Intel 80286, 80287 manual Glossary FLOATING-POINT Terminology, Glossary-1

GLOSSARY OF 80287

AND FLOATING-POINT TERMINOLOGY

This glossary defines many terms that have precise technical meanings as specified in the IEEE 754 Standard. Where these terms are used, they have been capitalized to emphasize the precision of their meanings. In reading these definitions, you may therefore interpret any capitalized terms or phrases as cross-references.

Affine Mode: a state of the 80287, selected in the 80287 Control Word, in which infinities are treated as having a sign. Thus, the values +INFINITY and - INFINITY are considered different; they can be compared with finite numbers and with each other.

Base: (1) a term used in logarithms and exponentials. In both contexts, it is a number that is being raised to a power. The two equations (y = log base b of x) and (bY = x) are the same.

Base: (2) a number that defines the representation being used for a string of digits. Base 2 is the binary representation; Base 10 is the decimal representation; Base 16 is the hexadecimal representation. In each case, the Base is the factor of increased significance for each succeeding digit (working up from the bottom).

Bias: the difference between the unsigned Integer that appears in the Exponent field of a Floating- Point Number and the true Exponent that it represents. To obtain the true Exponent, you must subtract the Bias from the given Exponent. For example, the Short Real format has a Bias of 127 whenever the given Exponent is nonzero. If the 8-bit Exponent field contains 10000011, which IS 131, the true Exponent is 131-127,or +4.

Biased Exponent: the Exponent as it appears in a Floating-Point Number, interpreted as an unsigned, positive number. In the above example, 131 is the Biased Exponent.

Binary Coded Decimal: a method of storing numbers that retains a base 10 representation. ,Each decimal digit occupies 4 full bits (one hexadecimal digit). The hex values A through F (1010 through 1111) are not used. The 80287~supports a Packed Decimal format that consists of 9 bytes of Binary Coded Decimal (18 decimal digits) and one sign byte.

Binary Point: an entity just like a decimal point, except that it exists in binary numbers. Each binary digit to the right of the Binary Point is multiplied by an increasing negative power of two.

C3-CO: the four "condition code" bits of the 80287 Status Word. These bits are set to certain values by the compare, test, examine, and remainder functions of the 80287.

Characteristic: a term used for some non-Intel computers, meaning the Exponent field of a Floatirtg- Point Number.

Chop: to set the fractional part of a real number to zero, yielding the nearest integer in the direction of zero.

Control Word: a 16-bit 80287 register that the user can set, to determine the modes of computation the 80287 will usc, and the error interrupts that will be enabled.

Glossary-1