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1–8 Alpha Architecture Handbook
Operations that produce UNPREDICTABLE results may also produce exceptions.
• An occurrence specified as UNPREDICTABLE may happen or not based on an arbi-
trary 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 current
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.
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.
1.6.4 Ranges and Extents
Ranges are specified by a pair of numbers separated by two periods and are inclusive. For
example, a range of integers 0..4 includes the integers 0, 1, 2, 3, and 4.
Extents are specified by a pair of numbers in angle brackets separated by a colon and are inclu-
sive. For example, bits <7:3> specify an extent of bits including bits 7, 6, 5, 4, and 3.
1.6.5 ALIGNED and UNALIGNED
In this document the terms ALIGNED and NATURALLY ALIGNED are used interchange-
ably to refer to data objects that are powers of two in size. An aligned datum of size 2**N is
stored in memory at a byte address that is a multiple of 2**N, that is, one that has N low-order
zeros. Thus, an aligned 64-byte stack frame has a memory address that is a multiple of 64.
If a datum of size 2**N is stored at a byte address that is not a multiple of 2**N, it is called
UNALIGNED.