PIC18F44J50-I Datasheet, PDF (113/562 Pages) Microchip Technology – 28/44-Pin, Low-Power, High-Performance USB Microcontrollers

PIC18F46J50 FAMILY
8.0 8 x 8 HARDWARE MULTIPLIER
8.1 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 the PIC18 devices to be used in many applica-
tions previously reserved for digital signal processors.
Table 8-1 provides a comparison of various hardware
and software multiply operations, along with the
savings in memory and execution time.
8.2 Operation
Example 8-1 provides 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 provides the instruction sequence for an
8 x 8 signed multiplication. 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, 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 8-1: PERFORMANCE COMPARISON FOR VARIOUS MULTIPLY OPERATIONS
Routine
Multiply Method
Program
Memory
(Words)
Cycles
(Max)
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 unsigned
Without hardware multiply
Hardware multiply
21
28
16 x 16 signed
Without hardware multiply
Hardware multiply
52
35
69
5.7 s
27.6 s
69 s
1
83.3 ns
400 ns
1 s
91
7.5 s
36.4 s
91 s
6
500 ns
2.4 s
6 s
242
20.1 s
96.8 s
242 s
28
2.3 s
11.2 s
28 s
254
21.6 s 102.6 s 254 s
40
3.3 s
16.0 s
40 s
 2011 Microchip Technology Inc.
DS39931D-page 113