PIC18F26K80-I Datasheet, PDF (145/622 Pages) Microchip Technology – 28/40/44/64-Pin, Enhanced Flash Microcontrollers with ECAN™ and nanoWatt XLP Technology

PIC18F66K80 FAMILY
9.0 8 x 8 HARDWARE MULTIPLIER
9.1 Introduction
All PIC18 devices include an 8 x 8 hardware multiplier
as part of the ALU. The multiplier performs an unsigned
operation and yields a 16-bit result that is stored in the
product register pair, PRODH:PRODL. The multiplier’s
operation does not affect any flags in the STATUS
register.
Making multiplication a hardware operation allows it to
be completed in a single instruction cycle. This has the
advantages of higher computational throughput and
reduced code size for multiplication algorithms and
allows PIC18 devices to be used in many applications
previously reserved for digital-signal processors. A
comparison of various hardware and software multiply
operations, along with the savings in memory and
execution time, is shown in Table 9-1.
9.2 Operation
Example 9-1 shows the instruction sequence for an 8 x 8
unsigned multiplication. Only one instruction is required
when one of the arguments is already loaded in the
WREG register.
Example 9-2 shows the sequence to do an 8 x 8 signed
multiplication. To account for the sign bits of the argu-
ments, each argument’s Most Significant bit (MSb) is
tested and the appropriate subtractions are done.
EXAMPLE 9-1:
MOVF ARG1, W
MULWF ARG2
8 x 8 UNSIGNED MULTIPLY
ROUTINE
;
; ARG1 * ARG2 ->
; PRODH:PRODL
EXAMPLE 9-2:
MOVF ARG1, W
MULWF ARG2
BTFSC ARG2, SB
SUBWF PRODH, F
MOVF
BTFSC
SUBWF
ARG2, W
ARG1, SB
PRODH, F
8 x 8 SIGNED MULTIPLY
ROUTINE
; ARG1 * ARG2 ->
; PRODH:PRODL
; Test Sign Bit
; PRODH = PRODH
;
- ARG1
; Test Sign Bit
; PRODH = PRODH
;
- ARG2
TABLE 9-1: PERFORMANCE COMPARISON FOR VARIOUS MULTIPLY OPERATIONS
Routine
Multiply Method
Program
Memory
(Words)
Cycles
(Max)
@ 64 MHz
Time
@ 48 MHz @ 10 MHz
@ 4 MHz
8 x 8 unsigned ^{Without hardware multiply}
13
Hardware multiply
1
8 x 8 signed
Without hardware multiply
Hardware multiply
33
6
16 x 16
Without hardware multiply
21
unsigned
Hardware multiply
28
Without hardware multiply
52
16 x 16 signed
Hardware multiply
35
69
4.3 s
5.7 s
27.6 s
69 s
1
62.5 ns
83.3 ns
400 ns
1 s
91
5.6 s
7.5 s
36.4 s
91 s
6
375 ns
500 ns
2.4 s
6 s
242
15.1 s
20.1 s
96.8 s 242 s
28
1.7 s
2.3 s
11.2 s
28 s
254
15.8 s
21.2 s 101.6 s 254 s
40
2.5 s
3.3 s
16.0 s
40 s
 2010-2012 Microchip Technology Inc.
DS39977F-page 145