Compaq ECQD2KCTE manual Tfloating Datum

This mapping preserves both normal values and exceptional values. Note that the mapping for all 1’s differs from that of F_floating load, since for S_floating all 1’s is an exceptional value and for F_floating all 1’s is a normal value.

The S_floating store instruction reorders register bits on the way to memory and does no checking of the low-order fraction bits. Register bits <61:59> and <28:0> are ignored by the store instruction. The S_floating load instruction does no checking of the input.

The S_floating store instruction does no checking of the data; the preceding operation should have specified an S_floating result.

An S_floating datum is specified by its address A, the address of the byte containing bit 0. The memory form of an S_floating datum is sign magnitude with bit 31 the sign bit, bits <30:23> an excess-127 binary exponent, and bits <22:0> a 23-bit fraction.

The value (V) of an S_floating number is inferred from its constituent sign (S), exponent (E), and fraction (F) fields as follows:

•If E=255 and F<>0, then V is NaN, regardless of S.

•If E=255 and F=0, then V = (–1)**S x Infinity.

•If 0 < E < 255, then V = (–1)**S x 2**(E–127) x (1.F).

•If E=0 and F<>0, then V = (–1)**S x 2**(–126) x (0.F).

•If E=0 and F=0, then V = (–1)**S x 0 (zero).

Floating-point operations on S_floating numbers may take an arithmetic exception for a vari- ety of reasons, including invalid operations, overflow, underflow, division by zero, and inexact results.

Note:

Alpha implementations will impose a significant performance penalty when accessing S_floating operands that are not naturally aligned. (A naturally aligned S_floating datum has zero as the low-order two bits of its address.)

2.2.6.2 T_floating

An IEEE double-precision, or T_floating, datum occupies 8 contiguous bytes in memory start- ing on an arbitrary byte boundary. The bits are labeled from right to left, 0 through 63, as shown in Figure 2–13.

Figure 2–13: T_floating Datum

31 30	20 19	0
		Fraction Lo
S	Exponent	Fraction Hi

:A

:A+4

2–8Alpha Architecture Handbook