The PC100 Standard

The memory chips used on memory modules are not the only determining factor when calculating the speed of memory. The speed of the memory chips is only one of several factors in the overall equation.

Other factors include the quality and accuracy of the individual chips, the printed circuit board (PCB), the engineering of the circuitry on the PCB and the presence of an SPD (serial presence detect) device, to name a few.

The PCB contains the memory module's circuitry and is a critical factor in determining how fast that module will run. As an example, if a module uses all 100MHz parts, that in itself does not mean that it will run reliably, or run at 100MHz. This is due to the demands placed on the memory module circuitry itself, beyond the chip. To ensure that 100MHz modules really are capable of running at 100MHz, Intel developed the PC100 Standard.

The PC100 Standard, presuming that all manufacturers adhere to it, ensures the compatibility of SDRAM subsystems with Intel's 100MHz FSB (front side bus) processors. The adherence to specifications set forth in the PC100 Standard significantly enhances data throughput, bringing system performance to a new level. This rigorous design specification, and the accompanying production and test requirements, have also created new challenges for semiconductor companies, as well as memory module manufacturers and suppliers.

Each PC100 SDRAM module requires specific key attributes in order to guarantee full compliance, such as the use of 8ns DRAM components (chips) that are capable of operating at 125MHz. This provides a margin of safety in ensuring that that the memory module will be able to run at PC100 (100MHz) speeds.

In addition, SDRAM chips must be used in conjunction with a correctly programmed EEPROM, a serial presence detect device that accurately defines the memory's characteristics, on a properly designed Printed Circuit Board. Properly engineered trace circuitry is also very important. The shorter the distance the signal needs to travel, the faster it runs. For this reason, there are additional layers of internal circuitry on PC100 modules, as opposed to 66MHz and EDO modules. SDRAM chips and modules that are not fully compliant with the PC100 specification severely compromise the quality and reliability of the entire PC system. At speeds of 100MHz and above, timing is so critical that every aspect of the memory Bus, from trace lengths, impedance, as well as the memory chips themselves, must be perfect.

The requirements and specifications called for in the PC100 Standard spells good news for end-users, in that users will experience increased Bus speeds, faster throughput, faster processors and a noticeable improvement in a PC's overall performance.

Not all 100MHz modules are created equally!

Unfortunately some module assemblers and vendors use 100MHz SDRAM chips to build modules and label them as PC100 compliant, when in fact they aren't compliant at all. You may want to see what we found in our PC100 Compliance review. A 100MHz SDRAM chip peaks at a Bus speed of 100MHz, and it cannot sustain the speed required in the PC100 specification. A module labeled as 100MHz may not even allow your system to run as fast as a qualified PC100 module would allow! A SDRAM chip that internally clocks at 125MHz is necessary to sustain a 100MHz local Bus speed. It is important for consumers to closely analyze SDRAM offerings, especially those that are "to good to be true", to ensure PC100 compliance. Since it is not easy to verify PC100 compliance, purchasers are well advised to do at least two things, (A) buy PC100 SDRAM modules that only use 125MHz or 8 nanosecond parts, and (B) only purchase modules manufactured by a reputable supplier willing and able to meet the PC100 specification.

As memory speeds increase, so will the speed with which modules manufacturers make modules available in the marketplace. Some unscrupulous suppliers will waste no time in caching in on the ignorance of buyers who do not know how to identify memory. You will find allot of information in our Introduction to Memory, our discussion of PC100 Compliance Issues as well as our discussion of How to Identify Memory in general and How to Identify PC-133 memory modules. With the recent break through in memory architecture and design, PC100 is being quickly replaced by PC133, Rambus® memory and DDR (double data rate) SDRAM. As a result, there is now a PC133 standard, as well as standards for Rambus® and DDR. Although all of the same principles exist as with PC100, these new standards create an even more stringent component and PCB design requirement upon chip and module manufacturers.

Should you upgrade to SDRAM?

Although most personal computers manufactured over the last eighteen months or so already use either PC100, PC133 SDRAM or Rambus DRAM, you may want to verify whether or not your computer supports the use of SDRAM. You may only use SDRAM if the chipset on your motherboard supports it. The motherboard chipset is the part of the your computer that moves data to and from the processor. SDRAM modules are typically in the form of a 168 pins DIMM for desktop use, 100 pin DIMM or 144 pin SODIMM for a laptop or printer use, or a 72 pin SIMM for most FPM and EDO modules. DDR SDRAM comes in the form of a new module type, a 184 pin DIMM, and Rambus® is in the form of a 184 pin RIMM. Simply said, you motherboard must have the on-board chipset and slots that support and accommodate DIMMs. There are also 80 pin SIMM modules used only for flash memory, 200 pin SODIMM's for use in next-generation DDR Laptop applications and 66 pin AIMM modules, known as a GPA or graphics performance accelerators, which are highly specially memory modules used only in systems which utilize Intel's 815E chipset and integrated graphics.

There are some motherboards that will support either or both EDO and SDRAM memory, however you must remove the EDO modules to take full advantage of the speed of SDRAM in your computer. Systems typically only run as fast as the slowest memory installed on the motherboard.

Are different speed SDRAM backwards compatible?

To a certain extent, yes. You can use PC133 memory in a PC100 system and you can use PC100 memory in a 66MHz system, subject to the specifications of the motherboard. However, the memory will only run as fast as (A) the capability of the motherboard, and (B) the slowest memory present in the system. As an example, a PC133 module will run at PC100 speed in a system with a 100MHz Bus, or if there is a *compatible PC100 module already present, it will only run at 100MHz.

*While we do not recommend mixing and matching PC100 with PC133, it can be done as long as the two modules are compatible.

Some people, in planning for a total future upgrade, will buy all of the total PC133 memory they anticipate needing and install it in their present system, even though it doesn't support PC133 SDRAM, with the intent to upgrade the system at a later date with a new motherboard that supports PC133 memory. In this way, you can simply take the memory out of your old system and use it in the new one.

Looking for more technical details? Contacts us, we will be more than happy to answer your questions.

Is SDRAM going to be around for a while?

Any upgrade is an investment, and we fully understand that you don't want to buy a new computer only to find it will be obsolete in the near future. Unlike many computer manufacturers, we do not pre-plan obsolescence into the computers we build. To the contrary, we pre-plan as much upgrade-ability into them as reasonably possible.

To answer the question, yes, SDRAM will be with us for several years. Memory upgrades, chosen carefully, are an investment and not wasted money. The DRAM industry continually develops better and faster products. Currently, there are a few new memory developments, such as DDR and RAMBUS, which you might want to read about.

If you would like to review more about memory related issues, you may want to follow these links:

Memory, Evolution or a Revolution?

How Memory Speeds Are Determined

How to Identify PC-133 Memory Modules

Frequently Asked Questions About Memory

Troubleshooting Memory Problems

Megabyte (MB) vs. Megabit (Mb)

Memory Trends in 2001

How Much Memory Do You Need?

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This page updated: 11/01/2000

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