PIC18F6585/8585/6680/8680
8.0
8.1
8 x 8 HARDWARE MULTIPLIER
Introduction
8.2
Operation
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.
enhanced devices using the single-cycle hardware
multiply and performing the same function without the
hardware multiply.
unsigned multiply. Only one instruction is required
when one argument of the multiply is already loaded in
the WREG register.
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:
MOVF
MULWF
ARG1, W
ARG2
8 x 8 UNSIGNED
MULTIPLY ROUTINE
;
; ARG1 * ARG2 ->
; PRODH:PRODL
EXAMPLE 8-2:
MOVF
MULWF
BTFSC
SUBWF
MOVF
BTFSC
SUBWF
ARG1, W
ARG2
ARG2, SB
PRODH
ARG2, W
ARG1, SB
PRODH
8 x 8 SIGNED MULTIPLY
ROUTINE
;
;
;
;
;
;
;
;
;
;
ARG1 * ARG2 ->
PRODH:PRODL
Test Sign Bit
PRODH = PRODH
- ARG1
Test Sign Bit
PRODH = PRODH
- ARG2
TABLE 8-1:
Routine
PERFORMANCE COMPARISON
Multiply Method
Without hardware multiply
Hardware multiply
Without hardware multiply
Hardware multiply
Without hardware multiply
Hardware multiply
Without hardware multiply
Hardware multiply
Program
Memory
(Words)
13
1
33
6
21
24
52
36
Cycles
(Max)
69
1
91
6
242
24
254
36
Time
@ 40 MHz
6.9
µs
100 ns
9.1
µs
600 ns
24.2
µs
2.4
µs
25.4
µs
3.6
µs
@ 10 MHz
27.6
µs
400 ns
36.4
µs
2.4
µs
96.8
µs
9.6
µs
102.6
µs
14.4
µs
@ 4 MHz
69
µs
1
µs
91
µs
6
µs
242
µs
24
µs
254
µs
36
µs
8 x 8 unsigned
8 x 8 signed
16 x 16 unsigned
16 x 16 signed
2004 Microchip Technology Inc.
DS30491C-page 107
MICROCHIP [ MICROCHIP TECHNOLOGY ]
