[Re: RAM question]

[Matt Graham]

Ben Pfaff <blp@cs.stanford.edu> wrote:

> Do typical modern motherboards have a cache?  I didn't realize
> that there was another level of caching beyond the 512 kB or so
> on the chip.

The last x86 motherboards that had L2 cache on them that I'm aware of were the
Super 7 ones (since not all the K6-?s had L2 cache on-chip, it had to go
*somewhere* in those boards.)  The problem that Ed/Jo were referring to arose
because of other design flaws, namely that the processor and/or chipset didn't
always know which pages of main memory were in the processor's caches.  This
information was stored in the "cache tag RAM", a small block of RAM on the
motherboard that was (usually) not upgradable.  

Pages of main memory were mapped to this cache tag RAM in a fixed way,
designed to minimize performance problems under DOS/Doze if someone put more
RAM in the board than the cache tag RAM could handle.  If you had a small
block of cache tag RAM, only 64M could get cached, and it would be the "lower"
64M if you had, say, 80M.  Since Linux puts the frequently-accessed kernel
data structures at "high" physical addresses, these structures would never be
cached, making Linux very slow in these circumstances.  (DOS and its
descendants apparently fill system RAM from the "bottom" up, so the important
kernel qoutines and structs would always be cacheable in DOS on one of these
boards.)

Information stolen from Scott Mueller's _Upgrading and Repairing PCs_, 11th
Edition, as well as some HOWTOs that I've forgotten the names of.  Corrections
welcome as it's been many months since I've had to remember this garbage.

-- 
Matt G / Dances With Crows
There is no Darkness in Eternity/But only Light too dim for us to see
"I backed up my brain to tape, but tar says the tape contains no data...."