Datasheet

 2011 Microchip Technology Inc. DS39932D-page 113
PIC18F46J11 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: 8 x 8 UNSIGNED MULTIPLY
ROUTINE
EXAMPLE 8-2: 8 x 8 SIGNED MULTIPLY
ROUTINE
TABLE 8-1: 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
@ 48 MHz @ 10 MHz @ 4 MHz
8 x 8 unsigned
Without hardware multiply 13 69 5.7 s27.6 s69 s
Hardware multiply 1 1 83.3 ns 400 ns 1 s
8 x 8 signed
Without hardware multiply 33 91 7.5 s36.4 s91 s
Hardware multiply 6 6 500 ns 2.4 s6 s
16 x 16 unsigned
Without hardware multiply 21 242 20.1 s96.8 s242 s
Hardware multiply 28 28 2.3 s 11.2 s28 s
16 x 16 signed
Without hardware multiply 52 254 21.6 s 102.6 s254 s
Hardware multiply 35 40 3.3 s16.0 s40 s