PIC18F84J11T-I/PT【PDF 15/332 页】IC PIC MCU FLASH 8KX16 80TQFP

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2010 Microchip Technology Inc.

DS39774D-page 111

PIC18F85J11 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 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 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

@ 40 MHz

@ 10 MHz

@ 4 MHz

8 x 8 unsigned

Without hardware multiply

6.9

s27.6 s69 s

Hardware multiply

100 ns

400 ns

8 x 8 signed

Without hardware multiply

9.1

s36.4 s91 s

Hardware multiply

600 ns

2.4

s6 s

16 x 16 unsigned

Without hardware multiply

242

24.2

s96.8 s242 s

Hardware multiply

2.8

11.2

s28 s

16 x 16 signed

Without hardware multiply

254

25.4

102.6

s254 s

Hardware multiply

4.0

s16.0 s40 s

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