64 bits native types atomicy and memory bus?

I have read at some point that native types are atomic as long as they are not wider than the memory bus and aligned. I may be very wrong on this, but isn't a memory bus usually 64-bits even for a 32bits CPU (http://en.wikipedia.org/wiki/Front-side_bus)? In which case the above statement would mean that a read or write or an aligned 64bits native type would be atomic right? I looked into intel's documentation and it says x86 is guaranteed to be atomic for 32 bits and below only - but I wanted to make sure I understand the memory bus width part correctly.



The width of the memory bus has nothing whatsoever to do with the atomicity of memory accesses. You could have a one-bit memory bus and still have atomic 64-bit memory accesses by holding the bus through 64 reads or writes. You could have a 64-bit bus but not atomic 64-bit memory accesses if the path between the CPU and the cache was 32-bits and the cache could not be locked.

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