KarbosGuide.com. Module 2e3.

About new fast RAM

The contents:

  • DIMMs
  • PC100 RAM
  • PC133 and VC133
  • Intel and PC133

    On the following pages:

  • Rambus
  • DDR
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  • DIMMs

    The most used modern RAM type, SDRAM is made in 64 bit wide modules called DIMMs (Dual Inline Memory Module).

    They have a 168 pin edge connector. Here you see one module:

    Since the DIMM modules are 64 bits wide, you can install one module at a time. They are available in 8, 16, 32, 64, 128, 256 MB, and 512MB with 6, 8, 10, and 12 ns speed. There are usually 2 -4 DIMM sockets on a motherboard.

    The advantage of SDRAM is increased speed. That allows you to increase system bus speed. With 60 ns EDO-RAM, you can run at a maximum of 75 MHz on the system bus, while SDRAM speed can increase to 133 MHz and above. Also the SDRAM work synchronous with the system bus for a better performance.

    Most chip sets are made for SDRAM. Some motherboards had both SIMM and DIMM sockets. The idea was that you could reuse old EDO RAM in the SIMM sockets, or choose to install SDRAM in the DIMM sockets. They were not designed to mix RAM types although it works at some boards.

    Above: a 64 MB DIMM-module holding 32 chips each of 16 Mbit (32 X 16 Mbit / 8 bit = 64 MB).

    It is better to use DIMMs made of the new 64 Mbit chips. A 64 MB module is this way made of only 8 chips (8 X 64 Mbit / 8 bit = 64 MB).

    Fast RAM

    Intel have managed to speed up the processors power by factor 200 times the last ten years. That is a lot, but it is a problem that RAM memory technology only has improved by factor 20 in the same period.

    Today we hope and dream of new fast RAM types, that will help us to get the full potential from our powerful PCs.

    PC100 RAM

    The first attempt to improving RAM speed was the PC100 standard. With chip sets like BX the system bus speed has come up to 100 MHz. Hence Intel has made a new standard called PC100. Only 8 ns SD-RAM modules that are constructed according to these standards are guaranteed to work at 100 MHz. In some articles this RAM is described at 125 SD-RAM.


    The new DIMM-modules include a EPROM-chip holding information about the module. This little 8-pin chip works as a SPD (Serial Presence Detect) - a unit storing information about the RAM type. The idea is that BIOS can read this information and this way tune the system bus and the timings for a perfect CPU-RAM performance.

    You can find a program, that tests the contents of the SPD at this c't homepage. It works with the Intel chip sets holding a 82371 south bridge like BX and GX.

    Another program is called DIMM_ID.


    The PC133 RAM running at 133 MHz is the latest version of SDRAM. Specifications are made by VIA, Micron, NEC, Samsung, SIS, Acer Labs and other vendors. The first production (from Corsair, June 1999) used 7.5 ns RAM modules from Micron.

    VIA supports the PC133 RAM with their Apollo Pro Plus chip set (693A). Later they launched support for PC266 DDR RAM!

    Also AMD's K7 Athlon may use PC133 RAM with the VIA KX133Pro chipset.


    Virtual Channel 133 is another flavour of the PC133 standard. The modules holds a small cache of superfast SRAM. According to tests, these modules perform very well, but due to unknown reasons, it never became popular.

    Intel and PC133

    Originally Intel planned to by-pass PC133 RAM in their roadmaps. They intended to migrate from PC100-based chip sets (like BX) to Rambus-based chip sets (like i820).

    For a period of 12 months in 1999-2000, Intel experienced several disastrous incidents from their attempt to implement Rambus in chip sets and motherboards. During this period they were forced (by taiwanese motherboard manufactures) to adapt the PC133 standard.

    The chip set i815 was the result of this revision of strategies.

    Intel's problem is that they have "sold their soul" to Rambus Inc. According to their agreement, until 2003 Intel can only implement other RAM types than RDRAM if the bandwidth is less than 1 GB/sec. This agreement does not include server chipsets, from what we understand.

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    Learn more

    Read module 5a about expansion cards, where we evaluate the I/O buses from the port side.

    Read module 5b about AGP and module 5c about Firewire.

    Read module 7a about monitors, and 7b on graphics card.

    Read module 7c about sound cards, and 7d on digital sound and music.

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    Copyright (c) 1996-2005 by Michael B. Karbo. www.karbosguide.com.