From Novice To a Masterpiece.

Rambus Dynamic Access Memory

Introduction

RDRAM or Rambus Dynamic Random Access Memory is the latest in the field of RAM. Though it is considered as a successor to the SDRAM, it is way too far away from it to be called as a successor. This is not only vis-a-vis reliability but also with respect to performance. Before we can proceed with the discussion about this latest entrant, let me give you a very brief intro to the other types of RAMs that are available. If you thought 133MHz was the fastest RAM speed that you could get, then get ready to enlighten yourself.

RDRAM is developed by Rambus Inc., who are the leading manufacturers of RAMs. Currently, SDRAM is the fastest, clocking speeds of 133MHz. But RDRAM completely eclipses its performance with a blitzkrieg 600MHz.

FPM RAM (Fast Page Mode DRAM)

A timing option that permits several bits of data in a single row on a DRAM to be accessed at an accelerated rate. Fast Page mode involves selecting multiple column addresses in rapid succession once the row address has been selected.

EDO DRAM (Extended Data Out DRAM)

A form of DRAM that has a 2 stage pipeline, which lets the memory controller read data off the chip while it is being reset for the next operation. While similar in performance to SDRAM, it cannot support bus speeds above 66MHz.

SDRAM (Synchronous DRAM)

A type of DRAM designed to deliver bursts of data at very high speeds using automatic addressing, multiple page interleaving, and a synchronous(or clocked) interface.

SIMM (Single Inline Memory Module)

On Pentium-class PCs, SIMM-style RAM chips replaced the Dual Inline Package(DIP) chips, identifiable by two rows of protruding legs, that were popular in the 1980s. They are themselves being replaced by the DIMMs (Dual Inline Memory Module).

DIMM (Dual Inline Memory Module)

A DIMM is a circuit board containing memory chips which plug into the motherboard via a row of connection tabs on the DIMM's lower edge.

RIMM (Rambus Inline Memory Module)

A form of chip packaging that is similar to DIMMs to be used with the next generation of RDRAM memory subsystems.

Behind The Scenes

The phenomenal performance of this RAM is achieved by a variation in the bus structure. Also, the way the signals are carried is different. Usually, 32 bits or 64 bits are sent in the conventional memory data path. But RDRAM sends less information in the data bus. This is usually 18 bits wide. But the data is sent frequently. Also data is read during both the rising and falling edges of the clock signal. Thus, it can achieve effective data transfer of over 800MHz.

The RAM architecture is totally revamped. It has a new pathway (the Rambus Channel) between memory devices and with bus mastering (Rambus Channel Master). At current speeds,a single channel is capable of data transfers at 1.6 GBps and multiple channels can be used in parallel to achieve an output upto 6.4GBps. The Rambus is a high peformance plug-in memory module for main memory.

The RIMM module conforms to the DIMM form factor, but is not pin compatible i.e. even if given a chance to insert a RIMM strip on a DIMM socket ( viz is actually not possible ) you'll not be able use it because the pins are not compatible. Since the architecture is based on the electrical requirements of the Direct Rambus Channel which enables a data rate of 800 MHz. A two byte wide data channel is used, resulting in a peak data transfer rate of 1.6GBps. The bus uses transmission line characteristics to maintain high signal integrity.

The Rambus Channel extends from the controller thru each RIMM module in a continuous flow until channel termination is reached. Upto 3 RIMMs can be used in any motherboard. Both RIMMs and DIMMs are 5.25 x 1.25 inches. However, the number of pins on the Rambus RIMM is 184 as against 168 for the SDRAM DIMM. Thus is not possible to connect the 184 pin RIMM to a 168 pin DIMM slot.

The indication that the power is on is provided by an onboard PROM( Programmable Read Only Memory ) chip called as Serial Presence Detect(SPD). This is the initialisation information.

Since Rambus Archiecture is a complete departure from the normal memory architecture, the leading chip manufacturers will have to design their microprocessors, chipsets and motherboards in order to use this. This will surely hurt the ego of large corporations, but it is inevitable. Don't use it. Lose.

For this same reason, the leading component manufacturers have entered into an alliance with Rambus Inc. In fact, the Pentium 1GHz completely depends on RDRAM. Hence, if RDRAM doesn't click, which is a possibility considering the fact that it is a very costly technology, then there is no way that Intel can sell its chips. Law of Averages, you see!!!