Does unaligned memory access always cause bus errors?

Question

According to this wiki page, bus error can be caused by unaligned memory access. The wiki page gives an example about how to trigger a bus error. In the example, we have to enable alignment checking to see the bus error. What if we disable such alignment checking? The program seems to work properly. I have a program access unaligned memory frequently and it is used by quite a few people, but no one reports bus error or other weird results to me. The question is if we disable alignment checking, what is the side effect of unaligned memory?

Answer 1

1. It is significantly slower to access unaligned memory (as in, several times slower).

2. Not all platforms even support unaligned access - x86 and x64 do, but ia64 (Itanium) does not, for example.

3. A compiler can emulate unaligned access (VC++ does that for pointers declared as __unaligned on ia64, for example) - by inserting additional checks to detect the unaligned case, and loading/storing parts of the object that straddle the alignment boundary separately. That is even slower than unaligned access on platforms which natively support it, however.

Answer 2

Very much depends on the chip architecture x86 and Power are very forgiving, Sparc, Itanium and VAX through different exceptions.