DDR/General Memory FAQ (2003)

[Floppyman]

DDR/General Memory FAQ:

What does DDR stand for?

The Acronym DDR stands for Double Data Rate.

How does DDR ram work?

Conventional sdram gets clocked once per cycle. DDR ram on the other hand gets clocked twice per cycle allowing for twice as much data to be transferred in one cycle.

What does this mean?

This means that essentially 100mhz DDR ram runs really as fast as 200mhz sdram, because it transfers twice as much data in one cycle.

Picture the following analogy:

Imagine a wheel with one dot somewhere near the edge. Each time it rotates one full revolution we’ll call this one cycle. Imagine the dot to be a chunk of data. Let's really stretch our mind and imagine that each time the wheel makes a full revolution and the dot returns to its starting position one "chunk" of data has been transferred. This would be a good comparison of how normal sdram works.

Now, how does this picture change for DDR?

Imagine two dots on the wheel now, spaced 180 degrees apart. Once again these dots are to be chunks of data. Essentially what happens now is that each time the wheel makes a complete revolution not one, but two chunks of data are being transferred.

This is how DDR memory works. Twice as much data gets transferred in one cycle.

(I think it was Demosthenes who posted this analogy (or one similar to it) once; hope you don't mind me using it)

What types of DDR memory are currently available?

As of 6/20/03

pc1600 = 100mhz DDR (200mhz)
pc2100 = 133mhz DDR (266mhz)
pc2400 = 150mhz DDR (300mhz)
pc2700 = 166mhz DDR (333mhz)
pc3000 = 183mhz DDR (366mhz)
pc3200 = 200mhz DDR (400mhz)
pc3500 = 216mhz DDR (433mhz)
pc3700 = 233mhz DDR (466mhz)


What do all the different memory timings mean?

Timings come in terms of latencies, which can be usually thought of as delays (which come in clock cycles). However, latency can also mean the time (or number of clock cycles) available for an operation (e.g. a data transfer), therefore its meaning can be somewhat ambiguous.

I will touch on both of these definitions of latency below when I talk in detail about memory timings. Until explicitly stated so though, assume latency means delay.


Some manufacturers list the timings on their ram in the form:

A-B-C-D-E (where A-E are numbers and E is followed by a "T")


A = CAS latency
B = tRCD (RAS-to-CAS delay)
C = RAS precharge
D = tRAS
E = CMD

CAS = Column Address strobe
RAS = Row Address Strobe.

This would be the correct order if A-E is present. Some manufactures might only list A, or only list A-C, etc.

Okay, fine, now what does all this mean?

First let's talk some memory basics:

Think of your memory as a matrix or spreadsheet where each cell or data chunk has a row AND column address. Now let's go through a really simplified description of how memory is read incorporating some of the terms above. The Row address is what is accessed first. After that there is a few clock cycles wait (the "B" timing above) and then the column address is accessed. Finally there is couple more cycles wait before the data appears on the ram pins (the "A" timing above).

Now a little bit more in depth:


CAS latency or Read latency:

In other words, CAS Latency is the ratio of the memory's column access time divided by the current system clock. The column access time turns out to be a constant value, so you can see as you in increase the system clock it becomes harder to achieve a lower CAS latency (denominator grows-- numerator stays the same-- you get the idea).

From this and the example up above you can conclude that the lower the CAS delay or latency, the better.

RAS Precharge:

I was not able to find a whole lot on this but here is some info:

This is the number of cycles that are necessary for the Row Address Strobe to accumulate charge before the ram refreshes. In general, the lower the lower this value the better.

RAS-to-CAS delay:

This is the latency between the Row Address Strobe and Column Address Strobe. Basically the delay between accessing the Row and the Column memory addresses. Once again here, the lower the better.

tRAS also known as Row Address Strobe (RAS) Pulse Width:

This is somewhat difficult to explain, but I was able to find some good info from Mushkin. In this case, latency now means the amount of time available. In simple terms, it is the number of clock cycles available for a memory read. This is usually calculated as B+A+2 extra cycles. If the pulse width is too short (i.e. the latency value is too low) it can result in a cut off data transfer. This would mean that the transfer has to be repeated-- hence too low of tRAS value would actually result in a performance decrease.

CMD or Command:

This is the time (here time being a clock cycle delay again) it takes the chipset to translate a virtual memory address into a physical memory address.


Memory Density:

To calculate the density of the individual dram chips on your memory module, first determine

a) the size of your memory stick
b) the amount of chips on your memory
c) How many megabytes each individual chip holds

For example let’s say you have 512mb stick of ram that has 8 memory chips on it. Simple math will tell you that each chip must hold 64mb. Now to determine the density of one dram chip multiply its capacity (in megabytes) by 8 bits. This will yield a density of 512 megabits. Now let’s assume the stick had 16 separate dram chips on it. 512mb divided 16 yields 32mb. Multiply this value 8 and you get 256 megabits. This makes sense. You have twice as many chips so half the density. Simply put, the number of dram chips on your ram stick, and the density of each chip, are inversely related.


I just bought a stick of 512mb 64x64 ram. What does 64x64 mean?

The first number refers to the capacity of each individual dram chip (in megabytes). The second number (in bits) refers to the width of the memory bus or how many words can fit through it. One word equals approximately 8bits= 1byte. So, in this case the bus would be about 8 words wide (8words x 8 bits = 64).


Now, parity/ECC ram uses a 9th bit for error correction. This is why you see this kind of memory advertised as 32x72, 64x72 etc. Now each word is made up of 9 bits instead of 8. The bus is still 8 words wide, but a 9th bit is used with this type of memory. (8 words x 9 bits =72)


---------------------------------------------------------------------------

This is information I gathered from various sites on the Internet. I don’t know how exactly accurate my descriptions are so please feel free to correct me— doing that will make this a better FAQ. My hope is that this will help people out that are little confused with this stuff.

Information was found at the following websites:'

www.corsairmicro.com
www.rojakpot.com
www.mushkin.com
www.sharkyextreme.com
www.lycos.com

[edit by Force Flow]:
Corsair put up this RAM timings performance guide just recently. It's a little complex, but interesting, nevertheless. I didn't want it to get lost in the rest of the thread, so I'm sticking it here.

http://www.corsairmemory.com/corsair...erformance.pdf

[Nuclear Krusader]

I have seen some talk about Dual DDR lately. That it has to be paired or something like that.

Can you shed some light on the matter?

[Spyda]

Dual DDR means there are dual channels for memory instead of just one, its supposed to theoretically double the memory bandwidth, so if you had 2X 256MB of PC3200 RAM you would have a theoretical bandwidth twice that the RAM is rated for.
PC3200 is rated for a max theoretical bandwidth of 3.2GB/sec or 3200MB/sec, hence its PC**** name.
so on a dual channel DDR motherboard, the theoretical max bandwidth would be 6.4GB/sec, or 6400MB/sec.
This theoretical bandwidth exactly matches the new 800MhzFSB P4s bandwidth, which is why all 800Mhz supporting motherboards have dual channel DDR, designed for PC3200 RAM so there will be no bottlenecks.

[TwoRails]

DualDDR is different than "Double-Pumped" or "Quad-Pumped."

800FSB is a Quad-pumped 200 FSB CPU, a 333FSB is a Double-pumped 166 FSB CPU, and a 266FSB is a Double-pumped 133 FSB CPU.

DuallDDR just give better performance than the same setup without it. The theoretical bandwidth does double, but benchmarks show a typical overall system gain of 5 - 20 % depending on benchmark.

[TwoRails]

Floppyman!! Very nice post!! I'll bookmark this one for sure!!

Thanks

TwoRails

[Joeberg]

Indeed

[jra64]

To use dualDDR do u HAVE to have pc 3200 cuz i just bought pc 2700 ram for my comp and i wanna know if it will run dual.

I know that i DOES support dual.

By the way NICE article =)

[glc]

2700 will run dual no problem.

[jra64]

ok heres what my manual says:

"This motherboard provides four 184-pin DDR DIMM slots for single/dual channel DDR 400 memory modules with memory expansion size up to 4GB"

ARE you sure that it will? It says DDR 400 =(

[Floppyman]

What motherboard is it?

[ComputerDragon79]

I would like to know more about the dual ddr myself, because my friend just brought a motherboard that has dual ddr but it says it supports PC2100, 2700, 3200. Will any type of retail RAM work (such as Centon)?

[PilotinCommand]

I haven't been here in a wile but I came ocroos Corsair's website and I found an interesting presentation covering RAM stuff. It's very similar to this thread it's just presented via clear ilustrations. I couldn't keep this to myself so check out the presentation via this link.

http://www.corsairmemory.com/memory_...707/index.html

PiC

[TwoRails]

Very nice link, PilotinCommand !!

[ultralkalie]

man i have 100% more knowledge about how memory memory works because of this thread, thanks guys

[Nuclear Krusader]

Does dual ddr have to be installed in pairs, as the rambus memory?

[Floppyman]

Originally posted by Nuclear Krusader Does dual ddr have to be installed in pairs, as the rambus memory?

Yes, you need two identical sticks for it to work. HTH

[Hdcgamer]

I have a question specifically related to my system: when I go into my bios to set my ram frequency, I set it to 100% and it says the resulting frequency is 166mhz. What's up with this? Shouldn't it run at 200 with DDR400? Does this have anything to do with the fact that my processor has only a 333 mhz fsb?

[TwoRails]

Yes: 166 x 2 - 333 fsb (don't ya love computer math??)

[Hdcgamer]

That wasn't my question.

[Floppyman]

In short, yes. Your cpu's FSB is 333mhz (166x2). That explains why your memory is running at 333mhz. Look for a cpu/memory ratio option (or something similar) in your board's bios. What is this currently set to? If your board has this type of option, changing it might allow you to run your ram at 400mhz even though your cpu's FSB is only 333mhz.

[Spyda]

i think its 5/4 or something

[Floppyman]

If he's using pc3200, shouldn't it be 5/6 (166/200)?

[Hdcgamer]

What should I change it to to run the memory at 200mhz?
5/6?

[Floppyman]

If it's cpu/memory ratio and you've got that option, I believe the answer is yes.

[Floppyman]

Quick Update 7/19/03:

pc4000 DDR ram is starting to become available:

pc4000 = 250mhz DDR (500mhz)

[dmt]

ok now, I have 3 DIMM slots in a dual channel board. I am running Mushkin dual cahnnel pc2700 2x256mb cas 222. I have them installed in DIMM slot 3 (master slot) and slot 1. If I add another stick of the same ram into the #2 slot, will it still run in dual channel?

[Spyda]

yes, it should, as long as the dimm3 master slot is occupied DC should be enabled

[DarkHorse]

Just for clarification, any 2 (near identical) DDR memory sticks should be able to run as a dual unit on a compatible mobo, right? The reason I ask is that I saw "paired" memory by Corsair, and wondered if there was some other requirement I didn't know.

I'm pretty sure the reason for pairing is so one stick isn't held back by the other's shortcomings, those caused by normal fluctuations in production.

I seem to remember a review mentioning that when benchmarking the Dual'd unit behaving as the slowest component...sort of the weakest link, while when separated one stick performed slightly better. Can anyone verify this?

[TwoRails]

Mixing memory is always risky, quite often they just don't work together. Newer mobos and memory is closing the gap pretty good, though. So, many mis-matches that wouldn't run just a year or three ago seem to run together OK now.

As for as mixing them up on DualDDR, I don't recall seeing anybody benching a mis-match. I would still recommend at least buying the same brand an part number to be safe.

So far I have not seen any compelling reason to spend the extra $$ for these newer "matched" sets that Corsair and others are selling.

And yes, any memory will run only as fast as the slowest stick in the system, if they are compatible enough to run at all.

HTH

TwoRails

[Spyda]

just as a DC sort of confirmation thing, if you run Sisoft SANDRAS memory bandwidth benchmark and its over the rated bandwidth for your memory, you know DC is enabled, if its under, you know it isnt.

My brothers board supposedly has DC, and in its current configuration, it should be DC, yet at POST it does not say DC enabled, and in SANDRAs bandwidth bench, he gets 2800MB/sec with PC3200 memory, i have the exact same memory but i know DC is enabled, and my bandwidth is close to 4000MB/sec.

There is also a new version of CPU-Z which tells you whether DC is enabled

http://www.cpuid.com/cpuz.php#download