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→Sub-word and Unaligned Access
== Sub-word and Unaligned Access ==
Most processors use a [[Word|word]] size that is multiple of width of some common data types -- for example, a system with a 32-bit [[Word#Hardware Word|hardware word size]] that is running applications which use UTF-8 character encoding may often need to read or write single [[Word#Byte|bytes]] of data. A byte-sized read will cause the CPU to perform a 32-bit read, followed by [[Bitwise Operations|masking and shifting operations]] to extract the desired byte from the 32-bit word. A single-byte write operation will cause the CPU to read the existing word, extract the unaffected bits within that word (with [[Bitwise Operations#AND|AND]]), [[Bitwise Operations#OR|OR]] in the new value, and then write the word back to memory.
Unaligned memory access causes similar issues. For example, to read a 32-bit value from the byte address 0x2, most 32-bit CPUs will read a 32-bit value starting at byte address 0x0 and perform an [[Bitwise Operations|AND]] to extract highest 16 bits, then a shift to move those bits to the lowest 16 bit positions. The CPU will then read a 32-bit value starting at byte address 0x4, perform an AND to extract the lowest 16 bits, shift those bits to the highest 16 bit positions, and then OR the high 16 bits and the low 16 bits together into a single 32 bit value. (Writing an unaligned 32-bit value is even worse!)
Note that even on systems that perform alignment fixups in hardware, unaligned access is [[Expensive|expensive]].
Note also that the native word size of the CPU may be different from the physical word size of main memory; for example, some 32-bit systems have a 64-, 128-, or 256-bit-wide memory configuration.
== Processor-specific Optimizations ==
