Chapter 2 -Lesson 4
- 1. chapter 2 – The Computer Lesson 4 – Memory
- 2. Lesson Objectives: • At the end of the lesson the students can: • Explain the memory of the computer. • Differentiate processing and networking.
- 3. memory short term and long term speed, capacity, compression formats, access
- 4. Short-term Memory - RAM • Random access memory (RAM) – on silicon chips – 100 nano-second access time – usually volatile (lose information if power turned off) – data transferred at around 100 Mbytes/sec • Some non-volatile RAM used to store basic set-up information • Typical desktop computers: 64 to 256 Mbytes RAM
- 5. Long-term Memory - disks • magnetic disks – floppy disks store around 1.4 Mbytes – hard disks typically 40 Gbytes to 100s of Gbytes access time ~10ms, transfer rate 100kbytes/s • optical disks – use lasers to read and sometimes write – more robust that magnetic media – CD-ROM - same technology as home audio, ~ 600 Gbytes – DVD - for AV applications, or very large files
- 6. Blurring boundaries • PDAs – often use RAM for their main memory • Flash-Memory – used in PDAs, cameras etc. – silicon based but persistent – plug-in USB devices for data transfer
- 7. speed and capacity • what do the numbers mean? • some sizes (all uncompressed) … – this book, text only ~ 320,000 words, 2Mb – the Bible ~ 4.5 Mbytes – scanned page ~ 128 Mbytes • (11x8 inches, 1200 dpi, 8bit greyscale) – digital photo ~ 10 Mbytes • (2–4 mega pixels, 24 bit colour) – video ~ 10 Mbytes per second • (512x512, 12 bit colour, 25 frames per sec)
- 8. virtual memory • Problem: – running lots of programs + each program large – not enough RAM • Solution - Virtual memory : – store some programs temporarily on disk – makes RAM appear bigger • But … swopping – program on disk needs to run again – copied from disk to RAM – s l o w s t h i n g s d o w n
- 9. Compression • reduce amount of storage required • lossless – recover exact text or image – e.g. GIF, ZIP – look for commonalities: • text: AAAAAAAAAABBBBBCCCCCCCC 10A5B8C • video: compare successive frames and store change • lossy – recover something like original – e.g. JPEG, MP3 – exploit perception • JPEG: lose rapid changes and some colour • MP3: reduce accuracy of drowned out notes
- 10. Storage formats - text • ASCII - 7-bit binary code for to each letter and character • UTF-8 - 8-bit encoding of 16 bit character set • RTF (rich text format) - text plus formatting and layout information • SGML (standardized generalised markup language) - documents regarded as structured objects • XML (extended markup language) - simpler version of SGML for web applications
- 11. Storage formats - media • Images: – many storage formats : (PostScript, GIFF, JPEG, TIFF, PICT, etc.) – plus different compression techniques (to reduce their storage requirements) • Audio/Video – again lots of formats : (QuickTime, MPEG, WAV, etc.) – compression even more important – also ‘streaming’ formats for network delivery
- 12. methods of access • large information store – long time to search => use index – what you index -> what you can access • simple index needs exact match • forgiving systems: – Xerox “do what I mean” (DWIM) – SOUNDEX – McCloud ~ MacCleod • access without structure … – free text indexing (all the words in a document) – needs lots of space!!
- 13. processing and networks finite speed (but also Moore’s law) limits of interaction networked computing
- 14. Finite processing speed • Designers tend to assume fast processors, and make interfaces more and more complicated • But problems occur, because processing cannot keep up with all the tasks it needs to do – cursor overshooting because system has buffered keypresses – icon wars - user clicks on icon, nothing happens, clicks on another, then system responds and windows fly everywhere • Also problems if system is too fast - e.g. help screens may scroll through text much too rapidly to be read
- 15. Moore’s law • computers get faster and faster! • 1965 … – Gordon Moore, co-founder of Intel, noticed a pattern – processor speed doubles every 18 months – PC … 1987: 1.5 Mhz, 2002: 1.5 GHz • similar pattern for memory – but doubles every 12 months!! – hard disk … 1991: 20Mbyte : 2002: 30 Gbyte • baby born today – record all sound and vision – by 70 all life’s memories stored in a grain of dust! /e3/online/moores-law/
- 16. the myth of the infinitely fast machine • implicit assumption … no delays an infinitely fast machine • what is good design for real machines? • good example … the telephone : – type keys too fast – hear tones as numbers sent down the line – actually an accident of implementation – emulate in deisgn
- 17. Limitations on interactive performance Computation bound – Computation takes ages, causing frustration for the user Storage channel bound – Bottleneck in transference of data from disk to memory Graphics bound – Common bottleneck: updating displays requires a lot of effort - sometimes helped by adding a graphics co- processor optimised to take on the burden Network capacity – Many computers networked - shared resources and files, access to printers etc. - but interactive performance can be reduced by slow network speed
- 18. Networked computing Networks allow access to … – large memory and processing – other people (groupware, email) – shared resources – esp. the web Issues – network delays – slow feedback – conflicts - many people update data – unpredictability