Intel 80286, 80287 manual Temporary Real Encodings Contd, Invalid Operation, Zero Divisor

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OVERVIEW OF NUMERIC PROCESSING

Table 1-16. Temporary Real Encodings (Cont'd.)

ClassSign

BiasedSignificand·

Exponentl,:lff...ff

		00	1	11 ... 11	100... 00
			1	11 ... 11	100... 00
			·	·	·
Q)			·	·	·
.~			·	·	·
III
iii	~I
z
Cl		Indefinite	1	11 ... 11	110...00
Q)
			·	0	0
			·	·	·
			1·	11	111 ... 11
				11 ...·	·

1..--15 bits ---.11-4..--- 64 bits --~,.~I

Numeric Exceptions

Whenever the 80287 NPX attempts a numeric operation with invalid operands or produces a result that cannot be represented, the 80287 recognizes a numeric exception condition. Altogether, the 80287 checks for the following six classes of exceptions while executing numeric instructions:

1.Invalid operation

2.Divide-by-zero

3.Denormalized operand

4.Numeric overflow

5.Numeric underflow

6.Inexact result (precision)

INVALID OPERATION

The 80287 reports an invalid operation if any of the following occurs:

•An attempt to load a register that is not empty (stack overflow).

•An attempt to pop an operand from an empty register (stack underflow).

•An operand is a NaN.

•The operands cause the operation to be indeterminate (square root of a negative number, 0/0).

An invalid operation generally indicates a program error.

ZERO DIVISOR

If an instruction attempts to divide a finite nonzero operand by zero, the 80287 will report a zero divide exception.

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