PIC17C7XX_13 Datasheet, PDF (67/306 Pages) Microchip Technology – High-Performance 8-bit CMOS EPROM Microcontrollers with 10-bit A/D

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.
PIC17C7XX
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
MOVFP
MULWF
ARG1, WREG ;
ARG2
; ARG1 * ARG2 ->
; PRODH:PRODL
EXAMPLE 9-2:
8 x 8 SIGNED MULTIPLY
ROUTINE
MOVFP
MULWF
BTFSC
SUBWF
MOVFP
BTFSC
SUBWF
ARG1, WREG
ARG2
ARG2, SB
PRODH, F
ARG2, WREG
ARG1, SB
PRODH, F
; ARG1 * ARG2 ->
; PRODH:PRODL
; Test Sign Bit
; PRODH = PRODH
;
- ARG1
; Test Sign Bit
; PRODH = PRODH
;
- ARG2
TABLE 9-1: PERFORMANCE COMPARISON
Routine
Multiply Method
8 x 8 unsigned
8 x 8 signed
16 x 16 unsigned
16 x 16 signed
Without hardware multiply
Hardware multiply
Without hardware multiply
Hardware multiply
Without hardware multiply
Hardware multiply
Without hardware multiply
Hardware multiply
Program
Memory
(Words)
13
1
—
6
21
24
52
36
Cycles
(Max)
69
1
—
6
242
24
254
36
Time
@ 33 MHz @ 16 MHz @ 8 MHz
8.364 s
0.121 s
—
0.727 s
29.333 s
2.91 s
30.788 s
4.36 s
17.25 s
0.25 s
—
1.50 s
60.50 s
6.0 s
63.50 s
9.0 s
34.50 s
0.50 s
—
3.0 s
121.0 s
12.0 s
127.0 s
18.0 s
 1998-2013 Microchip Technology Inc.
DS30289C-page 67