Intel mcs-48 manual Application Examples Double Exchange, Double Left Logical Shift

APPLICATION EXAMPLES

DOUBLE EXCHANGE

DEX:	DEC	RXO	;EXCHANGE A AND LOW BYTE
	XCH	A,@RXO
	INC	RXO	;EXCHANGE AEX AND HIGH BYTE
	XCH	A,AEX
	XCH	A,@RXO
	XCH	A,AEX
	RET		;RETURN
DOUBLE LEFT LOGICAL SHIFT
LLSH:	RLC	A	;SHIFT A
	XCH	A,AEX	;SHIFT AEX
	RLC	A
	XCH	A,AEX
	RET		;RETURN
DOUBLE RIGHT LOGICAL SHIFT
RLSH:	XCH	A,AEX	;SHIFT AEX
	RRC	A
	XCH	A,AEX
	RRC	A	;SHIFT A
	RET		;RETURN
DOUBLE RIGHT ARITHMETIC SHIFT
RASH:	CLR	C	;SET CARRY
	CPL	C
	XCH	A,AEX	;IF AEX[7]<>1 THEN
	JB7	$+3	;CLEAR CARRY
	CLR	C
	RRC	A	;SHIFT C INTO AEX
	XCH	A,AEX
	RRC	A	;SHIFT A
	RET		;RETURN

SINGLE PRECISION BINARY MULTIPLY

This routine assumes a one-byte multiplier and a one-byte multiplicand. The product, therefore, is two-bytes long.

The algorithm follows these steps:

1. The registers are arranged as follows:

ACC-O

R1 - Multiplier

R2 - Multiplicand

R3 - Loop Counter (=8)

The Accumulator and register R1 are treated as a register pair when they are shifted right (see Step 2)

2.The Accumulator and R1 are shifted right one place, thus the LSB of the multiplier goes into the carry.

3.The multiplicand. is added to the accumulator if the carry bit is a 'one'No. action if the carry is a 'zero'.

4.Decrement the loop counter and loop (return to Step 2) until it reaches zero.

5.Shift the result right one last time just before exiting the routine

*The result will be found in the Accumulator (MS Byte) and R1 (LS Byte).

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