PIC18F2X1X Datasheet, PDF (81/380 Pages) Microchip Technology – 28/40/44-PIN FLASH MICROCONTROLLERS WITH 10-BIT A/D AND NANO WATT 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 argu-
ments, each argument’s Most Significant bit (MSb) is
tested and the appropriate subtractions are done.
PIC18F2X1X/4X1X
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
 2004 Microchip Technology Inc.
Preliminary
DS39636A-page 79