Replies to post #3982 on Rambus (RMBS)

[cordob]

I wonder when Samsung will put up an XDR data sheet?

Isn't mass production a wonderful thing?
(especially if you don't know the specs)

Seriously, I was in touch with Samsung IR and asked them as I was communicating with them anyway what the 10K price for XDR is.

They replied that such prices fluctuate due to market conditions and could not give an answer. Will try again on a different tack sometime later.

Do we know anything about pricing? I.e. has it been published? I thought DramExchange was going to track them, but am not sure about that.

Cheers
Cor

[calbiker]

Seems to me the DRAM performance will be dictated by the serial bus.

That's probably correct. And seems to be a weak point of the design. Their design rule is to have the bus 6 times faster than the DRAM. This sounds like a potential bottleneck already. The max bus speed (right now) is designed for DDR 800. Six times 800 Mhz means the serial bus max speed is 4.8 Gbps. With 14 parallel lines, the peak data rate is:

4.8 Gbps/line * 14 lines * 8b/10b / 8b/B = 6.72 GB/s

A 64-bit DDR 800 DIMM has a peak bandwidth of 6.4 GB/s, just filling up the entire bus bandwidth. On top of that, this serial bus must also carry address and control data, not to mention streaming data from other FB-DIMMs on this bus. Sounds like a major bottleneck.

So, would XDR increase performance? Perhaps. Intel does state there's an upgrade path to DDR3. DDR3 would see the same bandwidth limitations that XDR would encounter. So there must be a fix. Perhaps increase the serial bus speed to 10 Gbps? That's awful fast. It's iffy if they can accomplish that. Don't know why they didn't increase the number of lanes from the get-go.