PIC18F23K22-ESLASHML datasheet(111/496 Pages) MICROCHIP

111 / 496 page

 2010 Microchip Technology Inc.

Preliminary

DS41412C-page 111

PIC18(L)F2X/4XK22

8.0

8 x 8 HARDWARE MULTIPLIER

8.1

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 applica-

tions 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 .

8.2

Operation

Example 8-1 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 8-2 shows the sequence to do an 8 x 8 signed

multiplication. 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 8-1:

8 x 8 UNSIGNED

MULTIPLY ROUTINE

EXAMPLE 8-2:

8 x 8 SIGNED MULTIPLY

ROUTINE

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

TABLE 8-1:

PERFORMANCE COMPARISON FOR VARIOUS MULTIPLY OPERATIONS

Routine

Multiply Method

Program

Memory

(Words)

Cycles

(Max)

Time

@ 64 MHz @ 40 MHz @ 10 MHz @ 4 MHz

8 x 8 unsigned

Without hardware multiply

4.3

s6.9 s27.6 s69 s

Hardware multiply

62.5 ns

100 ns

400 ns

s

8 x 8 signed

Without hardware multiply

5.7

s9.1 s36.4 s91 s

Hardware multiply

375 ns

600 ns

2.4

s6 s

16 x 16 unsigned

Without hardware multiply

242

15.1

s24.2 s96.8 s242 s

Hardware multiply

1.8

s2.8 s11.2 s28 s

16 x 16 signed

Without hardware multiply

254

15.9

s25.4 s

102.6

s254 s

Hardware multiply

2.5

s4.0 s16.0 s40 s

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