4Data storage Analogue signals are continuous wave forms, vary in frequency and amplitude to represent information from sound and data.Digital signals are described using two states: on (1) or off (0).Computers are digital and require digital data.
5Memory - overviewTo be able to run a program, a computer must first have the program in its memory.Main memory consists of memory chips, which are mounted within the computer itself.Secondary storage is used for long-term storage of computer programs and data.
6Memory - CacheCache memory is used to facilitate faster transfer of instructions and data to processor.Cache increases the rate at which work can be performed by a computer.It is a high speed holding area for program instructions and dataholds instructions and data most likely to be needed next by processor to reduce access to slower RAM.
7Main Memory – Primary storage RAM (Random Access Memory)is a read-and-write memory.It is electronic circuitry and has no moving parts, so access is at electronically fast speed.128 – 512 MB 1 GB
9Main Memory – Primary storage Common RAM technologies are:Dynamic RAM (DRAM), slow, must refreshStatic RAM (SRAM), faster, no refresh needed, but more expensive, needs more space and powerEnhanced Data Output (EDO) RAM, 50% faster than DRAMSynchronous dynamic RAM (SDRAM) enables data transfers at more than twice the speed of previous RAM technologies (standard memory on most new PCs).
10BusesBuses are used to transport electrical signals (bits) from one place to another. There are two types of buses:address busdata busSource and destination addresses are sent over address bus to access memory locationsdata and instructions are transferred using data bus.
11Secondary storage and files Information is stored permanently in Secondary Storage and retrieved in milliseconds.Within a computer system, programs and information (text, image, audio, video) are retrieved from secondary storage.stored temporarily in RAM (primary storage) for processing.
13Secondary storage and files A file is simply a recording of information.Everything we do on a computer involves a file and therefore secondary storage.The operating system manages where a file is stored in secondary memory.
14Data access – type of secondary storage Two methods for storing and accessing data which are sequential and random.Sequential Processing the computer searches the storage medium from the beginning to find the desired programs, data, or files.Magnetic tapes can be used for sequential access only.Random Processing the desired programs, data, and files are accessed directly from the storage medium.Magnetic disks have random or direct-access capabilitiesAlso allow sequential access
15Magnetic disks Two types of magnetic disks which are: Interchangeable Magnetic DisksCan be stored offline and loaded to the magnetic disk drives as they are neededFixed Magnetic Disks (hard disks) which are permanently installed, or fixed.These are rigid and usually made of aluminum with a surface coating of easily magnetized elements, e.g. iron, cobalt, nickel.
16Interchangeable disks Virtually all PCs today are configured with at least one hard disk drive and one interchangeable disk drive.There are three types of interchangeable disk drives are commonly used on PCs
17Interchangeable disks 3 1/2-inch Floppy DiskettesThis is a thin disk, permanently enclosed in a rigid plastic jacket and comes in two capacities KB or 1.44 MB.SuperDiskIt combines hard and floppy disk technology and can store 120 MB. It has the same size as floppy disks but different disk densities.Zip Drive & Zip Diskthe Zip drive also combines floppy and hard disk technology to read and write up to 100-MB Zip disks. Its storage capacity equivalent to 70 floppy diskettes.
19Hard disksHard Disk manufacturers are working to put more information in less disk space and to enable a more rapid transfer of information to/from RAM.There are two types of hard disks:Permanently installedInterchangeable hard disks
21Hard disks Permanently installed Storage capacities of 2 to 120 GB (~85,000 floppy disks!).It contains several disk platters stacked on a single rotating spindle.Data stored on all recording surfaces.Disks spin continuously at a high speed within a sealed enclosure which keeps the disk-face surfaces free from contaminants,Allows greater density than interchangeable diskettes.
22Hard disks Interchangeable hard disks Can store up to 1 GB of information.It is inserted and removed as easily as the 3 1/2-inch floppyits performance is almost as good as that of a permanently installed hard disk
23Magnetic tapesIn the 1950s and 60s, the foundation of many information systems was sequential processing using magnetic tape master files.Today, magnetic tape storage is no longer used for routine processing.Once loaded, the magnetic tape is online, i.e. the data on the tape is accessible to the computer system.
26Optical laser disksHigh density storage technology which may make magnetic disk and tape storage obsolete.One laser beam writes to the recording surface by scoring microscopic pits in the diskanother laser reads the data from the light-sensitive recording surface.A light beam is easily reflected to the desired place on the optical disk.
27Optical laser disksCurrently there are three main categories of optical laser disks:CD-ROM and DVDWORM disksRe-writable optical disks
28CD-ROM CD-ROM was introduced in 1980. The extraordinarily successful CD, or compact disc, is an optical laser disk designed to enhance the reproduction of recorded musicCD-ROM is a spin-off of audio CD technology and stands for Compact Disk-Read-Only Memory.Once inserted into the CD-ROM drive, the text, video images, etc can be read into RAM for processing or display.Data on the disk are fixed, i.e., cannot be altered with up to 680 MB of data can be stored on a CD.
30DVD DVDs (digital videodisks) are poised to replace CD-ROMs. The DVD looks like CD-ROM, but can store from 7 to 14 times as much information (up to 10 GB).It can store the video for a full-length movie.DVD drives are backwards compatible, i.e. they can also play CDs and CD-ROMs
31WORM DisksWrite Once, read many (WORM) optical laser disks are used by end user companies to store their own proprietary information.In this case, data can only be read, not updated or changed.WORM disk cartridges can store greater volumes of information than a CD-ROM.WORM applications involve image processing or archival storage (e.g. electronic catalogue) and a mainframe-based WORM disk has capacity of 200 GB.
32Re-writable Optical Disks Use several technologies, including magneto-optical (MO) technology, to integrate optical and magnetic disk technology to enable read-and-write storage.A 5 1/4-inch re-writable disk cartridge can store up to 5 GB.Usually used for applications using large volumes of storage with little update activity
33Summary Memory Cash memory RAM ROM Secondary memory Secondary storage and filesMagnetic disksOptical laser disks Cash memoryCD-ROMDVDWORM disksRewritable Optical Disks