Memory modules are computer chips used to add memory to a computer.

There are two basic distinctions of memory. One is volatile memory where the data is lost as soon as power is removed, and one is non-volatile that can store the data without power. Random access memory (RAM) is used as read-write memory, which the processor can use as a scratch pad and modify rapidly. It utilizes silicon transistors and capacitors to store data but is done so in a grid-like configuration with a transistor-capacitor pair at each intersection of the grid. The advantage of the grid architecture is that it allows any specific bit of information to be read and written at any time. Any point on the grid can be accessed by interrogating the two lines, which will read or write the spot at which they intersect. This type of volatile memory is very fast but the downfall is that the capacitors lose their charge over time so the data must constantly be refreshed. This constant refreshing aspect of the memory is called dynamic RAM or DRAM.

By contrast, static random access memory (SRAM) does not need to be refreshed. Because of this, SRAM is faster because it doesn’t require the time necessary to refresh each bit. SRAM is also more expensive and not used as often. Different types of SRAM may lose the memory after power is removed but some do not.

Non-volatile RAM memory, or NVRAM, is a class of memory that also has a grid architecture, but the data is retained even after the power is removed. Flash memory is an example of a type of NVRAM that utilizes a special type of metal oxide semiconductor field effect transistor (MOSFET) to store data. Other types of NVRAM are ferroelectric RAM (FeRAM) and magnetoresistive ram (MRAM).

Read only memory (ROM) is memory that contains preset instructions and data, often for controlling physical devices like disk drives associated with the PC. ROM is non-volatile so it does not lose what is stored without power. While the name suggests that it is read-only and certain types are, more often it is read-only during normal operation but can be written under the special circumstances. ROM also covers a wide array of memory types.

Programmable read-only memory (PROM) is a type of ROM that is typically programmed once and can’t be changed after that. This type of memory is used in things like firmware and RFID chips. It is often used in hardware that has a dedicated purpose that will not change.EPROM memory is similar to PROM with one key difference. It can be erased and reprogrammed although this is not expected to happen often. The memory chips have an optical window on them, which must be exposed to UV light that will erase the memory and allow them to be reprogrammed. With this method, the memory must be completely erased before any new information is written.

Since the UV light method is somewhat inconvenient, electrically erasable programmable read-only memory (EEPROM) was developed. Erasing and programming are typically done with a voltage higher than normal operation.

Types of Memory Modules

Choices for memory type include:

• RAM (random access memory)
• DRAM (direct random access memory)
• FPM RAM (fast page mode RAM)
• EDO RAM (extended data output RAM)
• BEDO RAM (burst extended data output RAM)
• SDRAM (synchronous DRAM)
• SRAM (static random access memory)
• L2 Cache (level 2 cache)
• Async RAM (asynchronous RAM)
• Sync RAM (synchronous RAM)
• PB SRAM (pipelined burst SRAM)
• VRAM (video RAM)
• WRAM (window RAM)
• SGRAM (synchronous graphics RAM)
• ROM (read only memory)
• PROM (programmable read only memory)
• EPROM (erasable programmable read only memory)
• EEPROM (electronically erasable programmable read only memory)
• Flash

Specifications

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.

Features

• Capacity is the amount of data that can be transmitted over a specific period of time.
• Clock speed is the raw MHz that the CPU (Central Processor Unit) operates at. For example, an AMD Athlon 1 GHz has an operating clock speed of 1,000 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 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 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.

Applications

Memory is used in lots of digital electronic devices from smart phones and watches to electronic toothbrushes and radios. Just about any electric device that needs to store information uses some form of memory. For a monitor or TV, this storage may be for the internal software that makes the device run, or as temporary storage for a processor. Many modern appliances such as refrigerators, thermostats, air conditioners, and automobiles use memory, as well as:

• Personal computing
• Motherboards and RAID cards
• Digital electronics
• Servers and networking
• Printers and imaging

Image credits:

Wintec Industries, Inc. (1,3) | Digi-Key Corporation (2)