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Last modified: Friday, January 04, 2002 

Internal storage areas in the computer. The term memory identifies data storage that comes in the form of chips, and the word storage is used for memory that exists on tapes or disks. Moreover, the term memory is usually used as a shorthand for physical memory, which refers to the actual chips capable of holding data. Some computers also use virtual memory, which expands physical memory onto a hard disk.

Every computer comes with a certain amount of physical memory, usually referred to as main memory or RAM. You can think of main memory as an array of boxes, each of which can hold a single byte of information. A computer that has 1 megabyte of memory, therefore, can hold about 1 million bytes (or characters) of information.

There are several different types of memory:

  • RAM (random-access memory): This is the same as main memory. When used by itself, the term RAM refers to read and write memory; that is, you can both write data into RAM and read data from RAM. This is in contrast to ROM, which permits you only to read data. Most RAM is volatile, which means that it requires a steady flow of electricity to maintain its contents. As soon as the power is turned off, whatever data was in RAM is lost.
  • ROM (read-only memory): Computers almost always contain a small amount of read-only memory that holds instructions for starting up the computer. Unlike RAM, ROM cannot be written to.
  • PROM (programmable read-only memory): A PROM is a memory chip on which you can store a program. But once the PROM has been used, you cannot wipe it clean and use it to store something else. Like ROMs, PROMs are non-volatile.
  • EPROM (erasable programmable read-only memory): An EPROM is a special type of PROM that can be erased by exposing it to ultraviolet light.
  • EEPROM (electrically erasable programmable read-only memory): An EEPROM is a special type of PROM that can be erased by exposing it to an electrical charge.
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    For pages about memory . Also check out the following links!

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    Memory Errors, Detection and Correction 
    Describes the facilities available for doing error detection and correction and discusses what some of the important factors are. This page is from "The PC Guide."

    PC Mechanic - online guide to computer hardware 
    Contains overviews, purchasing and installation information for PC motherboards, memory, drives, expansion cards, and power supplies. Also contains links to companies and manufacturers and PC hardware related links.

    System Memory Overviews 
    Describes memory technology types, memory speed, access and timing, DRAM technologies, memory size and packaging, errors, detection and correction, and logical memory layout. This page is from "The PC Guide".

    Logical Memory Layout
    Describes how memory is allocated and how it is used within the PC. This page is from "The PC Guide".

    Memory manual
    Provide general information about memory modules and how to identify them. It offers definitions, acronyms, types of DRAM and module packages, information on 2-, 3- and 4-chips, and identifying modules.

    Memory Packaging
    Describes the types of memory packaging available and the issues involved in selecting and using memory packages and DRAM chips in a PC. This page is from "The PC Guide."

    Memory Speed, Access and Timing
    Describes how the system memory works. Includes details on how memory is accessed and important issues related to system timing. This page is from "The PC Guide".

    Memory Technology Types
    Describes the main types of memory used in PCs (ROM, RAM, SRAM, and DRAM) and how they differ from each other. This page is from "The PC Guide".

    PC Guide's system memory reference
    Extensive reference covering the system memory. Covers technology types, memory speed and timing, packaging, error detection and correction, and the logical memory layout.

    PC memory management
    Phoenix Technologies, designer of the PhoenixCard Manager Plus, describes the evolution of PC memory management with a description of how this product works within the DOS Protected Mode Services (DPMS).

    Troubleshooting the System Memory
    Describes memory-related problems, referring specifically to the main system memory. This page is from "The PC Guide."

    The WebCrawler search engine allows you to search the Web by keyword or by subject category.

    Yahoo!'s memory retailers page
    Yahoo!'s directory of memory retailers.

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