Operating Systems

• Roles of an operating system
  • An Operating System is a program that manages the resources of a computer and provides an interface for system interaction.
    • Resources : memory, hard drive, cpu, ...
    • Examples: Windows, Unix, Linux, Max OS X, VM, VMS, ...
  • Each operating system has different goals
  • But each has common structures
  • Booting (bootstrapping)
    • 
    • The ROMS contain a program to load any other program from secondary storage
    • This is called the boot loader
    • Ths program loads either
      • An operating system
      • A secondary loader
        • The secondary loader may give you a choice of what operating system you wish to run.
        • In this case, you can have two or more operating systems
  • Programs/processes
    • Program - code, machine, assembly, higher level
    • Binary - code in machine language
    • Executable - a program that can be run
    • Executing - running a program
    • process - a running program
    • State - information about a process
      • Memory
      • Code
      • Registers
      • And Others
  • Resources
    • Only one program can use a resource at a time.
    • Multiprogramming - a technique of keeping multiple programs in memory at the same time.
    • Memory Management - Keeping track of where programs and data are stored in memory
    • Process management - tracking processes and their state
      • An os must perform both of these
      • It includes deciding which process has access to (CPU/Memory)
    • CPU Scheduling - deciding which process has access to the cpu.
• Some OS Terms
  • Batch processing
    • A job is an executable + control info + data
    • Often a set of cards
      • Instructions first
      • Code for program
      • data to be used
    • A human Operator
      • set up any devices required for the run
      • place the deck in the card reader
      • Press the "go" button
      • Deliver the output to the programmer/user
    • Later, multiple jobs were grouped into batches
      • Jobs that used the same resources
      • Set up time was minimized
    • Today
      • a batch more commonly refers to a set of system commands set up to run in a file
      • (a batch file, or .bat in dos)
      • this is also often called a script
      • These are traditionally thought of as not being interactive processes
  • Time Sharing
    • A system where multiple users each get a portion of CPU time
    • It appears that each has their own CPU/Memory/...
    • Or possibly each has their own Virtual Machine
  • Another term MultiTasking
• memory management
  • Programs would like to view memory as a single contiguous unit
  • But his is not handy for the OS
    • All programs want their address space to start at 0
    • We don't need all of the program laded into memory at the same time
  • Memory is divided into frames
    • Processes are divided into pages
    • A page fits into a frame
    • The OS maintains a page table to keep track of what pages are associated with what processes.
    • When another page is needed, an old page is written to disk and a new page is loaded
    • This is called pagging or a page swap
    • This allows the hard drive to act as virtual memory
    • But hard drives are much slower than memory
    • So processes are often suspended while a new page is loaded
  • If the system becomes overloaded
    • Too may processes may be trying to use memory
    • Every time a process tries to do something, it needs to bring in a new page of memory
    • But if this with every process thrashing occurs
    • And the system becomes unusable
• Process Management
  • The states of a process
    • New - just created
    • Ready - can be run
    • Running - using the cpu
    • Waiting - blocked, looking for a resource
    • Terminating -
  • When the OS changes from one process to another, a context switch occurs.
    • All information about the current process must be saved.
    • All information about the new process must be loaded.
• CPU Scheduling
  • Two types
    • Non-preemptive - let a process run as long as it can
    • preemptive - stop a process after it has run for a period of time
  • How to decide which process runs next
    • First come first served
    • Shortest Job Next
    • Round Robin