PIC18F6585 Datasheet, PDF (109/496 Pages) Microchip Technology – 64/68/80-Pin High-Performance, 64-Kbyte Enhanced Flash Microcontrollers with ECAN Module

PIC18F6585/8585/6680/8680
8.0 8 x 8 HARDWARE MULTIPLIER
8.1 Introduction
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.
Table 8-1 shows a performance comparison between
enhanced devices using the single-cycle hardware
multiply and performing the same function without the
hardware multiply.
8.2 Operation
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:
MOVF ARG1, W
MULWF ARG2
8 x 8 UNSIGNED
MULTIPLY ROUTINE
;
; ARG1 * ARG2 ->
; PRODH:PRODL
EXAMPLE 8-2:
MOVF ARG1, W
MULWF ARG2
BTFSC ARG2, SB
SUBWF PRODH
MOVF
BTFSC
SUBWF
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: PERFORMANCE COMPARISON
Routine
Multiply Method
Program
Memory
(Words)
Without hardware multiply
13
8 x 8 unsigned
Hardware multiply
1
8 x 8 signed
Without hardware multiply
33
Hardware multiply
6
Without hardware multiply
21
16 x 16 unsigned
Hardware multiply
24
Without hardware multiply
52
16 x 16 signed
Hardware multiply
36
Cycles
(Max)
69
1
91
6
242
24
254
36
Time
@ 40 MHz @ 10 MHz @ 4 MHz
6.9 µs
100 ns
9.1 µs
600 ns
24.2 µs
2.4 µs
25.4 µs
3.6 µs
27.6 µs
400 ns
36.4 µs
2.4 µs
96.8 µs
9.6 µs
102.6 µs
14.4 µs
69 µs
1 µs
91 µs
6 µs
242 µs
24 µs
254 µs
36 µs
 2004 Microchip Technology Inc.
DS30491C-page 107