Reserved And Ignored Registers And Fields - Intel ITANIUM ARCHITECTURE - SOFTWARE DEVELOPERS MANUAL VOLUME 1 REV 2.3 Manual

ignore the value written. In variable-sized register sets, registers which are
unimplemented in a particular processor are also reserved registers. An access to one
of these unimplemented registers causes a Reserved Register/Field fault.
Within defined registers, fields which are not defined are either reserved or ignored. For
reserved fields, hardware will always return a zero on a read. Software must always
write zeros to these fields. Any attempt to write a non-zero value into a reserved field
will raise a Reserved Register/Field fault. Reserved fields may have a possible future
use.
For ignored fields, hardware will return a 0 on a read, unless noted otherwise.
Software may write any value to these fields since the hardware will ignore any value
written. Except where noted otherwise some IA-32 ignored fields may have a possible
future use.
Table 3-1
fields.
Table 3-1.
Type
Reserved register
Ignored register
Reserved field
Ignored field
For defined fields in registers, values which are not defined are reserved. Software
must always write defined values to these fields. Any attempt to write a reserved
value will raise a Reserved Register/Field fault. Certain registers are read-only
registers. A write to a read-only register raises an Illegal Operation fault.
When fields are marked as reserved, it is essential for compatibility with future
processors that software treat these fields as having a future, though unknown effect.
Software should follow these guidelines when dealing with reserved fields:
• Do not depend on the state of any reserved fields. Mask all reserved fields before
testing.
• Do not depend on the state of any reserved fields when storing to memory or a
register.
• Do not depend on the ability to retain information written into reserved or ignored
fields.
• Where possible reload reserved or ignored fields with values previously returned
from the same register, otherwise load zeros.
1:24
summarizes how the processor treats reserved and ignored registers and

Reserved and Ignored Registers and Fields

Read
Illegal Operation fault
0
0
0 (unless noted otherwise)
Write
Illegal Operation fault
Value written is discarded
Write of non-zero causes Reserved Reg/Field fault
Value written is discarded
Volume 1, Part 1: Execution Environment