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Subject: Re: fsb
Name:
 ludicrous
Date: 11/1/2001 5:05:59 PM (GMT-7)
IP Address: 138.67.74.229
In Reply to: fsb posted by
kenny
Message:
as SickOfItAll noted, the "PC2100" designation refers to bandwidth.
(133MHz) x (2 for DDR) x (64-bit data path) = 17 024 000 000 bits/sec
(17024E6 bits/sec) / (8 bits/byte) = 2 128 000 000 Bytes/sec
Divide by 1024 twice and, with a little creative rounding, you get roughly 2.1GB/sec. You can do the same calculation at 100MHz and get a number close to 1.6GB/sec.
Unfortunately, with the introduction of RDRAM memory Intel and Rambus tried to confuse the issue with names like "PC800", implying that old "PC100" SDRAM was a lot slower. Where that number comes from is that Rambus operates at up to 400MHz double-data rate BUT its design has a peak bandwidth of 1.6GB/second. Not so impressive as the name implies.
Foregoing a long technical explanation, the trade-off is that you can have data transferring on more paths, but your top operating speed will be more limited (DDR), or you can achieve much higher speeds with fewer data lines (RDRAM). There are other aspects to consider but you get the idea.
So, to even things up on the marketing end, DDR memory spec'd for 100(200)MHz operation is "PC1600", and DDR memory spec'd for 133(266)MHz operation is "PC2100".
Other monikers like "PC2400" and "PC2700" are, as of yet, just marketing gags for promoting memory that supposedly can run 150 or 166MHz. There are no officially-released standards for these, yet, although PC2700 is in discussion IIRC.
The other option is to run two distinct memory channels and thus double your bandwidth. Thus the Intel i840/i850 chipsets could achieve a peak of 3.2GB/sec. The new NVidia nForce does the same thing, but with DDR, for a peak bandwidth of 4.2GB/sec. But remember that bandwidth is not directly proportional to performance. Again, there are other factors to consider.
Confused? 
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