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Synchronous DRAM (SDRAM)

A relatively new and different kind of RAM, Synchronous DRAM or SDRAM differs from earlier types in that it does not run asynchronously to the system clock the way older, conventional types of memory do. SDRAM is tied to the system clock and is designed to be able to read or write from memory in burst mode (after the initial read or write latency) at 1 clock cycle per access (zero wait states) at memory bus speeds up to 100 MHz or even higher. SDRAM supports 5-1-1-1 system timing when used with a supporting chipset. SDRAM accomplishes its faster access using a number of internal performance improvements, including internal interleaving, which allows half the module to begin an access while the other half is finishing one.

SDRAM is rapidly becoming the new memory standard for modern PCs. The reason is that its synchronized design permits support for the much higher bus speeds that have started to enter the market. SDRAM doesn't offer that much "real world" additional performance over EDO in many systems, due to the system cache masking much of that differential in speed, and the fact that most systems are running on relatively slow 66 MHz or lower system bus speeds. As 100 MHz bus system PCs become mainstream, SDRAM will largely replace older technologies, since it is designed to work at these higher operating speeds and conventional asynchronous DRAM is not.

There are several important characteristics and concerns regarding SDRAMs that are relatively unique to the technology. In addition to the notes below, you will want to read this informative article that goes into more depth on choosing SDRAM modules:

• Speed and Speed Matching: SDRAM modules are generally speed-rated in two different ways: First, they have a "nanosecond" rating like conventional asynchronous DRAMs, so SDRAMs are sometimes refered to as being "12 nanosecond" or "10 nanosecond". Second, they have a "MHz" rating, so they are called "83 MHz" or "100 MHz" SDRAMs for example. Because SDRAMs are, well, synchronous, they must be fast enough for the system in which they are being used. With asynchronous DRAMs such as EDO or FPM, it was common to add extra wait states to the access timing for the memory to compensate for memory that was too slow. With SDRAM however, the whole point of the technology is to be able to run with zero wait states. In order to do this, the memory must be fast enough for the bus speed of the system. One place where people run into trouble in this regard is that they take the reciprocal of the "nanosecond" rating of the module and conclude that the module can run at that speed. For example, the reciprocal of 10 ns is 100 MHz, so people assume that 10 ns modules will definitely be able to run on a 100 MHz system. The problem is that this allows absolutely no room for slack. In practice, you really want memory rated slightly higher than what is required, so 10 ns modules are really intended for 83 MHz operation. 100 MHz systems require faster memory, which is why the PC100 specification was developed (see below).
• Speed Rating: Due to the confusion inherent in the speed rating system described immediately above, and the likelihood of problems running slower SDRAM modules on new 100 MHz system bus motherboards, Intel created a formal specification for SDRAM capable of being used in these new PCs. Dubbed PC100, these modules generally are rated at 8 ns as previously mentioned, but there are other internal timing characteristics that must be met in order to have a module certified as PC100-compliant. While relying on a specification is never foolproof, it is definitely a good idea to ensure that any SDRAM you intend to use on a 100 MHz system bus motherboard is in fact PC100 specification compliant.
• Latency: SDRAMs are still DRAMs, and therefore still have latency. The fast 12, 10 and 8 nanosecond numbers that everyone talks about refer only to the second, third and fourth accesses in a four-access burst. The first access is still a relatively slow 5 cycles, just as it is for conventional EDO and FPM memory.
• 2-Clock and 4-Clock Circuitry: There are two slight variations in the composition of SDRAM modules; these are commonly called 2-clock and 4-clock SDRAMs. They are almost exactly the same, and they use the same DRAM chips, but they differ in how they are laid out and accessed. A 2-clock SDRAM is structured so that each clock signal controls 2 different DRAM chips on the module, while a 4-clock SDRAM has clock signals that can control 4 different chips each. You need to make sure that you get the right kind for your motherboard. The current trend appears to be toward 4-clock SDRAMs.
• Serial Presence Detect: Some motherboards are now being created that require the use of special SDRAM modules that include something called a Serial Presence Detect (SPD) chip. This is an EEPROM that contains speed and design information about the module. The motherboard queries the chip for information about the module and makes adjustments to system operation based on what it finds. A great idea in theory, but you won't think it's great if you buy an SDRAM module without the chip on it when your board requires SPD...
• CAS2 vs. CAS3: "CAS" stands for column address strobe, one of the main signals used in accessing DRAM chips; see here for an explanation of what CAS is all about. The terms "CAS2" and "CAS3" are used to distinguish between slight variants in SDRAM modules. In fact, the term is a misnomer; the "2" and "3" refer to the latency of the CAS line, so the terms should be "CL2" and "CL3". Theoretically a "CAS2" module is slightly faster than a "CAS3" module, making it more likely to function if the system bus is being overclocked beyond 100 MHz, but the whole matter of "CAS2" and "CAS3" has been overhyped to the Nth degree by many vendors.  Dean Kent's article on SDRAM terminology explains this in greater detail.
• Packaging Concerns: To make matters even more confusing, SDRAM usually comes in DIMM packaging, which itself comes in several different formats (buffered and unbuffered, 3.3 volts and 5 volts). You need to make sure you get the right type of packaging as well; see here for more on DIMMs.

Now that you've read that, do you feel a bit confused about exactly what type of SDRAM you need? I don't blame you! This is why I strongly advise working closely with your motherboard manufacturer and/or a trusted vendor in choosing your SDRAM. It's one thing to try to figure all of this out from specifications, but it's much better to contact the company that made your motherboard and have them say definitively that you need "10 ns, 4-clock, unbuffered, 3.3 volt SDRAM modules with serial presence detect", or whatever. You will want to examine at the very least, the bottom of the article Choosing Your SDRAM, for some specifics on what is important and what is not in selecting SDRAM modules.

Next: Double Data Rate SDRAM (DDR SDRAM)

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