RAM Guide

Memory Timings

Memory timings are also known as refresh rates. Since your memory is sustained electrically, it must be recharged constantly. Memory timing is how many rows of memory much be recharged at once. Without refreshing your memory, you'd lose the information stored in the memory addresses.

A useful technology used in your laptop is self-refreshing memory. This memory can, well, refresh itself. This means the CPU or other devices don't have to be involved, making less need to power. This helps conserve your battery, so you can play games longer on the airplane. :)

Burst Writes

Whenever data is moved from your main memory onto a disk (your hard disk, a floppy), it is called writing. There is a usual rate that your computer moves data from your memory to the disk. This is, naturally, the normal transfer rate. But there are certain times that something called burst mode exists.

Burst mode occurs when your memory temporarily "takes control of your computer." What happens is that the bus is reserved just for the transfer occurring between the disk and RAM. In this situation, your RAM produces a burst of information, getting transfer rates much higher than normal.

This burst speed cannot be sustained for long, as other devices in your system also need access to the bus. But during this tiny period of time, information shoots through your system with transfer rates skyrocketing.

Wait States

Generally your CPU is running much faster than your RAM. This means that your CPU cannot accesses your RAM every clock cycle. Though not limited to RAM, I'll discuss wait states in this context. As your CPU gets ahead of your RAM, if has to wait for the RAM to "catch up". This causes a temporary pause in your system. Though one maybe far too small to notice, as they begin to pile up, you'll find your applications running slower.

This is why SDRAM, and BEDO DRAM to a lesser degree, are so much more advanced than regular RAM. These two types of RAM are able to keep up with clock speed, allowing your CPU to interface with the memory at every clock cycle and eliminate wait states.

BEDO DRAM is capable of keeping up at this rate for only short periods of time, called bursts (hence the name Burst EDO DRAM). SDRAM, however, can keep up with your CPU for much longer periods of time, synchronizing itself with your CPU at bus speeds of up to 100 MHz, maybe even more! So be prepared to watch SDRAM or one of its successors take over the market.

RDRAM - The Future of Memory

RDRAM is new type of dynamic memory invented by Rambus. RDRAM is incredible break through in memory technology. It is an extremely high-speed type of memory, running at 64 megabits. RDRAM is capable of bursts as fast as 2 ns a byte! In addition, RDRAM can handle transfer rates in excess of 533 MHz, making it the obvious choice for anyone wanting to handle the next wave of P2's (see the CPU comparison page).

RDRAM can also handle multiple Rambus Channels. Each of these channels can achieve transfer rates of up to 533 MB a second. This means that if you're running four channels (the maximum supported for one controller), you could get transfer rates over 2 GB / sec!

After reading the SDRAM section, you might have thought EDO DRAM sounded ancient since it only supported bus speeds of up to 83.3 MHz (and only 66 MHz officially). SDRAM seems far superior with bus speeds supported up to 100 MHz. RDRAM can support bus speeds up to 267 MHz! RDRAM can synchronize itself with your CPU bus as high as 133 MHz! This means incredible performance.

RDRAM will soon be appearing in PCs everywhere. Don't be surprised if your next computer uses RDRAM.

SDRAM - The Disappointment

After reading so much able to virtues of SDRAM (see the entry above), you're probably wondering why anyone would want anything but SDRAM. FPM RAM can only get access speeds down to 60 ns, EDO DRAM can only get to 45 ns, but SDRAM has gotten the time it takes your processor to access a memory address down to only 10 ns!

And SDRAM almost eliminates those pesky wait states (see above). By synchronizing itself to your CPU, SDRAM can interface with your processor at ever clock cycle! It can even support bus speeds higher than any type of EDO DRAM. SDRAM sounds incredible.

If you've seen benchmarks on RAM though, you might have noticed something. SDRAM only gives you about a 5-10% performance increase over EDO DRAM. There are two main reasons why SDRAM fails to perform.

If you've read above, you know at least a little of EEPROM. This type of ROM is absent from the SDRAM DIMMs. Used in RAM, EEPROM helps the memory function properly. Without, SDRAM has taken a performance hit.

The second reason that SDRAM just doesn't perform is that it does not have proper chipset support. Like many other types of RAM, SDRAM needs the support of the chipset to function at its best. Since the chipsets used today are optimized for EDO and not SDRAM, SDRAM once again is put at a disadvantage.

Well, it looks like SDRAM isn't destined to take over the market like it should have. But I feel that one of its successors might. If only a better variant of SDRAM could be made…maybe they could tack on that wonderful "II" to the name (hint hint)

SDRAM II - A New Hope

We all know that today's SDRAM is rated at a 100 MHz speed but I'm sure a few of us gamers and 3d developers are wondering whether or not this RAM can keep up with our Deschutes coupled with our 4x AGP video cards which we all say we are going to buy (or K-6 with a VP3 AGP) ... Taking into consideration that Deschutes will run at 100 MHz bus speed and your AGP card at 2x mode is going to have a 200 MHz pipeline to your main RAM, it seems like SDRAM just isn't good enough. What are you looking for now? RDRAM you might say, but considering the standards are still being argued about and that the RDRAM won't be manufactured probably around 1Q or 2Q 98' and that you can't wait that long to buy RAM for your Deschutes which doesn't come out until Q4, what will you do??? Well thanks to the decisions of the Jedec Consortium (Joint Electron Devices Engineering Councils), we now have another alternative - DDR SDRAM. DDR SDRAM is a new type of SDRAM that simply put has a Double Data Rate (hence DDR) similar to the AGP 2x mode essentially doubling the speed of the memory and DDR is the predecessor to SLDRAM (Sync-Link DRAM) the only difference is that SLDRAM has burst transfers upping the transfer rate to 400 MB/s. The DDR SDRAM spec will have them rated at 83.3 MHz, 100 MHz, and 125 MHz - in my personal opinion they should drop the 83.3 MHz version (those 100 MHz bus speed CPUs!!) keep the 100 MHz and 125 MHz and add 133 MHz version and the chips will perform like a 200 MHz, 250 MHz and 266 MHz parts!. If Implemented properly you can expect to see DDR SDRAM supported in Intel's Deschutes chipset (i440BX and i450NX) and possible in a later revision of the VIA VP3 (for K6 supporters). DDR's expected average Data Transfer rate is ~256 MB/s versus the SDRAM data transfer rate of ~128 MB/s. the average access time is <10 ns for DDR SDRAM and SDRAM is 8-12 ns. Both the faster access times and faster transfer rates give you a high speed solution for those of us who demand the fastest processors NOW :).

Submitted by Stephen O'Neal