amd64 uses a different method for system calls than int 0x80, although that might still work with 32-bit libraries installed, etc. Whereas on x86 one would do:
mov eax, SYSCALL_NUMBER
mov ebx, param1
mov ecx, param2
mov edx, param3
int 0x80
on amd64 one would instead do this:
mov rax, SYSCALL_NUMBER_64 ; different from the x86 equivalent, usually
mov rdi, param1
mov rsi, param2
mov rdx, param3
syscall
For what you want to do, consider the following example:
bits 64
global _start
section .text
_start:
push 0x0a424242
mov rdx, 04h
lea rsi, [rsp]
call write
call exit
exit:
mov rax, 60 ; exit()
xor rdi, rdi ; errno
syscall
write:
mov rax, 1 ; write()
mov rdi, 1 ; stdout
syscall
ret
