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L14

Guilin Wang

School of Computer Science

The University of Birmingham

[adapted from Ata Kaban]

Memories, I/O devices and Networks

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Topics for This Lecture

• Internal (Primary/Main) Memories– Main memory, cache

• External (Secondary) Memories– disks, CD-ROMs,…

• Input/Output Devices– terminals, mice, printers, ...

• Computer Networks– LANs, WANs, MANs, WLANs

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CommunicationNetwork

Typical Computing Environment

A number of computers interconnected by a computer network, communicating with file servers, shared databases, printers, etc.

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Memory Hierarchy

• Memory is used to store data and programs

• Memory is organized into a hierarchy: • Internal memory

- registers in CPU (closest to the processor)

- cache (faster than main memory)

- main memory (usually RAM, not ROM)External memory

- magnetic disk (direct access) - optical disk, such as CD-ROM, CD-RW, DVD, … - magnetic tape

http://www.maximumpc.com/files/u69/Memory.jpg

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Memory Hierarchy

• d High speed & cost, small size

Low speed & cost, large size

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Memory

• The basic unit of memory is the binary digit, a bit. • Memories consist of a number of cells (or locations)• Each cell stores k bits (a word) • Each cell has its address, by which programs can refer to it. • Adjacent cells have consecutive addresses. • Some recent computers (e.g. IBM PC) use 8-bit cells. • A cell is the smallest addressable unit, though data can be

transferred in much larger units (blocks of words).

http://www.treehugger.com/files/2007/10/home_energy_usa.php

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Primary Memory

Example:

3 Ways of organising 96 bit memory

• N cells• Addresses

0…N-1

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Error-correcting Codes

• To avoid possible errors in memories, error-detecting or error-correcting codes are used.

(a) Encoding of 1100

(b) Even parity added

(c) Error in AC

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Cache Memory

• fast memory• more expensive• usually small

Stores most heavily used memory words. Logically, between CPU and main memory. First look in the cache, then main memory.

RegistersRegisters

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Magnetic Disks

http://www.ptdd.com/datarecovery/hard-drive.htm

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Magnetic Disks

• A disk consists of a few aluminum platters.• Each platter rotates under a head; magnetic coating reacts.• Bits stored in tracks (concentric circles), split into sectors.• Disks can be hard or flexible (floppy disk, diskette).• SCSI (scuzzy) disks have high transfer rates (5~320

MB/sec), though IDE disks are also popular.

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Magnetic Disks

Cylinder: The set of tracks at a given radial position.

A disk with four platters

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Optical disks

• CD-ROM (Compact Disk - Read Only Memory)• CD-Rs (Compact Disk - Recordables)• DVD (Digital Video/Versatile Disk); higher capacity.

Higher capacity than magnetic disks. High power infrared laser burns holes in master. Copy made with bumps (pits) where the laser holes were.

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A computer with I/O devices

• A single bus (often there are two or more).• A bus arbiter, to resolve conflicts over simultaneous

access.• A controller for each device.• Various I/O devices (keyboard/mouse, monitor, printer,

modem, camera, mike, CD, DVD, etc).

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Computer Networks

• Computer network: many connected computers for exchanging information.

• Essence: packet-switching• Topology architecture:

http://en.wikipedia.org/wiki/File:NetworkTopologies.png

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Internet

WiFi

computer

RouterLaptop RouterRouter

WiFi

Computer Networks

AP

Router

AP

• Network Protocols: rules to guarantee that packets transmission can be conducted properly.

• Combining networks: connect via repeater, bridge, switch and router.

• Routing information

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Types of Computer Networks

• LANs (Local Area Networks)– technology suitable for small area, usually

wire/fibre

• WANs (Wide Area Networks)– large distances, inter-city/country/continental– the Internet

• MANs (Metropolitan Area Networks)– intra-city, cable based, multimedia

• Wireless networks– WLANs, WPANs

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LANs

• Local Area Networks– within an institution, home, etc

• High bandwith(total amount of data per unit of time)

• Low latency(time taken for the first bit to reach destination)

• Technology– predominantly Ethernet, now 100~1000Mbps

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WANs

• Low bandwith, high latency• Satellite/wire/cable • Routers introduce delays

MANs• Wire/cable• Range of technologies (ATM, Ethernet)

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The Internet & WWW

• The Internet – large, heterogeneous and open-ended WAN– connects home users and businesses

• World-wide Web: resource sharing over the Internet

• Based on technologies:– HTML (HyperText Markup Language)– URL (Uniform Resource Locator)– client-server architecture

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Internet

Home intranetWAP gateway

Host siteLaptop

Mobile

PrinterCamera

Host intranetWireless LAN

phone

The future is mobile...

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The future is home intranet...

• Wireless LANs (WLANs)– connectivity for portable devices (laptops,

PDAs, mobile phones, video/dig. cameras, …)

• Home intranet– devices embedded in home appliances (hi-fi,

washing machines, …)– universal ‘remote control’ + communication

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Summary

• Conventional I/O devices– memory and external storage– increasing variety of I/O devices

(multimedia - sound, video, etc)• Networks for sharing and communication• Current & future developments

– increasing of personal & mobile devices– growth of home intranets