Physical Memory

Memory is an important part of any computer's system. Memory is used in every function of a computer, and it can have a major effect on computer performance. If you are going to get the most out of your computer, you must understand the types of physical memory and how to select the type that is best suited to your computing needs.

random access memory (RAM)

A temporary memory location that stores the operating system, applications, and files that are currently in use. The content in this type of memory is constantly changing. When you shut down the computer, all information in this type of memory is lost.

read-only memory (ROM)

A type of memory that has data pre-copied onto it. The data can only be read from and cannot be overwritten. ROM is used to store the BIOS software.

power-on self-test (POST)

A set of diagnostic tests that are used to determine the state of hardware installed in the computer. Some components that fail the POST, such as bad RAM or a disconnected keyboard, will prevent the computer from booting up properly.

Note 

In Windows XP and Windows Server 2003, memory is more important than ever. The minimum requirement for Windows XP Professional is 64MB, and 128MB is recommended. Windows Server 2003 has a minimum requirement of 128MB, and 256MB recommended.

Memory is basically a series of cells with an address. Each memory cell stores a small piece of information. And each memory cell is identified by a unique address so the processor knows where the cell resides and can easily access it. Computers use several types of memory, each serving a different purpose.

Random Access Memory (RAM)

Random access memory (RAM), often referred to as main memory, is a temporary type of memory that the computer uses as a work area. This type of memory is dynamic, meaning that it is constantly changing because of the activity of the processor. When you shut off the power to the computer, RAM loses everything stored in it. RAM stores program instructions and related data for the CPU to quickly access without having to extract data from a slower type of memory, such as the hard disk.

The hard disk and floppy disk are more permanent forms of data storage. Programs and their output data are stored on disks for future use. When you shut off the power to the computer, the data on the storage media is intact. Accessing data and program instructions from storage media can take over a hundred times longer than from RAM.

Read-Only Memory (ROM)

Read-only memory (ROM) is a special type of memory in which data is written onto a chip during manufacturing. Information stored in ROM is permanent and cannot be changed. ROM stores the BIOS, the set of instructions a computer uses during the first stages of initialization. Without the BIOS, the computer would not have a mechanism to verify that the main hardware components are installed and functioning properly.

RAM Types

Every computer needs RAM, but which type? Not all types of RAM will work in a computer. Some physically won't fit in the RAM socket, and others will fit but won't work, preventing the computer from passing the power-on self-test (POST).

Selecting the right type of RAM requires you to know your CPU type and motherboard. Some CPUs, such as the Pentium II, work only with motherboards designed for that chip. The motherboard is typically designed to meet the highest performance levels of a particular CPU and, therefore, determines which types of physical RAM can be used. RAM comes in one of two types: Single Inline Memory Modules (SIMMs) and Dual Inline Memory Modules (DIMMs).

parity

An extra bit found on some memory modules. Non-parity memory has 8 bits. Parity adds an extra bit that is used to keep track of the other 8 bits. This can help prevent memory errors and is recommended for use in servers.

SIMMs

SIMMs are physically different from DIMMs. Older SIMMs were designed with 30 pins that connected to a slot in the motherboard. These modules were slow and typically had to be added in groups of two or four identical SIMMs to be recognized by the BIOS. The current model is a 72-pin SIMM. On motherboards designed for the Pentium processor, the SIMMs must be added in pairs.

DIMMs

DIMMs have 168 pins with a data path of either 64 bits for non-parity memory or 72 bits for parity memory. DIMMs have the largest data path of any memory module. The wider data path makes the chip as fast as the data path on the CPU. This means that the DIMMs can be added one at a time and in varying sizes. It is because of this improved performance and flexibility that DIMMs have become popular in today's personal computers.

RAM Speed

Identifying the type of RAM that will physically fit into your computer is only one part of the selection process. You also should consider the performance of the RAM you select. Two types of RAM to choose from include Extended Data Out (EDO) and Synchronous Dynamic RAM (SDRAM). Each offers improved performance over older models. Check with your computer manufacturer to see which type of RAM is supported on your computer.

EDO RAM

EDO RAM uses dual-pipeline architecture that enables the unit to store data (write) at the same time it sends it out (reads). EDO RAM is limited to a bus speed of 66MHz, due to its non-parity design. EDO RAM can be purchased in 72-pin SIMMs or 168-pin DIMMs.

SDRAM

SDRAM is similar in design to EDO RAM in that it writes at the same time that it reads, vastly accelerating data along. SDRAM is a popular choice over EDO RAM due to its high bus speed of 100MHz and its low cost.

Other ROM Types

Besides the basic ROM chip discussed earlier, other ROM chips are used in computers and small computing devices. The ROM chips described next are programmable, meaning that information can be recorded onto them. These types of chips are important because they enable software that is critical to the computer start-up process to be updated.

PROM

Programmable Read-Only Memory (PROM) is a special type of chip that is manufactured without any configuration. Manufacturers can then burn in, or program, the chip to contain whatever configuration is needed.

EPROM

Erasable Programmable Read-Only Memory (EPROM) maintains its contents without the use of electrical power. The stored contents of an EPROM chip are erased and reprogrammed by removing the protective cover and using special equipment to reprogram the chip.

EEPROM

Electrically Erasable Programmable ROM (EEPROM) typically maintains the BIOS code, which can be updated with a disk that the BIOS manufacturer supplies.

expansion card

An add-on device, such as a sound or video card, that is installed directly into an expansion slot built into a motherboard. The card must be of the same bus architecture as the slot on the motherboard.

network interface card

A device that connects a computer to the physical cable media and produces signals for transferring data.