MacOS Assembly - Triangle

triangle.asm

    global start
    section .text

start:
    mov rdx, output         ; rdx holds address of next byte to write
    mov r8, 1               ; initial line length
    mov r9, 0               ; number of stars written on line so far

line:
    mov byte [rdx], '*'     ; write single star
    inc rdx                 ; advance pointer to next cell to write
    inc r9                  ; "count" number so far on line
    cmp r9, r8              ; did we reach the number of stars on this line?
    jne line                ; not yet, keep writing on this line

lineDone:
    mov byte [rdx], 10      ; write a new line char
    inc rdx                 ; and move pointer to where next char goes
    inc r8                  ; next line will be one char longer
    mov r9, 0               ; reset count of stars written on this line
    cmp r8, maxlines        ; wait, did we already finish the last line?
    jng line                ; if not, begin writing this line

done:
    mov rax, 0x2000004      ; system call for write
    mov rdi, 1              ; file handle 1 is stdout
    mov rsi, output         ; address of string to output
    mov rdx, dataSize       ; number of bytes
    syscall                 ; invoke operating system to do the write
    mov rax, 0x2000001      ; system call for exit
    xor rdi, rdi            ; exit code 0
    syscall

    section .bss
maxlines: equ 8
dataSize: equ 44
output: resb dataSize

To compile:

nasm -fmacho64 triangle.asm
gcc -o triangle.o triangle
./triangle