DM163006 datasheet(85/320 Pages) MICROCHIP

85 / 320 page

 2001 Microchip Technology Inc.

Advance Information

DS39541A-page 85

PIC18C601/801

7.0

8 X 8 HARDWARE MULTIPLIER

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

PIC18C601/801 devices. 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 regis-

ter pair (PRODH:PRODL). The multiplier does not

affect any flags in the STATUS 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 some applications previously reserved for Digital

Signal Processors.

Table 7-1 shows a performance comparison between

enhanced devices using the single cycle hardware mul-

tiply, and performing the same function without the

hardware multiply.

TABLE 7-1:

PERFORMANCE COMPARISON

Routine

Multiply Method

Program

Memory

(Words)

Cycles

(Max)

Time

@ 25 MHz

@ 10 MHz

@ 4 MHz

8 x 8 unsigned

Without hardware multiply

11.0

µs27.6 µs

69.0

µs

Hardware multiply

160.0 ns

400.0 ns

1.0

µs

8 x 8 signed

Without hardware multiply

14.6

µs36.4 µs

91.0

µs

Hardware multiply

960.0 ns

2.4

µs

6.0

µs

16 x 16 unsigned

Without hardware multiply

242

38.7

µs96.8 µs

242.0

µs

Hardware multiply

3.8

µs

9.6

µs

24.0

µs

16 x 16 signed

Without hardware multiply

254

40.6

µs102.6 µs

254.0

µs

Hardware multiply

5.8

µs

14.4

µs

36.0

µs

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