Use native div instructions for itoa

fasm/lib/itoa.inc

@@ -1,58 +1,49 @@
 ;; rax contains the integer
 ;; rbx contains a pointer to the string buffer
 itoa:
-   mov byte [rbx], '0'
-
    ;; Special case for the number '0'
    cmp rax, 0
    jne .not_zero
+   mov byte [rbx], '0'
    mov byte [rbx + 1], 0 
    ret
 
 .not_zero:
-   ;; Fill 6 digits with zeroes
+   mov rdx, 0
-   mov byte [rbx + 1], '0'
+   mov rcx, 100000
-   mov byte [rbx + 2], '0'
+   div rcx
-   mov byte [rbx + 3], '0'
+   add rax, '0'
-   mov byte [rbx + 4], '0'
+   mov [rbx], al
-   mov byte [rbx + 5], '0'
 
-   sub rax, 100000
+   mov rax, rdx
-   inc byte [rbx]
+   mov rdx, 0
-   cmp rax, 0
+   mov rcx, 10000
-   jge itoa
+   div rcx
-   add rax, 100000
+   add rax, '0'
-   dec byte [rbx]
+   mov [rbx + 1], al
-.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
 
+   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