From ECDL.web

Computer storage, computer memory, and often casually memory refer to computer components, devices and recording media that retain data for some interval of time. Computer storage provides one of the core functions of the modern computer, that of information retention. It is one of the fundamental components of all modern computers, and coupled with a Central Processing Unit (CPU), implements the basic Von Neumann computer model used since the 1940s.

In contemporary usage, memory usually refers to a form of solid state storage known as random access memory (RAM) and sometimes other forms of fast but temporary storage.

[edit] Primary storage

Primary storage, or internal memory, is computer memory that is accessible to the central processing unit of a computer without the use of computer's input/output channels. Primary storage is used to store data that is likely to be in active use. Primary storage is typically very fast, as in the case of RAM. RAM is also volatile, losing the stored information in an event of power loss, and quite expensive. ROM is not volatile, but is not suited to storage of large quantities of data because it is expensive to produce. Typically, ROM must also be completely erased before it can be rewritten, making large scale use impractical, if not impossible. Therefore, separate secondary storage, or external memory, is usually required for long-term persistent storage.

Primary storage may include several types of storage, such as main storage, cache memory, and special registers, all of which can be directly accessed by the processor. Primary storage can be accessed randomly, that is, accessing any location in storage at any moment takes the same amount of time. A particular location in storage is selected by its physical memory address. That address remains the same, no matter how the particular value stored there changes.

Today, primary storage is typically random access memory, a type of semiconductor memory. Over the history of computing hardware, a variety of technologies have been used for primary storage. Some early computers used mercury delay lines, in which a series of acoustic pulses were sent along a tube filled with mercury. When the pulse reached the end of the tube, the circuitry detected whether the pulse represented a binary 1 or 0 and caused the oscillator at the beginning of the line to repeat the pulse. Other early computers stored primary memory on rapidly rotating magnetic drums.

Modern primary storage devices include:

• Random access memory (RAM) - includes dynamic random access memory (DRAM), static random access memory (SRAM), Video RAM and others.
• Read-only memory (ROM)

[edit] Secondary storage

In computer storage, secondary storage, or external memory, is computer memory that is not directly accessible to the central processing unit of a computer, requiring the use of computer's input/output channels. Secondary storage is used to store data that is not in active use. Secondary storage is usually slower than primary storage, or internal memory, but also almost always has higher storage capacity and is non-volatile, which makes it perfect for the preservation of stored information in an event of power loss.

Storage devices in this category include:

• CD, CD-R, CD-RW
• DVD
• Flash memory
• Floppy disk
• Zip Drive
• Hard disk drive
• Magnetic tape
• Paper tape
• Punch card
• RAM disk
• External hard drive
• Blu-Ray Disc

Almost all storage devices are either magnetic or optic. A well known exception is provided by flash memories (used in digital audio players, USB stick memories, etc.) which uses a different approach.

The secondary storage is physically formatted according to a filesystem format, such as FAT, Ext3 or NTFS, which provides the abstraction necessary to organize data into files and directories, providing also additional information (called metadata) describing the owner of a certain file, the access time, permissions and other information.