PIC18LF4331 datasheet(95/392 Pages) MICROCHIP

95 / 392 page

 2010 Microchip Technology Inc.

DS39616D-page 95

PIC18F2331/2431/4331/4431

9.0

8 x 8 HARDWARE MULTIPLIER

9.1

Introduction

All PIC18 devices include an 8 x 8 hardware multiplier

as part of the ALU. The multiplier performs an unsigned

operation and yields a 16-bit result that is stored in the

product register pair, PRODH:PRODL. The multiplier’s

operation does not affect any flags in the STATUS

Making multiplication a hardware operation allows it to be

completed in a single instruction cycle. This has the

advantages of higher computational throughput and

reduced code size for multiplication algorithms, and

allows the PIC18 devices to be used in many applications

previously reserved for digital signal processors.

A comparison of various hardware and software multi-

ply operations, along with the savings in memory and

execution time, is shown in Table 9-1.

9.2

Operation

Example 9-1 shows the sequence to do an 8 x 8

unsigned multiply. Only one instruction is required

when one argument of the multiply is already loaded in

the WREG register.

Example 9-2 shows the sequence to do an 8 x 8 signed

multiply. To account for the sign bits of the arguments,

each argument’s Most Significant bit (MSb) is tested

and the appropriate subtractions are done.

EXAMPLE 9-1:

8 x 8 UNSIGNED MULTIPLY

ROUTINE

EXAMPLE 9-2:

8 x 8 SIGNED MULTIPLY

ROUTINE

TABLE 9-1:

PERFORMANCE COMPARISON

MOVF

ARG1, W

;

MULWF

ARG2

; ARG1 * ARG2 ->

; PRODH:PRODL

MOVF

ARG1, W

MULWF

ARG2

; ARG1 * ARG2 ->

; PRODH:PRODL

BTFSC

ARG2, SB

; Test Sign Bit

SUBWF

PRODH, F

; PRODH = PRODH

;

- ARG1

MOVF

ARG2, W

BTFSC

ARG1, SB

; Test Sign Bit

SUBWF

PRODH, F

; PRODH = PRODH

;

- ARG2

Routine

Multiply Method

Program

Memory

(Words)

Cycles

(Max)

Time

@ 40 MHz

@ 10 MHz

@ 4 MHz

8 x 8 Unsigned

Without Hardware Multiply

6.9

s

27.6

s

Hardware Multiply

100 ns

400 ns

s

8 x 8 Signed

Without Hardware Multiply

9.1

s

36.4

s

Hardware Multiply

600 ns

2.4

s6 s

16 x 16 Unsigned

Without Hardware Multiply

242

24.2

s

96.8

s242 s

Hardware Multiply

2.4

s9.6 s

s

16 x 16 Signed

Without Hardware Multiply

254

25.4

s102.6 s254 s

Hardware Multiply

3.6

s

14.4

s

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