saphalline
12-31-2003, 01:53 AM
I don't usually start a new thread, so listen up people! :eek: :p
I know a lot of people who come to the forums constantly have questions about how DDR or "quad-pumped" works. They often post questions about why their CPU has a 333MHz or 800MHz FSB, but only shows up as 166MHz or 200MHz in the BIOS. The difference is frequency vs speed, and I think I've come up with a good analogy for those who don't have a degree in electrical engineering. :cool:
Walking up stairs. Silly, I know, but let's go.
Let's say you can walk up the stairs at a rate of 100 foot-steps per minute. That's your frequency. Not bad, but you do it the old-fashioned way - one stair-step at a time. Someone else can only move at 60 foot-steps per minute, but they walk up two stair-steps at a time. That means they can actually go up 120 stair-steps per minute, because they do more work per foot-step than you. Starting to see the picture? Steps per minute is your frequency, but your speed is your frequency times the number of stair-steps you can walk at once.
Ok, in terms of DDR and other related technologies, this means that if your FSB's frequency is 166MHz, but it works double-time (two bits per clock cycle) so that's an effective speed of 333MHz. That's AMD's way. Intel has "quad-pumped" or QDR FSB's (four bits per clock cycle), so a 200MHz frequency for the P4 is really quadrupled to 800MHz. Now keep in mind these FSB's don't actually run at 800MHz, that's just how fast it seems to run because of their ability to pass multiple bits per clock cycle.
This is why most people find it totally confusing to match DDR RAM to FSB speed. Matching speeds, not frequencies, is most important. DDR RAM at a speed of 400MHz matches a 400MHz FSB perfectly, but what about Intel's 800MHz FSB? That's where dual-channel comes into play. By running two sticks of 400MHz DDR RAM in parallel, you get 800MHz total! :D See? It does work! But oh! The fun is just beginning...
CPU's of the same frequency can have different FSB speeds. It's not against the law. Intel's P4 2.4GHz, for instance. The "Northwood A" version had a 400MHz FSB while the "C" version has 800MHz. Even more confusing are AMD's AthlonXP's. The 2600+ "T-bred B" version run at 2133MHz for the 266MHz FSB, and 2083MHz for the 333MHz FSB. Huh? The 2600+ "Barton" version runs at 1917MHz for the 333MHz FSB. More huh? Ok, not as bad as it seems. The only thing you need to know is the FSB speed. 266MHz gets PC2100 DDR RAM, and 333MHz gets PC2700 DDR RAM. Got that? Good.
Ok, so let's recap the DDR RAM specs:
PC1600 = 100MHz frequency = 200MHz speed
PC2100 = 133MHz frequency = 266MHz speed
PC2700 = 166MHz frequency = 333MHz speed
PC3200 = 200MHz frequency = 400MHz speed
Those are the official ones. I recommend sticking to the official specs, but overclockers are wont for more...
PC2400 = 150MHz frequency = 300MHz speed
PC3000 = 187MHz frequency = 375MHz speed
PC3500 = 220MHz frequency = 440MHz speed
PC4000 = 250MHz frequency = 500MHz speed
These frequencies and speeds are estimates, and not being official you shouldn't expect RAM manufacturers to stick to them. Use at your own risk.
Ok, I'm done for now. If anyone else has anything to add, or any corrections, you know what to do. ;)
I know a lot of people who come to the forums constantly have questions about how DDR or "quad-pumped" works. They often post questions about why their CPU has a 333MHz or 800MHz FSB, but only shows up as 166MHz or 200MHz in the BIOS. The difference is frequency vs speed, and I think I've come up with a good analogy for those who don't have a degree in electrical engineering. :cool:
Walking up stairs. Silly, I know, but let's go.
Let's say you can walk up the stairs at a rate of 100 foot-steps per minute. That's your frequency. Not bad, but you do it the old-fashioned way - one stair-step at a time. Someone else can only move at 60 foot-steps per minute, but they walk up two stair-steps at a time. That means they can actually go up 120 stair-steps per minute, because they do more work per foot-step than you. Starting to see the picture? Steps per minute is your frequency, but your speed is your frequency times the number of stair-steps you can walk at once.
Ok, in terms of DDR and other related technologies, this means that if your FSB's frequency is 166MHz, but it works double-time (two bits per clock cycle) so that's an effective speed of 333MHz. That's AMD's way. Intel has "quad-pumped" or QDR FSB's (four bits per clock cycle), so a 200MHz frequency for the P4 is really quadrupled to 800MHz. Now keep in mind these FSB's don't actually run at 800MHz, that's just how fast it seems to run because of their ability to pass multiple bits per clock cycle.
This is why most people find it totally confusing to match DDR RAM to FSB speed. Matching speeds, not frequencies, is most important. DDR RAM at a speed of 400MHz matches a 400MHz FSB perfectly, but what about Intel's 800MHz FSB? That's where dual-channel comes into play. By running two sticks of 400MHz DDR RAM in parallel, you get 800MHz total! :D See? It does work! But oh! The fun is just beginning...
CPU's of the same frequency can have different FSB speeds. It's not against the law. Intel's P4 2.4GHz, for instance. The "Northwood A" version had a 400MHz FSB while the "C" version has 800MHz. Even more confusing are AMD's AthlonXP's. The 2600+ "T-bred B" version run at 2133MHz for the 266MHz FSB, and 2083MHz for the 333MHz FSB. Huh? The 2600+ "Barton" version runs at 1917MHz for the 333MHz FSB. More huh? Ok, not as bad as it seems. The only thing you need to know is the FSB speed. 266MHz gets PC2100 DDR RAM, and 333MHz gets PC2700 DDR RAM. Got that? Good.
Ok, so let's recap the DDR RAM specs:
PC1600 = 100MHz frequency = 200MHz speed
PC2100 = 133MHz frequency = 266MHz speed
PC2700 = 166MHz frequency = 333MHz speed
PC3200 = 200MHz frequency = 400MHz speed
Those are the official ones. I recommend sticking to the official specs, but overclockers are wont for more...
PC2400 = 150MHz frequency = 300MHz speed
PC3000 = 187MHz frequency = 375MHz speed
PC3500 = 220MHz frequency = 440MHz speed
PC4000 = 250MHz frequency = 500MHz speed
These frequencies and speeds are estimates, and not being official you shouldn't expect RAM manufacturers to stick to them. Use at your own risk.
Ok, I'm done for now. If anyone else has anything to add, or any corrections, you know what to do. ;)