Assembly x64 Intro - Arrays

1.  all1_64.asm

; call1_64.asm  a basic structure for a subroutine to be called from "C"
;
; Parameter:   long int *L
; Result: L[0]=L[0]+3  L[1]=L[1]+4

        global call1_64 ; linker must know name of subroutine

        extern printf ; the C function, to be called for demo

        SECTION .data ; Data section, initialized variables
fmt1:    db "rdi=%ld, L[0]=%ld", 10, 0 ; The printf format, "\n",'0'
fmt2:    db "rdi=%ld, L[1]=%ld", 10, 0 ; The printf format, "\n",'0'

SECTION .bss
a: resq 1 ; temp for printing

SECTION .text           ; Code section.

call1_64: ; name must appear as a nasm label
        push rbp ; save rbp
        mov rbp, rsp ; rbp is callers stack
        push rdx ; save registers
        push rdi
push rsi

mov rax,rdi ; first, only, in parameter
mov [a],rdi ; save for later use

mov rdi,fmt1 ; format for printf debug, demo
mov rsi,rax         ; first parameter for printf
mov rdx,[rax] ; second parameter for printf
mov rax,0 ; no xmm registers
        call    printf ; Call C function

mov rax,[a] ; first, only, in parameter, demo
mov rdi,fmt2 ; format for printf
mov rsi,rax         ; first parameter for printf
mov rdx,[rax+8] ; second parameter for printf
mov rax,0 ; no xmm registers
        call    printf ; Call C function

mov rax,[a] ; add 3 to L[0]
mov rdx,[rax] ; get L[0]
add rdx,3 ; add
mov [rax],rdx ; store sum for caller

mov rdx,[rax+8] ; get L[1]
add rdx,4 ; add
mov [rax+8],rdx ; store sum for caller

        pop rsi ; restore registers
pop rdi ; in reverse order
        pop rdx
        mov rsp,rbp ; restore callers stack frame
        pop rbp
        ret ; return

2.  call1_64.c

#include <stdio.h>

void call1_64(long int L[]);

int main()
{
  long int L[2];
  printf("test_call1_64.c using call1_64.asm\n");
  L[0]=1;
  L[1]=2;
  printf("address of L=L[0]=%p, L[1]=%p \n", &L[0], &L[1]);
  call1_64(L); // add 3 to L[0], add 4 to L[1]
  printf("L[0]=%ld, L[1]=%ld \n", L[0], L[1]);
  return 0;
}

3. run

address of L=L[0]=0x7fffec6057f0, L[1]=0x7fffec6057f8
rdi=0x7fffec6057f0, L[0]=1
rdi=0x7fffec6057f0, L[1]=2
L[0]=4, L[1]=6