Practical OS Considerations

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Practical OS Considerations

• There is a relatively small amount of choice in
  the consumer market - the popular OSs are
• Windows XP (Home and Pro versions) Vista soon
• Windows NT 4.0, Windows 2000, Windows 2003 Server
• MacOS various versions
• Linux various versions
• Other OSs are available but tend to be used in
  smaller niche markets

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Practical OS Considerations

• For palm-sized PCs
• Windows CE, Windows Mobile in various versions
• Palm OS (various versions 4.x onward are
  modern)
• Psion Epoc32 / Symbian (phone-PDAs)

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Practical OS Considerations

• Why dont palm-sized PCs run ordinary OSs?
• Some do for example, the ALPS Access Linux
  Platform

4
Size Matters

• Windows Pocket PC 2000 OS plus the Pocket Office
  software fitted into 16MB of ROM
• Most PDA or smartphone OSs are about 4 to 16MB
  in size
• An example Windows NT 4.0 system (late 1990s),
  set up as a workstation, used 236MB of hard disc

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Size Matters

• Windows XP (2001-2006) used a couple of gigabytes
• Some slimmed-down versions of Linux can still be
  installed in less than 256MB (2006)
• Why is XP so big? Windows is packed full of
  utilities and libraries of routines that you
  rarely use
• Windows is intricate, huge and growing...

6
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

7
Some DLLs in the Windows folder
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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
9
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

10
What is multitasking?

• Computers need to keep many things going
  simultaneously, yet until recently there was
  usually just one CPU (now 2 or more cores is
  common)
• Its the old spinning plates stage show - you
  can do one thing at a time, so fake it by
  swapping from one task to the next very quickly
• A good multitasking system will prioritise
  according to some static or dynamic criteria

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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)

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Summary

• There is some 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