Hello World! An Invitation to Computer Science

Lecture Slides

Operating Systems
Monday April 21, 2008

Recapping the Big Picture

What is an operating system?

It's a resource manager.

• system (as opposed to application) software
• supervising program; manages:
    processes (other programs)
    file system
    memory system
    I/O
    security/access control
• abstraction: software layer between hardware and application software
• performance and system utilization:
    squeezing more efficiency out of system
    reducing idle time

UI is not central to OS

• does have to allow user to select actions and dispatch them
• interfaces with I/O (resource management)
• see REPL in fig 6.15 (p 265)
• example of text shell v. GUI

access control/security

• more recent development in OS history
• increasingly important
• resource management: should user X be able to do Y?
• example in *nix terminal

Brief history of operating systems

• early on: "operator" refers to person
• 0th gen - no distinction between system software and application
• 1st gen - no real OS, just run system software as needed
• 2nd gen - batch systems: stacked punch cards, processing w/out intervention
3rd gen - multiprogrammed - multiple programs can exist in memory at one time
  introduces risk that bug in one program can effect another
    early pc systems - in particular MS DOS
  greatly increased complexity of situation
    real rise of OS begins with time-sharing systems
  networking begins to play a role
  remote access and illusion of multiprocessing
  time slices
• 4th gen - networked os

Why are batch systems inappropriate for something like an airline-reservation system?

Process management and multitasking

• a process: a running instance of a program
• state of process: memory, file descriptions, permissions/ownership
• context switching: save state of one process, (re)load another
• what is "real-time" here?
    single-user: appearance of simultaneous behavior
    multiuser: each user appears to have machine to herself
• scheduler: priorities and preemption
• protected memory

What if time slice too large or too small?