aoc_omni/fasm/lib/itoa.inc

;; rax contains the integer
;; rbx contains a pointer to the string buffer
itoa:
   ;; Fill 6 digits with zeroes
   mov byte [rbx], '0'
   mov byte [rbx + 1], '0'
   mov byte [rbx + 2], '0'
   mov byte [rbx + 3], '0'
   mov byte [rbx + 4], '0'
   mov byte [rbx + 5], '0'

   ;; Special case for the number '0'
   cmp rax, 0
   jne .not_zero
   mov byte [rbx + 1], 0 
   ret

.not_zero:
   sub rax, 100000
   inc byte [rbx]
   cmp rax, 0
   jge itoa
   add rax, 100000
   dec byte [rbx]
.ten_thousand:
   sub rax, 10000
   inc byte [rbx + 1]
   cmp rax, 0
   jge .ten_thousand
   add rax, 10000
   dec byte [rbx + 1]
.one_thousand:
   sub rax, 1000
   inc byte [rbx + 2]
   cmp rax, 0
   jge .one_thousand
   add rax, 1000
   dec byte [rbx + 2]
.one_hundred:
   sub rax, 100
   inc byte [rbx + 3]
   cmp rax, 0
   jge .one_hundred
   add rax, 100
   dec byte [rbx + 3]
.ten:
   sub rax, 10
   inc byte [rbx + 4]
   cmp rax, 0
   jge .ten
   add rax, 10
   dec byte [rbx + 4]
   add [rbx + 5], rax


   ;; 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