IEEE Floating-Point Conformance

The 21264/EV67 does not produce a denormal result for the underflow exception. Instead, a true zero (+0) is written to the destination register. In the 21264/EV67, the FPCR underflow to zero (UNDZ) bit must be set if the underflow disable (UNFD) bit is set. If desired, trapping on underflow can be enabled by the instruc- tion and the FPCR, and software may compute the denormal value as defined in the IEEE standard.

The 21264/EV67 records floating-point exception information in two places:

The FPCR status bits record the occurrence of all exceptions that are detected, whether or not the corresponding trap is enabled. The status bits are cleared only through an explicit clear command (MT_FPCR); hence, the exception information they record is a summary of all exceptions that have occurred since the last time they were cleared.

If an exception is detected and the corresponding trap is enabled by the instruction, and is not disabled by the FPCR control bits, the 21264/EV67 will record the condition in the EXC_SUM register and initiate an arithmetic trap.

The following items apply to Table A–11:

The 21264/EV67 traps on a denormal input operand for all arithmetic operations unless FPCR[DNZ] = 1.

Input operand traps take precedence over arithmetic result traps.

The following abbreviations are used: Inf: Infinity

QNaN: Quiet NaN

SNaN: Signalling NaN

CQNaN: Canonical Quiet NaN

For IEEE instructions with /S, Table A–11lists all exceptional input and output conditions recognized by the 21264/EV67, along with the result and exception gen- erated for each condition.

Table A–11 Exceptional Input and Output Conditions

 

21264/EV67 Hardware

 

Alpha Instructions

Supplied Result

Exception

 

 

 

ADDx SUBx INPUT

 

 

 

 

 

Inf operand

±Inf

(none)

QNaN operand

QNaN

(none)

SNaN operand

QNaN

Invalid Op

Effective subtract of two Inf operands

CQNaN

Invalid Op

 

 

 

ADDx SUBx OUTPUT

 

 

 

 

 

Exponent overflow

±Inf or ±MAX

Overflow

Exponent underflow

+0

Underflow

Inexact result

Result

Inexact

Alpha 21264/EV67 Hardware Reference Manual

Alpha Instruction Set A–15

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Compaq 21264, EV67 specifications Table A-11 Exceptional Input and Output Conditions, Alpha Instruction Set A-15