1 L14 Guilin Wang School of Computer Science The University of Birmingham [adapted from Ata Kaban] Memories, I/O devices and Networks

2 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

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

4 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 emory.jpg

5 Memory Hierarchy d High speed & cost, small size Low speed & cost, large size

6 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).

7 Primary Memory Example: 3 Ways of organising 96 bit memory N cells Addresses 0…N-1

8 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

9 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. Registers

10 Magnetic Disks

11 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.

12 Magnetic Disks Cylinder: The set of tracks at a given radial position. A disk with four platters

13 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.

14 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).

15 Computer Networks Computer network: many connected computers for exchanging information. Essence: packet-switching Topology architecture:

16 Internet WiFi computer Router Laptop Router 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

17 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

18 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

19 WANs Low bandwith, high latency Satellite/wire/cable Routers introduce delays MANs Wire/cable Range of technologies (ATM, Ethernet)

20 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

21 Internet Home intranet WAP gateway Host site Laptop Mobile Printer Camera Host intranet Wireless LAN phone The future is mobile...

22 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

23 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