Datasheet
2003-2013 Microchip Technology Inc. DS39609C-page 85
PIC18F6520/8520/6620/8620/6720/8720
8.0 8 X 8 HARDWARE MULTIPLIER
8.1 Introduction
An 8 x 8 hardware multiplier is included in the ALU of
the PIC18FXX20 devices. By making the multiply a
hardware operation, it completes in a single instruction
cycle. This is an unsigned multiply that gives a 16-bit
result. The result is stored in the 16-bit product register
pair (PRODH:PRODL). The multiplier does not affect
any flags in the ALUSTA register.
Making the 8 x 8 multiplier execute in a single cycle
gives the following advantages:
• Higher computational throughput
• Reduces code size requirements for multiply
algorithms
The performance increase allows the device to be used
in applications previously reserved for Digital Signal
Processors.
Table 8-1 shows a performance comparison between
enhanced devices using the single-cycle hardware
multiply and performing the same function without the
hardware multiply.
8.2 Operation
Example 8-1 shows the sequence to do an 8 x 8
unsigned multiply. Only one instruction is required
when one argument of the multiply is already loaded in
the WREG register.
Example 8-2 shows the sequence to do an 8 x 8 signed
multiply. 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
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
@ 40 MHz @ 10 MHz @ 4 MHz
8 x 8 unsigned
Without hardware multiply 13 69 6.9 s27.6 s69 s
Hardware multiply 1 1 100 ns 400 ns 1 s
8 x 8 signed
Without hardware multiply 33 91 9.1 s36.4 s91 s
Hardware multiply 6 6 600 ns 2.4 s6 s
16 x 16 unsigned
Without hardware multiply 21 242 24.2 s96.8 s242 s
Hardware multiply 28 28 2.8 s 11.2 s28 s
16 x 16 signed
Without hardware multiply 52 254 25.4 s 102.6 s254 s
Hardware multiply 35 40 4.0 s16.0 s40 s