Computer Systems – Storage and Memory. Units. The basic unit of memory is called a BYTE . A byte consists of 8 bits ( bi nary digi ts ). Each keyboard character occupies one byte. Units. a KILOBYTE (Kb) is 1024 bytes (2 10 bytes) a MEGABTYE (Mb) is 1024 Kb or 1048576 bytes
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Computer Systems – Storage and Memory
Units • The basic unit of memory is called a BYTE. A byte consists of 8 bits (binary digits). • Each keyboard character occupies one byte.
Units • a KILOBYTE (Kb) is 1024 bytes (210 bytes) • a MEGABTYE (Mb) is 1024 Kb or 1048576 bytes • a GIGABYTE (Gb) is 1024 Mb or 1073741824 bytes
Primary Storage • Primary storage or Main Memory refers to integrated circuits that store program data and instructions. • Memory chips are installed in the system unit. • The two common types of main memory are ROM and RAM
ROM • Read Only Memory • Used for storing information and instructions that do not change. • This information is retained even when the power is turned off, this type of memory is called non-volatile. • Chips used in some games, cars and appliances use ROM.
ROM • ROM is used to store computer start-up instructions. • ROM cannot be altered. • Other types of ROM include PROM (Programmable ROM) and EPROM (Erasable Programmable ROM)
RAM • Random Access Memory • This is where information and data can be stored temporarily. This type of memory is volatile - once the power is off then all information is lost. • RAM chips (SIMMs - Single In-line Memory Module) are mounted directly on to the main circuit board.
RAM • SRAM (Static RAM) holds information as long as current flows through the circuit. • DRAM (Dynamic RAM) needs high and low voltages at regular intervals. • SRAM is faster but more expensive.
RAM • Flash RAM or flash memory can retain data even when the power is turned off. • RAM and ROM are both measured in Mb.
Cache Memory • High speed RAM memory between the CPU and main memory used to improve processing efficiency and overall system performance. • Cache stores frequently used data and is easily accessible by the CPU.
Secondary Storage • Secondary storage or data storage, stores programs and data when they are not being processed. • The two main types of secondary storage are • Magnetic • Optical
Magnetic • Magnetic disks are the most common form of secondary storage. • Capacities range up to several gigabytes.
Magnetic • Magnetic disks are coated with a magnetic material. • Magnetic types include tape, diskettes, hard disks and removable disk cartridges.
Magnetic Tape • Similar to an audio tape. They are a cheap form of storage and are now mostly used for backing up data. • Very slow. • Sequential access storage medium.
Hard Disks • Hard disks consist of one or more solid disks (or platters) that are coated with an oxide material. • The disks are permanently mounted, generally inside the computer.
Hard Disks • Data can be written to both sides of the platters. • The platters spin at high speed and access time is much faster - although the drives mechanical components make it the slowest component on the computer.
Hard Disks • Smaller hard disks can be connected into an integrated group called RAID • Redundant Array of Inexpensive Disks
RAID • RAID disks have the advantage of being able to read and write data faster. • There is also a much less risk of losing data.
Removable Disk Cartridges • This type of storage includes Zip disks and removable disks. • These have a high capacity and also provide portability.
Optical • A rigid disk of plastic onto which data is recorded by lasers. • Similar in concept to ROM. • Data is directly accessed by an optical disk device (not unlike a stereo CD player)
CD ROM • Compact Disk ROM • once data is recorded it cannot be modified • these disks are now very inexpensive • CD burners are now part of most standard computer systems • CD-RW (CD-rewritable) allows data to be written many times over
Magneto-Optical • The Magnetic-Optical disk (MO) is a almost a cross between magnetic and optical disks. • They have large storage capacity but have slow access time.
Digital Video Disk • DVD’s look like CD’s but can store up to 135 mins of digital video. • They can also be used as computer storage disk and can hold up 17 Gb. • Current DVD players can read CD ROMs but current CD players cannot read DVDs
Media Capacity Cost Speed Use Floppy disk Up 2Mb Cheap 20 Kb/sec Backup, data transfer Hard Disk > 1000 Mb ~ $500 500 Kb/sec Main secondary storage CD ~ 650 Mb Cheap 200 Kb/sec Large portable storage, software distribution Tape > 1000 Mb Cheap 200 Kb/sec Backup, archiving Secondary Storage Comparison