Random Access Memory

RAM - A Brief History:

Random Access Memory (RAM) is now virtually all supplied in the form of Dual In - Line Memory Modules (DIMM) and Single In - Line Memory Modules (SIMM).

DIMM has 168 pins and is 64 bits wide. Originally SIMM was 8 bits wide, with 30 pins. This standard has just about been completely supplanted by a 32 bit wide configuration (a much more suitable arrangement for 386 and 486 processors), with 72 pins.

An additional confusion is the matter of error detection and / or correction. In the original I.B.M. personal computer, the memory was configured with a width of 9 bits. The ninth bit was not accessible to the user, but used in conjunction with a block of hardware logic called a parity generator, to detect random single bit errors which may occur in the memory system. This standard carried over into later designs. Parity SIMM as a result are either 9 bits wide (for 30 pin modules) or 36 bits wide (for 72 pin modules).

The parity system was limited in as much as it was not guaranteed to find multiple bit errors within a single byte, and could flag, but not correct an error. Many manufacturers, in an effort to cut costs, have removed this parity memory chip, and in most cases the parity detection logic from the motherboard.

Recently, recognising that this was a perilous course, particularly for high end systems like corporate file servers, a new Error Checking and Correcting standard (ECC) has appeared. Each 8 bit wide byte has four additional error checking bits calculated and stored, making a total width of 12 bits. This system not only allows detection of a single bit error, as did parity, but by utilisation of the 4 error checking bits, a correction can be performed. Multiple bit errors in a single byte can usually also be detected, but NOT successfully corrected. Addition of the extra 'internal' bits for storage of error checking data results in an ECC DIMM being 96 bits wide instead of the standard 64 bits.

Memory Types:

The current types which you are likely to encounter are:

Static
FPM (Fast Page Mode)
EDO (Enhanced Data Output)
SDRAM (Synchronous Dynamic RAM)

Static RAM is used in areas such as processor cache and video memory on video cards. (not exclusively however: video card design is an extremely specialised area and other technologies are employed here as well) Static RAM has the advantage of extreme speed, very low power consumption and requires very little in the way of external support hardware. It can hold it's contents merely by the maintenance of it's external power supply. Some specialised chips even have an INBUILT lithium battery, enabling the entire device in which it is fitted to be powered off (and the memory device even removed from the board if needs be) without losing the contents. (An example is the CMOS setup memory chips used on some manufacturers motherboards) The downside is higher cost and smaller capacities for a given physical size.

FPM (Fast Page Mode) RAM was the first generation of dynamic RAM. It's advantage is low cost, and compact size. Unfortunately there is the requirement for external hardware to perform 'refresh'. Without constant 'refreshing' the memory would lose it's contents. This also means that retention of contents without a system being fully operational is very difficult.

EDO (Enhanced Data Output) RAM is a redesign of conventional dynamic RAM to improve it's speed. The speed increase is not dramatic (usually only 10 - 20%) but as processors have become faster, the 'RAM bottleneck' was reduced by this innovation.

SDRAM (Synchronous Dynamic RAM) is the latest development in dynamic RAM. This design gives you the speed advantage of static memory (very close to) with the compactness and low cost advantages of traditional dynamic RAM. The name synchronous implies the memory is capable of operating at the full bus speed of the processor. (usually 66MHz or 100MHz in the latest designs)

The Pitfalls - and How to Avoid Them:

If you are intending to upgrade or upsize the memory on your system, you need to know what configuration of memory you currently have, and what your motherboard will support.

A Few Guidelines (far from an exhaustive list):

Module Type. (ie: SIMM 30 pin / 72 pin or DIMM)
The modules ARE NOT interchangeable. The mounting arrangements are all completely different. Some motherboards provide the option of installing either SIMM or DIMM or in some cases, a combination of both. The use of combinations of both types, or exclusively SIMM on a modern motherboard which offers the option of DIMM, will impose system speed penalties.
DIMM Type.
- Speed: 100MHz DIMM is available for the latest BX motherboards which utilise a higher memory bus speed. Standard 66MHz DIMM WILL NOT function correctly with a 100MHz RAM Bus speed.
- ECC: For situations where Error Checking and Correcting (ECC) systems are supplied on the motherboard appropriate ECC DIMM must be installed.
- Speed EEPROM: Most DIMM is now supplied with a small EEPROM used by the system to select optimal memory speed. Some older motherboards will not recognise this new form of DIMM, and either will not run at all, or will incorrectly report the memory size.
DIMM Voltage.
DIMM are available in either 5 Volt or 3.3 Volt versions. It is normally NOT recommended that versions be mixed on a single motherboard even if this arrangement is allowed. Some motherboards provide jumpers or similar to allow you to configure the board to the desired voltage. Be EXTREMELY careful with this adjustment, and incorrect setting may cause memory and / or motherboard damage.
Parity. (ie: parity or non - parity)
If you wish to utilise the features of parity checking supplied on your motherboard, all memory installed must support it. In most cases you can install non - parity RAM, if your motherboard offers a feature of disabling parity checking. Unfortunately in doing so, you are sacrificing the benefits of any parity RAM you may have fitted.
Module Configuration. (Check your motherboard handbook)
Some motherboards require a specific configuration of modules.
Examples:
- Modules must be fitted in identical pairs (nearly always the case on Pentium motherboards using SIMM).
- The smaller of a selection of modules must be fitted first or last.
- In the case of a single module, it must occupy a particualr socket (bank).
- On motherboards that allow mixture of both SIMM and DIMM, some restrictions may apply to module sizes and which sockets can be simultaneously in use.
Modules Sizes Supported. (Check your motherboard handbook)
Another trap you can fall into is that some motherboards (particularly some 386 and 486 motherboards) do not support all memory module sizes. Examples seen include no support for 8MB modules, only 4 and 16MB. Fortunately, this particular problem appears to have been largely overcome in more recent designs.
Mixing Module Types.
It is possible on some motherboards to combine the use of both EDO and FPM modules with different timing specifications. I do not recommend that this be attempted, since the result is usually unstable. The advantages of the faster memory installed will most likely not be realised since the overall system will be limited to the slowest memory type.
Specialised Memory Types.
Some motherboards gain speed or performance improvements by the use of specialised memory modules, for example Dynami-Cache RAM. Depending on the motherboard design it may not be possible to mix with other standard type RAM , or you may be able to do so only with an extreme performance penalty.

WARNING:

The mounting and / or removal of SIMM and DIMM memory can also be tricky. Depending upon placement and the type of mounting socket employed, extreme difficulty may be encountered when trying to install or remove a module. If you have doubts about your ability to successfully undertake this activity, it is probably best if you take your machine to a suitably qualified technical support centre. Failure to correctly mount a module, or non-observance of anti-static handling procedures may result in failure of the module and / or your motherboard. This type of damage is normally expressly excluded in warranty provisions.

If in doubt, ask your motherboard supplier for assistance. They should be happy to help. If they are not cooperative, email ZCM Services, and I will attempt to assist you in your choice.

**Price Guide - Memory**
Code	Item Description	Retail Price	Warranty
SIMM4P30	4MB FPM 9 bit parity, 30 pin SIMM, 70ns	$35.00	Limited Lifetime
SIMM8NP	8MB FPM 32 bit non - parity, 72 pin SIMM, 70ns	$31.00	Limited Lifetime
SIMM16NP	16MB FPM 32 bit non - parity, 72 pin SIMM, 70ns	$73.00	Limited Lifetime
SIMM32NP	32MB FPM 32 bit non - parity, 72 pin SIMM, 70ns	$161.00	Limited Lifetime
SIMM8P	8MB FPM 36 bit parity, 72 pin SIMM, 70ns	$54.00	Limited Lifetime
SIMM16P	16MB FPM 36 bit parity, 72 pin SIMM, 70ns	$114.00	Limited Lifetime
SIMM32P	32MB FPM 36 bit parity, 72 pin SIMM, 70ns	$215.00	Limited Lifetime
EDO4NP	4MB EDO 32 bit non - parity, 72 pin SIMM, 60ns	$20.00	Limited Lifetime
ED08NP	8MB EDO 32 bit non - parity, 72 pin SIMM, 60ns	$32.00	Limited Lifetime
ED016NP	16MB EDO 32 bit non - parity, 72 pin SIMM, 60ns	$66.00	Limited Lifetime
ED032NP	32MB EDO 32 bit non - parity, 72 pin SIMM, 60ns	$145.00	Limited Lifetime
ED064NP	64MB EDO 32 bit non - parity, 72 pin SIMM, 60ns	$204.00	Limited Lifetime
DIMM32NP	32MB Sync DRAM 64 bit non - parity, 168 pin DIMM, 10ns, with Speed EEPROM	$88.00	Limited Lifetime
DIMM64NP	64MB Sync DRAM 64 bit non - parity, 168 pin DIMM, 10ns, with Speed EEPROM	$178.00	Limited Lifetime
DIMM128NP	128MB Sync DRAM 64 bit non - parity, 168 pin DIMM, 10ns, with Speed EEPROM	$348.00	Limited Lifetime
DIMM32ECC	32MB Sync DRAM 96 bit Error Checking and Correcting, 168 pin DIMM, 10ns, with Speed EEPROM	$119.00	Limited Lifetime
DIMM64ECC	64MB Sync DRAM 96 bit Error Checking and Correcting, 168 pin DIMM, 10ns, with Speed EEPROM	$208.00	Limited Lifetime
DIMM128ECC	128MB Sync DRAM 96 bit Error Checking and Correcting, 168 pin DIMM, 10ns, with Speed EEPROM	$399.00	Limited Lifetime
BXDIMM32NP	32MB Sync DRAM 64 bit non - parity, 168 pin DIMM, PC100	$92.00	Limited Lifetime
BXDIMM64NP	64MB Sync DRAM 64 bit non - parity, 168 pin DIMM, PC100	$182.00	Limited Lifetime
BXDIMM128NP	128MB Sync DRAM 64 bit non - parity, 168 pin DIMM, PC100	$356.00	Limited Lifetime
BXDIMM64ECC	64MB Sync DRAM 96 bit Error Checking and Correcting, 168 pin DIMM, PC100	$211.00	Limited Lifetime
BXDIMM128ECC	128MB Sync DRAM 96 bit Error Checking and Correcting, 168 pin DIMM, PC100	$410.00	Limited Lifetime

Notes:

Prices listed above are for individual items.
Because of extreme market volatility, no guarantees can be offered as to the price or availability of any item. The list above is merely a comparative guide.
A freight charge of $7.00 per order is payable for items not purchased with a motherboard, processor or complete system.
Some reductions in price are possible for bulk purchases.
Please email ZCM Services to receive a current price or further details.

All promotional photographs and advertising material, corporate names and logos, product names, trade names, trademarks and registered trademarks are the property of their respective owners, and are acknowledged as such. Whilst every care is taken in the preparation of the information presented in these pages, ZCM Services can take no responsibility for errors or omissions.