Underclocking memory

[Nuno J. Silva]

I have a computer with a Celeron Coppermine which is supposed to work
at 800 Mhz. Unfortunately, as I have no 100Mhz DIMMs around, I had to
underclock it to a 66Mhz FSB (now it's 533Mhz).

As I've not yet found any 100Mhz module in the stores I looked for, I'd
like to know if it is okay to just underclock 133 Mhz modules (I found
a store selling 256 MB 133 Mhz SDRAM DIMMs) to 100 Mhz to get the CPU
working at its normal speed.

That's, will it work and cause no stability problems?

--
Nuno J. Silva
LEIC student at Instituto Superior Técnico
Lisbon, Portugal
Homepage: <http://njsg.no.sapo.pt/>
Registered Linux User #402207 - http://counter.li.org

Using Claws Mail 3.2.0

Gentoo Base System release 1.12.10
Linux 2.6.17-gentoo-r7 i686 Pentium II (Deschutes)

-=-=-
Air is water with holes in it.

 

[kony]

I have a computer with a Celeron Coppermine which is supposed to work
at 800 Mhz. Unfortunately, as I have no 100Mhz DIMMs around, I had to
underclock it to a 66Mhz FSB (now it's 533Mhz).

Check the motherboard bios to see if it has an asynchronous
memory setting feature, it might read something like
+-33MHz. If it has that feature you can keep 100MHz FSB and
reduce memory bus to 66MHz.

As I've not yet found any 100Mhz module in the stores I looked for, I'd
like to know if it is okay to just underclock 133 Mhz modules (I found
a store selling 256 MB 133 Mhz SDRAM DIMMs) to 100 Mhz to get the CPU
working at its normal speed.

That's, will it work and cause no stability problems?

There is no problem underclocking PC133 to 100MHz, that
alone will not cause instability although adding any memory
to a system would still have a potential to be instable
depending on the quality of the memory, motherboard, and
bios settings.

However some motherboards can only use a lower density
memory than the most common PC133 modules, in which case it
would either not run or only see half of the capacity of
each module. If it only saw half the capacity it might
still run fine and often the lower density memory is twice
as expensive (but I haven't priced it recently and don't
know the market where you are).

If the system has integrated video, check the bios for the
asynchronous memory setting I referred to above, but instead
of setting it to run the memory slower than the FSB, set it
to run the PC133 memory at +33MHz faster than the FSB.
Doing this has minimal to no benefit without integrated
video but with integrated video (assuming it uses main
system memory) it may improve performance.

 

[Nuno J. Silva]

Check the motherboard bios to see if it has an asynchronous
memory setting feature, it might read something like
+-33MHz. If it has that feature you can keep 100MHz FSB and
reduce memory bus to 66MHz.

I just found that the option to change the DRAM clock has two choices:
HOST CLK and HOST CLK + 33. No "HOST CLK - 33" option, but this is good
enough, at least I can have 133 Mhz DRAM and 100 Mhz CPU

There is no problem underclocking PC133 to 100MHz, that
alone will not cause instability although adding any memory
to a system would still have a potential to be instable
depending on the quality of the memory, motherboard, and
bios settings.

However some motherboards can only use a lower density
memory than the most common PC133 modules, in which case it
would either not run or only see half of the capacity of
each module. If it only saw half the capacity it might
still run fine and often the lower density memory is twice
as expensive (but I haven't priced it recently and don't
know the market where you are).

At least in the store website, I can find no information on the density.

It's this:
http://www.chiptec.net/incs/verproduto.php?id=2011&session=7c6c18caa2b78d60355f65e244aacf1b

As that is in portuguese, I'll try to translate the description:

KINGSTON SDRAM 256MB PC133 DSIDE
Manufacturer: Kingston
Description:
-Memory for older pcs
-256MB memory
-clock:133MHz
-Differential 1x
-Bus 64Bits/8 Bytes
-Bandwidth:1066MB/s

I think it's worth a try. I can always take one of the 66 Mhz modules
which works to check if the module is fully compatible (IIRC, the
position of marks (or holes) in the modules says if they're unbuffered
or not, and specifies the voltage).

If the system has integrated video, check the bios for the
asynchronous memory setting I referred to above, but instead
of setting it to run the memory slower than the FSB, set it
to run the PC133 memory at +33MHz faster than the FSB.
Doing this has minimal to no benefit without integrated
video but with integrated video (assuming it uses main
system memory) it may improve performance.

It has no integrated video, but has that option

--
Nuno J. Silva
LEIC student at Instituto Superior Técnico
Lisbon, Portugal
Homepage: <http://njsg.no.sapo.pt/>
Registered Linux User #402207 - http://counter.li.org

Using Claws Mail 3.2.0

Gentoo Base System release 1.12.10
Linux i686 Pentium II (Deschutes)

-=-=-
``Experience is the name everyone gives to their mistakes.''
-- Oscar Wilde

 

[GT]

Check the motherboard bios to see if it has an asynchronous
memory setting feature, it might read something like
+-33MHz. If it has that feature you can keep 100MHz FSB and
reduce memory bus to 66MHz.

I just found that the option to change the DRAM clock has two choices:
HOST CLK and HOST CLK + 33. No "HOST CLK - 33" option, but this is good
enough, at least I can have 133 Mhz DRAM and 100 Mhz CPU



Someone in this group once advised against using this feature as it might
actually slow things down. As the CPU and RAM are out of sync, transfers
to/from memory might actually be occasionally hindered by this setting. I
personally feel that this doesn't make sense, but have no evidence either
way!

 

[kony]

I just found that the option to change the DRAM clock has two choices:
HOST CLK and HOST CLK + 33. No "HOST CLK - 33" option, but this is good
enough, at least I can have 133 Mhz DRAM and 100 Mhz CPU

133MHz is not a requirement, it is only the upper speed it
is guaranteed to be able to run stabily. Similarly, your
car tires might be rated to 125MPH but that doesn't mean you
have to drive that fast!

Someone in this group once advised against using this feature as it might
actually slow things down. As the CPU and RAM are out of sync, transfers
to/from memory might actually be occasionally hindered by this setting. I
personally feel that this doesn't make sense, but have no evidence either
way!

The question is one of the inherant bandwidth each has at
any given frequency or clock rate, for example some (like
that era) are SDR or single data rate while later we saw DDR
or Intel's "quad pumped" being Quad Data Rate or QDR.

The memory bus would ideally be able to move the data as
fast as the FSB can accept it (or in cases of on-die CPU
memory controller, that bus instead of FSB) meaning that in
some cases like Intel QDR but using DDR(n) memory the memory
bus might be faster for best benefit. In the case of SDR on
both FSB and memory bus, raising the memory bus to a faster
speed than FSB can accept it does little and when there is
asynchronous speed there is a latency to buffer it at the
chipset.

Essentially it means until either the memory bus or FSB went
to a data rate that is a multiple of that buss clock rate,
the tiny benefit of raising one of the clock rates higher
than the other is offset by the inherant latency, EXCEPT
that if it's FSB raised on a CPU with locked multiplier then
of course FSB rate determines CPU frequency as well.

The difference in performance is not much, but back to the
main point: There is no need to try to run PC133 memory at
133MHz merely because it's PC133 memory. As mentioned
previously if there is actually integrated video in use the
higher bandwidth may help but that doesn't seem to be the
case here.

OP should benchmark running it both ways as sometimes
running the lower memory bus speed allows using tighter
memory timings which in some situations is more of a
performance increase.

More significant today is that memory sold as PC133 is
typically higher density than some chipsets use. It's not
that PC100 memory can't run at 133MHz, as yields of chips
that could run at 133MHz were great many years ago, today
it's mostly marketing purposes that cause new (not old
stock/used) memory to be sold as PC100 to denote it's lower
density - particularly from major retail sellers like
Kingston as it was common that chipsets which used 100MHz
FSB as the max the chipset supported where about where the
line was drawn between not supporting high density memory
and later 133MHz FSB capable (as spec'd vs overclocked)
chipsets that could.