[H-GEN] Tag RAM and lots of memory

[Matthew Tippett]

Martin Pool wrote:
> 
> I read something a while ago which more or less said:
> 
>    If you put more than 64MB in a PC, you have to buy more tag
>    RAM/cache/something else or it will slow right down.  PCs
>    can't normally cache the memory above 64MB.
> 
> Perhaps somebody on the list with a studly machine or a bit of general
> knowledge can enlighten me?
> 
> Martin Pool
I don't have a studly machine, but....

How much cache you need for a given amount of memory depends
on your MB manufacturer.  Most that I have seen let 256k
support
up to 64 MB.  

Some newer motherboards have 512k on by default.  You should
be able to get the person who sold you the board supply the
information from their suppliers.  It may also be in the
motherboard manual.

WR2 TAG RAM memory, that typically is associated with the
amount
of cache you have.  (IIRC it is used to say what is dirty
and what
is clean).  If you get cache modules you will get it as a
all in
one package (as in ASUS boards for example).  If you need to
get
individual chips then follow your motherboard manual's
directions.

Now for the theory behind it all. 

On a 486 when you press the turbo button you generally
disable 
the cache (the 486 doesn't like having it's clock speed
changed).
It runs like a dog without the cache (makes you wonder how 
much of performance increase there was between the 386 and
486
apart from the cache size...).

When you have more memory than the cache architecture is
designed
to handle you start to get misses (read the system gets the 
data from the memory, not the cache).  This has a similar
effect to have little or no cache.  Add more cache memory
and then the misses go down and things go back to normal.

So in reality it doesn't slow things down, it just doesn't
work
efficiently.  I could come up with some interesting
analogies if 
people are interested.  

WR2 the last line PC's can't normally cache above 64M.  That
would
most likely mean an 'out of the box PC in its standard 
configuration' can't cache above 64M.

All in all get you MB manual, check in that, check out you
MB 
makers website or beat the door down of the person you
bought
the system off.

Hope this helps.

Matt
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