PIC18F2450 Datasheet, PDF (85/320 Pages) Microchip Technology – 28/40/44-Pin, High-Performance, 12 MIPS, Enhanced Flash, USB Microcontrollers with nanoWatt Technology

7.0 8 x 8 HARDWARE MULTIPLIER
7.1 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.
A comparison of various hardware and software
multiply operations, along with the savings in memory
and execution time, is shown in Table 7-1.
7.2 Operation
Example 7-1 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 7-2 shows the sequence to do 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.
PIC18F2450/4450
EXAMPLE 7-1:
MOVF ARG1, W
MULWF ARG2
8 x 8 UNSIGNED
MULTIPLY ROUTINE
;
; ARG1 * ARG2 ->
; PRODH:PRODL
EXAMPLE 7-2:
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 7-1: PERFORMANCE COMPARISON FOR VARIOUS MULTIPLY OPERATIONS
Routine
Multiply Method
Program
Memory
(Words)
Cycles
(Max)
Time
@ 40 MHz @ 10 MHz @ 4 MHz
Without hardware multiply
13
8 x 8 unsigned
Hardware multiply
1
Without hardware multiply
33
8 x 8 signed
Hardware multiply
6
Without hardware multiply
21
16 x 16 unsigned
Hardware multiply
28
Without hardware multiply
52
16 x 16 signed
Hardware multiply
35
69
6.9 μs
27.6 μs
69 μs
1
100 ns
400 ns
1 μs
91
9.1 μs
36.4 μs
91 μs
6
600 ns
2.4 μs
6 μs
242
24.2 μs
96.8 μs 242 μs
28
2.8 μs
11.2 μs
28 μs
254
25.4 μs 102.6 μs 254 μs
40
4.0 μs
16.0 μs
40 μs
© 2006 Microchip Technology Inc.
Advance Information
DS39760A-page 83