Putting It Together

Part of the Fundamentals section of Coddy's Assembly journey — lesson 39 of 45.

You have learned all four pieces of the write syscall:

Register	What it holds	Value for screen output
`rax`	Syscall number	1 (write)
`rdi`	File descriptor	1 (stdout)
`rsi`	Buffer address	Address of your string
`rdx`	Byte count	Length of your string

Now it is time to put them all together.

The complete write syscall pattern:

mov rax, 1          ; syscall number for write
mov rdi, 1          ; file descriptor (stdout)
mov rsi, msg        ; address of the string
mov rdx, msgLen     ; number of bytes to print
syscall             ; ask the OS to print

A complete working program:

section .data
    msg db 'Hello, World!', 0x0a
    msgLen equ $ - msg

section .text
    global _start

_start:
    mov rax, 1
    mov rdi, 1
    mov rsi, msg
    mov rdx, msgLen
    syscall

    mov rax, 60
    mov rdi, 0
    syscall

What this program does:

Step	What happens
1	Prints "Hello, World!" to the screen
2	Moves to a new line
3	Exits cleanly with code 0

The order matters:

You must set up the registers before executing syscall. The order of the mov instructions does not matter as long as all four are set before syscall.

Challenge

Write a complete program that prints the message "Assembly is fun" followed by a newline, then exits cleanly.

The string and its length are already defined. You only need to write the code inside _start:.

Try it yourself

section .data
    msg db 'Assembly is fun', 0x0a
    msgLen equ $ - msg

section .text
    global _start

_start:
    ; TODO: Set up and call the write syscall
    ; Use rax=1, rdi=1, rsi=msg, rdx=msgLen

    ; TODO: Exit cleanly with code 0