I found an example and was editing it for gas.
extern printf .global _start .data hello: db "Hello", 0xa, 0 .text _start: mov %rdi, hello mov %rax, 0 call printf mov %rax, 0 ret But it doesn't work. What's wrong? What does this mean:
hello: db "Hello", 0xa, 0 I understand what it scope of memory, but I don't understand this string
db "Hello", 0xa, 0 And here
_start: mov %rdi, hello mov %rax, 0 call printf mov %rax, 0 ret os: linux (debian). intel 64-bit
It's is the null-byte-terminattor. Well-know as C-string.Such byte at end-of-string say where the string ends. For example,you pass the pointer to your string to call a routine,the routine will understand that the area of such string on memory is from begging a[0](in C terminology) until a[x] == 0 is seen.
All that does is place bytes into the program. The bytes are the characters "Hello", followed by 0xa (which is the line termination), and finally a NULL byte. In C it would be something like "char *hello = "Hello\n";"
At your _start: label, you place the address of the hello label into register %rdi, you place 0 into %rax, and you call the printf function.
The following declares a string with Hello followed by a line feed and null terminator. The null terminator is required for C strings
db "Hello", 0xa, 0 To call printf, you need to pass the parameter on the stack so it would be something like
mov hello, (%esp) call printf As far as I know, the convention is mov source, destination. You seem to have coded it the other way round.
rdi for the first argument is correct on a 64-bit Linux/UNIX. Windows on x86-64 uses rcx, rdx, r8 and r9 for the first four integer/pointer arguments, and the stack for the rest. Linux/UNIX uses rdi, rsi, rdx, rcx, r8 and r9 for the first six arguments. The source/destination order does look reversed though if this is supposed to be AT&T syntax code.
push $hello to push the address as an immediate. You don't want to load bytes from the string, and mov from memory to memory isn't allowed anyway. In 64-bit code, lea hello(%rdi), %rdi.
dbis a NASM directive. In GAS you want.asciiz "Hello\n". Also, AT&T syntax ismov $0, %eax, with the destination last.