PIC17C752-08SLASHCL datasheet(65/328 Pages) MICROCHIP

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DS30289A-page 65

PIC17C7XX

9.0

HARDWARE MULTIPLIER

All PIC17C7XX devices have an 8 x 8 hardware multi-

plier included in the ALU of the device. By making the

multiply a hardware operation, it completes in a single

instruction cycle.

This is an unsigned multiply that

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

PRODuct register (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 algo-

rithms

The performance increase allows the device to be used

in applications previously reserved for Digital Signal

Processors.

Table 9-1 shows a performance comparison between

PIC17CXXX devices using the single cycle hardware

multiply, and performing the same function without the

hardware multiply.

Example 9-1 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 9-2 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 9-1:

8 x 8 UNSIGNED MULTIPLY

ROUTINE

EXAMPLE 9-2:

8 x 8 SIGNED MULTIPLY

ROUTINE

MOVFP

ARG1, WREG

;

MULWF

ARG2

; ARG1 * ARG2 ->

;

PRODH:PRODL

MOVFP

ARG1, WREG

MULWF

ARG2

; ARG1 * ARG2 ->

;

PRODH:PRODL

BTFSC

ARG2, SB

; Test Sign Bit

SUBWF

PRODH, F

; PRODH = PRODH

;

- ARG1

MOVFP

ARG2, WREG

BTFSC

ARG1, SB

; Test Sign Bit

SUBWF

PRODH, F

; PRODH = PRODH

;

- ARG2

TABLE 9-1:

PERFORMANCE COMPARISON

Routine

Multiply Method

Program

Memory

(Words)

Cycles

(Max)

Time

@ 33 MHz

@ 16 MHz

@ 8 MHz

8 x 8 unsigned

Without hardware multiply

8.364

µs

17.25

µs

34.50

µs

Hardware multiply

0.121

µs

0.25

µs

0.50

µs

8 x 8 signed

Without hardware multiply

—

Hardware multiply

0.727

µs

1.50

µs

3.0

µs

16 x 16 unsigned

Without hardware multiply

242

29.333

µs

60.50

µs

121.0

µs

Hardware multiply

2.91

µs

6.0

µs

12.0

µs

16 x 16 signed

Without hardware multiply

254

30.788

µs

63.50

µs

127.0

µs

Hardware multiply

4.36

µs

9.0

µs

18.0

µs

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