The main "competition" to the proposed DRDRAM standard is a new standard called Synchronous-Link DRAM or SLDRAM. This new technology is being developed by the SLDRAM Consortium, a group of about 20 major computer industry manufacturers, working to establish SDRAM as the next standard for high-speed PC memory.
SLDRAM is an evolutionary design that greatly improves the performance of the memory subsystem over SDRAM, without a completely new architecture such as that used by DRDRAM. The initial specifications for SLDRAM call for a 64-bit bus running at a 200 MHz clock speed. As with DDR SDRAM, transfers are made twice on each clock cycle, for an effective speed of 400 MHz. This yields a net theoretical bandwidth of about 3.2 Gbytes/second, double that of DRDRAM. Finally, SLDRAM is an open standard, meaning that no royalties need be paid to anyone in order to make use of it.
Interestingly enough, the DRDRAM and SLDRAM battle seems to be playing out in a manner similar to many prior technological skirmishes. One that comes immediately to mind is the fight for dominance between BEDO and SDRAM in the mid-90s; though many thought that BEDO was better technologically, Intel single-handedly sealed its fate by deciding to go with SDRAM instead. Today, we have Intel going with DRDRAM, against a consortium of companies trying to push SLDRAM as a better solution. However, as we enter 1999 we have more non-Intel choices in processors and chipsets than we did in 1996, so it is not clear at all if Intel will have its way in
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establishing DRDRAM over SLDRAM as the next standard. Another factor that will support SLDRAM is that it does not require the payment of royalties
the way DRDRAM does, something that could seriously harm the DRDRAM camp despite the presence of Intel.
Video RAM (VRAM) and Other Video DRAM Technologies
Modern video adapters use their own, specialized RAM that is separated from the main system memory. The demands placed on video memory are far greater than those placed on system memory. In addition to the video image being accessed and changed by the processor on a continual basis (many times a second when you are running a game for instance), the video card also must access the memory contents between 50 and 100 times per second to display the information on the monitor. Video cards have therefore spawned the creation of several new, innovative memory technologies, many of them designed to allow the memory to be accessed by the processor and read by the video card's refresh circuitry simultaneously. This is called dual porting and is found on Video RAM or VRAM memory. Cards using this type of memory are faster and more expensive than ones using FPM or EDO DRAM.
In addition to VRAM, several other new memory technologies and designs have evolved to maximize performance with video cards.
Memory Errors
Memory is an electronic storage device, and all electronic storage devices have the potential to incorrectly return information different than what was originally stored.
Some technologies are more likely than others to do this. DRAM memory, because of its nature, is likely to return occasional memory errors. DRAM memory stores ones and zeros as charges on small capacitors that must be continually refreshed to ensure that the data is not lost. This is less reliable than the static storage used by SRAMs.
Every bit of memory is either a zero or a one, the standard in a digital system.
This in itself helps to eliminate many errors, because slightly distorted values are usually recoverable. For example, in a 5 volt system, a "1" is +5V and a "0" is 0V. If the sensor that is reading the memory value sees +4.2V, it knows that this is really a "1", even though the value isn't +5V. Why? Because the only other choice would be a "0"
and 4.2 is much closer to 5 than to 0. However, on rare occasions a+5V might be read as +1.9V and be considered a "0" instead of a "1". When this happens, a memory error has occurred.
There are two kinds of errors that can typically occur in a memory system. The first is called a repeatable or hard error. In this situation, a piece of hardware is broken and will consistently return incorrect results. A bit may be stuck so that it always returns "0" for example, no matter what is written to it. Hard errors usually indicate loose memory modules, blown chips, motherboard defects or other physical problems.
They are relatively easy to diagnose and correct because they are consistent and repeatable.
The second kind of error is called a transient or soft error. This occurs when a bit reads back the wrong value once, but subsequently functions correctly. These problems are, understandably, much more difficult to diagnose! They are also, unfortunately, more common. Eventually, a soft error will usually repeat itself, but it can take anywhere from minutes to years for this to happen. Soft errors are sometimes caused
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by memory that is physically bad, but at least as often they are the result of poor quality motherboards,
memory system timings that are set too fast, static shocks, or other similar problems that are not related to the memory directly. In addition, stray radioactivity that is naturally present in materials used in PC systems can cause the occasional soft error.
On a system that is not using error detection, transient errors often are written off as operating system bugs or random glitches.
The exact rate of errors returned by modern memory is a matter of some debate. It is agreed that the DRAMs used today are far more reliable than those of five to ten years ago. This has been the chief excuse used by system vendors who have dropped error detection support from their PCs. However, there are factors that make the problem worse in modern systems as well. First, more memory is being used; 10 years ago the typical system had 1 MB to 4 MB of memory; today's systems usually have 16 MB to 64 MB--or much more, since RAM prices have fallen dramatically in the last three years. Second, systems today are running much faster than they used to; the typical memory bus is running from 3 to 10 times the speed of those of older machines.
Finally, the quality level of the average PC is way down from the levels of 10 years ago.
Cheaply thrown-together PCs, made by assembly houses whose only concern is to get the price down and the machine out the door, often use RAM of very marginal quality.
Regardless of how often memory errors occur, they do occur. How much damage they create depends on when they happen and what it is that they get wrong. If you are playing your favorite game and one of the bits controlling the color of the pixel at screen location (520, 277) is inverted from a one to a zero on one screen redraw, who cares, right? However, if you are defragmenting your hard disk and the memory location containing information to be written to the file allocation table is corrupted, it's a whole different ball game...
The only true protection from memory errors is to use some sort of memory detection or correction protocol. (Well, that's not totally true. The other form of protection is prevention: buying quality components and not abusing or neglecting your system.) Some protocols can only detect errors in one bit of an eight-bit data byte;
others can detect errors in more than one bit automatically. Others can both detect and correct memory problems, seamlessly.
Causes
The memory errors that your PC is likely to suffer fall into two broad classes, soft errors and hard errors. Either can leave you staring at an unflinching screen, sometimes but not always emblazoned with a cryptic message that does nothing to help you regain the hours’ work irrevocably lost. The difference between them is transience.
Soft errors are little more than disabling glitches that disappear as fast as they come.
Hard errors linger until you take a trip to the repair shop.
Soft Errors
For your PC, a soft memory error is an unexpected and unwanted change.
Something in memory turn up different than it is supposed to be. One bit in a memory chip may suddenly, randomly change state. Or a glitch of noise inside your system may get
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stored as if it were valid data. In either case, one bit becomes something other than what it’s supposed to be, possibly changing an instruction in a program or a data value.
With a soft error, the change appears in your data rather than hardware.
Replace or restore the erroneous data or program code, and your system will operate exactly as it always has. In general, your system needs nothing more than a reboot—a cold boot being best to gain the assurance of your PC’s self-test of its circuits (including memory). The only damage is the time you waste retracing your steps to get back to the place in your processing at which the error occurred. Soft errors are the best justification for the sage advice, "Save often."
Most soft errors result from problems either within memory chips themselves or in the overall circuitry of your PC. The mechanisms behind these two types of soft errors is entirely different.