Computer memory: DRAM types

This installment of Computer Memory for your Windows Environment discusses the various DRAM types and their affect on systems performance

As we've discovered, most computer memory today uses a DIMM memory module using DRAM. The problem at this stage is that the type of DRAM on the module can greatly affect performance and cause the most confusion when designing or configuring a system.


  • FPM (fast page mode DRAM). Introduced in 1987, this is an early form of DRAM that was once very common because it was slightly faster than DRAM. This computer memory type was frequently mounted on a SIMM in 486 and early Pentium computers.
  • EDO (extended data output). EDO offers a slight performance boost over FPM DRAM by cutting a few steps when addressing memory. Introduced in 1995, EDO requires a specific chip set and is limited to bus speeds of 66 MHz. Once again, these chips are mounted on SIMM modules.
  • BEDO (burst EDO RAM). This is a slightly faster variant of the EDO RAM chip in which read and write operations send bursts of data in batches of four. This type of computer memory is not widely used.
  • SDRAM (synchronous DRAM). SDRAM synchronizes itself with the microprocessor clock speed allowing faster access to memory. These chips are mounted on DIMM memory modules and are classified according to the CPU speed they are designed to support. For example, a PC133 SDRAM DIMM is designed for a Pentium 133 CPU.
  • ESDRAM (enhanced SDRAM). This version of SDRAM includes a small SRAM cache in order to reduce latency and speed up operations. This standard is not widely used.
  • SLDRAM (synchronous link dynamic RAM). Another version of SDRAM, which was designed as a royalty-free, open-industry standard design alternative to RDRAM. It is not widely used.
  • DDR (double data rate SDRAM). DDR allows data transfers on both the rising and falling edges of the clock cycle, which doubles the data throughput. DDR SDRAM chips are mounted on 184-pin DIMM modules and are typically available in 128 Mb to 1 GB capacity. They operate at bus speeds up to 400 MHz. DDR memory is very common, but the technology behind it is at its limits, and it is being replaced by DDR2.
  • DDR2. These chips are the next generation of DDR SDRAM memory. They are mounted on 240-pin DIMM modules, can operate at higher bus speeds and have a capacity to hold 256 Mb to 2 GB of memory. DDR2 has twice the latency of DDR but delivers data at twice the speed of DDR, theoretically performing at the same level.
  • RDRAM (Rambus DRAM). A proprietary protocol-based, high-speed memory technology developed by Rambus Inc., RDRAM has current frequencies of 800 MHz to 1200 MHz, and planned chip sets can expect to reach 1600 MHz. RDRAM RIMMs can only be used on motherboards or systems specifically designed for them. There are 184 pins for 16-bit RDRAM RIMMs; 32-bit RDRAM RIMMs have 232 pins.


Computer Memory for your Windows Environment

  Introduction
  Form Factors: Computer memory modules and pin configurations
  Computer memory types: Static RAM vs. dynamic RAM
  Computer memory: DRAM types
  Error Correcting Code and Parity

ABOUT THE AUTHOR:
Bernie Klinder, MVP, consultant
Bernie Klinder is a technology consultant for a number of Fortune 500 companies. He is also the founder and former editor of LabMice.net, a comprehensive resource index for IT professionals who support Microsoft Windows NT/2000/XP/2003 and BackOffice products. For his contributions to the information technology community, Bernie was selected as an MVP (Most Valuable Professional) by Microsoft. Copyright 2004 TechTarget

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