Intel 80287, 80286 manual Glossary-4

GLOSSARY OF 80287 AND FLOATING-POINT TERMINOLOGY

Precision Exception: an 80287 error condition that results when a calculation does not return an exact answer. This exception is usually Masked and ignored; it is used only in extremely critical applications, when the user must know if the results are exact.

Projective Mode: a state of the 80287, selected in the 80287 Control Word, in which infinities are treated as not having a sign. Thus the values +INFINITY and - INFINITY are considered the same. Certain operations, such as comparison to finite numbers, are illegal in Projective Mode but legal.in Affine Mode. Thus Projective Mode gives you a greater degree of error control over infinite inputs.

Pseudo Zero: a special value of the Temporary Real format. It is a number with a zero significand and an Exponent that is neither all zeros or all ones. Pseudo zeros can come about as the result of multiplication of two Unnormal numbers; but they are very rare.

Real: any finite value (negative, positive, or zero) that can be represented by a decimal expansion. The fractional part of the decimal expansion can contain an infinite number of digits. Reals can be repre- sented as the points of a line marked off like a ruler. The term Real can also refer to a Floating-Point Number that represents a Real value.

Short Integer: an Integer format supported by the 80287 that consists of a 32-bit Two's Complement quantity. Short Integer is not theshortest 80287 Integer format-the 16-bit Word Integer is.

Short Real: a Floating-Point Format supported by the 80287, which consists of a sign, an 8-bit Biased Exponent, an Implicit Integer Bit, and a 23-bit Significand-a total of 32 explicit bits.

Significand: the part of a Floating-Point Number that consists of the most significant nonzero bits of the number, if the number were written out in an unlimited binary format. The Significand alone is considered to have a Binary Point after the first (possibly Implicit) bit; the Binary Point is then moved according to the value of the Exponent.

Single Extended: a Floating-Point format, required by the Standard, that provides greater precision than Single; it also provides an explicit Integer Significand bit. The 80287's Temporary Real format meets the Single Extended requirement as well as the Double Extended requirement.

Single Floating-Point Number: the Standard's term for the 80287's 32-bit Short Real format.

Standard: "a Proposed Standard for Binary Floating-Point Arithmetic," Draft 10.0 of IEEE Task P754,

December 2, 1982. '.

Status Word: A 16-bit 80287 register that can be manually set,.but which is usually controlled by side effects to 80287 instructions. It contains condition codes, the 80287 stack pointer, busy and interrupt bits, and error flags.

Tag Word: a 16-bit 80287 register that is automatically maintained by the 80287. For each space in the 80287 stack, it tells if the space 1S occupied by a number; ii so, it gives infunll<liiull <luuui wliiit kind of number.

Temporary Real: the main Floating-Point Format used by the 80287. It consists of a sign, a 15-bit Biased Exponent, and a Significand with an explicit Integer bit and 63 fractional-part bits.

Transcendental: one of a class of functions for which polynomial formulas are always approximate, never exact for more than isolated values. The 80287 supports trigonometric, exponential, and logarith- mic functions; all are Transcendental.

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