%include "IO_DEFS.asm"

; Because my memory is not working
;   x /11g $rsp

section .data
        base  dd  2
        power dd  10

section .bss

        buffer resb 16

section .text
   global _start


Power:
    ; parameters:
    ;    rdi: the base
    ;    esi: the power
    ; set up for the function.
    push rbp      ; save the current base pointer
    mov rbp, rsp  ; set the base pointer to point to the current stack pointer
    push rsi      ; not needed but we want to look at it.

    ; if power == 0
    test esi, esi
    jnz power_recurse

    mov rax, 1  ; rax = 0
    jmp power_exit

power_recurse:
    
    dec esi    ; power--
    call Power ; rax = power(base, power)
    mul edi    ; rax = base * rax

power_exit:
    ; at this point, rax contains the return value
    ;   right now the stack contains    ra, rbp, esi

    pop rsi    ; remove esi from the stack
    pop rbp    ; restore the base pointer 
    ret       

_start:
    
    mov edi, [base]
    mov esi, [power]
    call Power

    ; print the result
    lea edi, [buffer + 15]
    mov byte [edi], 0x0A

convert:
    dec edi                ; edi--  Carter page 23
    mov edx, 0             ; edx = 0
    mov ecx, 10            ; ecx = 10
    div ecx                ; eax = edx:eax/ecx edx = edx:eax%ecx
                           ; carter page 44
    add dl, '0'            ; dl + '0'   add ascii 0 to the digit
    mov [edi], dl          ; store the new character in edi buffer[14] = digit
    test eax, eax          ; check for any bits of eax not zero
                           ; carter page 51
    jnz convert

    mov esi, edi         ; edi needs the start position of the string.
                          
    lea edx, [buffer+16] ; edx needs the length of the string
                         ; the last character in the buffer.
    sub edx, edi         ; starting poisition - end position

    mov eax, SYS_WRITE
    mov edi, STDOUT      ; set the destination to stdout

    syscall


    ; Exit
    mov eax, SYS_EXIT
    mov edx, SUCCESS
    syscall