Intel 80286, 80287 manual Data Types and Formats, Number System

OVERVIEW OF NUMERIC PROCESSING

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Figure 1-8. 80287 Number System

From a practical standpoint, the 80287's set of real numbers is sufficiently large and dense so as not to limit the vast majority of microprocessor applications. Compared to most computers, including mainframes, the NPX provides a very good approximation of the real number system. It is important to remember, however, that it is not an exact representation, and that arithmetic on real numbers is inherently approximate.

Conversely, and equally important, the 80287 does perform exact arithmetic on integer operands. That is, an operation on two integers returns an exact integral result, provided that the true result is an integer and is in range. For example, 4 + 2 yields an exact integer. I + 3 does not, and 240 X 230 + 1 does not, because the result requires greater than 64 bits of precision.

Data Types and Formats

The 80287 recognizes seven numeric data types, divided into three classes: binary integers, packed decimal integers, and binary reals. A later section describes how these formats are stored in memory (the sign is always located in the highest-addressed byte). Figure 1-9 summarizes the format of each data type. In the figure, the most significant digits of all numbers (and fields within numbers) are the leftmost digits. Table 1-5 provides the range and number of signficant (decimal) digits that each format can accommodate.

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