Intel 64 and IA-32 Architectures Software Developers Manual Volume 2A, Instruction Set Reference, A-M

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1.3.2 Reserved Bits and Software Compatibility
In many register and memory layout descriptions, certain bits are marked as
reserved. When bits are marked as reserved, it is essential for compatibility with
future processors that software treat these bits as having a future, though unknown,
effect. The behavior of reserved bits should be regarded as not only undefined, but
unpredictable. Software should follow these guidelines in dealing with reserved bits:
• Do not depend on the states of any reserved bits when testing the values of
registers which contain such bits. Mask out the reserved bits before testing.
• Do not depend on the states of any reserved bits when storing to memory or to a
register.
• Do not depend on the ability to retain information written into any reserved bits.
Figure 1-1. Bit and Byte Order
Byte 3
Data Structure
Byte 1
Byte 2
Byte 0
31
24
23
16
15
8
7
0
Lowest
Bit offset
28
24
20
16
12
8
4
0
Address
Byte Offset
Highest
Address