;; rax contains the integer
;; rbx contains a pointer to the string buffer
itoa:
   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'
   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
   ret