I'm learning assembly on linux, and [] notation is confusing me. (NASM)

Question

I'm working with the following code:

section .text
    global _start
_start:
    mov ebx, testing
    mov [ebx], byte 0x4A
    add ebx, byte 1
    mov [ebx], byte 0x4B
    add ebx, byte 1
    mov [ebx], byte 0x4C

    ;sets var1 as '*'
    mov eax, 0x2A
    mov [var1], eax

    ;sets var2 as 'N'
    mov eax, 0x4E
    mov [var2], eax

    ;sets var3 = var2
    mov eax, [var2]
    mov [var3], eax

    ;Prints var1
    mov eax, 4
    mov ebx, 1
    mov ecx, var1
    mov edx, 1
    int 0x80

    ;Jumps a line
    mov eax, 4
    mov ebx, 1
    mov ecx, line_break
    mov edx, 2
    int 0x80

    ;Prints var2
    mov eax, 4
    mov ebx, 1
    mov ecx, var2
    mov edx, 1
    int 0x80

    ;Jumps a line
    mov eax, 4
    mov ebx, 1
    mov ecx, line_break
    mov edx, 2
    int 0x80

    ;Prints var3
    mov eax, 4
    mov ebx, 1
    mov ecx, var3
    mov edx, 1
    int 0x80

    ;Jumps a line
    mov eax, 4
    mov ebx, 1
    mov ecx, line_break
    mov edx, 2
    int 0x80

    ;Prints variable
    mov eax, 4
    mov ebx, 1
    mov ecx, variable
    mov edx, 3
    int 0x80

    ;Jumps a line
    mov eax, 4
    mov ebx, 1
    mov ecx, line_break
    mov edx, 2
    int 0x80

    ;Prints testing
    mov eax, 4
    mov ebx, 1
    mov ecx, testing
    mov edx, 3
    int 0x80

    ;Jumps a line
    mov eax, 4
    mov ebx, 1
    mov ecx, line_break
    mov edx, 2
    int 0x80


    ;Finishes program
    mov eax, 1
    mov ebx, 1
    int 0x80

section .data
    variable times 3 db 0x2A
    line_break dw 0x0D0A

section .bss
    testing resb 3
    var1 resb 1
    var2 resb 1
    var3 resb 1

It prints out:

*
N
N
***
JKL

What i don't understand is: why in the first line "mov ebx, testing" there is no brackets arround "testing" and why those brackets are present in [var2] in the line "mov eax, [var2]". How exactly does this notation work? Shouldn't [var2] be the effective address of var2?

score 2 · Answer 1 · answered Jul 05 '15 at 20:39

2

The brackets around an identifier indicates that the value should be used as a pointer. So without the brackets:

mov ebx, testing

moves the address of testing into the eax register, where

mov eax, [var2]

moves the value pointed to by var2 into eax.

answered Jul 05 '15 at 20:39

Craig S. Anderson

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Michael · Answer 2 · 2015-07-05T20:44:30.007

0

why in the first line "mov ebx, testing" there is no brackets arround "testing"

The code in your question is written in NASM syntax. There are other x86 assemblers that use a slightly different syntax (e.g. MASM and TASM).

Anyway, mov ebx, testing means "place the address of testing in register ebx". That address is then used on the very next line, where the instruction mov [ebx], byte 0x4A writes the value 0x4A to the first byte pointed to by ebx (i.e. the first byte at testing).

In MASM/TASM syntax you would have written this as mov ebx, OFFSET testing.

why those brackets are present in [var2] in the line "mov eax, [var2]"

Because here you want to get the value at var2, not var2's address. And in NASM syntax you do that using brackets, as in mov eax,[var2]. In MASM/TASM syntax both mov eax,[var2] and mov eax,var2 would've read the value at var2.

Note that var1, var2 and var3 are one byte each. So trying to read and write 32-bit registers from/to those variables looks suspiciously like a bug. You might want to change the declarations from resb 1 to resd 1, or just read/write 8-bit registers from/to them.

edited Jul 05 '15 at 20:44

answered Jul 05 '15 at 20:39

Michael

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http://www.tutorialspoint.com/assembly_programming/assembly_addressing_modes.htm in this page, in the part of "Indirect Memory Addressing" shouldn't it be "MOV EBX, MY_TABLE"? Since MOV EBX, [MY_TABLE] would transfer the value of MY_TABLE to EBX? The operands of the MOV mnemonic are both addresses? – RFiischer Jul 05 '15 at 20:45
Yes, [the tutorial is wrong](http://stackoverflow.com/questions/20608930/addressing-modes-in-assembly-language-ia-32-nasm/20609569#20609569). I'd suggest finding another one. – Michael Jul 05 '15 at 20:52
I really wouldn't know. The only two books I've ever read on assembly language programming were a couple of old ones on DOS programming that I found at the local library many years ago. The rest I've managed to pick up by experimenting and writing lots of code. – Michael Jul 05 '15 at 21:01

I'm learning assembly on linux, and [] notation is confusing me. (NASM)

2 Answers2