CODEWITHSUNDEEP
assemblyx86nasm
Yoinksawse
Yoinksawse

Reputation: 49

Is this instruction redundant? Taken from x86 assembly tutorial code

I have recently been following a tutorial on x86 programming. Here is a program written in x86 NASM, that is supposed to display all 256 (displayable) ASCII characters.

code-here

However, I observe a possibly redundant line of code, the mov instruction in line 17, and the cmp instruction in line 18. As you can see, there are no other instructions in the code that use the zero flag that it activates, such as control loops.

enter image description here

I tested the code with and without that cmp instruction and mov instruction, here is with it:

└─$ ./example_displayascii
0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~��������������������������������������������������������������������������������������������������������������������������������

Here is without (either of those 2 instructions):

└─$ ./example_displayascii
0123456789:;<=>?@ABCDEFGHIJKLMNOPQRSTUVWXYZ[\]^_`abcdefghijklmnopqrstuvwxyz{|}~��������������������������������������������������������������������������������������������������������������������������������

Does exactly the same thing. I suspect that this is a debug line left there for no reason. Please take a look and see if it has any intent. Thanks!

Here is the text code if you need it:

section .data
    achar db '0'

section .text
    global main:
main:
    call display
    mov eax, 1
    int 0x80

display:
    mov ecx, 256

next:
    push ecx
    mov eax, 4
    mov ebx, 1
    mov ecx, achar
    mov edx, 1
    int 80h
    pop ecx
    ;mov dx, [achar]
    ;cmp byte[achar], 0dh
    inc byte[achar]
    loop next
    ret

Upvotes: 2

Views: 64

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