Write to file in ARM Assembly

问题

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.

回答1:

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"
  

来源：https://stackoverflow.com/questions/28817574/write-to-file-in-arm-assembly