PIC18F2420/2520/4420/4520
EXAMPLE 8-1:
8 x 8 UNSIGNED MULTIPLY
ROUTINE
8.0
8.1
8 x 8 HARDWARE MULTIPLIER
Introduction
MOVF
MULWF
ARG1, W
ARG2
;
; ARG1 * ARG2 ->
; PRODH:PRODL
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.
EXAMPLE 8-2:
8 x 8 SIGNED MULTIPLY
ROUTINE
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 the PIC18 devices to be used in many applica-
tions 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 8-1.
MOVF
MULWF
ARG1, W
ARG2
; ARG1 * ARG2 ->
; PRODH:PRODL
; Test Sign Bit
; PRODH = PRODH
BTFSC
SUBWF
ARG2, SB
PRODH, F
;
- ARG1
MOVF
BTFSC
SUBWF
ARG2, W
ARG1, SB
PRODH, F
; Test Sign Bit
; PRODH = PRODH
;
- ARG2
8.2
Operation
Example 8-1 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 8-2 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.
TABLE 8-1:
Routine
PERFORMANCE COMPARISON FOR VARIOUS MULTIPLY OPERATIONS
Program
Memory
(Words)
Time
Cycles
(Max)
Multiply Method
@ 40 MHz @ 10 MHz @ 4 MHz
Without hardware multiply
Hardware multiply
13
1
69
1
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
69 μs
1 μs
91 μs
6 μs
8 x 8 unsigned
8 x 8 signed
Without hardware multiply
Hardware multiply
33
6
91
6
Without hardware multiply
Hardware multiply
21
28
52
35
242
28
254
40
96.8 μs
11.2 μs
102.6 μs
16.0 μs
242 μs
28 μs
254 μs
40 μs
16 x 16 unsigned
16 x 16 signed
Without hardware multiply
Hardware multiply
© 2008 Microchip Technology Inc.
DS39631E-page 89
MICROCHIP [ MICROCHIP ]
