System DRAM Technologies

System DRAM Technologies

DRAM Technology Types

As discussed earlier, DRAM was originally created to act as a fast middle-man to slow storage devices. Once the small section of data actually being used was offloaded into memory, it could work with the microprocessor much faster and nearly eliminate delays.

As time passed, however, hard disks and microprocessors began to hit the market in faster and faster models, to the point where the DRAM itself could be considered a major bottleneck. In order to eliminate that bottleneck, DRAM itself needed to evolve.

Today, there are 10 different commercially available types of system DRAM (excluding parity / ECC functions).

Page Mode DRAM - (DRAM)

The first brand of DRAM. It provided a fine solution for ancient ISA Video cards and 286-486 PCs. However, when the performance of CPU's began to increase more rapidly, it was quickly thrown out to make way for faster and more efficient DRAM technology.

Fast Page Mode DRAM - (FPM DRAM)

The first popular redesign of DRAM used a new feature called Fast Page Mode, which by default receives performance gains if the next access is identical or concurrent to the prior access. This form of DRAM was also manufactured at sub 60 ns ratings for various video cards.

Extended Data Output DRAM - (EDO DRAM)

EDO DRAM provides another performance boost by decreasing latency timing and providing quicker sequential reads. EDO DRAM is also seemingly more capable of handling 60 ns, which makes it ideal for 66 MHz motherboards.

Burst Extended Data Output DRAM - (BEDO DRAM)

BEDO provided a performance increase much like the small evolution that Fast Page mode does. BEDO improves over EDO by allowing bursts of data to be read, rather than one set at a time. It is hard to find, however, as it was only useable with certain 3rd party chipsets, and quickly overshadowed by SDRAM.

Enhanced DRAM - (EDRAM)

An older product from Enhanced Memory Systems, inc. It combines SRAM caches with FPM DRAM to provide excellent performance for 50 MHz bus speeds. By hiding DRAM charge time, it also improves page miss latency.

Multibank (Burst) Extended Data Output Enhanced DRAM - (Multibank EDO EDRAM) (Multibank BEDO EDRAM)

A specific product available from Enhanced Memory Systems, Inc. It combines the multibank SRAM caches of MDRAM with the read/write advancements of EDO/BEDO DRAM, combined into a SIMM/DIMM package for system memory.

Synchronous DRAM - (SDRAM)

System Memory finally took the needed step up into high performance with SDRAM. Unlike prior DRAMs, SDRAM operates synchronously with the System Clock Speed! Combined with the incredibly low timing rates, it nearly matches SRAM for speed.

PC100 Specification Synchronous DRAM - (PC100 SDRAM)

Technically, there is no difference between this and normal SDRAM - the only change is the better (lower) nanosecond rates. There are actually several levels of PC100 specification, dealing with latency and PCB layers, but anyone who wants to use it with a 100 MHz motherboard should stick with 8 ns or lower. Most PC100 SDRAMs also include an EEPROM with Serial Presence Detection, which helps the motherboard automatically adjust settings.

Enhanced Synchronous DRAM - (ESDRAM)

Another company decided to try a new memory technology, called ESDRAM.

It is supposedly capable of reducing internal latency and cycle times by 2x while offering twice the peak bandwidth of RDRAM. Like CDRAM, it apparently uses hybrid technology to achieve the gains in performance. Specific information on this memory technology can be found at the ESDRAM Website.

Double Data Rate Synchronous DRAM - (DDR SDRAM)

The first evolution to SDRAM. It doubles potential bandwidth by sending data on the rise and the fall of each clock signal. Because it is compatible with traditional SDRAM architecture it is expected to supplant SDRAM in the near future.

Direct Rambus DRAM - (DRDRAM)

In an attempt to provide the memory market with a high speed RAM capable of staying in the marketplace for years, RDRAM was born. It uses 16bit DRAM chips clocked at 400-800 MHz combined with DDR technology to achieve its unmatched clock speed. Specific information on this memory technology can be found at the Rambus Website.

Synchronous Link DRAM - (SLDRAM)

The third powerful next generation memory is known as SLDRAM. It utilizes multiplexing technology to achieve higher bandwidth through each of the pins. It is also considered to be the strongest competitor against RDRAM. SLDRAM, Inc. has more information on this emerging technology.

What are Parity and ECC (Error Checking and Correction)?

Early on, RAM was not as stable a solution as it is today. Irregularities could cause the data in memory to corrupt or alter in ways that often led to a system crash or hard disk data damage. This problem was first solved with Parity RAM. Through additional or modified chips, it added an additional bit to each byte of RAM which verified the validity of each byte. If the data did not check out properly, your computer would typically halt to avoid further problems.

ECC added a further process to the cycle. Instead of merely checking the bytes, it can correct most errors with an extra bit. It is fairly popular with the CAD crowd, as it helps maintains strict accuracy. For most consumers, however, it is not necessary due to the low rate of errors in today's memory, and actually involves a slight performance hit.

What are RAS and CAS?

They stand for Row Access Strobe (RAS) and Column Access Strobe (CAS). Each describes how long it takes to read a row or column of memory cells, known as the CAS/RAS Latency. Each is described with a rating number, where lower numbers are better. The rating is also dependant on the front side bus speed of your motherboard so the rating may rise on higher speeds.

Direct RDRAM versus SLDRAM

The speed at which technology gets faster is increasing exponentially as the millenium approaches, with the invent of three distinct new memory technologies: Direct RDRAM and SLDRAM.

They both offer clock speeds of 400 MHz and beyond (4 times the speed of today's system bus!) with better latencies and timing rates than today's SDRAM. On paper, the differences seem to be extreme enough to give hesitation to people buying memory today.

Whether these two new types can coexist is seriously in doubt, however. While Rambus has Intel on its side (Intel is responsible for motherboard chipsets which control what types of RAM can be used), memory manufacturers around the industry have complained about the licensing fees, patent clauses and new form factor involved.

Companies like Micron are pointing to SLDRAM as an easily comparable technology that doesn't require fees (SLDRAM, inc. is non profit) or patent clauses, while still using today's DIMM form factor and interface.

Currently only Fujitsu is a member of the Rambus consortium - only time will tell if Intel or the world's memory manufacturers will prevail.