CODEWITHSUNDEEP
linuxassemblyx86nasmx86-64
DayzZombie
DayzZombie

Reputation: 57

Assembly 'Segmentation Fault' When Trying to Print Decimal Number

I am trying to create a macro that will print a decimal number for me. My idea was to divide the number by ten, convert the remainder to ASCII and push it into the stack until the the ratio is 0. I then print the digits one by one but for some reason I get a 'Segmentation fault'.

I am using Linux, nasm, and a x86_64 processor.

%macro printInt 1
    mov rax, %1
    mov rbx, 0
%%loop1:                  ;Convert all digits to ascii
    inc rbx               ;Count of the digits in number
    mov rdx, 0
    div rax
    add rdx, 48           ;Convert remainder to ascii
    push rdx              ;Push remainder into stack
    cmp rax, 0            ;Compare ratio to 0
    jne %%loop1
%%loop2:                  ;Print the number digit by digit
    dec rbx

    mov rax, 1            ;Sys_Write
    mov rdi, 1            ;STDOUT
    pop rsi               ;Pop digit
    mov rdx, 1
    syscall

    cmp rbx, 0
    jne %%loop2
%endmacro

section .data
    number1 db 123

section .text
    global _start
_start:

    printInt number1

    mov rax, 60
    mov rdi, 0
    syscall

Any help is appreciated, thanks in advance!

Upvotes: 1

Views: 238

Answers (1)

rkhb
rkhb

Reputation: 14409

First mistake:

number1 db 123

number1 is defined as BYTE, but will be treated as QUADWORD (mov rax, %1). Change it to

number1 dq 123

Second mistake:

printInt number1

The macro gets the argument as string. So, mov rax, %1 will be solved to mov rax, number1. In NASM, RAX will get the address of number1, not the value. Change it to

printInt [number1]

Third mistake:

div rax

means: Divide RDX:RAX by RAX, store the result in RAX and the remainder in RDX. This will always result in RAX=1 and RDX=0. Use another register with the value 10.

Fourth mistake:

pop rsi

The SYSCALL/RAX=1 routine of the kernel expects a pointer in RSI, not a number. There are several ways to address the problem. I leave that to your imagination.

Upvotes: 2

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