Factorial in assembly (M6800)

factorial

; Course:     1007ICT - Griffith University
; Assignment: 2
; Author:     Tate Johnson
; Created:    2008-05-30
; Processor:  Motorola 6800

; QUESTION 6
; Calculate factorial of a 
; number ranging from 1 - 8, 
; depending on desired value 
; inputted by user on their
; keyboard

; Store a 16 bit result as a
; high byte and low byte.
; ByteHi $0120 and ByteLo $0121

; ### START OF PROGRAM ###

; To save resources, we'll check
; user input with FactorMin to
; calculate 1! = 1
FactorMin	.equ 1

; KeyInput will setup X register 
; to correctly capture user 
; input from keyboard
KeyInput	.equ $FFFF

			clra
			clrb

; Wait for user input from
; keyboard interrupt
			wai
			ldx #KeyInput
			ldaa 0,x

; Decode ASCII value in to
; desired factorial
			ldab #48
			sba

; If input is 0, save resources
; and skip to end of program
			tsta
			beq SkipToEnd

; If input is 1, save resources
; by storing 1! = 1 at ByteLo and
; skipping to end of program
			cmpa #FactorMin

; Input = 1 skip to IsOne
			beq IsOne
; Input > 1 continue with
; program
			jmp NotOne
IsOne		ldaa #1
			staa ByteLo
			jmp SkipToEnd

; Initialise initial ByteLo and
; subsequent factorial number			
NotOne		staa ByteLo
			deca
			staa FactorX
			clra
			clrb

; Load current factorial 
; and initialise counts for
; Low and High bytes
Loop		ldaa FactorX
			staa CountLo
			staa CountHi
			
; Load previous ByteHi value
; in to TempByteHi. Used for
; calculating subsequent ByteHi
			ldaa ByteHi
			staa TempByteHi

; Reset HiOrLo
			ldaa #0
			staa HiOrLo

; Test if previous result is
; an 8 or 16 bit number
			tst ByteHi

; Previous result = 8 bit number
			beq IfNoHiCalc

; Previous result = 16 bit number
			jmp LoopLo
IfNoHiCalc	inc HiOrLo

; Peform multiplication on low
; byte by adding ByteLo to itself
; CountLo times. IE, current
; factorial
LoopLo		addb ByteLo

; If there is no carry, do not
; calculate HiByte and branch
; to NoCarry
			bcc NoCarry
			inca
NoCarry		dec CountLo

; While LoopLo > 0, branch back 
; to LoopLo and repeat
			tst CountLo
			bne LoopLo

; Store values of result at
; final result locations in 
; memory
			stab ByteLo
			staa ByteHi
			clra
			clrb

; If previous number was 8 bit,
; don't calculate ByteHi and
; go branch back to Loop			
			tst HiOrLo
			bne SkipLoopHi

; Peform multiplication on high
; byte by adding ByteHi to itself
; CountHi times. IE, current
; factorial
LoopHi		addb TempByteHi
			dec CountHi

; While LoopHi > 0, branch back 
; to LoopHi and repeat
			tst CountHi
			bne LoopHi

; Load current ByteHi value
; together with previous
; ByteHi value to calculate
; new ByteHi value
			ldaa ByteHi
			aba
			staa ByteHi
			clra
			clrb

; While FactorX > 0, branch back
; Loop and repeat
SkipLoopHi	dec FactorX
			tst FactorX
			bne Loop

; Shortcut for terminating the
; program
			.org $80
SkipToEnd	jmp Terminate

; Used to determine if previous
; result was an 8 or 16 bit
; number
			.org $90
HiOrLo		.byte 0

; Used in a loop to perform
; multiplication on current
; factorial for low byte
			.org $A0
CountLo		.byte 0

; Used in a loop to perform
; multiplication on current
; factorial for high byte
			.org $B0
CountHi		.byte 0

; Current factorial
			.org $C0
FactorX		.byte 0

; Temporary location to store
; high byte from previous result.
; Used to calculate new high byte
; value
			.org $D0
TempByteHi	.byte 0

; Final result where high byte
; is stored
			.org $0120
ByteHi		.byte 0

; Final result where low byte is
; stored
			.org $0121
ByteLo		.byte 0

Terminate	.end

; ### END OF PROGRAM ###