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Data structure alignment is the way data is arranged and accessed in computer memory. It consists of two separate but related issues: ''data alignment'' and ''data structure padding''. When a modern computer reads from or writes to a memory address, it will do this in word sized chunks (e.g. 4 byte chunks on a 32-bit system) or larger. ''Data alignment'' means putting the data at a memory address equal to some multiple of the word size, which increases the system's performance due to the way the CPU handles memory. To align the data, it may be necessary to insert some meaningless bytes between the end of the last data structure and the start of the next, which is ''data structure padding''. For example, when the computer's word size is 4 bytes (a byte means 8 bits on most machines, but could be different on some systems), the data to be read should be at a memory address which is some multiple of 4. When this is not the case, e.g. the data starts at address 14 instead of 16, then the computer has to read two or more 4 byte chunks and do some calculation before the requested data has been read, or it may generate an alignment fault. Even though the previous data structure end is at address 13, the next data structure should start at address 16. Two padding bytes are inserted between the two data structures at addresses 14 and 15 to align the next data structure at address 16. Although data structure alignment is a fundamental issue for all modern computers, many computer languages and computer language implementations handle data alignment automatically. Ada, PL/I, certain C and C++ implementations, D,〔(【引用サイトリンク】 title = Attributes - D Programming Language: Align Attribute )〕 and assembly language allow at least partial control of data structure padding, which may be useful in certain special circumstances. ==Definitions== A memory address ''a'', is said to be ''n-byte aligned'' when ''a'' is a multiple of ''n'' bytes (where ''n'' is a power of 2). In this context a byte is the smallest unit of memory access, i.e. each memory address specifies a different byte. An ''n''-byte aligned address would have ''log2(n)'' least-significant zeros when expressed in binary. The alternate wording ''b-bit aligned'' designates a ''b/8 byte aligned'' address (ex. 64-bit aligned is 8 bytes aligned). A memory access is said to be ''aligned'' when the datum being accessed is ''n'' bytes long and the datum address is ''n''-byte aligned. When a memory access is not aligned, it is said to be ''misaligned''. Note that by definition byte memory accesses are always aligned. A memory pointer that refers to primitive data that is ''n'' bytes long is said to be ''aligned'' if it is only allowed to contain addresses that are ''n''-byte aligned, otherwise it is said to be ''unaligned''. A memory pointer that refers to a data aggregate (a data structure or array) is ''aligned'' if (and only if) each primitive datum in the aggregate is aligned. Note that the definitions above assume that each primitive datum is a power of two bytes long. When this is not the case (as with 80-bit floating-point on x86) the context influences the conditions where the datum is considered aligned or not. Data structures can be stored in memory on the stack with a static size known as ''bounded'' or on the heap with a dynamic size known as ''unbounded''. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Data structure alignment」の詳細全文を読む スポンサード リンク
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