Hello World for syscall in assembly

syscall.S

#Hello World example of syscall

#What follows after this are not comments. It is actually instructions to the cpp - c pre processor to include those files and definition for converting this to .s file.

#include <asm/unistd.h>
#include <syscall.h>
#define STDOUT_FILENO 1

.file "syscall.S"
.section        .rodata
L1:
.string "Testing syscalls \n"
L2:

.text
.globl _start
_start:
        movl  $(SYS_write), %eax           # a constant defined to indicate write to
	movl  $(STDOUT_FILENO), %ebx       # the standard output = 0
	movl  $L1, %ecx                    # starting address of string to be written 
	movl  $(L2-L1), %edx               # size of item to be written
	int   $128

	movl  $(SYS_exit), %eax
	movl  $0, %ebx
	int   $128

Syscall_readme.md

#Very Important Instructions for this file

A simple hello world x86 program demonstrating syscalls.

###Build Instructions: You need to run the cpp to convert the .S file to .s file. Then instead of manually assembling and linking the file, you can run cc as shown.

###Main vs _start Here note the use of the 'dont include std lib' flag. This is because we do not have a main but a direct _start. If you want to use main instead, replace all instances of _start with main and remove this flag. It should work the same.

/lib/cpp -m32 syscall.S syscall.s
cc -nostdlib -m32 syscall.s
./a.out

The expected output is :

Testing syscalls