[arch-dev-public] Processor Generation++

eliott eliott at cactuswax.net
Tue Mar 18 23:07:29 EDT 2008

> 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..

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