[mythtv-users] Mythbuntu 32bit - how much RAM can it use?

[Simon Hobson]

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

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