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Introduction 1–7
1.6.1 Numbering
All numbers are decimal unless otherwise indicated. Where there is ambiguity, numbers other
than decimal are indicated with the name of the base in subscript form, for example, 10
16.
1.6.2 Security Holes
A security hole is an error of commission, omission, or oversight in a system that allows pro-
tection mechanisms to be bypassed.
Security holes exist when unprivileged software (software running outside of kernel mode)
can:
• Affect the operation of another process without authorization from the operating sys-
tem;
• Amplify its privilege without authorization from the operating system; or
• Communicate with another process, either overtly or covertly, without authorization
from the operating system.
The Alpha architecture has been designed to contain no architectural security holes. Hardware
(processors, buses, controllers, and so on) and software should likewise be designed to avoid
security holes.
1.6.3 UNPREDICTABLE and UNDEFINED
The terms UNPREDICTABLE and UNDEFINED are used throughout this book. Their mean-
ings are quite different and must be carefully distinguished.
In particular, only privileged software (software running in kernel mode) can trigger UNDE-
FINED 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;
it continues to execute instructions in its normal manner. In contrast, UNDEFINED operation
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 may vary from moment to
moment, implementation to implementation, and instruction to instruction within
implementations. Software can never depend on results specified as UNPREDICT-
ABLE.
• An UNPREDICTABLE result may acquire an arbitrary value subject to a few con-
straints. Such a result may be an arbitrary function of the input operands or of any state
information that is accessible to the process in its current access mode. UNPREDICT-
ABLE results may be unchanged from their previous values.