1. IA32 Assembly Programming in Linux
작성자: 박창범

2. Sample Code (AT&T syntax)
/* ssize_t write(int fd, const void *buf, size_t count); */
/* write(fileno(stdout), "hello", 6); */
.include "defines.h"
.data
hello:
.string "hello world\n"
.text
.globl _start
_start:
movl $SYS_write,%eax // SYS_write = 4
movl $STDOUT,%ebx // fd = fileno(stdio)
movl $hello,%ecx // buf = str
movl $12,%edx // count = 0x6
int $0x80
movl $SYS_exit,%eax
xorl %ebx,%ebx
ret
Section Declaration
.data section
var_name: .var_type [value]
.text section
.globl function_name
function_name:
[codes]

3. System Call System call number goes into %eax
The args go in %ebx,%ecx,%edx,%esi,%edi in order
The return value of the syscall is stored in %eax
Calling: int $0x80
The syscall number can be found in /usr/include/sys/syscall.h
The macros are defined as SYS_<syscall name> i.e. SYS_exit, SYS_close, in provided sample code and “defines.h”

4. Syscalls with > 5 args
Same as Syscalls with < 6 args but except argument passing
the args are arranged in memory and the pointer to the first arg is stored in %ebx
/* mappedptr=mmap(NULL,filelen,PROT_READ,MAP_SHARED,fd,0); */
movl %edx,(%esp)
movl %eax,4(%esp)
movl $PROT_READ,8(%esp)
movl $MAP_SHARED,12(%esp)
movl $fd,%ebx
movl (%ebx),%eax
movl %eax,16(%esp)
movl %edx,20(%esp)
movl $SYS_mmap,%eax
movl %esp,%ebx
int $0x80

5. Mixing C-Assembly Make object file Use Makefile
%> gcc –c xxx.S
Use Makefile
We can use another assembler
GAS(AT&T syntax), NASM(Intel syntax)

6. Tips Use gcc –S xxx.c We can get xxx.S
Referring assembly code generated by compiler could be helpful (don’t just copy generated code)

7. References http://www.linuxassembly.org/