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Printing Integers in x86_64 Assembly

I finally got the assembly for printing numbers working. It was somewhat tricky as each individual digit needs to be extracted, converted to ASCII, and then printed. Here's how it works:

Suppose we want to print the number 123.
We need to divide that number by 10 and put the remainder in the stack. We do this until our result equals 0:
```
123 / 10 = 12 with a remainder or 3
12 / 10 = 1 with a remainder of 2
1 / 10 = 0 with a remainder of 1
```
We need to add 48 to the digit because 48 is the ASCII value for 0. So we're offsetting the ASCII character for 0 by the value of the digit.
Now we can iterate through each value and print it.

Here's the final code with some comments to help make whats happening clearer:

section .text
    global _start

_start:
    ;; Move the number we want to print into rax
    mov rax, 123
    call _printInt

    ;; Exit
    mov rax, 60
    mov rdi, 0
    syscall

_printInt:
    ;; r8 will keep a count of how many bytes to print
    mov r8, 0
    call _pushDigitsOnStack
    ret

_pushDigitsOnStack:
    ;; Divide rax by 10 and add 48. 
    ;; This extracts the digit and converts it to ascii.
    ;; 48 is the ascii value for 0.
    xor rdx, rdx
    mov rbx, 10
    div rbx
    add dl, 48

    ;; Allocate 1 byte on the stack for the digit
    ;; and place it in the stack.
    ;; Then increment the count
    sub rsp, 1
    mov byte [rsp], dl
    inc r8

    ;; If rax is equal to 0 then we're done
    cmp rax, 0
    jne _pushDigitsOnStack

_printDigitsOnStack:
    ;; Add the newline character to the stack
    ;; and increment the number of bytes we need to print
    ;; to take into account this newline character
    mov byte [rsp+r8], 10
    inc r8

    ;; Print all the bytes in the stack
    mov rax, 1
    mov rdi, 1
    mov rsi, rsp
    mov rdx, r8
    syscall

    ;; Move the stack pointer back to the location
    ;; of the return instruction
    dec r8
    add rsp, r8
    ret