Hello World Assembly language program

Since our last post described how we could use Unix system calls in unistd.h header file and then putting the system call and its options into the registers.

Suppose we want to write "Hello World" to the screen using assembly language.

1. We will use the write() syscall to print the “Hello World”
2. System call for write is #define __NR_write 4
3. We need to store 4 into EAX register
4. We would need to store the  file descriptor (FD) which is STDOUT =1
5. Remember in Linux FD STDIN = 0, FD STDERR = 2

When we do a man 2 write in Linux we get the format of the write call which is

ssize_t write(int fd, const void *buf, size_t count);

int fd – is the stdout FD which is 1

Const void *buf – pointer to the memory location which holds the “hello world” string

size_t count – Size of the string which is 12

Now we are finally ready to write out our program. Given below is our hello world program.

# Hello World assembly program

.section .data

message: .ascii "Hello World\n"

.section .text

.global _start

_start:

#Load all arguments for write () system call

  movl $4, %eax

  movl $1, %ebx

  movl $message,%ecx

  movl $12, %edx

  int $0x80

# We now need to exit the program

  movl $1, %eax

  movl $0, %ebx

  int  $0x80

We will now compile and link this program in Ubuntu Linux by using the following commands.

% as -o helloworld.o helloworld.s  # Compile the object file

% ld -o helloworld helloworld.o  # Link the Hello World program