Compaq ECQD2KCTE manual Ffloating Load Exponent Mapping Mapf, Gfloating

Table 2–1: F_floating Load Exponent Mapping (MAP_F)

Memory <14:7>	Register <62:52>
1 1111111	1 000 1111111
1 xxxxxxx	1 000 xxxxxxx	(xxxxxxx not all 1’s)
0 xxxxxxx	0 111 xxxxxxx	(xxxxxxx not all 0’s)
0 0000000	0 000 0000000

The F_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.

An F_floating datum is specified by its address A, the address of the byte containing bit 0. The memory form of an F_floating datum is sign magnitude with bit 15 the sign bit, bits <14:7> an excess-128 binary exponent, and bits <6:0> and <31:16> a normalized 24-bit fraction with the redundant most significant fraction bit not represented. Within the fraction, bits of increasing significance are from 16 through 31 and 0 through 6. The 8-bit exponent field encodes the val- ues 0 through 255. An exponent value of 0, together with a sign bit of 0, is taken to indicate that the F_floating datum has a value of 0.

If the result of a VAX floating-point format instruction has a value of zero, the instruction always produces a datum with a sign bit of 0, an exponent of 0, and all fraction bits of 0. Expo- nent values of 1..255 indicate true binary exponents of –127..127. An exponent value of 0, together with a sign bit of 1, is taken as a reserved operand. Floating-point instructions pro- cessing a reserved operand take an arithmetic exception. The value of an F_floating datum is in the approximate range 0.29*10**–38 through 1.7*10**38. The precision of an F_floating datum is approximately one part in 2**23, typically 7 decimal digits. See Section 4.7.

Note:

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

2.2.5.2 G_floating

A G_floating datum in memory is 8 contiguous bytes starting on an arbitrary byte boundary. The bits are labeled from right to left, 0 through 63, as shown in Figure 2–7.

Figure 2–7: G_floating Datum

31	16 15 14		4	3	0
Fraction Midh		S	Exp.	Frac.Hi		:A
Fraction Lo			Fraction Midl			:A+4

2–4Alpha Architecture Handbook