XDR DRAM

XDR DRAM or extreme data rate dynamic random-access memory is a high-performance RAM interface and successor to the Rambus RDRAM it is based on, competing with the rival DDR2 SDRAM and GDDR4 technology.

Overview

XDR was designed to be effective in small, high-bandwidth consumer systems, high-performance memory applications, and high-end GPUs. It eliminates the unusually high latency problems that plagued early forms of RDRAM. Also, XDR DRAM has heavy emphasis on per-pin bandwidth, which can benefit further cost control on PCB production. This is because fewer lanes are needed for the same amount of bandwidth. Rambus owns the rights to the technology. XDR is used by Sony in the PlayStation 3 console.

Technical specifications

Performance

Features

XDR2 DRAM

XDR2 DRAM is a type of dynamic random-access memory that is offered by Rambus. It was announced on July 7, 2005 and the specification for which was released on March 26, 2008. Rambus has designed XDR2 as an evolution of, and the successor to, XDR DRAM.

XDR2 DRAM is intended for use in high-end graphics cards and networking equipment.

As a fabless semiconductor company, Rambus only produces a design; it must make deals with memory manufacturers to produce XDR2 DRAM chips, and there has been a notable lack of interest in doing so.

Changes from XDR DRAM

Signaling

In addition to a higher clock rate (up to 800 MHz), the XDR2 differential data lines transfer data at 16 times the system clock rate, transferring 16 bits per pin per clock cycle. This "Hexadecimal Data Rate" is twice XDR's 8× multiplier. The basic burst size has also doubled.

Unlike XDR, memory commands are also transmitted over differential point-to-point links at this high data rate. The command bus varies between 1 and 4 bits wide. Even though each bit requires 2 wires, this is still less than the 12-wire XDR request bus, but it must grow with the number of chips addressed.