PIC18F258 MICROCHIP [Microchip Technology], PIC18F258 Datasheet - Page 77

• Manufacturer Part Number: PIC18F258
• Description: High Performance, 28/40-Pin Enhanced FLASH Microcontrollers with CAN
• Manufacturer: MICROCHIP [Microchip Technology]

7.0
7.1
An 8 x 8 hardware multiplier is included in the ALU of
the PIC18FXX8 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 into the 16-bit product regis-
ter 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
The performance increase allows the device to be used
in applications previously reserved for Digital Signal
Processors.
Table 7-1 shows a performance comparison between
enhanced devices using the single cycle hardware mul-
tiply, and performing the same function without the
hardware multiply.
TABLE 7-1:
 2002 Microchip Technology Inc.
16 x 16 unsigned
algorithms
8 x 8 unsigned
16 x 16 signed
8 x 8 signed
Routine
8 X 8 HARDWARE MULTIPLIER
Introduction
PERFORMANCE COMPARISON
Without hardware multiply
Without hardware multiply
Without hardware multiply
Without hardware multiply
Hardware multiply
Hardware multiply
Hardware multiply
Hardware multiply
Multiply Method
Program
Memory
(Words)
13
33
21
24
52
36
1
6
7.2
Example 7-1 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 7-2 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 7-1:
EXAMPLE 7-2:
MOVF
MULWF
MOVF
MULWF
BTFSC
SUBWF
MOVF
BTFSC
SUBWF
Cycles
(Max)
242
254
69
91
24
36
1
6
Operation
ARG1, W
ARG2
ARG1,
ARG2
ARG2,
PRODH
ARG2,
ARG1,
PRODH
@ 40 MHz
24.2 µs
25.4 µs
100 ns
600 ns
6.9 µs
9.1 µs
2.4 µs
3.6 µs
SB
SB
W
W
8 x 8 UNSIGNED
MULTIPLY ROUTINE
8 x 8 SIGNED MULTIPLY
ROUTINE
PIC18FXX8
;
; ARG1 * ARG2 ->
;
; ARG1 * ARG2 ->
; PRODH:PRODL
; Test Sign Bit
; PRODH = PRODH
;
; Test Sign Bit
; PRODH = PRODH
;
@ 10 MHz
102.6 µs
PRODH:PRODL
27.6 µs
36.4 µs
96.8 µs
14.4 µs
Time
400 ns
2.4 µs
9.6 µs
DS41159B-page 75
- ARG1
- ARG2
@ 4 MHz
242 µs
254 µs
69 µs
91 µs
24 µs
36 µs
1 µs
6 µs