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I'm attempting to write the integer contents of a register to a text file in ARM Assembly. I have been reading with little success - I have deduced from reading the technical manual I will require some SWI instruction, but I cannot for the life of me find a concrete example of how to do this.

Operating system is Raspbian running on Pi B model.

Thank you in advance for any help, if you require any clarifications please let me know!

EDIT : To clarify, I am assembling and executing my code successfully using GCC. I want to store the values of my data registers to a text file.

Example : 

.global main .func main main: mov r1, #19 mov r2, #11 add r0, r1, r2 bx lr

In the above case, r0 would hold the value 30. I want to write that value to a text file.

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  • It is much easier to get help when you ask concrete questions. Commented Mar 2, 2015 at 19:23
  • @auselen - what more could I add? Assume a simple assembly program which stores the value 3 in r0. I now want to write the contents of r0 to a txt file. I am not sure what else you would like, but please tell me. Commented Mar 2, 2015 at 19:28
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    Writing a '3' to a file is not as writing any integer. Are you able to write ~itoa in C? Do you know how to write '3' to a text file? Why not to just STDOUT? Do you know the difference between a file and STDOUT? Are you able to use the tools to produce an executable from an assembly file? Since I don't know your foundations, I don't know where to start answering. Commented Mar 2, 2015 at 19:54
  • Anyway I added answer to other question: stackoverflow.com/a/28818270/1163019 Commented Mar 2, 2015 at 19:54
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    Assembly language itself has no concept of anything beyond instructions, registers and memory addresses. Since you'd essentially end up implementing fopen(), fwrite(), fclose() and possibly various other bits of the C library that's already right there, there's a strong argument for just linking the C library and calling its I/O functions directly (learning the details of calling conventions in the process). Of course, that's not to say you shouldn't learn the details of the OS interface, just that it's possibly something that can wait. Commented Mar 2, 2015 at 20:15

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I did give this a try, thinking about things you need to know beforehand. Of course there are many ways to utilize assembly programming but I aimed for no help from the compiler or libraries.

  • Be able to use toolchain correctly
  • Linker expects a _start symbol
  • Define a string, address it
  • Constants for O_WRONLY, O_CREAT
  • How to encode permission
  • Find out call signatures of open, write, fsync, close, exit
  • How to make a syscall
  • Some kind of memory management, I used stack
  • Pointers
  • ASCII representation of a digit
  • Load / Store of registers, addressing modes

  • How to debug! Funny thing I started thinking this should be straight forward only to find out hard way that you need to call fsync before exiting the process to ask kernel for flushing. Probably O_DIRECT | O_SYNC is an easier choice. 

    $ as open_e.s -o open_e.o $ ld open_e.o -o open_e $ cat open_e.s .global _start _start: @open syscall ldr r0, =filename mov r1, #0101 @ O_WRONLY | O_CREAT ldr r2, =0666 @ permissions mov r7, #5 @ 5 is system call number for open svc #0 cmp r0, #0 blt exit @r0 contains fd (file descriptor, an integer) mov r5, r0 @write syscall @use stack as buffer sub sp, #8 @ stack is full descending, this is how we leave some space mov r1, #'4' strb r1, [sp] mov r1, #'2' strb r1, [sp, #1] mov r1, #'\n' strb r1, [sp, #2] mov r1, sp mov r2, #3 mov r7, #4 @ 4 is write svc #0 @fsync syscall mov r0, r5 mov r7, #118 svc #0 @close syscall mov r7, #6 @ 6 is close svc #0 exit: mov r0, #0 mov r7, #1 svc #0 .data .align 2 filename: .asciz "out.txt"
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