Data structure alignment: Difference between revisions

Revision as of 17:56, 31 May 2006

Revision as of 20:34, 1 June 2006

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== Definition ==

'''Data structure alignment''' is the way [[Data (computing)|data]] is arranged in [[physical memory]].

== Definition ==

An address ''a'', is said to be ''n''-byte aligned when ''a'' is a multiple of ''n'' bytes.

== Problems ==

Because of the way [[computer memory]] works, it is highly desirable for all memory accesses to meet certain alignment requirements. As a rule of thumb, the alignment for a primitive data type should be the same as the size of the data to be accessed, rounded up to a power of two. This avoids crossing any [[word (computer science)|word]], [[cache-line]], or [[paging|page]] boundaries.

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==Compatibility==

The advantage to supporting unaligned access is that is is easier to write compilers that

do not need to align memory, at the expense of the cost of slower access.

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architecture had no such restrictions.

It would also cause difficulties in porting Microsoft Office to Windows NT on [[MIPS]]

It would also

cause difficulties in porting Microsoft Office to Windows NT on [[MIPS]]

and [[PowerPC]] for [[NEC]] and [[IBM]]. Since the software was not written with such restrictions

in mind, designers had to set a bit in the O/S to enable non-aligned data.

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==Typical aligment of C structs on x86==

[[Data structure]] members are stored sequentially in a memory so that in the structure below the member Data1 will always precede Data2 and Data2 will always precede Data3:

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*[http://www-306.ibm.com/chips/techlib/techlib.nsf/techdocs/852569B20050FF778525699600719DF2 PowerPC Microprocessor Family: The Programming Environments for 32-Bit Microprocessors]

[[Category:ComputerData programmingstructures]]

[[Category:ComputerCompiler sciencetheory]]