
Glossary-14
MPC7400 RISC Microprocessor Users Manual
Transient memory
. Memory that has poor locality and is likely to be
referenced very few times or over a very short period of time.
UISA (user instruction set architecture)
. The level of the architecture to
which user-level software should conform. The UISA deTnes the
base user-level instruction set, user-level registers, data types,
oating-point memory conventions and exception model as seen by
user programs, and the memory and programming models.
Underow
. A condition that occurs during arithmetic operations when the
result cannot be represented accurately in the destination register.
For example, underow can happen if two oating-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.
VEA (virtual environment architecture)
. The level of the
architecture
that
describes the memory model for an environment in which multiple
devices can access memory, deTnes aspects of the cache model,
deTnes cache control instructions, and deTnes 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.
Vector
. The spatial parallel processing of short, Txed-length, one-
dimensional matrices performed by an execution unit.
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
.
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
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