Print a string

So far, you have read each example's result in a register or in memory. This program puts text in the console panel. It sets up two write requests, one for each line, and uses syscall to send each request.

default rel
global _start

section .rodata
greeting:   db "Hello, world!", 10      ; 10 is the newline
GLEN        equ $ - greeting
answer:     db "The answer is 42", 10
ALEN        equ $ - answer

section .text
_start:
    mov rax, 1              ; syscall 1: write
    mov rdi, 1              ; to file descriptor 1, standard output
    lea rsi, [greeting]     ; the bytes to write
    mov rdx, GLEN           ; how many of them
    syscall

    mov rax, 1
    mov rdi, 1
    lea rsi, [answer]
    mov rdx, ALEN
    syscall

    mov rax, 60             ; syscall 60: exit
    xor rdi, rdi            ; with status 0
    syscall

Run the program. The console shows:

Hello, world!
The answer is 42

syscall takes no written operands. Before each write, rax holds call number 1, rdi holds standard output descriptor 1, rsi points to the first byte, and rdx holds the requested byte count. The second request uses the same setup with answer and ALEN.

write requests up to rdx bytes; its result in rax says how many were actually written and can be smaller. For these short messages in the playground, the requested bytes appear in the console. write uses the byte count rather than searching for an ending byte, and it adds no line break. The 10 after each message is its newline byte. GLEN equ $ - greeting counts the bytes from greeting to the current position, including that newline. Remove , 10 from the first message and its length loses one byte too: the console then shows Hello, world!The answer is 42 on one line.

The second message already contains the characters 4 and 2. This program sends those stored characters; it does not convert a numeric value held in a register into text.

The final block requests exit with status 0, ending the program after the two lines.

Your turn

Add a third line, I can print too!, after the first two. Put its bytes and newline in .rodata, give it a length with equ, and make one more write request before exit. The console should show all three lines in this order:

Hello, world!
The answer is 42
I can print too!

default rel
global _start

section .rodata
greeting:   db "Hello, world!", 10
GLEN        equ $ - greeting
answer:     db "The answer is 42", 10
ALEN        equ $ - answer
; Add the third message and its length here.

section .text
_start:
    mov rax, 1
    mov rdi, 1
    lea rsi, [greeting]
    mov rdx, GLEN
    syscall

    mov rax, 1
    mov rdi, 1
    lea rsi, [answer]
    mov rdx, ALEN
    syscall

    ; Write the third message here.

    mov rax, 60
    xor rdi, rdi
    syscall

default rel
global _start

section .rodata
greeting:   db "Hello, world!", 10
GLEN        equ $ - greeting
answer:     db "The answer is 42", 10
ALEN        equ $ - answer
third:      db "I can print too!", 10
TLEN        equ $ - third

section .text
_start:
    mov rax, 1
    mov rdi, 1
    lea rsi, [greeting]
    mov rdx, GLEN
    syscall

    mov rax, 1
    mov rdi, 1
    lea rsi, [answer]
    mov rdx, ALEN
    syscall

    mov rax, 1
    mov rdi, 1
    lea rsi, [third]
    mov rdx, TLEN
    syscall

    mov rax, 60
    xor rdi, rdi
    syscall