Specifications

xvi
Subject to Change – 17 January 1997
Operations that produce UNPREDICTABLE results might also produce
exceptions.
– An occurrence specified as UNPREDICTABLE may or may not happen
based on an arbitrary choice function. The choice function is subject to the
same constraints as are UNPREDICTABLE results and 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.
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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.