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 1.

Table 1. Performance Comparison for Various Multiply Operations
Routine Multiply Method Program
Memory
(Words) Cycles
(Max) Time
@ 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
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
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
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
Hardware multiply 35 40 2.5 μs 4.0 μs 16.0 μs 40 μs