Glossary
e200z3 Power Architecture Core Reference Manual, Rev. 2
Glossary-10
Freescale Semiconductor
Throughput. The measure of the number of instructions that are processed per clock
cycle.
U
Underflow. A condition that occurs during arithmetic operations when the result cannot
be represented accurately in the destination register. For example, underflow can
happen if two floating-point fractions are multiplied and the result requires a
smaller exponent and/or mantissa than the single-precision format can provide. In
other words, the result is too small to be represented accurately.
User mode. The operating state of a processor used typically by application software. In
user mode, software can access only certain control registers and can access only
user memory space. No privileged operations can be performed. Also referred to
as problem state.
V
VEA (virtual environment architecture). The level of the architecture that describes the
memory model for an environment in which multiple devices can access memory,
defines aspects of the cache model, defines cache control instructions, and defines
the time-base facility from a user-level perspective. Implementations that conform
to the PowerPC VEA also adhere to the UISA, but may not necessarily adhere to
the OEA.
Virtual address. An intermediate address used in the translation of an effective address to
a physical address.
Virtual memory. The address space created using the memory management facilities of
the processor. Program access to virtual memory is possible only when it coincides
with physical memory.
W
Way. A location in the cache that holds a cache block, its tags and status bits.
Word. A 32-bit data element.
Write-back. A cache memory update policy in which processor write cycles are directly
written only to the cache. External memory is updated only indirectly, for
example, when a modified cache block is cast out to make room for newer data.
Write-through. A cache memory update policy in which all processor write cycles are
written to both the cache and memory.
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