TITLE: GreatestCommonDivisor<GCD> (GCD.asm)
 
;------------------------------------------------------------
; Name: 
;
; Filename: GCD.asm
;
; Project #: 5
;
; Completed: March 9, 2003
;
; Description: My fifth assembly program. A test program that
;              will call the PROC findGCD, passing in the integer
;              values input by the user and then it will diplay
;              the results to the screen.
;              From Assembly Language For Intel-Based Computers,
;              4th Ed. by Kip R. Irvine. Problem #6 on pg. 257.
;
; Reference:   Assembly Language For Intel-Based Computers,
;              4th Ed. by Kip R. Irvine
;------------------------------------------------------------
INCLUDE Irvine32.inc
 
.data
int1 sdword ?
int2 sdword ?
caseTable byte '1'                ;lookup value
          dword enterInt          ;address of lookup value
entrySize = ($ - caseTable )
          byte '2'
          dword endTest
          numberOfEntries = 2
msgIntro  byte "This is Your Name's fifth assembly program. A test program that",0dh,0ah
          byte "will call the PROC findGCD, passing in the integer values input by",0dh,0ah
          byte "the user and then it will display the results to the screen.",0dh,0ah,0
msgSelMn  byte "---------------------------------",0dh,0ah
          byte "Select 1 to Run Prog or 2 to Quit ",0dh,0ah
          byte "---------------------------------",0dh,0ah
          byte "Enter Selection: ",0
msgInt1   byte "Enter first integer: ",0
msgInt2   byte "Enter second integer: ",0
msgAns    byte "Greatest common divisor is: ",0
msgEnd    byte "GoodBye",0dh,0ah,0
 
.code
main PROC
;///////Intro Message/////////////////////////////////
	mov edx,OFFSET msgIntro  ;intro message into edx
        call WriteString         ;display msgIntro
        call Crlf                ;endl
        call WaitMsg             ;pause message
        call Clrscr              ;clear screen
        call Menu                ;menu procedure
 
ExitProg::exit                   ;global label to exit
main ENDP
 
;------------------------------------------------
Menu PROC
;
; Receives: Nothing
; Returns: Nothing
;------------------------------------------------
	mov  edx,OFFSET msgSelMn ;ask user for input
	call WriteString         ;display msgSelMn
	call ReadChar		 ;read one character
	call Clrscr
        mov  ebx,OFFSET caseTable;point EBX to the table
	mov  ecx,numberOfEntries ;loop counter
 
        L1:
	cmp  al,[ebx]		 ;match found?
	jne  L2		         ;no: continue
	call NEAR PTR [ebx + 1]	 ;yes: call the procedure
	call WriteString	 ;display message
	call Crlf                ;endl
	call Clrscr              ;clear screen
	jmp  L3		         ;exit the search
 
        L2:
	add  ebx,5		 ;point to the next entry
	loop L1		         ;repeat until ECX = 0
 
        L3:
	jmp Menu	         ;run Menu again
 
Menu ENDP
 
;------------------------------------------------
enterInt PROC
;
; Finds the GCD
; Receives: EAX = 1, EBX = 1
; Returns: EAX = GCD
;------------------------------------------------
        push edx
 
        mov  edx,OFFSET msgInt1  ;ask user for input
	call WriteString         ;display msgInt1
	call ReadInt		 ;read integer
        mov  int1,eax
	mov  edx,OFFSET msgInt2  ;ask user for input
        call WriteString         ;display msgInt2
	call ReadInt		 ;read integer
        mov  int2,eax
 
        mov eax,int1
        mov ebx,int2
        call findGCD
	mov  edx,OFFSET msgAns
	call WriteString
	call WriteDec
	call Crlf
 
        call WaitMsg
        pop edx
 
enterInt ENDP
 
;------------------------------------------------
findGCD PROC
;
; Finds the GCD
; Receives: EAX = 1, EBX = 1
; Returns: EAX = GCD
;------------------------------------------------
	push ebx
	push edx
 
	or eax,eax                ;explicitly set sign flag
	.IF SIGN?                 ;in the event of a neg int
	  neg eax
	.ENDIF
	or ebx,ebx
	.IF SIGN?
	  neg ebx
	.ENDIF
 
        L1:
        mov  edx,0
	div ebx		          ;divide int1 by int2
	cmp  edx,0		  ;does remainder = 0 ?
	je   L2		          ;yes: quit
	mov  eax,ebx		  ;no: prepare for
	mov  ebx,edx		  ;next iteration
	jmp  L1
 
        L2:
        mov eax,ebx		  ;EAX = GCD
 
	pop edx
	pop ebx
	ret
 
findGCD ENDP
 
;------------------------------------------------
endTest PROC
;
; Receives: Nothing
; Returns: EDX = offset of message
;          Sets CF = 1 to signal end of program
;------------------------------------------------
        mov  edx,OFFSET msgEnd   ;msgEnd into edx
	call WriteString	 ;display message
	call Crlf                ;endl
	stc	                 ;CF = 1
	jc ExitProg    	         ;if CF=1 then jump to Global ExitProg
 
endTest ENDP
 
END main