Why is RDRAM so expensive?

[No_Tone]

Currently I am running a P4 2.66ghz and have 512mb of PC1066. I've recently been using alot of apps to require more memory so the logical thing to do is get more RAM. But the thing is, another 512mb of RDRAM costs about $175.

Should I just upgrade to DDR400? Will I get a performance boost? Also if I decide to upgrade, should I get dual channel DDR? Is there that much more of a boost with dual channel? Thanks in advance!

 

[BrentUnitedMem]

PC1066 = 533MHz at 8.4GB/s (dual channel)

PC3200 = 200MHz at 6.4GB/s (dual channel)

PC-1066 actually performs better. To match PC-1066 with DDR, you'll need:
PC4200 = 266Mhz at 8.4GB/s (dual channel)

Contrary to popular belief, dual-channel doesnt offer too much of a performance increase over single-channel. I think it's only about 5-7% increase over single channel.

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[fishmahn]

As Brent said, DDR SDRAM is actually slower than RDRAM, but its a lot cheaper, so consumer pressure made Intel drop RDRAM.

For that 175, maybe a little more, you should be able to buy a new mobo and 1gig of RAM for it. If you go this route, definitely get Dual channel (a board based on the 865 chipset) - Intel CPUs can use the extra bandwidth, though, as Brent said, it doesn't make a huge difference.

Mike.

 

[Crashman]

NO, PC1066=4.2GB/s (dual channel). Most people refer to the i850E chipset, which used 2 16-bit DIMMs or 1 32-bit DIMM. The 32-bit DIMMs were in effect dual-channel on a single slot.

RIMM4200 is not PC1066. RIMM4200 is 32-bit, PC1066 is 16-bit. Yes, I realize there are lots of companies calling RIMM4200 the wrong name (PC1066), but the difference in name is there for a reason, so people don't confuse the two.

Now, RIMM4200 is named after 4200MB/s (rounded number). And because it pairs the channels, a single module does the same dual-channel configuration as two PC1066 modules does. Therefore, RIMM4200 is run singly and has 4.2GB/s. A pair of RIMM4200 is still 32-bits, and still the same 4.2GB/s, the second RIMM is in series with the first.

Do the math if you're still in doubt and get back to me!

533MHz, using double data rate (yes, RAMBUS does that too), is 1066MHz data rate, across a 16-bit bus (convert to 2 bytes) is 2133MB/s. Double that for two modules and you get 4266MB/s. RIMM4200 is already doubled...

The reason I mention the i850E specifically is that SiS made a chipset that was QUAD CHANNEL capable, using 2 RIMM4200 modules.

I've seen significant advantages to dual-channel operation for my P4. Over 15% in games, as much as 30%.

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[jammydodger]

The reason I mention the i850E specifically is that SiS made a chipset that was QUAD CHANNEL capable

What it offered 8.4GB/sec bandwidth?

 

[Crashman]

Yes, SiS had a "dual channel" board that used 2 32-bit RIMMs. The reason I say quad channel is that 32-bit RIMMs were originally intended at dual-channel on a single chip. But the SiS chipset was actually 32-bits per channel so far as I know. I believe Abit offered a high-end board using it, but I've never seen that product go retail.

That SiS chipset (was it the 656?) was the last major effort of any motherboard product to use RDRAM for improved performance. 8.4GB/s would have been possible in theory using two RIMM4200 modules, but I've never seen any reviews to test it.

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[BrentUnitedMem]

Sorry for not clarifying:

The bandwidths are as follows:

2 x16 bit
PC1066 = 533MHz = 4.2GB/s

2 x32 bit
PC1066 = 533MHz = 8.4GB/s

In which case DDR-400 dual channel at 6.4GB/s has better performance over x16 bit, but not x32 bit modules.

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[Crashman]

But the i850E doesn't DO 64-bit memory, it only does 32, so...
2 x16 bit = 32-bit
PC1066 = 533MHz = 4.2GB/s

2 x32 bit = 32
RIMM4200 = 533MHz = 4.2GB/s

Nobody here remembers the special circumstance for the SiS RDRAM chipset!

Yes, I realize it's a chipset issue, but a darned good one for making the comparison.

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[BrentUnitedMem]

The bandwidths are based on the data bus:

2 x16 bit <b>data bus</b>
PC1066 = 533MHz = 4.2GB/s

2 x32 bit <b>data bus</b>
PC1066 = 533MHz = 8.4GB/s

<A HREF="http://www.tomshardware.com/motherboard/20040119/index-01.html" target="_new">source</A>

i850 specific chipset:
<A HREF="http://www.intel.com/design/chipsets/850/pix/850_800.gif?iid=ipp_850chpst+diag_lg&" target="_new">Intel® 850 Chipset System Diagram</A>

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[Crashman]

Yes, so I'm right. The chipset diagram even shows there are only 2 16-bit channels. So 4 16-bit RIMMs has the same bandwidth as 2 16-bit RIMMs, since the second pair is in series to the first. And 2 32-bit RIMMs provide the same bandwidth as 1 32-bit RIMM, because a 32-bit RIMM occupies both channels.

Now as for your source, I "know people" who write for Tom's...BTW, in a different article Tom's even defined the proper differentiation in naming, in a different article, where they said 32-bit 533MHz RDRAM is supposed to be called RIMM4200, not RDRAM, so sourcing this particular mistake (calling 32-bit modules PC1066) doesn't help.

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[BrentUnitedMem]

Yes that's right. It shows 4.2GB in the diagram.

Not sure what TOMS is talking about here. But if the bus for another system is 2x32bit then the bandwidth would be 8.4GB/s maximum like it is for RIMM 4200.

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[Crashman]

Yes, I was just saying that the only place anyone would find that 64-bit RDRAM bus would be on the magically disappearing SiS 656 chipset.

Tom's might have been speaking in theoretical numbers, but then again, they called 32-bit 533MHz DDR RDRAM...PC1066 rather than RIMM4200, which an article previous to that one explained shouldn't be done (because it confuses the buyer).

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