7.3 8x8 Hardware Multiplier

This device includes an 8x8 hardware multiplier as part of the ALU within the CPU. The multiplier performs an unsigned operation and yields a 16-bit result that is stored in the product register, PROD. 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 device to be used in many applications 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 7-2.

Table 7-2. Performance Comparison for Various Multiply Operations
Routine	Multiply Method	Program Memory (Words)	Cycles (Max)	Time
Routine	Multiply Method	Program Memory (Words)	Cycles (Max)	@ 64 MHz	@ 40 MHz	@ 10 MHz	@ 4 MHz
8x8 unsigned	Without hardware multiply	13	69	4.3 μs	6.9 μs	27.6 μs	69 μs
8x8 unsigned	Hardware multiply	1	1	62.5 ns	100 ns	400 ns	1 μs
8x8 signed	Without hardware multiply	33	91	5.7 μs	9.1 μs	36.4 μs	91 μs
8x8 signed	Hardware multiply	6	6	375 ns	600 ns	2.4 μs	6 μs
16x16 unsigned	Without hardware multiply	21	242	15.1 μs	24.2 μs	96.8 μs	242 μs
16x16 unsigned	Hardware multiply	28	28	1.8 μs	2.8 μs	11.2 μs	28 μs
16x16 signed	Without hardware multiply	52	254	15.9 μs	25.4 μs	102.6 μs	254 μs
16x16 signed	Hardware multiply	35	40	2.5 μs	4.0 μs	16.0 μs	40 μs