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2010-2012 Microchip Technology Inc.

DS39977F-page 145

PIC18F66K80 FAMILY

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 PIC18 devices to be used in many applications

previously reserved for digital-signal processors. A

comparison of various hardware and software multiply

operations, along with the savings in memory and

execution time, is shown in Table 9-1.

9.2

Operation

Example 9-1 shows the instruction sequence for an 8 x 8

unsigned multiplication. Only one instruction is required

when one of the arguments is already loaded in the

WREG register.

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

multiplication. To account for the sign bits of the argu-

ments, 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 FOR VARIOUS MULTIPLY OPERATIONS

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

@ 64 MHz

@ 48 MHz

@ 10 MHz

@ 4 MHz

8 x 8 unsigned

Without hardware multiply

4.3

s5.7 s27.6 s69 s

Hardware multiply

62.5 ns

83.3 ns

400 ns

8 x 8 signed

Without hardware multiply

5.6

s7.5 s36.4 s91 s

Hardware multiply

375 ns

500 ns

2.4

s6 s

16 x 16

unsigned

Without hardware multiply

242

15.1

s20.1 s96.8 s

242

Hardware multiply

1.7

s2.3 s11.2 s28 s

16 x 16 signed

Without hardware multiply

254

15.8

s21.2 s

101.6

254

Hardware multiply

2.5

s3.3 s16.0 s40 s

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