Structure of Assembly Program

My Guru for this is Vivek Ramachand

Structure of an Assembly Program

.data - is where all initialized data resides.
.bss - is where all uninitialized data resides.
.text - is where the program instructions resides.
.globl _start - Will call external callable routines.
_start: is comparable with main() function of C programming.

Linux System calls

It is a library which kernel exposes to get various tasks done.
List of all calls are available in
bala@lappi:~/ASM$ cat /usr/include/asm/unistd.h
# ifdef __i386__
#  include "unistd_32.h"
# else
#  include "unistd_64.h"
# endif
bala@lappi:~/ASM$ head -15 /usr/include/asm/unistd_32.h
#ifndef _ASM_X86_UNISTD_32_H
#define _ASM_X86_UNISTD_32_H
/*
 * This file contains the system call numbers.
 */
#define __NR_restart_syscall      0
#define __NR_exit          1
#define __NR_fork          2
#define __NR_read          3
#define __NR_write          4
#define __NR_open          5
#define __NR_close          6
#define __NR_waitpid          7
We can call these system calls by invoking INT 0x80

Passing arguments to syscalls
EAX - System Call Number
EBX - First Argument
ECX - Second Argument
EDX - Third Argument
ESI - Fourth Argument
EDI - Fifth Argument

Coding Simple exit program in Assembly
Calling exit(0) to exit a program
Function definition
void_exit(int status);

1.Sys call # for exit() is 1, so we need to load EAX with 1 with the command

movl $1, %eax

2."Status" is "0" if program exited normally, so we need to load EBX with 0

movl $0, %ebx

3.Raise the software interrupt 0x80
int 0x80

Simple exit Program starts here

bala@bala-laptop:~/ASM$ cat exit.s
.text
.globl _start
_start:
    movl $1, %eax
    movl $0, %ebx
    int $0x80
bala@bala-laptop:~/ASM$
Now to compile the Assemble program we use the "as" compiler, to create the exit object file.
bala@bala-laptop:~/ASM$ as -o exit.o exit.s
This alone cannot do any thing we need to have a linker

bala@bala-laptop:~/ASM$ ld -o exit exit.o

Now we can run the program
bala@bala-laptop:~/ASM$ ./exit
The exit status is zero :-)
bala@bala-laptop:~/ASM$ echo $?
0
bala@bala-laptop:~/ASM$

Now we will attempt to write a Hello Earth program :-)

The syscall used to print "Hello Earth" is called write()
Then we need to exit the program gracefully with exit()

The write() syscall needs three parameters
ssize_t write(int fd, const void *buf, size_t count);
The syscall number for Write is 4 load it in EAX
The File Descriptor of STDOUT is 1 load in EBX
Buf -> is a pointer to the mem location containing "Hello Earth" in ECX
count = string length in EDX

Simple write Program starts here

bala@bala-laptop:~/ASM$ cat hello.s
# This is my 1st print program
.data
HelloEarthString:
    .ascii "Hello Earth\n"
.text
.globl _start
_start:
    #Let us load all argument for write in registers
    movl $4, %eax
    movl $1, %ebx
    movl $HelloEarthString, %ecx
    movl $12, %edx
    int $0x80

    #Now let us exit this program gracefully
    movl $1, %eax
    movl $0, %ebx
    int $0x80
bala@bala-laptop:~/ASM$ as -o hello.o hello.s
Now in the step above I have create the object file
bala@bala-laptop:~/ASM$ ld -o hello hello.o
Now in the step above I have create the linked executable
bala@bala-laptop:~/ASM$ ./hello
Hello Earth
bala@bala-laptop:~/ASM$ cat hello.s