; GCD.asm
; Author: A.Pinck
; Date written: 3 November 1997
; Purpose:
;  Enter two numbers (unsigned, maximum of 4 decimal digits)
;  Calculate and display their Greatest Common Divisor

GCD     Segment
        Assume  CS:GCD,DS:GCD
        Org     100h

start:  mov     dx,offset askFor1st
        mov     ah,09
        int     21h     ;prompt for first number

        call    getNum  ;value returned in AX
        jc      inDataErr ;Carry flag set if input invalid
        cmp     ax,0
        je      inDataZero
        mov     large,ax   ;save value if OK

        mov     dx,offset askFor2nd
        mov     ah,09
        int     21h     ;prompt for second number

        call    getNum
        jc      inDataErr
        cmp     ax,0
        je      inDataZero
        mov     small,ax

while:  mov     ax,large ;Loop until large and small are equal
        cmp     ax,small
        jz      done

        jnc     reduce   ;if large > small don't switch
        mov     bx,small ;otherwise, switch large and small values
        mov     small,ax
        mov     large,bx

reduce: mov     ax,large  ;reduce large by small amount
        sub     ax,small
        mov     large,ax

        jmp     while       ;loop back

done:   call    showNum
        mov     al,00       ;no error indicator
        jmp     exit

;--- error message handling ---
inDataErr:
        mov     dx,offset dataErr
        mov     al,04       ;invalid data error code
        jmp     showErr
inDataZero:
        mov     dx,offset dataZero
        mov     al,02       ;zero data value code
showErr:
        mov     ah,09
;-------------------------------
exit:
        mov     ah,4Ch ;terminate with code
        int     21h

;-- Data area --------------------------------------
large   dw      ?
small   dw      ?
askFor1st db    0Dh,0Ah,"Enter first number: $"
askFor2nd db    0Dh,0Ah,"Enter second number: $"
dataErr   db    0Dh,0Ah,"Invalid/Non-numeric character in input.$"
dataZero  db    0Dh,0Ah,"Zero input data value is not legal.$"

;==========================================

; showNum displays the 2's complement number
;  in AX as a decimal number

showNum proc    near
        push    ax
        push    bx
        push    cx
        push    dx

        cmp     ax,0    ;test for negative
        jp      positive;skip ahead if positive
        mov     ah,02   ;display character code
        mov     dl,'-'  ;  minus sign
        int     21h
        mov     bx,ax
        sub     ax,bx   ;ax now zero
        sub     ax,bx   ;ax now positive

positive:        
        mov     bx,10   ;divide by 10 to convert to decimal
        mov     cl,0    ;count of number of digits produced
div_loop:
        mov     dx,0    ;setup to divide dx|ax by bx
        idiv    bx      ;unsigned division: 
                        ;quotient in ax, remainder in dx
        push    dx      ;save the digit (remainder)
        inc     cl      ;keep track of number of digits
        cmp     ax,0    ;until nothing left to divide
        jnz     div_loop;  loop back

        mov     dx,offset newline ;ensure display is on a newline
        mov     ah,09
        int     21h
show_digit:
        pop     dx
        add     dl,30h  ;convert digit to ASCII code
        mov     ah,02   ;display character
        int     21h
        dec     cl      ;loop until all digits shown
        jnz     show_digit

        pop     dx
        pop     cx
        pop     bx
        pop     ax
        ret

;-- local data --
newline   db    0Dh,0Ah,"$"
showNum endp

;==========================================

; getNum returns number input (converted to
;   binary) in ax
; input is assumed to be a positive, decimal
;   value with a maximum of 4 digits, terminated
;   with a carriage return
; Carry flag is set (on) if an input error is
;   detected (Carry flag is clear (off) if the
;   input is valid

getNum  proc    near
        push    bx
        push    cx
        push    dx
        
        mov     num,0
        mov     bh,'9'  ;setup for valid range testing
        mov     bl,'0'
        mov     cl,4    ;maximum number of digits acceptable

get_loop:
        mov     ah,01   ;get key (with echo)
        int     21h
        cmp     al,0Dh  ;exit loop on carriage return
        je      get_exit

        cmp     bh,al   ;check that it's < '9'
        jc      bad_digit ;error if its not
        sub     al,bl
        jc      bad_digit ;error if its < '0'

        mov     ah,0
        push    ax
        mov     ax,num
        mov     dx,10   ;decimal based system
        imul    dx      ;multiply ax by dx results in dx|ax
        mov     num,ax  ;value of prior digits times 10
        pop     ax      ;add this digit's value
        add     num,ax

        dec     cl
        jnc     get_loop
bad_digit:  ;-- if we get here something is wrong
            ;-- by careful coding we have ensured that
            ;-- the carry flag will be set
            ;-- this is the "error" return indicator
get_exit:
        mov     ax,num
        pop     dx
        pop     cx
        pop     bx
        ret

;--- local data ---
num     dw      ?

getNum  endp
;==========================================

GCD     EndS
        End     start