PIC18F2331/2431/4331/4431
9.2
Operation
9.0
9.1
8 x 8 HARDWARE MULTIPLIER
Introduction
Example 9-1 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.
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 9-2 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.
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 applications
previously reserved for digital signal processors.
EXAMPLE 9-1:
8 x 8 UNSIGNED MULTIPLY
ROUTINE
MOVF
ARG1, W
ARG2
;
MULWF
; ARG1 * ARG2 ->
; PRODH:PRODL
A comparison of various hardware and software multi-
ply operations, along with the savings in memory and
execution time, is shown in Table 9-1.
EXAMPLE 9-2:
8 x 8 SIGNED MULTIPLY
ROUTINE
MOVF
MULWF
ARG1, W
ARG2
; ARG1 * ARG2 ->
; PRODH:PRODL
BTFSC
SUBWF
ARG2, SB
PRODH, F
; Test Sign Bit
; PRODH = PRODH
;
- ARG1
MOVF
BTFSC
SUBWF
ARG2, W
ARG1, SB
PRODH, F
; Test Sign Bit
; PRODH = PRODH
;
- ARG2
TABLE 9-1:
Routine
PERFORMANCE COMPARISON
Program
Time
@ 40 MHz @ 10 MHz @ 4 MHz
Cycles
(Max)
Multiply Method
Memory
(Words)
Without Hardware Multiply
Hardware Multiply
13
1
69
1
6.9 s
100 ns
9.1 s
600 ns
24.2 s
2.4 s
25.4 s
3.6 s
27.6 s
400 ns
36.4 s
2.4 s
69 s
1 s
8 x 8 Unsigned
8 x 8 Signed
Without Hardware Multiply
Hardware Multiply
33
6
91
6
91 s
6 s
Without Hardware Multiply
Hardware Multiply
21
24
52
36
242
24
254
36
96.8 s
9.6 s
242 s
24 s
254 s
36 s
16 x 16 Unsigned
16 x 16 Signed
Without Hardware Multiply
Hardware Multiply
102.6 s
14.4 s
2010 Microchip Technology Inc.
DS39616D-page 95
MICROCHIP [ MICROCHIP ]
