Understanding RAM.... quesitons inside

[LordTerrin]

Dear learned reader,

Of all the parts of the computer, I understand RAM the least. Lately I have been hearing so much more about CAS timing, these 2-3-1-2 things that I do not understand ONE bit, I don't know what CAS stands for, or RAS for that matter. I would a more technical approach, not like "oh well 2-2-2 faster than x-y-z-4... I don't know what the numbers mean, nor what their places mean. For example, I see "valueRAM" rated with a CAS timing of 3. What exactly does that mean, and does 3 differ from 3-3-3-3, or 3-3-3? I would love an explanation. Why is 3 faster or slower than 2? and why is 2-3-2-3 better than x-y-z?

I'm also wondering about the speeds of different ram types. I remember when pc2100 was the standard, and it seems as though that standard has moved to pc2700 or pc3200, but what are the exact differences between these, besides just the mhz? I understand that for socket 754, pc3200 is the max the boards say they support, but I see people using pc3500 or pc3700 on them, why?

I am looking for... a long discussion to really try and understand ram better, thanks so much, we can talk on AIM too or over PM if anyone has the patience to help me learn.

my sn is sempaikeitaro

 

[Matthias99]

There was one of these recently in HT (I responded to it), and several (I think) in GH. Search first, then ask questions.

 

[LordTerrin]

really? strange... maybe I didn't search the right terms because I didn't find anything.. lemme try again

 

[LordTerrin]

http://forums.anandtech.com/messageview.cfm?catid=50&threadid=1358108&highlight_key=y&keyword1=CAS

well I'll be....

 

[LordTerrin]

I don't think anyone answered what CAS stands for.
So let me see if I have this correct, a setting like 2-2-3-2 would mean what?

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Things in bold are the specifcs that I'd like answered
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That it takes 2 milliseconds? for something to travel from point a to point b?

What exactly is travelling? Come to think of it, what exactly is RAM's job? I have always just kinda thought that faster ram was better, but I never have understood what it does and its function. It's my guess that RAM stores parts of information, but... is it bringing that information to a place, or holding it for another part of the computer to pick it up? What's it do?


edit: I'm going to try reading into this article, http://www.pcguide.com/ref/ram/timing.htm and see if it can shed some light on my questions, I would still LOVE for another person to help explain it tho.

 

[LordTerrin]

I have begun to be my own question answerer....

CAS stands for Column Address Select and RAS stands for Row Address Select.

Let me think back to beginning calculus when we studied matrices. If I have 1 stick of 512mb of ram, how is the ram itself organized into a matrix, then called upon for use? The topic on pcgude that I just posted pointed out that because of the ability to burst transfer, the CAS rating is really the FIRST number in the x-y-y-y series, and the other subsequent ones are not really as important. How am I doing so far?

LT

 

[Matthias99]

Originally posted by: LordTerrin
I have begun to be my own question answerer....

CAS stands for Column Address Select and RAS stands for Row Address Select.

Let me think back to beginning calculus when we studied matrices. If I have 1 stick of 512mb of ram, how is the ram itself organized into a matrix, then called upon for use? The topic on pcgude that I just posted pointed out that because of the ability to burst transfer, the CAS rating is really the FIRST number in the x-y-y-y series, and the other subsequent ones are not really as important. How am I doing so far?

LT

Better.

Here's a more basic article on RAM: howstuffworks article.

The numbers are in terms of *clock cycles*, not milliseconds (and, man, 2ms RAM would be ass-slow -- cycle time on DDR400 is five nanoseconds, and you get two pieces of data/cycle).

CAS Latency is basically how many clock cycles it takes to start up a read or write. If you want to read and write sequential addresses in the same 'column', the subsequent ones are very very fast.

 

[LordTerrin]

Ahhh, I see. So if my ram wants to do a read or write request, the CAS latency is how long it takes the RAM (or is it the RAM controller) to find the correct column? Why is the column important but not the row? I never see anyone mention RAS timings....

oh, and I meant to put 2ns, not 2ms

 

[Matthias99]

Originally posted by: LordTerrin
Ahhh, I see. So if my ram wants to do a read or write request, the CAS latency is how long it takes the RAM (or is it the RAM controller) to find the correct column? Why is the column important but not the row? I never see anyone mention RAS timings....

oh, and I meant to put 2ns, not 2ms

RAM is addressed by row and column. The reason the CAS is more important (they both play a role in speed) is just due to the way the cells are organized. They could have easily done it the other way (so that the 'row' was the one that was more important), but this is the design that JEDEC settled on for DRAM. If you dig around you should be able to find more details on exactly how the addressing works, but this may be lower-level than you're interested in.

 

[Ryoga]

http://www.corsairmicro.com/corsair/products/tech/memory_basics/