Mootsie baby

[#686578]

I need you to help me understand RAM.

Basically, why is DDR3 twice as fast as DDR2 which is twice as fast as DDR1? I understand the concepts of CAS, all of the latency numbers. I understand frequency (I think) as well. And I understand that the actual speed is a function of latency and frequency.


What I dont understand is that the stereotypes would mean that the effective speed is the same.

DDR1 speeds are usually like 3-3-3-6
DDR2 speeds are usually like 6-6-6-18
DDR3 speeds are usually like 9-9-9-24

Or something roughly like that

But what I think explains the doubling is something that I dont understand... prefetch buffer depth...

From what I can figure out this would be the actual... amount transferred? I dont remember the exact number now but for simplicity let's just say DDR1 was 2, DDR2 4, DDR3 8.

And if I understand that right then that would explain why DDR3 is twice as fast as DDR2 etc...?

Something like (a very simplistic wrong equation)

cas/frequency * amount
DDR1 - 3/200 * 2
DDR2 - 6/400 * 4
DDR3 - 9/600 * 8

And that kind of makes an example of what I was going on about, where it looked like the combination of latency and frequency would give the same speed for each, but then what I might understand is that they transfer MORE at the same speeds?

I really need someone to explain this to me 'cause I just dont get it.

[Mizango]

Casual.

[#686578]

You dont even know what a bit is

[netz]

And that kind of makes an example of what I was going on about, where it looked like the combination of latency and frequency would give the same speed for each, but then what I might understand is that they transfer MORE at the same speeds?

This is all it really boils down to. DDR2 to DDR3 doubled the amount of data transferred per clock cycle. DDR3 also has an I/O bus running twice as fast as DDR2's, so the very slight increase in latency as measured by the number of clock cycles is somewhat misleading. The result is that actual latency is lower with DDR3.

[#686578]

The result is that actual latency is lower with DDR3.

How is the latency reduced when it's an increased amount of clock cycles per transfer?

[mootsfox]

The latency of RAM has been pretty constant over the years.

All you should worry about is price unless you are doing something that is very memory heavy. The difference between DDR2-800 and DDR3-1600 is not noticeable for 99% of applications.

[netz]

How is the latency reduced when it's an increased amount of clock cycles per transfer?

With a higher frequency, each clock cycle takes less time to complete.

[#686578]

Uh I thought you meant as in the actual term latency.

[#686578]

The latency of RAM has been pretty constant over the years.

All you should worry about is price unless you are doing something that is very memory heavy. The difference between DDR2-800 and DDR3-1600 is not noticeable for 99% of applications.

Duh! I'm not worried bout practicality or applicability, I just want to understand it.

[netz]

Uh I thought you meant as in the actual term latency.

Two latencies are at work here. The actual real-world time that passes, and the number of clock cycles that an operation takes. You can directly compare clock cycles if two devices run at the same frequency; in the case of DDR2 and DDR3, this can't be done -- DDR3 has double the I/O bus frequency over DDR2. The reason DDR3's latency numbers are higher is because more cycles do indeed pass while waiting for an operation to complete. However, it's still waiting the same amount of real-world time, because those cycles are shorter than DDR2's.

[_chuchu]

http://i48.tinypic.com/ekgvw5.png