Does unaligned memory access always cause bus errors?

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 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 errors or other weird results to me. If we disable alignment checking, what is the side effect of unaligned memory?

Platforms: I am working on x86/x86-64. I also tried my program by compiling it with "gcc -arch ppc" on a Mac and it works properly.

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.

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

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.