Data structure alignment: Difference between revisions

Revision as of 07:54, 14 December 2006

Revision as of 15:19, 17 December 2006

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The advantage to supporting unaligned access is that it is easier to write compilers that do not need to align memory, at the expense of the cost of slower access. One way to increase performance in [[RISC]] processors which are designed to maximize raw performance is to require data to be 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 need to 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 ST]]. The Intel [[8086]] architecture had no such restrictions. {{fact}}

It would also cause difficulties in porting Microsoft Office to Windows NT on [[MIPS]], [[DEC Alpha|Alpha]] and [[PowerPC]] for [[NEC]], [[Digital Equipment Corporation|DEC]] and [[IBM]] respectively. Since the software was not written with such restrictions in mind, designers had to set a bit in the operating system to enable non-aligned data. However since this bit was masked with other flags which were used elsewhere, it was impossible to keep the operating system in a state from faulting on non-aligned data. These platforms ultimately failed as platforms for hosting Windows applications. {{fact}}

==Typical alignment of C structs on x86==