> As far as I know, one of the biggest 'wins' for 64bit is in > addressing, for anything above a 32 bit address space (more than 4gb). > > You have to have pae support on a 32bit proc (and enable it in the os) > to hit more than 4gb, which does some type of mapping from 64 bit onto > a 32 bit address space. Memory addresses have to be translated, and > not a straightforward mapping. Memory access becomes a bit slower. > Add to this that the mapping can be different between processes. So if > you have processes sharing memory (ie for ipc), you get even more > overhead as it has to be translated, then translated again, to be > referenced by each site of a shared memory block. > > more about pae: http://en.wikipedia.org/wiki/Physical_Address_Extension > > 64 bit has some downsides. I think the hybrid x86_64 chips have weird > pipelines to the cpu to support both 32bit and 64bit mode..where it > does some weird split into two 32bit words, and uses part of each > chunk to tell which mode it is in.. thus reducing you by addressible > memory by some factor. Not sure about this one..not really that much > of a hardware guy. > > Also, the word size is larger, so there is some overhead. Memory > pointers take up more space, etc. So random app X might use a bit more > ram on 64bit than on 32bit. *shrug* > > I am sure some of the above is off, and I am sure the more hardware > centric folks in the audience will point out my inaccuracies. >
oh. also.. http://en.wikipedia.org/wiki/64-bit
