Why does 800 mhz RAM require a 1600 mhz FSB?

http://www.dell.com/content/products...=04&l=en&s=bsd

Speed 667MHz and 800MHz fully buffered ECC DIMMs
Note: 800MHz memory requires 1600MHz FSB processor

Aren't the front side bus and memory speed supposed to
be matched?

"Peter Olcott" wrote in message
...

http://www.dell.com/content/products...=04&l=en&s=bsd

Speed 667MHz and 800MHz fully buffered ECC DIMMs
Note: 800MHz memory requires 1600MHz FSB processor

Aren't the front side bus and memory speed supposed to be matched?



That is a persistent and totally untrue rumor which just refuses to die.
Computers haven't had a single clock speed in so long that I can't even
remember when that stopped being the case. But it was a
loooooooonnnnnnnnngggggggggg time ago (decades?). In general, the
motherboard chipset determines what RAM is supported, and the RAM has NO
RELATION at all to the installed CPU.

However, certain CPUs have memory controllers built into them. I don't know
about the Xeon like in the link you listed. It's possible that the Xeon has
a memory controller. That might explain why it says 800MHz memory requires
1600MHz FSB processor. That would make sense, as it would simply divide the
clock speed by two to run the RAM.

But obviously, even if you have a 1600MHz FSB processor with 800MHz RAM, the
CPU and RAM are not matched. So even in THIS case, you don't need to match
RAM to FSB. -Dave

On Thu, 15 May 2008 06:33:00 -0500, "Peter Olcott"
wrote:

http://www.dell.com/content/products...=04&l=en&s=bsd

Speed 667MHz and 800MHz fully buffered ECC DIMMs
Note: 800MHz memory requires 1600MHz FSB processor

Aren't the front side bus and memory speed supposed to
be matched?


Often they are matched, but often modern chipsets can run
asynchronous clock rates for the two and then use a divider
to determine the ratio like 1:1, 1:1.2, 1:1.25, 1:1.5
(FSB:Memory)


However, in this case they are "matched", they just used the
wrong terms in explaining it.

800MHz in DDR2 terms really means 400MHz bus clock x 2 (2
as-in DOUBLE data rate memory) = 800

1600MHz in FSB terms really means 400MHz bus clock x 4 (4
as-in quad pumped data rate) = 1600

So you do actually have synchronous clock rates in this
case, 400:400 or 1:1.

kony wrote in
:

Often they are matched, but often modern chipsets can run
asynchronous clock rates for the two and then use a divider
to determine the ratio like 1:1, 1:1.2, 1:1.25, 1:1.5
(FSB:Memory)


However, in this case they are "matched", they just used the
wrong terms in explaining it.

800MHz in DDR2 terms really means 400MHz bus clock x 2 (2
as-in DOUBLE data rate memory) = 800

1600MHz in FSB terms really means 400MHz bus clock x 4 (4
as-in quad pumped data rate) = 1600

So you do actually have synchronous clock rates in this
case, 400:400 or 1:1.


Yea, this is how it works but you can set any ram to any speed in the mb
bios so there is no requirement that you have to have 800mhz ram. You just
have to make sure you buy ram that can do 800mhz or better if you want the
cpu and ram to run synchronously.

Peter Olcott wrote:
http://www.dell.com/content/products...=04&l=en&s=bsd

Speed 667MHz and 800MHz fully buffered ECC DIMMs
Note: 800MHz memory requires 1600MHz FSB processor

Aren't the front side bus and memory speed supposed to
be matched?



That motherboard uses FBDIMMs, a technology that adds an extra
layer of buffering, between the memory chips and the Northbridge.

The motherboard uses the 5400 chipset. Page 202 shows how the
Northbridge is dictating a relationship between the FSB and
the memory. Notice that dividers other than 1:1 are available,
so in fact it doesn't have to stay at 1:1. The information
in the table was particularly poorly written (the table existed
in another document, and a couple lines were added, presumably
by a different tech writer).

http://download.intel.com/design/chi...hts/318610.pdf (page 202)

Page 8 here, shows a single FBDIMM channel. The 5400 has four
of these.

http://download.micron.com/pdf/prese...icron_2004.pdf

The datasheet here for an FBDIMM, shows an ordinary array of
memory chips like on desktop DIMMs (see page 5 - the diagram is
not actually for the product in question, but I'll ignore that
- it still illustrates the concepts). So at least the bandwidth
of the memory chips themselves will be similar (64 bit wide rank
times the DDR2-xxx rate in terms of actual user data).

http://www.corsair.com/_datasheets/CM72FB1024.pdf

The thing is, by the time the data has been moved from the memory side of
the 5400, to the FSB side, the relationship between the bandwidths
has kinda been lost. On the one hand, two processors at
FSB1600 * 8 = 12.8GB/sec, matches four (DDR2-800 * 8 or 6400GB/sec)
DIMMs, but the bus between the 5400 and AMB has an impact on the
transfer of data. Memory busses are seldom 100% efficient (even if the
AMB was missing, it might be 65%), which means you don't get nearly
the bandwidth expected. That is why, on desktop systems, there is
still some performance improvement as the memory clock is increased
past the "balance" point clock-wise. So more theoretical memory
bandwidth should be available than is present on the FSB, to
get closer to a true balance.

10pair
Proc --- FSB --- 5400 ---- AMB --- Memory
1 of 2 1 of 4 ---- Chips
shown channels 14pair DDR2-800
Half duplex

|--------- FBDIMM ----------|

I think this fits in the catagory of "don't worry about it" :-)

To read an article about another product using 5400 Northbridge,
try this one. I would have preferred they use four FBDIMMs
to test, or compare two FBDIMMs to four FBDIMMs, to see if
there is a real difference. With two FBDIMMs installed, they
measured 7147MB/sec. (The chipset has two branches, and two
channels per branch. The two DIMMs would be installed on the
same branch, one per channel, for a sort of "dual channel"
operation.)

http://www.hothardware.com/articles/..._x _2/?page=2
http://www.hothardware.com/articleim...sandra_mem.jpg

Tomshardware used four DIMMs. And what is interesting, is there
is little difference in the Sandra bandwidth number, both when
comparing to two sticks from the Hothardware article, and when
the CPU core clock rate changes. Of course, comparing benchmarks
between sites is a "bad thing", because they might not have tested
exactly the same way.

http://www.tomshardware.com/reviews/...3,1770-17.html

Paul

800 MHz RAM does NOT require a 1600 MHz FSB. Where did you hear that?

--
--DaveW


"Peter Olcott" wrote in message
...

http://www.dell.com/content/products...=04&l=en&s=bsd

Speed 667MHz and 800MHz fully buffered ECC DIMMs
Note: 800MHz memory requires 1600MHz FSB processor

Aren't the front side bus and memory speed supposed to be matched?

On Thu, 15 May 2008 14:03:58 GMT, Gumby
wrote:

kony wrote in
:

Often they are matched, but often modern chipsets can run
asynchronous clock rates for the two and then use a divider
to determine the ratio like 1:1, 1:1.2, 1:1.25, 1:1.5
(FSB:Memory)


However, in this case they are "matched", they just used the
wrong terms in explaining it.

800MHz in DDR2 terms really means 400MHz bus clock x 2 (2
as-in DOUBLE data rate memory) = 800

1600MHz in FSB terms really means 400MHz bus clock x 4 (4
as-in quad pumped data rate) = 1600

So you do actually have synchronous clock rates in this
case, 400:400 or 1:1.


Yea, this is how it works but you can set any ram to any speed in the mb
bios so there is no requirement that you have to have 800mhz ram.

On many systems you can, but on that specific OEM
workstation? I would not assume it, and one of the reasons
for it's higher cost is it support more memory which may
reduce the flexibility in choosing different combinations of
speeds and timings (if again, they let you choose at all
instead of it all being hidden and unsettable "auto"
configured).

You just
have to make sure you buy ram that can do 800mhz or better if you want the
cpu and ram to run synchronously.

IMO, anyone should buy -800 memory or better anyway, the
cost different is minor and if they can set speed and/or
timings they may have the potential for a faster system as a
result.

"DaveW" wrote in message
...
800 MHz RAM does NOT require a 1600 MHz FSB. Where did
you hear that?

--
--DaveW


"Peter Olcott" wrote in message
...

http://www.dell.com/content/products...=04&l=en&s=bsd

Speed 667MHz and 800MHz fully buffered ECC DIMMs
Note: 800MHz memory requires 1600MHz FSB processor

Aren't the front side bus and memory speed supposed
to be matched?





It is on the second tab of the link above, it is also
directly quoted immediately above:
Note: 800MHz memory requires 1600MHz FSB processor
The Intel website seems to say the same thing as a
requirement of their motherboards.

On 15 May, 14:14, kony wrote:
snip
800MHz in DDR2 terms really means 400MHz bus clock x 2 (2
as-in DOUBLE data rate memory) = 800


I knew how it worked for DDR.. Not for DDR2. So, just asking regarding
this DDR2

isn't 800 in DDR2 terms, 200MHz*4 ?

i.e. 200Mhz bus

snip

On Sat, 17 May 2008 14:41:50 -0700 (PDT),
"
wrote:

On 15 May, 14:14, kony wrote:
snip
800MHz in DDR2 terms really means 400MHz bus clock x 2 (2
as-in DOUBLE data rate memory) = 800


I knew how it worked for DDR.. Not for DDR2. So, just asking regarding
this DDR2

isn't 800 in DDR2 terms, 200MHz*4 ?

i.e. 200Mhz bus

snip

Depends on where the clock rate comes from, AFAIK.
On DDR2-800 the memory clock is 200MHz, the bus clock
400MHz. This contrasts DDR1 where the bus clock is the same
as the memory clock frequency.