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PIC18F2520E/ML 数据表(PDF) 91 Page - Microchip Technology |
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PIC18F2520E/ML 数据表(HTML) 91 Page - Microchip Technology |
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91 / 390 page  2004 Microchip Technology Inc. Preliminary DS39631A-page 89 PIC18F2420/2520/4420/4520 8.0 8 x 8 HARDWARE MULTIPLIER 8.1 Introduction All PIC18 devices include an 8 x 8 hardware multiplier as part of the ALU. The multiplier performs an unsigned operation and yields a 16-bit result that is stored in the product register pair, PRODH:PRODL. The multiplier’s operation does not affect any flags in the Status register. Making multiplication a hardware operation allows it to be completed in a single instruction cycle. This has the advantages of higher computational throughput and reduced code size for multiplication algorithms and allows the PIC18 devices to be used in many applica- tions previously reserved for digital signal processors. A comparison of various hardware and software multiply operations, along with the savings in memory and execution time, is shown in Table 8-1. 8.2 Operation Example 8-1 shows the instruction sequence for an 8 x 8 unsigned multiplication. Only one instruction is required when one of the arguments is already loaded in the WREG register. Example 8-2 shows the sequence to do an 8 x 8 signed multiplication. To account for the sign bits of the argu- ments, each argument’s Most Significant bit (MSb) is tested and the appropriate subtractions are done. EXAMPLE 8-1: 8 x 8 UNSIGNED MULTIPLY ROUTINE EXAMPLE 8-2: 8 x 8 SIGNED MULTIPLY ROUTINE TABLE 8-1: PERFORMANCE COMPARISON FOR VARIOUS MULTIPLY OPERATIONS MOVF ARG1, W ; MULWF ARG2 ; ARG1 * ARG2 -> ; PRODH:PRODL MOVF ARG1, W MULWF ARG2 ; ARG1 * ARG2 -> ; PRODH:PRODL BTFSC ARG2, SB ; Test Sign Bit SUBWF PRODH, F ; PRODH = PRODH ; - ARG1 MOVF ARG2, W BTFSC ARG1, SB ; Test Sign Bit SUBWF PRODH, F ; PRODH = PRODH ; - ARG2 Routine Multiply Method Program Memory (Words) Cycles (Max) Time @ 40 MHz @ 10 MHz @ 4 MHz 8 x 8 unsigned Without hardware multiply 13 69 6.9 µs27.6 µs69 µs Hardware multiply 1 1 100 ns 400 ns 1 µs 8 x 8 signed Without hardware multiply 33 91 9.1 µs36.4 µs91 µs Hardware multiply 6 6 600 ns 2.4 µs6 µs 16 x 16 unsigned Without hardware multiply 21 242 24.2 µs96.8 µs 242 µs Hardware multiply 28 28 2.8 µs 11.2 µs28 µs 16 x 16 signed Without hardware multiply 52 254 25.4 µs 102.6 µs 254 µs Hardware multiply 35 40 4.0 µs16.0 µs40 µs |
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