question about ddr sdram (this may sound ridiculous)

[tokamak]

ok i havent followed the pc hobby much lately, and the last time i was still interested plain old sdram was king. i understood that - pc100 meant that it was running at 100mhz bus, pc133 meant 133 bus, etc. but ddr ram really confuses me. do the numbers mean anything at all? does it matter what ram you pair with a certain processor/fsb any more? i read the faq but i didnt clear everything up for me. does an athlon xp (133x2=266mhz bus) require pc2100 ram? would using, say, pc2400 increase performance? i've totally lost track of how the whole pc-ram architecture works. am i rambling 😀?

 

[ShadowFox]

PC1600 = 100mhz x2
PC2100 = 133mhz x 2
PC2700 = 166mhz x 2

No, you dont have to use ddr with an Athlon XP and you can use PC1600 rather than PC2100, but that would decrease performance, currently, the Athlon XP would get no benefit from PC2700. PC2400 is for overclockers and is rated stable at higher speeds than PC2100.

And, yes, you were rambling 😉

 

[ElFenix]

there should be a wee bit of increase from running your RAM at +33MHz to your bus speed but thats only because anything that isn't the processor doesn't have to wait quite as much for its writes and reads.

 

[kgraeme]

The numbers correspond to memory bandwidth. The PC2100 is capable of 2.1GB/s. This number is figured by the following:

8 bytes
2 bytes per clock cycle (aka DDR or "double pumped")
133 MHz front side bus

8x2x133=2128 (2100)

So it still corresponds to bus speed, but the number shows actual peformance now. With other bus speeds you get the other numbers:

PC1600 = 8x2x100
PC2100 = 8x2x133
PC2700 = 8x2x166

Where it will get interesting is with DDR II. DDR II is capable of 4 bytes/clock. You may have heard the term "quad-pumped". It's a 100MHz bus running at 4x. DDR II for a 400 MHz board will be PC3200 (8x4x100) or capable of 3.2GB/s.

(BTW, credit goes to bpt8056 for pointing out some of the DDR II stuff to me.)