Computer storage

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.

CLASSIFICATION OF COMPUTER STORAGE

Primary storage - is directly connected to the central processing unit of the computer. It must be present for the CPU to function correctly.

Secondary storage is typically of much greater capacity than primary storage but it is also very much slower. In modern computers, hard disks are usually used for mass storage.

Tertiary storage - is a system where a robotic arm will "mount" (connect) or "dismount" off-line mass storage media according to the computer operating system's demands

Off-line storage is a system where the storage medium can be easily removed from the storage device. Off-line storage is used for data transfer and archival purposes.

Primary Storage typically consists of three kinds of storage:

Processor registers - contain information that the arithmetic and logic unit needs to carry out the current

instruction. They are technically the fastest of all forms of computer storage.

Main memory - contains the programs that are currently being run and the data the programs are operating on.

Cache memory - is a special type of internal memory used by many central processing units to increase their performance or "throughput".

Virtual memory is implemented by many operating systems using terms like swap file or "cache file". The main historical advantage of virtual memory was that it was much less expensive than real memory.

Characteristics of storage

I. Volatility of information

Volatile memory requires constant power to maintain the stored information. Volatile memory is typically used only for primary storage.

Non-volatile memory will retain the stored information even if it is not constantly supplied with electric power. It is suitable for long-term storage of information, and therefore used for secondary, tertiary, and off-line storage.

Dynamic memory is volatile memory which also requires that stored information is periodically refreshed, or read and rewritten without modifications.

II. Ability to access non-contiguous information

Random access means that any location in storage can be accessed at any moment in the same, usually small, amount of time

Sequential access means that the accessing a piece of information will take a varying amount of time, depending on which piece of information was accessed last.

III. Ability to change information

Read/write storage, or mutable storage, allows information to be overwritten at any time. A computer without some amount of read/write storage for primary storage purposes would be useless for many tasks.

Read only storage retains the information stored at the time of manufacture, and write once storage (WORM) allows the information to be written only once at some point after manufacture.

Slow write, fast read storage is read/write storage which allows information to be overwritten multiple times, but with the write operation being much slower than the read operation.

IV. Capacity and performance

Storage capacity is the total amount of stored information that a storage device or medium can hold. It is expressed as a quantity of bits or bytes.

Storage density refers to the compactness of stored information. It is the storage capacity of a medium divided with a unit of length, area or volume

Latency is the time it takes to access a particular location in storage. The relevant unit of measurement is typically nanosecond for primary storage, millisecond for secondary storage, and second for tertiary storage.

Throughput is the rate at which information can read from or written to the storage. In computer storage, throughput is

usually expressed in terms of megabytes per second or MB/s, though bit rate may also be used.

Technologies, devices and media

Magnetic storage storage uses different patterns of magnetization on a magnetically coated surface to store information. Magnetic storage is non-volatile

Forms of Magnetic Surface

Magnetic disk

Hard disk, used for secondary storage

Optical Disc, used for off-line storage

Floppy disk, used for off-line storage

Magnetic tape, used for tertiary and off-line storage

Hard Disk Drive Components

Disk platters

Read/Write Heads

Spindle motor

Cables and Connectors

Head actuator mechanism

Logical Board

Configuration items (such as jumpers or switches)

Nonvolatile Storage or permanent storage

Hard Disk Platters (Disks) Recording Media

Hard Disk Drive Components

Hard Disk Platters (Disks)

– 5 ¼-inch (actually 130mm, or 5.12 inches)

– 3 ½-inch (actually 95mm, or 3.74 inches)

– 2 ½-inch

– 1.8-inch

– 1-inch (MicroDrive)

Recording Media

– Oxide Media

– Thin-Film Media

Disk Formatting

Partitioning

– FAT (File Allocation Table)

• Standard file system use by DOS, win9x, win NT

• Support filenames of up to 11 characters max.

• 2Gb maximum volume, because of 12-16bit numbers to identify clusters.

– FAT32 (File Allocation Table, 32-bit)

• supported by win95 OSR2, Win2k

• 2TB or 2,048GB maximum volume, because of 32-bit numbers to identify clusters

Partitioning

– NTFS (Windows NT File System)

• supports filenames of up to 256 characters long

• Provides extended attributes and file system security

Logical or High-Level Formatting

– writes the structures necessary for managing files and data on the disk.

The magnetic surface and how it operates

Hard Disk Drive Components Read/Write Heads

Read/Write Heads

Hard Disk Drive Components

Head actuator mechanism

– Two Basic categories:

• Stepper Motor Actuator & Voice Coil

– an electrical motor that can “step,” or move from position to position.

– affected by a variety of problems such as temperature.

Stepper Motor

Forms of Optical Disc Storage

Magneto-optical disc storage is optical disc storage where the magnetic state on a ferromagnetic surface stores information. The information is read optically and written by combining magnetic and optical methods. Magneto-optical disc storage is non-volatile, sequential access, slow write, fast read storage used for tertiary and off-line storage.

Tracks & Sectors

Track- is a single ring of data on one side of a disk.

Sectors- represent a portion of a track.

7,200rpm

Other early methods

Paper tape and punch cards have been used to store information for automatic processing since the 1890s, long before general-purpose computers existed. Information was recorded by punching holes into the paper or cardboard medium, and was read by electrically (or, later, optically) sensing whether a particular location on the medium was solid or contained a hole.

Williams tube used a cathode ray tube, and Selectron tube used a large vacuum tube to store information. These primary storage devices were short-lived in the market, since Williams tube was unreliable and Selectron tube was expensive.