; This asm program sums a number using recurrsion
;
; Source: http://courses.ics.hawaii.edu/ReviewICS312/modules/subprograms/
;

%include "asm_io.inc"

segment .data
    msg1        db    "Enter n: ", 0
    msg2        db    "The sum is: ", 0

segment .text
    global    asm_main
asm_main:
    enter       0,0         ; set up
    pusha                   ; set up

    mov     eax, msg1       ; eax = address of msg1
    call    print_string    ; print msg1
    call    read_int        ; get an integer from the keyboard (in EAX)
    push    eax             ; put the integer on the stack (parameter #1)
    call    recursive_sum   ; call recursive_sum
    pop     ebx             ; remove the parameter from the stack
    mov     ebx, eax        ; save the value returned by recursive_sum
    mov     eax, msg2       ; eax = address of msg2
    call    print_string    ; print msg2
    mov     eax, ebx        ; eax = sum
    call    print_int       ; print the sum
    call    print_nl        ; print a new line

    popa                    ; clean up
    mov     eax, 0          ; clean up
    leave                   ; clean up
    ret                     ; clean up

;       FUNCTION: recursive_sum
;     Takes one parameter:  an integer
;     Destroys values of eax, ebx, ecx, and edx!! (eax = return value)
segment .bss
    value        resd  1    ; to store the return value temporarily

segment .text
recursive_sum:
    push    ebp             ; save ebp
    mov     ebp, esp        ; set EBP = ESP
    pusha                   ; save all registers (probably overkill)
    mov     ebx, [ebp+8]    ; ebx = integer (parameter #1)
    cmp     ebx, 0          ; ebx = 0 ?
    jnz     next            ; if (ebx != 0) go to next
    xor     ecx, ecx        ; ECX = 0
    jmp     end             ; Jump to end
next:
    mov     ecx, ebx        ; ECX = EBX
    dec     ecx             ; ECX = ECX - 1
    push    ecx             ; put ECX on the stack
    call    recursive_sum   ; recursive call to recursive_sum!
    pop     edx             ; pop the parameter from the stack
    add     ebx, eax        ; EBX = EBX + recursive_sum(EBX -1)
    mov     ecx, ebx        ; ECX = EBX
end:                        ; at this point, ECX contains the result
    mov     [value], ecx    ; save ECX, the return value, in memory
    popa                    ; restore all registers
    mov     eax, [value]    ; put the saved returned value into eax

    pop     ebp             ; restore EBP
    ret                     ; return