PIC18F4610T-I/ML【PDF 212/234 页】IC MCU FLASH 32KX16 44QFN

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2009 Microchip Technology Inc.

DS39636D-page 81

PIC18F2X1X/4X1X

7.0

8 x 8 HARDWARE MULTIPLIER

7.1

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 7-1.

7.2

Operation

Example 7-1 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 7-2 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 7-1:

8 x 8 UNSIGNED

MULTIPLY ROUTINE

EXAMPLE 7-2:

8 x 8 SIGNED MULTIPLY

ROUTINE

TABLE 7-1:

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

μs27.6 μs69 μs

Hardware multiply

100 ns

400 ns

μs

8 x 8 signed

Without hardware multiply

9.1

μs36.4 μs91 μs

Hardware multiply

600 ns

2.4

μs6 μs

16 x 16 unsigned

Without hardware multiply

242

24.2

μs96.8 μs242 μs

Hardware multiply

2.8

μs

11.2

μs28 μs

16 x 16 signed

Without hardware multiply

254

25.4

μs

102.6

μs254 μs

Hardware multiply

4.0

μs16.0 μs40 μs

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