[mythtv-users] Mythbuntu 32bit - how much RAM can it use?
Simon Hobson
linux at thehobsons.co.uk
Sun Dec 18 08:44:46 UTC 2011
Martin Moores wrote:
> > I moved my workstation motherboard into my mythbox, so now the
>32-bit OS has
>> access to 8G ram.
> > However it only "sees" 2.5G
>As far as I know, the technical limit of 3GB applies to Linux OS's as
>well as Windows.
Close. They can access a full 4GB of address space, but some of that
is taken up with ROMs, memory mapped I/O, video, etc, etc. How much
is lost depends very much on what's in the machine (especially
chipset), but it generally restricts you to about 3GB give or take a
bit. IIRC some Intel chipsets should leave you with about 3.2G or RAM.
While it may seem wasteful, if the chipset supports memory
interleaving, then you may get a faster machine by fitting 4G (2x 2G
or 4x 1G, matching specs) than by fitting 3G (2G + 1G, or 3x 1G).
This is because some chipsets support (if you have the right
combination of ram) interleaving memory usage - word 0 in A, word 1
in B, word 2 inA, and so on. Thus when reading sequential words (as
it very common), the chipset can setup the address for both banks of
ram, and the delay fetching word1, word3, etc is very much less than
having to reset the address after reading word0, word2, etc.
Address setup and settling time is major limitation in memory access times.
--
Simon Hobson
Visit http://www.magpiesnestpublishing.co.uk/ for books by acclaimed
author Gladys Hobson. Novels - poetry - short stories - ideal as
Christmas stocking fillers. Some available as e-books.
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