RAM 101 (And it begins…)

4 07 2008

I own a Dell Dimension 4500S. I’ve been trying to tweak it and change parts to make it preform better. I got it for free off of some woman my father works with. She didn’t want/need it, so it came to me.

This is how it was shipped to her: WinXP Home Edition, 128mb RAM, 1.78GHz processor (single core), some USB2.0 ports, a 20gb hard drive, 8mb of emulated on-board video RAM.

<sarcasm>Terrific.</sarcasm>

First things first… the RAM. I can’t run the computer without actually watching it redraw the windows on the left and then on the right of existing windows. Forget gaming, recording music, video editing, or even listing to music or surfing something like YouTube.

RAM is getting cheaper these days, so I wanted a lot, but I needed to check how much RAM the machine could handle. The old home computer that my family owns maxes out at 512mb… that’s a whopper of a total, weighing in at only 256mb per RAM slot. The max is important, because the slot won’t recognize / can’t utilize more RAM than its max.

RAM max for a single Dell Dimension 4500S slot: 1024 (1gb)

RAM slots on the standard 4500S motherboard: 2

Total: 1024mb x 2 = 2048mb (2gb)

So, http://www.tigerdirect.com is one of my preffered sources for such computer parts. Problem is, I need to know what kind of RAM my computer can make use of. The motherboard is what decides what I’m cabable of using. Skip over the terms section if you don’t care about it.

Terms to be familiar with: SDRAM, DDR, DDR2, Dual Channel

  • SDRAM stands for “synchronous dynamic RAM” and is an advancement of DRAM. RAM works by having some address of memory open for reading/writing. The processor can then request information from the RAM and the RAM will faithfully deliver it. DRAM only keeps one address open at any given time, while SDRAM keeps two open. Why? This cuts down on the delays caused by accessing various parts of the RAM rapidly (which is virtually always the case). The RAM must close one address before opening another. SDRAM cuts the delays by supporting two open addresses. The RAM can then send/write data to one address, while a second address is being prepared. When the second one is being accessed, the first one is closed and then adjusted to point to where the processor wants to go next. The first one gets the spotlight again, and then the second closes and goes and finds its next target. You get an overlapping effect. The addresses aren’t being read at the same time, but rather, the second address is filling in the gaps where delays (caused by closing and opening addresses) are occuring. The processor effectively doesn’t even need to care if the RAM is SD or just plain D.  In effect, this method of doubling speed is very internal.
  • DDR stands for “double data rate“, and is SDRAM. If you know much about how computer circuitry works, then you’ll know that the electronics’ speed is regulated by the internal clock. That’s the processor. The faster it can go, the faster the parts can operate. In the modern age, the clock regulates just about every part, making the design “synchronous”. All changes in the computer’s memory can only occur once per clock cycle, either at the beginning or the end of the cycle. DDR revolutionized this idea by allowing changes to the RAM at both the beginning and the end of the clock cycle, thus doubling the number of changes that could be made in a single clock cycle. This method brings an increase in speed that nears the external side of improvements.
  • DDR2 is an improvement on DDR which makes it twice as fast. Normal SDRAM pushes a single bit of data out each clock cycle (remember, it has two addresses open simultaneously, but only one is read/written at a time), while DDR SDRAM pushes a 2-bit line of data out (once on rising edge, once on falling edge of the clock), and then DDR2 SDRAM pushes out 4-bit data per clock cycle.
  • Dual Channel RAM is capable of yet again doubling the data transfer rate. Dual Channel memory starts to complicate things. According to most sources, mixing memory types can work, but the stability is often contested. It depends on some things… things that you really don’t have much control over, like how good your motherboard’s memory controller is. Ideally, you should match capacities and types. Ideally, you don’t want to mix memory types because the slowest one will determine the speed at which the computer will operate its memory processing. Consequently, the faster of the two modules would have reduced performance. Motherboards that support Dual Channel memory will divide its RAM slots into “banks” of (at least?) two slots. They’re usually color coded. The idea is that two identical memory modules can be put into the same bank, and then the CPU will know how to inferface with them most effectively
  • Other terms include DIMM and RIMM, but you likely don’t need to worry about that.  do a google search for “define: DIMM” if you want to know more about it.  Memory today works on DIMM technology.

So, the 4500S can only handle DDR SDRAM. Not DDR2, not Dual Channel. Just DDR. And since the slot max is 1gb, I have to buy two 1gb sticks of DDR SDRAM.

Now, the other variable here is the speed of the RAM. RAM comes in varying speeds. In fact, the speed of the RAM is most usually the actual bottleneck of speed. The processors of our computers are blazing fast, but the RAM can’t so easily match that speed. (That’s why we keep trying to double the speed of our RAM these days through varying techniques.) Check out this chart (taken from http://www.buildorbuy.org/ramchart.html):

DDR SDRAM DIMMs Data Rate FSB Peak Bandwidth
PC1600 = DDR200 200MHz 100MHz 1.6 GBps
PC2100 = DDR266 266MHz 133MHz 2.1 GBps
PC2700 = DDR333 333MHz 166MHz 2.7 GBps
PC3200 = DDR400 400MHz 200MHz 3.2 GBps
PC3500 = DDR400 433MHz 217MHz 3.5 GBps
PC3700 = DDR466 466MHz 233MHz 3.7 GBps
PC4000 = DDR500 500MHz 250MHz 4.0 GBps
PC4200 = DDR533 533MHz 266MHz 4.2 GBps

As you can see, the number postfixing “PC” in the left column corresponds to the data transfer rate.  The secret is that the speed of the RAM is bolstered by all of this DDR and Dual Channel nonsense.  The middle column is how fast the RAM can go, after applying all of the doubling factors like DDR or Dual Channel.  The fourth column is the speed of the broken-down parts of the RAM.  For instance, PC3200 RAM can operate at 400MHz total, but if you break it out of its DDR context (which allows the RAM to work on both the beginning and end of a clock cycle), then the RAM is really only running at 200MHz per clock cycle.  DDR capabilities (be it DDR or DDR2) allow the processor to interface with the RAM twice as much.

Anyway. My motherboard documentation says that I needed PC3200-type RAM. I only bought one stick for now, since that’s plenty for an old XP Home edition installation.

Perhaps my music recording will push that limit though… *shrug*

(Awesome list of terms and brief definitions: http://www.satech.com/glosofmemter.html)

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