DEC Digital Alpha VME 4/224 User Manual page 27

Operations that produce UNPREDICTABLE results might also produce
exceptions.
An occurrence specifed 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. They
must also not 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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