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Basic Architecture 2–5
A G_floating operand occupies 64 bits in a floating register, arranged as shown in Figure 2–8.
Figure 2–8: G_floating Register Format
A G_floating datum is specified by its address A, the address of the byte containing bit 0. The
form of a G_floating datum is sign magnitude with bit 15 the sign bit, bits <14:4> an excess-
1024 binary exponent, and bits <3:0> and <63:16> a normalized 53-bit fraction with the redun-
dant most significant fraction bit not represented. Within the fraction, bits of increasing
significance are from 48 through 63, 32 through 47, 16 through 31, and 0 through 3. The 11-bit
exponent field encodes the values 0 through 2047. An exponent value of 0, together with a sign
bit of 0, is taken to indicate that the G_floating datum has a value of 0.
If the result of a floating-point 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. Exponent values of
1..2047 indicate true binary exponents of –1023..1023. An exponent value of 0, together with a
sign bit of 1, is taken as a reserved operand. Floating-point instructions processing a reserved
operand take a user-visible arithmetic exception. The value of a G_floating datum is in the
approximate range 0.56*1 0**–308 through 0.9*10**308. The precision of a G_floating datum
is approximately one part in 2**52, typically 15 decimal digits. See Section 4.7.
Note:
Alpha implementations will impose a significant performance penalty when accessing
G_floating operands that are not naturally aligned. (A naturally aligned G_floating datum
has zero as the low-order three bits of its address.)
2.2.5.3 D_floating
A D_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 2–9.
Figure 2–9: D_floating Datum
A D_floating operand occupies 64 bits in a floating register, arranged as shown in Figure 2–10.
Figure 2–10: D_floating Register Format
0
6362
S
32 31
Exp. Fraction Hi Fraction Lo :Fx
52 51
S
Exp. Frac.HiFraction Midh :A
:A+4Fraction MidlFraction Lo
60
715
16 14
31
0
6362
S
4847 32 31 1615
Exp. Fraction Midh Fraction Midl Fraction Lo :Fx
5554
Frac. Hi