PIC18FXX20
8.2
Operation
8.0
8.1
8 X 8 HARDWARE MULTIPLIER
Introduction
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.
An 8 x 8 hardware multiplier is included in the ALU of
the PIC18FXX20 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 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
EXAMPLE 8-1:
8 x 8 UNSIGNED
MULTIPLY ROUTINE
MOVF
ARG1, W
ARG2
;
MULWF
; ARG1 * ARG2 ->
;
PRODH:PRODL
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 mul-
tiply, and performing the same function without the
hardware multiply.
EXAMPLE 8-2:
8 x 8 SIGNED MULTIPLY
ROUTINE
MOVF
MULWF
ARG1,
ARG2
W
; ARG1 * ARG2 ->
; PRODH:PRODL
; Test Sign Bit
; PRODH = PRODH
BTFSC
SUBWF
ARG2, SB
PRODH, F
;
- ARG1
MOVF
ARG2,
W
BTFSC
SUBWF
ARG1, SB
PRODH, F
; Test Sign Bit
; PRODH = PRODH
;
- ARG2
TABLE 8-1:
Routine
PERFORMANCE COMPARISON
Program
Time
@ 40 MHz @ 10 MHz @ 4 MHz
Cycles
Multiply Method
Memory
(Words)
(Max)
Without hardware multiply
Hardware multiply
Without hardware multiply
Hardware multiply
Without hardware multiply
Hardware multiply
Without hardware multiply
Hardware multiply
13
1
33
6
21
28
52
35
69
1
91
6
242
28
254
40
6.9 µs
100 ns
9.1 µs
600 ns
24.2 µs
2.8 µs
25.4 µs
4.0 µs
27.6 µs
400 ns
36.4 µs
2.4 µs
96.8 µs
11.2 µs
102.6 µs
16.0 µs
69 µs
1 µs
91 µs
6 µs
242 µs
28 µs
254 µs
40 µs
8 x 8 unsigned
8 x 8 signed
16 x 16 unsigned
16 x 16 signed
2003 Microchip Technology Inc.
Advance Information
DS39609A-page 85
MICROCHIP [ MICROCHIP ]
