Data structure alignment: Difference between revisions

Revision as of 06:58, 29 May 2006

Revision as of 17:52, 31 May 2006

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==Compatibility==

Because the main (only?) advantage to supporting unaligned access is that some memory can be used which would otherwise be lost as padding, and because the cost of supporting it is so high, many modern [[computer architecture]] designs don't support it at all. Most (all?) of the ones that do, require the [[operating system|OS]] to do most of the work. Older designs (notably [[x68]]) stilldo need tonot implement full support in hardware.

Some programs which were written for an architecture that did not enforce aligmnent may use unaligned memory access (either intentionally or not). This can be a problem when porting said programs.

One way to increase 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, but this would result in a slower call to 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. Both platforms ulimately

failed as platforms for hosting Windows applications. This may be

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

==Typical aligment of C structs on x86==