I am trying to make a simple program in assembler, but I do not understand why, I get a fault. I' ve a 64 bit machine running Ubuntu 12.04, and "as" as a assembly compiler. My goal merely is to print the string "Hello" on screen.
I wrote this:
#print.s
.section .data
.globl StringToPrint
StringToPrint: .asciz "Hello"
.globl _start
_start:
movq $4, %rax
movq $1, %rbx
movq $StringToPrint, %rcx
movq $5, %rdx
int $0x80
_done:
ret
But that's what I get:
$ as print.s -o print.o
$ ld print.o -o print
$ ./print
Hello[1] 10679 segmentation fault (core dumped) ./print
Why do you think this happens? Any idea?
Here is the fix :
#print.s
.section .data
.globl StringToPrint
StringToPrint: .asciz "Hello"
.globl _start
_start:
movl $5, %edx # string length
movl $StringToPrint, %ecx # pointer to string to write
movl $1, %ebx # file handle (stdout)
movl $4, %eax # system call number (sys_write)
int $0x80 # Passes control to interrupt vector
#sys_exit (return_code)
movl $1, %eax #System call number 1: exit()
movl $0, %ebx #Exits with exit status 0
int $0x80 #Passes control to interrupt vector
As Michael has already said you need to call sys_exit to avoid segmentation fault .
Edit :
Here is good to mention that int 0x80 invokes 32-bit system calls .
Using int 0x80 for syscall on x64 systems is used for backward compatibility to allow 32-bit applications to run .
On 64-bit systems will be correct to use syscall instruction .
Here is a working version :
.section .data
StringToPrint: .asciz "Hello"
.section .text
.globl _start
_start:
movq $1, %rax # sys_write
movq $1, %rdi # stdout
movq $StringToPrint, %rsi # pointer to string to write
movq $5, %rdx # string length
syscall
movq $60, %rax # sys_exit
movq $0, %rdi # exit code
syscall
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