Intel 80286, 80287 manual Infinity, positive

OVERVIEW OF NUMERIC PROCESSING

Table 1-4. Interpreting the NPX Condition Codes

Instruction	C.	C.	C,	C.	Interpretation
Type
Compare, Test	0	0	X	0	ST > Source or 0 (FTST)
	0	0	X	1	ST < Source or 0 (FTST)
	1	0	X	0	ST = Source or 0 (FTST)
	1	1	X	1	ST is not comparable
Remainder	Q,	0	Q.	Q.	Complete reduction with three
					low bits of quotient in Co, C3 ,
					and C,
	U	1	U	U	Incomplete Reduction
Examine	0	0	0	0	Valid, positive unnormalized
	0	0	0	1	Invalid, positive, exponent =	0
	0	0	1	0	Valid, negative, un normalized
	0	0	1	1	Invalid, negative, exponent = 0
	0	1	0	0	Valid, positive, normalized
	0	1	0	1	Infinity, positive
	0	1	1	0	Valid, negative, normalized
	0	1	1	1	Infinity, negative
	1	0	0	0	Zero, positive
	1	0	0	1	Empty Register
	1	0	1	0	Zero, negative
	1	0	1	1	Empty Register
	1	1	a	0	Invalid, positive, exponent =	0
	1	1	0	1	Empty Register
	1	1	1	0	Invalid, negative, exponent = 0
	1	1	1	1	Empty Register

NOTES:

1.ST = Top of stack

2.X = value is not affected by instruction

3.U = value is undefined following instruction

4.Qn = Quotient bit n following complete reduction (C.=O)

The low-order byte of this control word configures the 80287 error and exception masking. Bits 0-5 of the control word contain individual masks for each of the six exception conditions recognized by the 80287. The high-order byte of the control word configures the 80287 processing options, including

•Precision control

•Rounding control

•Infinity control

The Precision control bits (bits 8-9) can be used to set the 80287 internal operating precision at less than the default precision (64-bit significand). These control bits can be used to provide compatibility with the earlier-generation arithmetic processors having less precision than the 80287, as required by the IEEE 754 standard. Setting a lower precision, however, will not affect the execution time of numeric calculations.

The rounding control bits (bits 10-11) provide for directed rounding and true chop as well as the unbiased round-to-Ilearest-even mode specified in the IEEE 754 standard.

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