Computer Memory Tutorial

This tutorial discusses the various types and features of computer memory. For information on installing memory, read How to Install Desktop Memory.

Every PC or laptop must have RAM (Random Access Memory). RAM is temporary storage, meaning that once a computer is turned off, everything stored in it is lost. When an application is opened, it is copied into RAM. Having adequate computer memory is important. Even with a fast processor, not having enough memory can result in a slower running computer. RAM is sold in modules (more commonly called sticks). Just as motherboards are designed for certain CPUs, they are also designed to use certain types of RAM. There have been several types of computer memory over the years, but as of this writing, the prevailing standard is some form of DDR SDRAM.

Before explaining DDR, let's take a quick look at what SDRAM is.

SDRAM was developed many, many years ago and stands for Synchronous Dynamic Random Access Memory. It means that RAM was synchronized with the motherboard's speed. In other words, the speed of the RAM matched the speed of the motherboard (The front side bus speed is discussed in the motherboard section). Therefore, if a motherboard's FSB speed was 66 MHz, the memory ran at 66 MHz (called PC66 memory). If the FSB speed was 100 MHz, the memory ran at 100 MHz (called PC100 memory), and so on. This type was called Single Data Rate and is basically found on old motherboards.


To speed things up, Double Data Rate SDRAM (DDR SDRAM) was developed. As mentioned above, practically every motherboard today supports some type of DDR RAM. As the name implies, it was designed to run at twice the speed of the front-side bus. On a motherboard having a speed of 133 MHz, DDR memory would run at 266MHz (called DDR266 or PC 2100 memory). Eventually came DDR2 and DDR3, each running at twice the clock speed of its predecessor. DDR2 running at speeds from 400 to 1066 MHz and DDR3 from 800-2000MHz.

In addition to speed, DDR2 and DDR3 were developed to run at lower voltages than regular DDR RAM, resulting in less power consumption. DDR used 2.5v. DDR2 runs off 1.8v and DDR3 1.5v.


Physical Characteristics:
SDR RAM has 168 pins and two notches (holes). Regular DDR memory has 184 pins and DDR2 and DDR3 have 240 pins. Each type of DDR has one notch that is in a different location to prevent installing the wrong type.

Just like processors fit into certain sockets, different types of computer memory fit into certain slots called banks. Banks are designed to match the number of pins on a stick of RAM, much like cpu sockets are designed to match the number of pins on different processors. All types of SDRAM are DIMM modules and fit into DIMM slots. DIMM stands for Dual Inline Memory Module. DIMMs have a 64-bit bus width which means data can be accessed 64 bits at a time.


Laptops use a smaller version of DIMMS called SODIMM (Small Outline DIMM).


Dual Channel & Three Channel (tri-channel):
Dual channel is a process that allows twice as much information to be sent to and from memory at the same time. As the name implies, two memory controllers are used for data transfer instead of one. Three channel moves data much faster with three memory controllers.

To take advantage of this technology, a motherboard's chipset must support it, and the memory has to be installed in pairs (dual Channel) or in threes (tri channel) in the proper banks, which are usually colored coded. To avoid possible compatibility problems with motherboards, it's best to use identical memory sticks.

There is no such thing as dual channel or three channel memory. You may see matched sticks of RAM advertised as such, sometimes called memory "kits", but this is simply memory that has been tested to work on a dual or three channel board. Remember, the technology is on the motherboard, not on the memory itself.

Dual channel uses DDR, DDR2, or DDR3 RAM and is supported by most motherboards. Tri-channel only uses DDR3.


Capacity:
Memory capacity is the total amount of RAM that can be used in a PC. It also refers to the amount of data an individual memory module can store (also called density memory size). Some RAM module capacities are 1GB and 4GB.

All the slots on a particular board have the same maximum capacity. The sum of these capacities determines the total amount of memory a system can support. For example, a motherboard may have four banks, each with a capacity of 1GB (gigabyte). If you add these then the total amount of memory that can be used for that PC is 4GB of memory.

Bank capacities vary from board to board. You do not have to use the maximum amount supported, but generally the more RAM the better.

CAS Latency:
CAS (Column Address Strobe) latency is the time between when the processor asks for data and when that data is sent.The smaller the CAS latency, the quicker data is retrieved.

SRAM:
Among other types of RAM is SRAM (Static RAM). SRAM is a lot faster because it doesn't have to constantly be refreshed like DRAM. But since it is expensive, SRAM is limited to being used in small amounts as CPU cache.

What is Flash Memory?
Flash memory is another type of computer memory and a popular method of storage. Unlike DRAM, flash memory is non-volatile, meaning it doesn't require electricity to retain its contents. It is a type of EEPROM (Electrically Erasable Programmable Read Only Memory) that is erased and rewritten in chunks called blocks. Regular EEPROM is erased and reprogrammed a byte at a time. Using chunks makes erasing and rewriting much faster. This is why flash memory is the primary choice for so many devices: USB flash drives, memory cards for digital cameras, notebook cards, and more. The BIOS program that allows your computer to boot, resides on a flash memory chip on the motherboard-making updating very simple.