6809_MPRL.s

	.macro	CLC
		ANDCC	#$FE
	.endm
	.macro	SEC
		ORCC	#1
	.endm

;	Title:			Multiple Precision Rotate Left
;	Name:			MPRL
;
;	Purpose:		Rotate left a multi-byte operand N bits.
;
;	Entry:			TOP OF STACK 
;				High byte of return address
;				Low  byte of return address
;				Number of bits to rotate
;				Length of the operand in bytes 
;				High byte of operand base address
;				Low  byte of operand base address
;
;				The operand is stored with ARRAY[0] as its 
;				Least significant byte and ARRAY[LENGTH-1]
;				as its most significant byte
;
;				Operand rotated left
;				Carry Last := bit shifted from least significant position.
;
;	Registers Used:		A,B,CC,U,X
;
;	Time:			50 cycles overhead plus
;				((13 * Length) + 34) cycles per shift
;
;	Size:			Program 41 bytes
;

MPRL:	LDU	,S		; SAVE RETURN ADDRESS
;
;	EXIT IF LENGTH OF OPERAND OR NUMBER OF BITS TO ROTATE
;	IS ZERO. CARRY IS CLEARED IN EITHER CASE
;
	CLC			; CLEAR CARRY INITIALLY
	LDA	2,S		; GET NUMBER OF BITS TO ROTATE
	BEQ	EXITRL		; EXIT IF NUMBER OF BITS TO ROTATE IS ZERO
	LDA	3,S		; GET LENGTH OF OPERAND
	BEQ	EXITRL		; EXIT IF LENGTH OF OPERAND IS ZERO
;
;	SAVE POINTER TO MOST SIGNIFICANT BYTE OF OPERAND
;
	DECA			; OFFSET OF MOST SIGNIFICANT BYTE = 
				; LENGTH OF OPERAND - 1
	LDX	4,S		; GET BASE ADDRESS OF OPERAND
	LEAX	A,X		; POINT TO MOST SIGNIFICANT BYTE OF OPERAND
	STX	,S		; SAVE POINTER TO MOST SIGNIFICANT BYTE

				; ROTATE ENTIRE OPERAND LEFT ONE BIT 
				; USE PREVIOUS LEAST SIGNIFICANT BIT AS INITIAL 
				; CARRY INPUT TO PRODUCE ROTATION
RLLP:
	LDX	,S		; POINT TO MOST SIGNIFICANT BYTE
	LDA	,X		; GET MOST SIGNIFICANT BYTE
	LSRA			; SHIFT BIT 0 TO CARRY FOR USE IN ROTATION
	LDB	3,S		; GET LENGTH OF OPERAND IN BYTES
	LDX	,S		; GET BASE ADDRESS OF OPERAND
				;
				;	SHIFT EACH BYTE OF OPERAND LEFT ONE BIT
				;	START WITH LEAST SIGNIFICANT BYTE
RLLP1:
	ROR	,-X		; SHIFT NEXT BYTE LEFT
	DECB
	BNE	RLLP1		; CONTINUE THROUGH ALL BYTES

				; COUNT NUMBER OF ROTATES

	DEC	2,S		; DECREMENT NUMBER OF ROTATES
	BNE	RLLP		; CONTINUE UNTIL DONE

				; REMOVE PARAMETERS FROM STACK AND EXIT
EXITRL:
	LEAS	6,S		; REMOVE PARAMETERS FROM STACK
	JMP	,U		; EXIT TO RETURN ADDRESS

				; SAMPLE EXECUTION
SC4G:
	LDA	ROTATS		; GET NUMBER OF ROTATES
	LDB	#SZAY		; GET LENGTH OF OPERAND IN BYTES
	LDX	AYADR		; GET BASE ADDRESS OF OPERAND
	PSHS	A,B,X		; SAVE PARAMETERS IN STACK
	JSR	MPRL		; ROTATE RIGHT

				; RESULT OF ROTATING AY=EDCBA087654321H
				; 4 BITS IS AY=DCBA987654321EH, C=0
				;  IN MEMORY	AY   = 001H
				;		AY+1 = 032H
				;		AY+2 = 054H
				;		AY+3 = 076H
				;		AY+4 = 098H
				;		AY+5 = 0BAH
				;		AY+6 = 0DCH
;
; DATA SECTION
;
SZAY	EQU	7		; LENGTH OF OPERAND IN BYTES
ROTATS:	FCB	4		; NUMBER OF ROTATES
AYADR:	FDB	AY		; BASE ADDRESS OF OPERAND
AY:	FCB	$21,$43,$65,$87,$A9,$CB,$ED
	END