Compaq EV68A specifications Undefined

AlphaSignal[n:n] Boldface, mixed-case type denotes signal names that are assigned internal and external to the 21264/EV68A (that is, the signal traverses a chip interface pin).

AlphaSignal_x[n:n] When a signal has high and low assertion states, a lower- case italic x represents the assertion states. For example, SignalName_x[3:0] represents SignalName_H[3:0] and SignalName_L[3:0].

UNDEFINED

Operations specified as UNDEFINED may vary from moment to moment, implementa- tion to implementation, and instruction to instruction within implementations. The operation may vary in effect from nothing to stopping system operation.

UNDEFINED operations may halt the processor or cause it to lose information. How- ever, 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.

UNPREDICTABLE

UNPREDICTABLE results or occurrences do not disrupt the basic operation of the pro- cessor; it continues to execute instructions in its normal manner. Further:

•Results or occurrences specified as UNPREDICTABLE may vary from moment to moment, implementation to implementation, and instruction to instruction within implementations. Software can never depend on results specified as UNPREDICT- ABLE.

•An UNPREDICTABLE result may acquire an arbitrary value subject to a few con- straints. Such a result may 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 may also produce exceptions.

•An occurrence specified as UNPREDICTABLE may 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 may produce UNPREDICTABLE results must not:

–Write or modify the contents of memory locations or registers to which the cur- rent process in the current access mode does not have access, or

–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.

xxii 21264/EV68A Hardware Reference Manual