memory-understanding ram technologies and memory upgrades

RAM is the single most upgraded component in computer systems. The installation is easy, but you need to understand the issues surrounding RAM to ensure that an upgrade goes smoothly. In addition, errors, such as General Protection faults and paging errors can be caused by bad RAM.

RAM Basics
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- Bus Speed

The speed at which the system moves data from one place to another. Measured in megahertz (mHz). RAM bus speed should typically equal the motherboard bus speed. You can install RAM with a faster bus speed than the m/b, but the RAM will typically run at the m/b speed.

- Bandwidth

The amount of data the RAM can process per unit of time. RDRAM can process 1.8 gigabytes of data/second. Bandwidth depends on the architecture of the module.

- Latency'

The amount of time it takes for the processed data to go from one location to its destination on a system. Some types of RAM have high bandwidth, but becasue of other factors (such as architecture and location on the m/b).

- Parity and Error Correction Code

Parity is a method of detecting 'sibgle bit' errors in data. Abyte of data contains 8 bits. Parity memory adds an additional bit to each byte (processing 9 bits of data for a byte). Parity memory uses this method to detect, but not correct memory errors. This is an issue mostly in older computers ( of the Pentium - Pentium II class). You do not mix non-parity RAM with parity RAM. In an older system, parity RAM can only function with parity if the BIOS is set to parity checking. You will not need to worry about parity for DDR and RDRAM.

Error Correction Code (ECC) memory corrects single bit errors and detects (but does not correct) multi-bit errors. RAM is still sold in ECC and non-ECC modes. When looking up RAM for any particular system, you will also be told if the RAM is ECC or non-ECC.
Note: Rambus RIMM's will directly state, on the copper cover, whether the module is ECC or Non-ECC.

RAM Types
screenshots - requires flash 6

- EDO (Extended Data Out)
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RAM popular in systems running 486 and Pentium I processors. Modules are called SIMM's (Single Inline Memory Modules), and must be installed in identical pairs. This type of RAM is obsolete and has not been used in systems built after 1997.

- SDRAM (Synchronous Dynamic RAM)
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The most common type of RAM, though quickly being eclipsed by newer technologies. Modules are called DIMM's (Dual Inline Memory Modules), and can be installed singly. Typically run at bus speeds of 100 mHz or 133 mHz.

- DDR SDRAM (Double Data Rate SDRAM)
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Built on SDRAM technology, but processes data at a higher rate of speed. Modules are known as DDR DIMMS and can be installed singly. Typically run at 266 mHz, but newer incarnations will run at bus speeds of 333 mHz. Typically associated with AMD Athlon CPU's, and newer Macintosh Systems

- RDRAM (Rambus DRAM)
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Considered the highest performance RAM currently available. Basic architecture is different from SDRAM and DDR SDRAM. Typically run at bus speeds of 400 mHz or higher. All memory slots on an RDRAM motherboard must be populated. Continuity RIMMS (CRIMMS) fill unused slots, but don't add RAM. RIMM's must be installed in non-contiguous pairs. Thus In one pair of slots, there will be a RIMM and an associated CRIMM. In the next bank will be a second RIMM, with its associated CRIMM. RIMM's are never installed next to each other. Associated with Pentium 4 systems, though Pentium III systems with bios updates will also support it.

Before Upgrading
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- Motherboard support

Type: The motherboard has to be able to support the type of RAM in question. The support a motherboard provides depends on its chipset. The following guidelines are very general, and you should do some research if you're going to recommend RAM to someone:

- RDRAM is supported by Pentium motherboards with Intel chipsets.
- DDR SDRAM is typically supported by AMD Athlon motherboards with chipsets by non-Intel vendors ( ie Via). However, Motherboards with Intel chipsets have recently been produced that support DDR SDRAM. In addition, new Macs use DDR RAM.
- SDRAM is still widely supported by both Pentium and AMD Athlon motherboards.
- A motherboard that supports either RDRAM or DDR SDRAM will not work with regular SDRAM, and vice versa.

Amount: The motherboard has to be able to recognize the amount of RAM that has been installed. Not all motherboards, especially older ones, will recognize the massive amounts of RAM typicall in new computers. This is dependent on the BIOS and the chipset drivers. If a motherboard does not recognize the full amount of RAM that is inserted, consider an update to the BIOS and/or chipset drivers.

- BIOS
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As stated before, the BIOS has to be able to recognize and support the RAM in question. BIOS updates can ease the installation issues, allowing a BIOS to recognize the new RAM. A BIOS upgrade training will be given soon.

- OS
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The OS affects the amount of RAM that should be installed, not the type. Operating systems manage RAM, and not all operating systems can handle large amounts of RAM. Use the information belkow as a guide tot he following fact: While you may be able to install more RAM than is listed below for each OS, installing more than the max listed will not result in significant performance increases.

- Windows 95: 64 megabytes
- Windows 98/ME: 128 megabytes
- Windows 2000/XP: 4 gigabytes
- Mac OS 9: 1.5 gigabytes
- Mac OS 10 and above: 4 gigabytes

- Compatibility
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In this case, compatibility refers to how compatible will a new RAM module be with one already in the system. If the student installs incompatible RAM, the result could be numerous memory errors. Follow these general guidelines:

- Do not mix bus speeds, if possible.
- Do not mix parity and ECC RAM with non-parity/non-ECC RAM
- If a computer is experiencing general protection errors, ask the student if (s)he has recently upgraded RAM. If yes, open the comp, and remove the recently installed module. If the errors disappear, the student had bad or incompatible RAM.

Web sites for reference

- Computer Vendor Sites
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Gateway: An excellent support section on finding the right RAM and installing it properly

Dell: A support page which answers general questions about RAM upgrades. Also provides instructions on how to find the appropriate RAM for the particular system.

Compaq: Allows you to choose the memory for desktops or laptops, according to system model.

E-machines: Basic info on what kind of RAM the e-machine will accept.

- Macintosh: Provides RAM info specific to different models

iMac Flat Panel | G3 Desktops | G4 Desktops

- Online RAM vendors/resalers

Kingston RAM (vendor): the main page allows you to search for motherboards and brand name computer systems, after which you will be provided with the appropriate RAM for that system.

Crucial RAM (vendor): This vendor's main page has a search engine in which you can choose a system, and receive info regarding the appropriate RAM.

Memorytogo.com (online memory resaler): Non-vendor specific. Includes the same kind of search capabilities as the sites above.

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