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.
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.
EXAMPLE 9-1: 8 x 8 UNSIGNED MULTIPLY
ROUTINE
MOVFP
MULWF
ARG1, WREG
ARG2
;
Making the 8 x 8 multiplier execute in a single cycle
gives the following advantages:
; ARG1 * ARG2 ->
;
PRODH:PRODL
• Higher computational throughput
• Reduces code size requirements for multiply algo-
rithms
EXAMPLE 9-2: 8 x 8 SIGNED MULTIPLY
ROUTINE
The performance increase allows the device to be used
in applications previously reserved for Digital Signal
Processors.
MOVFP
MULWF
ARG1, WREG
ARG2
; ARG1 * ARG2 ->
PRODH:PRODL
; Test Sign Bit
; PRODH = PRODH
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.
;
BTFSC
SUBWF
ARG2, SB
PRODH, F
;
- ARG1
MOVFP
BTFSC
SUBWF
ARG2, WREG
ARG1, SB
PRODH, F
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.
; Test Sign Bit
; PRODH = PRODH
;
- ARG2
TABLE 9-1:
PERFORMANCE COMPARISON
Multiply Method
Time
Program Memory
(Words)
Routine
Cycles (Max)
@ 33 MHz
8 x 8 unsigned
8 x 8 signed
Without hardware multiply
Hardware multiply
13
1
69
1
8.364 µs
0.121 µs
—
Without hardware multiply
Hardware multiply
—
6
—
6
0.727 µs
29.333 µs
2.91 µs
30.788 µs
4.36 µs
16 x 16 unsigned
16 x 16 signed
Without hardware multiply
Hardware multiply
21
24
52
36
242
24
254
36
Without hardware multiply
Hardware multiply
1997 Microchip Technology Inc.
Preliminary
DS30264A-page 61
MICROCHIP [ MICROCHIP ]
