User`s guide

Data Units

The following data unit terminology, common within Digital, is used throughout

this guide:

Term Words Bytes Bits

Word 1 2 16

Longword 2 4 32

Quadword 4 8 64

Octaword 8 16 128

Hexword 16 32 256

Note

Notes provide additional information.

Numbering

All numbers are decimal or hexadecimal unless otherwise indicated. In case

of ambiguity, a subscript indicates the radix of nondecimal numbers. For

example, 19 is a decimal number, but 19

and 19A are hexadecimal numbers.

UNPREDICTABLE and UNDEFINED

Throughout this manual, the terms UNPREDICTABLE and UNDEFINED are

used. Their meanings are quite 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.