aoc_omni/fasm/lib/itoa.inc

;; rax contains the integer
;; rbx contains a pointer to the string buffer
itoa:
   ;; Special case for the number '0'
   cmp rax, 0
   jne .not_zero
   mov byte [rbx], '0'
   mov byte [rbx + 1], 0 
   ret

.not_zero:
   mov rdx, 0
   mov rcx, 100000
   div rcx
   add rax, '0'
   mov [rbx], al

   mov rax, rdx
   mov rdx, 0
   mov rcx, 10000
   div rcx
   add rax, '0'
   mov [rbx + 1], al

   mov rax, rdx
   mov rdx, 0
   mov rcx, 1000
   div rcx
   add rax, '0'
   mov [rbx + 2], al

   mov rax, rdx
   mov rdx, 0
   mov rcx, 100
   div rcx
   add rax, '0'
   mov [rbx + 3], al

   mov rax, rdx
   mov rdx, 0
   mov rcx, 10
   div rcx
   add rax, '0'
   mov byte [rbx + 4], al
   add dl, '0'
   mov byte [rbx + 5], dl

   ;; Time to shift everything left
   ;; End the current string with a null byte
   mov byte [rbx + 6], 0
.shift_left_loop:   
   cmp byte [rbx], '0'
   jne .done

   mov rcx, rbx
   inc rcx
   mov rdx, [rcx]
   and rdx, 255
   mov [rbx],     dx

   inc rcx
   mov rdx, [rcx]
   and rdx, 255
   mov [rbx + 1], dx

   inc rcx
   mov rdx, [rcx]
   and rdx, 255
   mov [rbx + 2], dx

   inc rcx
   mov rdx, [rcx]
   and rdx, 255
   mov [rbx + 3], dx

   inc rcx
   mov rdx, [rcx]
   and rdx, 255
   mov [rbx + 4], dx

   inc rcx
   mov rdx, [rcx]
   and rdx, 255
   mov [rbx + 5], dx

   jmp .shift_left_loop

.done:
   ret