[Re: RAM question]

[Matt Graham]

Edward Glowacki <glowack2@msu.edu> wrote:
> Quoted from Jo Dillon on Fri, Nov 16, 2001 at 03:31:50PM +0000:
>> Another possibility is it only has enough cache for 512MB worth of 
>> memory.
>> In that case it'd boot and recognise the memory but any access that 
>> happened to hit the high 512MB of memory would be extremely slow. So 
>> the system would appear (possibly intermittently) about as fast as a 
>> 386 ;)
> 
> I don't think this is correct, it doesn't make much sense.  Cache
> and main memory are independent, and caching algorithms generally
> work fine for any combination of the two.  You'll get better
> performance with bigger cache, but you won't get worse performance
> with bigger memory.

Um, nope.  The boards that Jo was talking about are certain older Pentium and
K6-? boards, which only had enough "cache tag RAM" on the motherboard to keep
track of a certain number of pages (64M, 256M, 512M).  If a page fell outside
this boundary, it would be fetched from main memory, even if the processor or
board had the page in the L1/L2 cache.  There are too many accounts and
examples of people stuffing > 64M into an old Pentium board and finding out
that things got very slow for this to be an urban legend.

The caching algorithms Ed's talking about are hardwired into the board here
and rather difficult to change.  Fortunately, the PII, PIII, PIV, and Athlon
designs have eliminated this problem or at least put the ceiling up relatively
high (~1G).

The board that the OP was referring to is probably not limited to 512M if the
manuals said that it can only take 128M DIMMs and it's working fine with a
256M DIMM.  PIIIs have the cache tag RAM on the CPU itself, not on the
motherboard, so even if it's limited to 512M, a slightly newer CPU should help
things out.

The manual's description of the limit may also be due to the strange problems
Lose9x/ME have with > 512M of RAM.  (Blame the board, not the OS, customer
upgrades board, customer still has problems with OS, customer upgrades OS,
{$COMPANY,Microsoft} make money.) 

-- 
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...."