Question
Is there really no way to print an ascii string in assembly to standard output without using up all four general purpose registers?
Solution
Right, it takes three registers for the parameters plus one for the system call number...
But, x86 has pusha
and popa
, which will push and pop all the registers in one instruction.
$ cat hwa.S
write = 0x04
exit = 0xfc
.text
_start:
pusha
movl $1, %ebx
lea str, %ecx
movl $len, %edx
movl $write, %eax
int $0x80
popa
xorl %ebx, %ebx
movl $exit, %eax
int $0x80
.data
str: .ascii "Hello, world!\n"
len = . -str
.globl _start
$ as -o hwa.o hwa.S
$ ld hwa.o
$ ./a.out
Hello, world!
OTHER TIPS
Well.. If you linked against libc you can call puts
, then you'd have some callee-save registers... :-)
But yeah. The syscall interface is pass-by-register. Sorry.
Don't be so shocked. It'd be the same way if you were doing a function call on some calling conventions. For many platforms that's pretty standard. (Including all amd64 compilers I know of...)
You could write a function that takes the needed arguments from the stack.