Does unaligned memory access always cause bus errors?

Question

According to the Wikipedia page Segmentation fault, a bus error can be caused by unaligned memory access. The article gives an example about how to trigger a bus er

Answer 1

It very much depends on the chip architecture. x86 and POWER are very forgiving, Sparc, Itanium and VAX throw different exceptions.

Answer 2

Consider the following example I have just tested on ARM9:

//Addresses       0     1     2    3     4     5     6     7      8    9
U8 u8Temp[10] = {0x11,0x22, 0x33, 0x44, 0x55, 0x66, 0x77, 0x88, 0x99, 0x00};

U32 u32Var;

u32Var = *((U32*)(u16Temp+1));  // Let's read four bytes starting from 0x22

// You would expect that here u32Var will have a value of 0x55443322 (assuming we have little endian)
// But in reallity u32Var will be 0x11443322!
// This is because we are accessing address which %4 is not 0.

Answer 3

1. It may be 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.