Datasheet
2012 Microchip Technology Inc. DS30684A-page 115
PIC18(L)F2X/45K50
9.0 8 x 8 HARDWARE MULTIPLIER
9.1 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 Tabl e 9 -1.
9.2 Operation
Example 9-1 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 9-2 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.
EXAMPLE 9-1: 8 x 8 UNSIGNED
MULTIPLY ROUTINE
EXAMPLE 9-2: 8 x 8 SIGNED MULTIPLY
ROUTINE
MOVF ARG1, W ;
MULWF ARG2 ; ARG1 * ARG2 ->
; PRODH:PRODL
MOVF ARG1, W
MULWF ARG2 ; ARG1 * ARG2 ->
; PRODH:PRODL
BTFSC ARG2, SB ; Test Sign Bit
SUBWF PRODH, F ; PRODH = PRODH
; - ARG1
MOVF ARG2, W
BTFSC ARG1, SB ; Test Sign Bit
SUBWF PRODH, F ; PRODH = PRODH
; - ARG2
TABLE 9-1: PERFORMANCE COMPARISON FOR VARIOUS MULTIPLY OPERATIONS
Routine Multiply Method
Program
Memory
(Words)
Cycles
(Max)
Time
@ 48 MHz @ 40 MHz @ 10 MHz @ 4 MHz
8 x 8 unsigned
Without hardware multiply 13 69
5.7 s
6.9 s27.6 s69 s
Hardware multiply 1 1 83.3 ns 100 ns 400 ns 1 s
8 x 8 signed
Without hardware multiply 33 91 7.5 s9.1 s36.4 s91 s
Hardware multiply 6 6 500 ns 600 ns 2.4 s6 s
16 x 16 unsigned
Without hardware multiply 21 242 20.1 s24.2 s96.8 s242 s
Hardware multiply 28 28 2.3 s2.8 s11.2 s28 s
16 x 16 signed
Without hardware multiply 52 254 21.6 s25.4 s 102.6 s254 s
Hardware multiply 35 40 3.3 s4.0 s16.0 s40 s