Data structure alignment: Difference between revisions

Revision as of 14:28, 9 May 2006

Revision as of 21:31, 23 May 2006

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A common problem in computer programming is called word alignement.

One way to increase performance, especially in [[RISC]] processors which

are designed to maximize raw performance is to require data to loaded or

stored on a word boundary. So though memory is commonly addressed by

8 bit bytes, loading a 32 bit integer or 64 bit floating point number

would be required to be start at every 64 bits on a 64 bit machine.

The processor could flag a fault if it were asked to load a number

which was not on such a boundary, or call a routine which would effectively

figure out which word or words contained the data and extract the equivalent

value. This caused difficulty when the team from [[Mosaic Software]] ported

their [[Twin Spreadsheet]] to the [[68000]] based Atari. The Intel [[8086]]

architecture had no such restrictions. 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.

However since this bit was masked with other flags, it was impossible to

keep the O/S from faulting on non-aligned data, and both platforms ulimately

failed as platforms for hosting Windows applications. This is a classic

instance of programming convenience winning out over careful design for speed, and

one reason for the continued dominance of the x86 architecture over rivals.

== Technical View==

[[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: