Dedicated Digital Processors: Methods in Hardware/Software Co-Design

4

Operations on Numbers

In order to perform numerical computations with a digital system, numbers need to be encoded as bit strings as explained in section 1.1.1. Then arithmetic operations can be implemented as special Boolean functions on the number codes and in turn be used as building blocks in numeric algorithms. The general scheme to arrive at the Boolean function realizing an arithmetic operation for the given encoding is as shown in Listing 3.2 for the add operation. For binary and signed binary numbers, the circuit design from gates or CMOS switches is actually carried out for single bit numbers only. For larger numbers, the arithmetic operations have fairly simple algorithms reducing them to single bit operations. There is no need to resort to ROM tables for their realization as one might consider for more irregular Boolean functions, an exception being the so-called distributed arithmetic where a complex arithmetic operation is realized with the aid of small tables. To study the materials in this chapter, the exercises section plays a special role. Several of the arithmetic circuits and algorithms are taken up there, and VHDL implementations are given that also help the reader to become more familiar with that language.

4.1 SINGLE BIT BINARY ADDERS AND MULTIPLIERS

If the Boolean 0 and 1 elements encode and are identified with the numbers 0 and 1, then the result of the arithmetic add operation applied to two numbers r, s ∊ B ranges from 0 to 2. Its binary encoding is ...