Practical OS Considerations

1
Practical OS Considerations

• There is a relatively small amount of choice in
  the consumer market - the popular OSs are
• Windows 98, Windows XP (Home and Pro versions)
• Windows NT 4.0, Windows 2000
• MacOS various versions
• Linux various versions
• Other OSs are available but tend to be used in
  smaller niche markets

2
Practical OS Considerations

• For palm-sized PCs
• Windows CE, Windows Pocket PC 2000/2002/2003
• Palm OS
• Psion Epoc32 / Symbian (phone-PDAs)
• There are other OSs but these are the popular
  ones
• Why dont palm-sized PCs run ordinary OSs?

3
Size Matters

• Windows Pocket PC plus the Pocket Office software
  fits into 16MB of ROM
• Most palm-PC OSs are about 4 to 8 MB in size
• An example Windows NT 4.0 system, set up as a
  workstation, uses 236MB of hard disc! Windows XP
  uses much more
• Windows is packed full of utilities and libraries
  of routines that you rarely use
• Windows is intricate, huge and growing...

4
Windows libraries

• Libraries are collections of program chunks, all
  in one file
• Dynamically Linkable Libraries are libraries
  that a program can load, use and then discard
• Windows, MacOS and some UNIX OSs use DLLs
• They reduce duplication of program code
• But they can cause version control nightmares

5
Libraries - Reduce Duplication
App One
Common - Lots of little bits of code
App Two
Common - Lots of little bits of code
App One
DLL - Lots of little bits of code
App Two
6
Windows device drivers

• Windows (and other OSs) use standards-based
  libraries to access hardware and software
  services
• Imagine a situation where every electrical goods
  manufacturer stuck to its own individual
  standards for power socket design and voltage
• You would need lots of different types of power
  points in your house
• It is far easier to have one common standard
  interface per device type

7
What is multitasking?

• Computers need to keep many things going
  simultaneously, yet there is usually just one CPU
• Its the old spinning plates stage show - you
  only do one thing at a time but swap from one to
  the next very quickly
• A good multitasking system will prioritise
  according to some static or dynamic criteria

8
Types of Multitasking

• Multitasking is usually one of two types
  round-robin (cooperative) or pre-emptive
• A round-robin system passes control from one
  process to the next, each process taking as much
  time as it needs (e.g. Windows 3.11)
• A pre-emptive system has a central time-keeper
  that allows each process a limited time-slice
  (e.g. Windows XP)

9
Summary

• There is a little choice in major operating
  systems
• We have seen a little about the relative scales
  of different types of OS
• And the idea of re-usability library files and
  dynamic libraries (DLLs)
• The concept of adapter software (device drivers)
  was introduced
• Two types of multitasking were introduced