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PIC18F258-E/PSQTP 数据表(PDF) 77 Page - Microchip Technology |
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PIC18F258-E/PSQTP 数据表(HTML) 77 Page - Microchip Technology |
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77 / 402 page  © 2006 Microchip Technology Inc. DS41159E-page 75 PIC18FXX8 7.0 8 x 8 HARDWARE MULTIPLIER 7.1 Introduction 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 in the 16-bit product register 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 algorithms 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 multiply and performing the same function without the hardware multiply. 7.2 Operation 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: 8 x 8 UNSIGNED MULTIPLY ROUTINE EXAMPLE 7-2: 8 x 8 SIGNED MULTIPLY ROUTINE TABLE 7-1: PERFORMANCE COMPARISON 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 ; PRODH = PRODH ; - ARG1 MOVF ARG2, W BTFSC ARG1, SB ; Test Sign Bit SUBWF PRODH ; 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 μs 27.6 μs 69 μs Hardware multiply 1 1 100 ns 400 ns 1 μs 8 x 8 signed Without hardware multiply 33 91 9.1 μs 36.4 μs 91 μs Hardware multiply 6 6 600 ns 2.4 μs6 μs 16 x 16 unsigned Without hardware multiply 21 242 24.2 μs 96.8 μs 242 μs Hardware multiply 24 24 2.4 μs9.6 μs 24 μs 16 x 16 signed Without hardware multiply 52 254 25.4 μs102.6 μs 254 μs Hardware multiply 36 36 3.6 μs 14.4 μs 36 μs |
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