Datasheet

 2009-2011 Microchip Technology Inc. DS39960D-page 139
PIC18F87K22 FAMILY
10.0 8 x 8 HARDWARE MULTIPLIER
10.1 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 PIC18 devices to be used in many applications
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 10-1.
10.2 Operation
Example 10-1 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 10-2 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 10-1: 8 x 8 UNSIGNED
MULTIPLY ROUTINE
EXAMPLE 10-2: 8 x 8 SIGNED MULTIPLY
ROUTINE
TABLE 10-1: PERFORMANCE COMPARISON FOR VARIOUS MULTIPLY OPERATIONS
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
Routine Multiply Method
Program
Memory
(Words)
Cycles
(Max)
Time
@ 64 MHz @ 48 MHz @ 10 MHz @ 4 MHz
8 x 8 Unsigned
Without Hardware Multiply 13 69 4.3 s5.7 s27.6 s 69 s
Hardware Multiply 1 1 62.5 ns 83.3 ns 400 ns 1 s
8 x 8 Signed
Without Hardware Multiply 33 91 5.6 s7.5 s36.4 s 91 s
Hardware Multiply 6 6 375 ns 500 ns 2.4 s6 s
16 x 16
Unsigned
Without Hardware Multiply 21 242 15.1 s20.1 s96.8 s242 s
Hardware Multiply 28 28 1.7 s2.3 s 11.2 s 28 s
16 x 16 Signed
Without Hardware Multiply 52 254 15.8 s21.2 s 101.6 s254 s
Hardware Multiply 35 40 2.5 s3.3 s16.0 s 40 s