User`s guide

– 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.

Operations that produce UNPREDICTABLE results might also produce

exceptions.

– An occurrence speciﬁed as UNPREDICTABLE might happen or not

based on an arbitrary choice function. The choice function is subject

to the same constraints as are UNPREDICTABLE results and, in

particular, must not constitute a security hole.

Speciﬁcally, UNPREDICTABLE results must not depend upon, or be

a function of the contents of memory locations or registers that are

inaccessible to the current process in the current access mode.

Also, operations that might produce UNPREDICTABLE results must

not:

• Write or modify the contents of memory locations or registers to

which the current process in the current access mode does not have

access.

• Halt or hang the system or any of its components.

For example, a security hole would exist if some UNPREDICTABLE

result depended on the value of a register in another process, on

the contents of processor temporary registers left behind by some

previously running process, or on a sequence of actions of different

processes.

• UNDEFINED

– Operations speciﬁed as UNDEFINED can vary from moment to

moment, implementation to implementation, and instruction to

instruction within implementations. The operation can vary in effect

from nothing, to stopping system operation.

– UNDEFINED operations can halt the processor or cause it to lose

information. However, UNDEFINED operations must not cause the

processor to hang, that is, reach an unhalted state from which there

is no transition to a normal state in which the machine executes

instructions. Only privileged software (that is, software running in

kernel mode) can trigger UNDEFINED operations.

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