Accessing C Structs in Assembler
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1. More about sections http www nongnu org avr libc user manual mem_sections html at while 1 return 0 One final note the structure must be declared volatile because the compiler could otherwise optimize the member assignments away they are not used in any part of code that the compiler is aware of Putting it all Together After everything is more or less explained we can put the whole thing together First create a C file for each structure you need the offsets of The macros defined above can be put into a header file call it asmDefine h for example The newly created C files need to be compiled every time the structure definition changes using the command line that first creates object code and then extracts the offsets This can be done with an IDE that support pre build steps a batch file or built into a makefile your choice Second include the generated header file in any assembler file that needs access to the offsets Any structure that is used must be imported as an external symbol for the linker to find it Third take into account that the compiler is not aware of the assembler code s functionality Depending on optimisation settings it might delete assignments in your main code if they are of no effect to the compiler2. a valid assembler instruction The compiler needs to be stopped before it calls the assembler For gcc this is done with the S option If you haven t done this before create the files myStruct h and myStruct_offsets c each with the content shown above open a terminal and execute avr gcc S myStruct_offsets c o The trailing is important as it tells the o option to write the output to stdout You should see the object file output including two lines starting with gt Here is a sample myStruct_defineOffsets push r29 push r28 reall in r28 SP_L__ in r29 __SP_H__ prologue function frame size 2 APP 53 myStruct_offsets c 1 gt offsetof_TMyStruct_first 0 As you can see the function myStruct_defineOffsets is cre ated and at the end of the above piece of object code the offset of the first member of TMyStruct is shown we re almost there The gt lines now have to be extracted from the object code which can be done using gawk avr gcc S myStruct_offsets c o gawk 1 gt print define 2 3 gt myStruct_offsets h The compiler output is piped to gawk which replaces gt with define and writes the resulting line to its own output gawk s output is written to myStruct_offsets h That file now con tains define offsetof_TMyStruct_first 0 define offsetof_TMyStruct_second 1 Using the Structure Offsets Now it s time to include this heade
3. h include lt stdint h gt struct SMyStruct_impl uint8_t first uint8_t second typedef struct SMyStruct_impl TMyStruct The defined type has two members first and second both are an unsigned 8 bit integer first starts at offset 0 and second starts at offset 1 This file is included by our main file main c include myStruct h volatile TMyStruct tMyStruct void asm_out void prototype x int main void tMyStruct first OxAA tMyStruct second 0x55 here we call the asm function asm_out while 1 return 0 gawk is available at http www gnu org s gawk a win dows port is available at http gnuwin32 sourceforge net packages gawk htm Two things must be considered here the compiler has no control over the code behind asm_out It only knows the prototype of this function but not what variables it might touch This is why tMyStruct is declared volatile it prevents the compiler from optimizing away the two assignments to the struct in main asm_out will later use the values assigned to the struct mem bers This code is written as if there was no interface between C and Assembler in fact one could know write asm_out in C Extracting the Structure Offsets The structure offsets are known when TMyStruct is typedef ed so myStruct h will be needed for this process Only the com piler can turn the type definition into offsets so compiler call is necessary in which the com
4. piler is used to calculate the offsets and write them into a seperate file The following is the file myStruct_offsets c include lt stddef h gt include myStruct h define _ASMDEFINE sym val asm volatile These macros as well as the 2 n n o Tr a following procedures are CANE eam Bs ae Awa based on those presented at define ASMDEFINE s m http docs blackfin uclinux org _ASMDEFINE of fsetof_ s _ m offsetof s m doku phprid tgolehain gas structs myStruct_defineOffsets ASMDEFINE TMyStruct first ASMDEFINE TMyStruct second Two macros are defined _ASMDEFINE sym val and ASMDEFINE s m ASMDEFINE uses s a structure name and m a structure mem ber to assemble a symbol string Assuming that s is TMyStruct and mis first the string is of fsetof_TMyStruct_first Then _ASMDEFINE is used to insert a line of assembler into the compiler output using asm volatile This is a fantasy instruction starting with gt Afterwards val is added as an integer value at the end of the line val was supplied by the calling macro ASMDEFINE and is equal the the member s offset in the struct At the end of myStruct_offsets c the two offsets are ac tually created by calling ASMDEFINE in a function This is a dummy function and will not show up in the final binary The assembler which is gas in this case cannot assemble the lines starting with gt it would emit an error because gt is not
5. r file in an asm file called asm_out S include lt avr io h gt include myStruct_offsets h define work 18 our working register extern tMyStruct import tMyStruct from main c section text global asm_out export for the linker asm_out directly accessing the first struct member as in PORTD tMyStruct first lds work tMyStruct offsetof_TMyStruct_first out _SFR_IO_ADDR PORTD work directly accessing the second member lds work tMyStruct offsetof_TMyStruct_second out _SFR_IO_ADDR PORTD work indirect access Z gt tMyStruct as in TMyStruct Z amp tMyStruct ldi ZL lo8 tMyStruct ldi ZH hi8 tMyStruct indirectly accessing the first member as in PORTD Z gt first ldd work Ztoffsetof_TMyStruct_first out _SFR_IO_ADDR PORTD work indirectly accessing the second member ldd work Ztoffsetof_TMyStruct_second out _SFR_IO_ADDR PORTD work ret The assembler file includes the headers imports the symbol tMyStruct and places asm_out in the text section asm_out writes the values of each structure member to PORTD in two different ways you should be familiar with them It is exported as a global symbol so that the linker can find it Finally it is possible to call the asm function from C main c include myStruct h volatile TMyStruct tMyStruct void asm_out void prototype int main void tMyStruct first OxAA tMyStruct second 0x55 asm_out
6. www avrbeginners net C Assembler Tutorial Accessing C Structs in Assembler Author Christoph Redecker Version 1 0 2 e e e ae Accessing C Structs in Assembler e e e N N 7 Interrupts e e e e Port I O Ra o Conditional Branches and Loops a e N E Jumps Calls and the Stack Registers and Memories Introduction to AVRs This tutorial is licensed under a Creative Commons Attribution NonCommercial OQO NoDerivs 3 0 Unported License EE http creativecommons org licenses by nc nd 3 0 Permissions beyond the scope of this license may be available at http www avrbeginners net ACCESSING C STRUCTS IN ASSEMBLER PROGRAMS is not an AVR issue but a generic programming problem However knowing how to do it is especially useful when an ISR or any other function must be hand coded This tutorial outlines the steps necessary for making C structs and their members offsets visible to assembler code and how to use them The procedure of making C structs usable in assembler code is as it is presented here neither system nor compiler independent Nonetheless it is very easy to port between systems I m not sure about compilers Furthermore a text processing tool such as gawk is needed For this tutorial it is assumed that gcc is used Initial Situation The first thing we need when assembler code must access a C struct is the C struct itself including a type definition These are in myStruct
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