Compaq AlphaPC 264DP Technical Reference Manual page 14

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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 con-

stitute a security hole.

Specifically, UNPREDICTABLE results must not depend upon, or be a

function of, the contents of memory locations or registers that are inacces-

sible 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 con-

tents of processor temporary registers left behind by some previously run-

ning process, or on a sequence of actions of different processes.

UNDEFINED

•

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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 stop-

ping system operation.

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UNDEFINED operations can halt the processor or cause it to lose infor-

mation. 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 privi-

leged software (that is, software running in kernel mode) can trigger

UNDEFINED operations.

xiv

12 February 1999 – Subject To Change

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