CY1001 Introduction to Computer Systems

Storage Devices

Lecture objectives:

• Introduce types of memory and data storage systems.

Input Output

Auxiliary(secondary)

Storage

Arithmetic Unit

The (very) basic architecture

Main(primary) Memory

Overview

Data 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.

Memory - overview

• To 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.

• Cache 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 data – holds instructions and data most likely to be needed

next by processor to reduce access to slower RAM.

Memory - Cache

• 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

Main Memory – Primary storage

Main Memory – Primary storage

• Common RAM technologies are:

– Dynamic RAM (DRAM), slow, must refresh

– Static RAM (SRAM), faster, no refresh needed, but more expensive, needs more space and power

– Enhanced Data Output (EDO) RAM, 50% faster than DRAM

– Synchronous dynamic RAM (SDRAM) enables data transfers at more than twice the speed of previous RAM technologies (standard memory on most new PCs).  

Buses

• Buses are used to transport electrical signals (bits) from one place to another. There are two types of buses:– address bus– data bus

• Source and destination addresses are sent over address bus to access memory locations

• data and instructions are transferred using data bus.

Secondary 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.

Secondary 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.

Secondary storage and files

Data 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

capabilities

– Also allow sequential access

Magnetic disks

• Two types of magnetic disks which are:

• Interchangeable Magnetic Disks – Can be stored offline and loaded to the magnetic disk

drives as they are needed

• Fixed 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.

Interchangeable 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

• 3 1/2-inch Floppy Diskettes• This is a thin disk, permanently enclosed in a rigid

plastic jacket and comes in two capacities - 720 KB or 1.44 MB.

• SuperDisk• It 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 Disk• the 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.

Interchangeable disks

Hard disks

• Hard 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 installed

– Interchangeable hard disks

Magnetic 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.

Hard 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 floppy – its performance is almost as good as that of a

permanently installed hard disk

Hard disks

Magnetic tapes

• In 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.

Optical laser disks

• High 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 disk

• another laser reads the data from the light-sensitive recording surface.

• A light beam is easily reflected to the desired place on the optical disk.

• Currently there are three main categories of optical laser disks:– CD-ROM and DVD– WORM disks– Re-writable optical disks

Optical laser disks

CD-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 music

• CD-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.

CD-ROM

DVD

• 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

WORM Disks

• Write 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.

Re-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

Summary

• Memory– Cash memory – RAM – ROM– Secondary memory

• Secondary storage and files • Magnetic disks• Optical laser disks Cash memory

– CD-ROM – DVD– WORM disks– Rewritable Optical Disks