Datasheet
1998-2013 Microchip Technology Inc. DS30289C-page 67
PIC17C7XX
9.0 HARDWARE MULTIPLIER
All PIC17C7XX devices have an 8 x 8 hardware multi-
plier included in the ALU of the device. 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 into the 16-bit
Product register (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 algo-
rithms
The performance increase allows the device to be used
in applications previously reserved for Digital Signal
Processors.
Table 9-1 shows a performance comparison between
PIC17CXXX devices using the single cycle hardware
multiply and performing the same function without the
hardware multiply.
Example 9-1 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 9-2 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 9-1: 8 x 8 UNSIGNED
MULTIPLY ROUTINE
EXAMPLE 9-2: 8 x 8 SIGNED MULTIPLY
ROUTINE
TABLE 9-1: PERFORMANCE COMPARISON
MOVFP ARG1, WREG ;
MULWF ARG2 ; ARG1 * ARG2 ->
; PRODH:PRODL
MOVFP ARG1, WREG
MULWF ARG2 ; ARG1 * ARG2 ->
; PRODH:PRODL
BTFSC ARG2, SB ; Test Sign Bit
SUBWF PRODH, F ; PRODH = PRODH
; - ARG1
MOVFP ARG2, WREG
BTFSC ARG1, SB ; Test Sign Bit
SUBWF PRODH, F ; PRODH = PRODH
; - ARG2
Routine Multiply Method
Program
Memory
(Words)
Cycles
(Max)
Time
@ 33 MHz @ 16 MHz @ 8 MHz
8 x 8 unsigned Without hardware multiply 13 69 8.364 s17.25 s 34.50 s
Hardware multiply 1 1 0.121 s0.25 s0.50 s
8 x 8 signed Without hardware multiply — — — — —
Hardware multiply 6 6 0.727 s1.50 s3.0 s
16 x 16 unsigned Without hardware multiply 21 242 29.333 s60.50 s 121.0 s
Hardware multiply 24 24 2.91 s6.0 s 12.0 s
16 x 16 signed Without hardware multiply 52 254 30.788 s63.50 s 127.0 s
Hardware multiply 36 36 4.36 s9.0 s 18.0 s