PIC17C75X Datasheet, PDF (61/320 Pages) Microchip Technology – High-Performance 8-Bit CMOS EPROM Microcontrollers

9.0 HARDWARE MULTIPLIER
All PIC17C75X 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.
PIC17C75X
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
8 x 8 unsigned
8 x 8 signed
16 x 16 unsigned
16 x 16 signed
Multiply Method
Without hardware multiply
Hardware multiply
Without hardware multiply
Hardware multiply
Without hardware multiply
Hardware multiply
Without hardware multiply
Hardware multiply
Program Memory
(Words)
Cycles (Max)
13
69
1
1
—
—
6
6
21
242
24
24
52
254
36
36
Time
@ 33 MHz
8.364 µs
0.121 µs
—
0.727 µs
29.333 µs
2.91 µs
30.788 µs
4.36 µs
© 1997 Microchip Technology Inc.
Preliminary
DS30264A-page 61