CODEWITHSUNDEEP
carraysassemblynasm
user3697569
user3697569

Reputation:

How to properly write an array of bytes in Assembly?

I need to write an array of bytes but i don't get it, only [matrix + 0] and [matrix + 1] is written(what i mean, EAX should be equal to 0301070F), what i am doing wrong?

This is my code:

%include "asm_io.inc"
segment .data
    matrix times 4 db 0

segment .bss

segment .text

  global asm_main

asm_main:
    enter 0,0
    pusha
    mov eax, 0

    mov dword [matrix + 3], 15
    mov dword [matrix], 3
    mov dword [matrix + 1], 1
    mov dword [matrix + 2], 7

    mov ah, [matrix]
    mov al, [matrix + 1]
    rol eax, 16
    mov ah, [matrix + 2]
    mov al, [matrix + 3]

    dump_regs 1

  popa
  mov eax, 0
  leave
  ret

and this is the output of dump_regs 1:

Register Dump # 1
EAX = 03010000 EBX = B774FFF4 ECX = BFF7C924 EDX = BFF7C8B4
ESI = 00000000 EDI = 00000000 EBP = BFF7C868 ESP = BFF7C848
EIP = 080484D8 FLAGS = 0282       SF

The caller is a simple C program:

int main() {
  int ret_status;
  ret_status = asm_main();
  return ret_status;
}

Edited: I found something wrong here, if i execute this code, all is working:

    mov dword [matrix], 3
    mov dword [matrix + 1], 1
    mov dword [matrix + 2], 7
    mov dword [matrix + 3], 15

but if i execute this, doesn't work:

    mov dword [matrix + 3], 15
    mov dword [matrix], 3
    mov dword [matrix + 1], 1
    mov dword [matrix + 2], 7

Why?

Upvotes: 3

Views: 655

Answers (1)

Frerich Raabe
Frerich Raabe

Reputation: 94279

It's been some time, but shouldn't

  mov dword [matrix], 3

rather be something like

  mov byte [matrix], 3

to avoid that the constant 3 is treated as a 32bit value?

Upvotes: 3

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