18F8680 datasheet(109/496 Pages) MICROCHIP

109 / 496 page

 2004 Microchip Technology Inc.

DS30491C-page 107

PIC18F6585/8585/6680/8680

8.0

8 x 8 HARDWARE MULTIPLIER

8.1

Introduction

An 8 x 8 hardware multiplier is included in the ALU of

the PIC18F6585/8585/6680/8680 devices. By making

the multiply a hardware operation, it completes in a sin-

gle instruction cycle. This is an unsigned multiply that

gives a 16-bit result. The result is stored in the 16-bit

product register pair (PRODH:PRODL). The multiplier

does not affect any flags in the ALUSTA register.

Making the 8 x 8 multiplier execute in a single cycle

gives the following advantages:

• Higher computational throughput

• Reduces code size requirements for multiply

algorithms

The performance increase allows the device to be used

in applications previously reserved for Digital Signal

Processors.

Table 8-1 shows a performance comparison between

enhanced devices using the single-cycle hardware

multiply and performing the same function without the

hardware multiply.

8.2

Operation

Example 8-1 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 8-2 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 8-1:

8 x 8 UNSIGNED

MULTIPLY ROUTINE

EXAMPLE 8-2:

8 x 8 SIGNED MULTIPLY

ROUTINE

TABLE 8-1:

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

; PRODH = PRODH

;

- ARG1

MOVF

ARG2, W

;

BTFSC

ARG1, SB

; Test Sign Bit

SUBWF

PRODH

; 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

µs

27.6

µs

Hardware multiply

100 ns

400 ns

µs

8 x 8 signed

Without hardware multiply

9.1

µs

36.4

µs

Hardware multiply

600 ns

2.4

µs6 µs

16 x 16 unsigned

Without hardware multiply

242

24.2

µs

96.8

µs

242

µs

Hardware multiply

2.4

µs9.6 µs

µs

16 x 16 signed

Without hardware multiply

254

25.4

µs102.6 µs

254

µs

Hardware multiply

3.6

µs

14.4

µs

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