How to Select Memory Modules

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Memory modules are computer chips used to add memory to a computer.  Memory comes in two basic forms, random access memory (RAM) and read-only memory (ROM).  The name "random access memory" is a bit misleading, because both kinds of memory are random access; that is, the CPU can call up any arbitrary address directly without reading sequentially through lower addresses. RAM is really read-write memory, which the processor can use as a scratch pad and modify rapidly. ROM is memory that contains preset instructions and data, often for controlling physical devices like disk drives associated with the PC. Configuration, performance specifications, and applications and features are all important to consider when selecting memory modules.

Memory Module Configuration

Important configuration parameters to consider for memory modules include memory type and form factor. 

Memory Type

Choices for memory type include:

• RAM
• DRAM
• FPM RAM
• EDO RAM
• BEDO RAM
• SDRAM
• SRAM
• L2 Cache
• Async RAM
• Sync RAM
• PB SRAM
• VRAM
• WRAM
• SGRAM
• ROM
• PROM
• EPROM

• EEPROM

• Flash. 

Memory Form Factor

The form factor of any memory module describes its size and pin configuration. Most computer systems have memory sockets that can accept only one form factor.  Choices for form factor include:

• SIMM- Single in-line memory module (SIMM) offers a data path of 32 bits. Because Pentium® memory modules are designed to handle a much wider data path than that, SIMMs must be used in pairs on Pentium motherboards (they can be used singly on boards based on 486 or slower processors).

• DIMM-Dual in line memory module (DIMM), which are of more recent origin, offer a 64-bit path, which makes them more suitable for use with the Pentium and other more recent processors. One DIMM will handle the work of two SIMMs and thus can be used singly on a Pentium motherboard. DIMMs are more economical in the long run, because they can be added one at a time to a system.

Memory Module Specifications

Important performance specifications to consider when searching for memory modules include capacity, clock speed, and cycle time.  

• Capacity is the amount of data that can be transmitted over a specific period of time. 

• Clock speedis the raw MHz that the CPU (Central Processor Unit) operates at. For example, an AMD Athlon 1GHz has an operating clock speed of 1000 MHz; this is the processor's clock speed. 

• The cycle time is the length of time it takes to transmit data expressed in terms of the minimum amount of time required for a memory to complete a cycle such as read, write, read/write, or read/modify/write.

Error Checking

Error checking and correction features of memory cards include parity, error checking parity, and nonparity.  As data moves through a computer (e.g. from the CPU to the main Memory), the possibility of errors can occur, particularly in older 386 and 486 machines. Parity error detection was developed to notify the user of any data errors. By adding a single bit to each byte of data, this bit is responsible for checking the integrity of the other 8 bits while the byte is moved or stored. Once a single-bit error is detected, the user receives an error notification; however, parity checking only notifies, and does not correct a failed data bit. If your SIMM module has 3, 6, 9, 12, 18, or 36 chips then it is more than likely parity. Error Checking and Correction (ECC) modules have an extra chip that detects if the data was correctly read or written by the memory module. If the data wasn't properly written, the extra chip will correct it in many cases (depending on what type of error).  Non-parity (also called non-ECC) modules do not have an error-detecting feature.

Memory Module Application

Common applications for memory modules include personal computing, motherboards and RAID cards, digital electronics, servers and networking, and printers and imaging.