PIC17C44-08SLASHL datasheet(49/240 Pages) MICROCHIP

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DS30412C-page 49

PIC17C4X

8.0

HARDWARE MULTIPLIER

All PIC17C4X devices except the PIC17C42, have an

8 x 8 hardware multiplier 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

stored

into

the

16-bit

PRODuct

(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

the PIC17C42 and all other PIC17CXX devices, which

have the single cycle hardware multiply.

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 MULTIPLY ROUTINE

MOVFP

ARG1, WREG

MULWF

ARG2

; ARG1 * ARG2 ->

;

PRODH:PRODL

EXAMPLE 8-2:

8 x 8 SIGNED MULTIPLY

ROUTINE

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 8-1:

PERFORMANCE COMPARISON

Routine

Device

Program Memory

(Words)

Cycles (Max)

Time

@ 25 MHz

@ 33 MHz

8 x 8 unsigned

PIC17C42

11.04

µs

N/A

All other PIC17CXX devices

160 ns

121 ns

8 x 8 signed

PIC17C42

—

N/A

All other PIC17CXX devices

960 ns

727 ns

16 x 16 unsigned

PIC17C42

242

38.72

µs

N/A

All other PIC17CXX devices

3.84

µs

2.91

µs

16 x 16 signed

PIC17C42

254

40.64

µs

N/A

All other PIC17CXX devices

5.76

µs

4.36

µs

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