5 run the program You have the choice of running the program without interruptions other than those caused by errors in your code by choosing ei
70. t the machine s fetch sequence Implementation All micros pc gt mar gt TransferRtoR Main mar gt mdr gt TransferAtoR mdr gt ir gt TransferRtoA Inc2 pc gt Set decode ir b Test gt Increment gt Arithmetic gt Shift gt Branch gt Logical gt gt delete gt Decode ne gt MemoryAccess gt IO gt SetCondBit End Help OK Cancel Figure 7 The dialog for editing the fetch sequence To remove a microinstruction from the fetch sequence click on it in the left scroll box to highlight it and then click gt gt delete You can reorder the microinstructions in the list on the left by dragging them up or down to a new position When you are done click OK Now the 12 machine instructions of the Wombat1 need to be created Choose Machine Instructions from the Modify menu The dialog box that appears allows you to edit machine instructions including the name opcode field lengths and the list of microinstructions that form the execution sequence of each instruction The name you specify for a machine instruction is used in assembly language programs to execute that instruction The opcode is specified in hexadecimal notation The field lengths give the number of bits in each field of the instruction including the opcode field the first field and all operand fields For example if an instruction has field lengths of 4 and 12 then the opcode occupies the leftmost 4 bits and the remaining
71. the Wombat is shown in Figure 4 The status register is 3 bits wide the pc and mar are 12 bits wide and the other three registers are 16 bits wide The Wombat also has a main memory RAM consisting of 128 bytes The arrows in the figure indicate the movement of data by the microinstructions The pc contains the address of the main memory location that contains the next instruction to be executed In the fetch sequence the first part of every machine cycle the next instruction to be executed is copied into the ir where the instruction is decoded Then the instruction is executed which completes the machine cycle This is followed repeatedly by more machine cycles each consisting again of the execution of the fetch sequence followed by the execution of a machine instruction All computations are done in the acc The mar and mdr are the registers through which data is transferred to and from the main memory There are 12 machine instructions for the Wombat1 each associated with a 4 bit opcode These instructions are HALT stop READ get input from the user WRITE send output to the user LOAD transfer data from the main memory to the acc STORE transfer data from the acc to the main memory ADD add a value from the main memory to the value in the acc putting the result in the acc SUBTRACT MULTIPLY DIVIDE all similar to ADD JMPZ if the value in the acc is 0 jump to a new location to obtain the next instruction to be executed
72. ther Assemble load amp run or Run from the Execute menu see Figure 12 or you can step through the program one fetch sequence and execute sequence or one microinstruction at a time by using debug mode T Debug Mode Debug Mode Assemble Assemble ble amp load a Assemble amp load L To Rei Assemble load amp run 3G Assemble load amp run Ctrl G Clear assemble load amp run E Clear assemble load amp run Ctrl E Clear registers amp run M Run Clear registers amp rur Ctrl M Ue sis i Stop Period Run Clear all registers amp arrays Sto Ctrl Period a inh Clear all RAMs Clear all registers amp arrays Clear everything R Clear all RAMs Options j Clear everything Ctrl R Options gt Figure 12 The Execute menu The Debug mode menu item causes the debug toolbar to be displayed see Figure 3 above The use of the debug toolbar is described below The Assemble menu item checks your code for legality If the code is legal nothing happens If the code is illegal an error message will appear and the offending line of assembly code will be highlighted for you to edit Note that the code in any text window must be saved to a file before it can be assembled If you try to assemble it before saving it a dialog box will ask you to save it first Note also that CPU Sim does not create executable files or object files Every time you want to run a program that program must be reassembl
73. tination register in the array to or from which the data is to be transferred For example a TransferRtoA microinstruction might be created to transfer bits from a register r to a register array A using bits 3 5 of an index register to specify which register in the array is to receive the data If bits 3 5 of the index register contain the value 0 then the data in register r will be transferred to the first register in array A Parameters source a register or register array from which data is to be transferred source start bit this number designates the leftmost bit that is to be transferred It must be a number from 0 to one less than the width of the source register or register array destination a register or register array to which data is to be transferred 44 destination start bit this number designates where the leftmost transferred bit is to be placed in the destination register number of bits this specifies how many bits are to be transferred The source start bit added to the number of bits must yield a sum that is at most the width of the source register or register array Similarly for the destination register or register array index a register which contains the index of the source or destination register in the array to or from which data is to be transferred This parameter is only used in transfers of type TransferRtoA or TransferAtoR The value of the bits in the index register that are used to compute the index are t
74. ting and assembling programs The editor will highlight the offending section of the program when the assembler finds an error Assembled programs can be executed without stopping or they can be executed in debugging mode which allows the users to step through the execution of a program one machine instruction or microinstruction at a time After each step users can edit the contents of any register or memory and then continue stepping or even back up one microinstruction or machine instruction at a time all the way back to the initial state of the machine The ideas for this simulator came from the STARTLE simulator developed by J M Kerridge of the University of Sheffield STARTING CPU Sim CPU Sim is a Java application and so requires that you have a Java runtime environment JRE installed on your computer CPU Sim should run with JRE 1 5 or later After having successfully downloaded and unzipped CPU Sim from the CPU Sim web pagel you can start up the program using a command line or terminal window as described in the InstallationInstructions txt file that is included in the download e Open a command or terminal window and navigate to the CPUSim3 4 X folder e Type in one of the following commands all on one line o Windows users java cp CPUSim3 4 jar jhall jar CPUSimHelp3 4 jar cpusim Main o Mac or Linux users java cp CPUSim3 4 jar jhall jar CPUSimHelp3 4 jar cpusim Main e You can also add four optional
75. truction has the following parts label include text file name comment The word include and the text file name must be included but the comment and the label are optional The text file name must be a string of 1 or more characters surrounded by double quotes or chevron quotes The string cannot include the end of line or return or double quote characters For example suppose a file F contains the include directive include W1 0 h Then at the point in F at which the include directive appears the contents of file W1 0 h are temporarily inserted for the purpose of assembly The quoted part of the Include statement must be the pathname of the file to be inserted This pathname is relative to the file F For example the Include directive above assumes that the file W1 0 h is in the same directory as F If the file W1 0 h is an a subdirectory called defs for example then the include directive should say include defs W1 0 h If the file W1 0 h is in the parent directory of F then the include directive should say include W1 0 h If the file F contains the include directive include lt W1 0 h gt then CPU Sim will search for the file W1 0 h relative to the directory containing the current machine that is the cpu file containing the current machine instead of relative to the directory containing F Macro declarations and calls Macro declarations give a name to a block of code
76. tructions must be given names You can name them anything you wish under the following conditions You must use at least one non space character and you have different names for each unit of the same type Machine instruction names must be valid symbols in the assembly language syntax which means they must consist of one or more letters or digits or or or _ characters and they must start with a letter Only ASCII characters are allowed in Machine instruction names It is strongly recommended but not required that all units of all types have unique names Note that if you give a register array of length 4 the name A then the 4 registers will automatically be given the names A 0 A 1 ARI ABY SPECIFICATION OF THE SIMULATED HARDWARE UNITS There are four different types of hardware units that can be specified by the user 1 Registers are storage locations for one value or bit string They can hold one bit or many bits Numbers are stored using 2 s complement notation Register arrays are indexed lists of registers Condition bits are specific bits of registers 4 RAMs are indexed lists of bytes The index of a byte is referred to as the address of the data in that location Data is accessed in the RAM through MemoryAccess microinstructions Saale td The parameters associated with each of these hardware components are discussed in each section below Note that every components must also have a name as
77. u wish This will create no problems because CPU Sim uses the names of hardware items and microinstructions only for display purposes The remaining sections of this manual include descriptions of the main desktop window s menu items as well as a complete specification of the hardware microinstructions and machine instructions that can be created using CPU Sim FILE MENU OPENING AND SAVING YOUR SIMULATED MACHINE AND PROGRAMS The File menu see Figure 9 provides the usual items for loading new or existing machines opening or closing text windows containing assembly language programs printing and displaying information about the machine or programs and exiting CPU Sim 18 New text N Open text 0 Reopen text gt Close text W Save text 5 New text CHIN Save text as Open text Ctri O Reopen text Load RAM Close text acu Store RAM Save text ctrl S Save text as New machine Open machine Load RAM re RAM Reopen machine gt New machine Save machine Open machine Save machine as Reopen machine gt Save machine Save machine in HTML Save machine it Save machine in HTML Print Preview Print setup Print Preview Print 3gp as setup Print Ctrl P Exit CPU Sim Exit CPU Sim Figure 9 The File menu Note 1 When a CPU Sim machine is saved to a file all the parts are saved including the hardware components registers register
78. ute menu Parameters direction either input or output indicating whether to get data from the user or from a file input or send data to the user or to a file output type either integer ascii or Unicode indicating the type of data that is to be input or output buffer the register to or from which data is to be input or output 48 MemoryAccess The MemoryAccess microinstruction transfers copies data between a register and a RAM The number of bits transferred is equal to the width of the register Parameters direction either read or write indicating whether data is to go from RAM toa register read or from a register to RAM write memory the RAM to or from which data is to be transferred data the register to or from which data is to be transferred The width of this register must be divisible by 8 address the register that contains the address indicating where in the RAM the data is to be read or written If multiple bytes are to be read or written the address is the first smallest address of the bytes Set The Set microinstruction sets a sequence of consecutive bits of a register to a specified value Parameters register the register whose bits are to be set start the leftmost bit of the register that is to be set This must be an integer from 0 to one less than the width of the register numBits the number of consecutive bits in the register to be set value the
79. will be stored using the given number of bytes e an integer between 1 and 8 indicating the cell size in bytes followed by a bracketed list of values to be put in successive cells of the given size Therefore the number of values in the bracketed list must equal the total number of bytes specified in the first operand divided by the cell size specified in the second operand The list items are optionally separated by commas e a bracketed list of values whose length is equal to the number of bytes specified in the first operand These values are optionally separated by commas Note that this case is just a special case of the preceding case in which the cell size is omitted and is implicitly assumed to be 1 byte If any of the numbers are negative integers they will be treated by CPU Sim in 2 s complement form Note that the values can be either integer literals EQU names or variables corresponding to labels In the last case the value of the variable is the address of the label For examples consider the following pseudoinstructions data 5 0 fills 5 bytes with O s data 5 0 0 0 0 0 fills 5 bytes with O s data 5 1 0 0 0 0 0 fills 5 bytes with O s data 5 5 0 fills 5 bytes with O s data 5 1 fills 5 bytes with all 1 bits the 2 s complement form for 1 data 5 3 fills the first 4 bytes with O s and the 5th byte with the bits 00000011 data 5 1 2 3 4 5 fills 5 bytes with the five 1 byt values 1 2 3 4
80. xt file using the m and t flags In that case the CPU Sim GUI windows never appear Any input or output channels in the machine normally directed to a console window or to a dialog are redirected to the command line Input and output channels directed to a file are not redirected to the command line As always users can create short cuts to avoid typing such a long command For example Linux users can put the above command in a script file Windows users can create a bat file containing the command above that they can double click to start up CPU Sim Such a file named CPUSim bat is including in the CPU Sim installation package Macintosh users can create AppleScript documents that do the same thing RUNNING PROGRAMS IN CPU SIM A Tour using the Wombat1 This section demonstrates how to use CPU Sim to run a program ona hypothetical machine In this tour we will a load into CPU Sim the hypothetical machine Wombat1 that has already been defined and saved in a file b open an assembly language program for the Wombat1 c assemble the program d load it into the Wombat1 s memory and e run it To begin start up CPU Sim without any of the four optional flags mentioned in the preceding section The window that will appear is the main desktop window for CPU Sim Except for some dialog boxes all windows used by CPU Sim are windows internal to the desktop window Such windows include text windows for writing and editing
81. y microinstruction in the list on the left before clicking the lt lt insert lt lt button To reorder the microinstructions in the fetch sequence just drag them up or down in the list on the left side of the dialog box A typical fetch sequence loads into an instruction register the contents of the memory location whose address is in a program counter register Then it increments 24 the program counter and decodes the instruction in the instruction register An instruction register is specified as a parameter of each Decode microinstruction as described below It is typically the case that the last microinstruction in the fetch sequence is a Decode microinstruction because the execution of a Decode microinstruction causes CPU Sim to a cease executing the fetch sequence b decode the contents of the instruction register and c begin executing the execute sequence of the machine instruction that was just decoded It is often the case that when you are modifying the fetch sequence you need to modify or create a new microinstruction and so you need to bring up the dialog box for modifying microinstructions To do so you can double click on the list of current microinstructions on the right side of the dialog box which will bring up the dialog for editing microinstructions When you are finished editing the microinstructions you can close that dialog box and can resume editing the fetch sequence Important note Any changes that you
82. y which RAM is the code store i e the RAM in which assembled code is to be loaded The user can also specify which address of the code store is to be the starting address for loading The Options IO connections menu item is discussed in the next section IO Options The Options IO connections menu item brings up a dialog box in which the user can specify where each IO microinstruction will get or put data The dialog box has a table with one row for each IO microinstruction in the current machine The name column gives the name of the IO microinstruction The connection column gives the current source or destination for the data when the microinstruction is executed If the connection is User then the data is sent to or read from the user via a dialog box one data value at a time If the connection is Console then the data is sent to or read from the user via the console one data value at a time If the connection is a file name then the data is sent to or read from that file The file must be a text file To change the connection for an IO microinstruction click on the connection entry for that microinstruction to bring up a popup menu and select User Console or File If you select File a dialog box will appear in which you can choose a new text file to be the source or destination of the data If two IO microinstructions both read from the same file or both write to the same file they do so

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