在 ARM 程序集中写入文件 [英] Write to file in ARM Assembly

问题描述

我正在尝试将寄存器的整数内容写入 ARM 汇编中的文本文件.我一直在阅读，但收效甚微 - 我从阅读技术手册中推断出我需要一些 SWI 指导，但我一生都找不到具体的例子来说明如何做到这一点.

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.

操作系统是运行在 Pi B 模型上的 Raspbian.

Operating system is Raspbian running on Pi B model.

在此先感谢您的帮助，如果您需要任何说明，请告诉我！

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

澄清一下，我正在使用 GCC 成功组装和执行我的代码.我想将数据寄存器的值存储到文本文件中.

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.

示例:

.global main
.func main

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

在上述情况下，r0 将保存值 30.我想将该值写入文本文件.

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

推荐答案

我确实试了一下，想了一些你需要事先知道的事情.当然，使用汇编编程的方法有很多，但我的目标是不提供编译器或库的帮助.

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.

• 能够正确使用工具链
• 链接器需要一个 _start 符号
• 定义一个字符串，寻址它
• O_WRONLY、O_CREAT 的常量
• 如何编码权限
• 找出open、write、fsync、close、exit的调用签名代码>
• 如何进行系统调用
• 某种内存管理，我使用了堆栈
• 指针
• 数字的 ASCII 表示
• 加载/存储寄存器、寻址模式

• 如何调试！有趣的是，我开始认为这应该是直截了当的，只是为了找出在退出进程之前需要调用 fsync 以请求内核刷新的困难方式.可能 O_DIRECT |O_SYNC 是一个更简单的选择.

• 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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