DEC Digital Alpha VME 4/224 User Manual page 26

Syntax
The following syntax elements are used throughout this manual. Do not type the
syntax elements when entering information.
Element Example
[ ] [-file filename]
| - | + | =
{} {- | + | =}
( ) —
. . . arg . . .
UNPREDICTABLE and UNDEFINED
In this manual, the terms UNPREDICTABLE and UNDEFINED are used. Their
meanings are different and must be carefully distinguished.
In particular, only privileged software (that is, software running in kernel mode)
can trigger UNDEFINED operations. Unprivileged software cannot trigger
UNDEFINED operations. However, either privileged or unprivileged software can
trigger UNPREDICTABLE results or occurrences.
UNPREDICTABLE results or occurrences do not disrupt the basic operation
of the processor. The processor continues to execute instructions in its normal
manner. In contrast, UNDEFINED operations can halt the processor or cause it
to lose information.
The terms UNPREDICTABLE and UNDEFINED can be further described as
follows:
• UNPREDICTABLE
Results or occurrences specified as UNPREDICTABLE might vary from
moment to moment, implementation to implementation, and instruction
to instruction within implementations. Software can never depend on
results specified as UNPREDICTABLE.
An UNPREDICTABLE result might acquire an arbitrary value subject
to a few constraints. Such a result might be an arbitrary function of the
input operands or of any state information that is accessible to the process
in its current access mode. UNPREDICTABLE results may be unchanged
from their previous values.
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Description
The enclosed items are optional.
Choose one of two or more items. Select
one of the items unless the items are
optional.
You must specify one (and only one) of the
enclosed items.
You must specify the enclosed items
together.
You can repeat the preceding item one or
more times.