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Little-endian. A byte-ordering method in memory where the address n of a
word corresponds to the least-significant byte. In an addressed
memory word, the bytes are ordered (left to right) 3, 2, 1,0, with 3
being the most-significant byte. See Big-endian.
M
MESI (modifiedlexclusivelsharedlinvalid). Cache coherency protocol used
to manage caches on different devices that share a memory system.
Note that the PowerPC architecture does not specify the
implementation of a MESI protocol to ensure cache coherency.
Memory access ordering. The specific order in which the processor
performs load and store memory accesses and the order in which
those accesses complete.
Memory-mapped accesses. Accesses whose addresses use the page or block
address translation mechanisms provided by the MMU and that
occur externally with the bus protocol defined for memory.
Memory coherency. An aspect of caching in which it is ensured that an
accurate view of memory is provided to all devices that share system
memory.
Memory consistency. Refers to agreement of levels of memory with respect
to a single processor and system memory (for example, on-chip
cache, secondary cache, and system memory).
Memory management unit (MMU). The functional unit that is capable of
translating an effective (logical) address to a physical address,
providing protection mechanisms, and defining caching methods.
Modified state. MEl state (M) in which one, and only one, caching device
has the valid data for that address. The data at this address in external
memory is not valid. See MESI.
Most-significant bit (msb). The highest-order bit in an address, registers,
data element, or instruction encoding.
Most-significant byte (MSB). The highest-order byte in an address,
registers, data element, or instruction encoding.
N
NaN. An abbreviation for not a number; a symbolic entity encoded in
floating-point format. There are two types ofNaNs-signaling NaNs
and quiet NaNs.
No-op. No-operation. A single-cycle operation that does not affect registers
or generate bus activity.
Glossary of Terms and Abbreviations
Glossary-7
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