Operating System

  • Software that manages computer resources
    • I/O devices, memory, etc.
  • Provides an interface for system interaction
  • Uses multiprogramming
    • Technique of keeping multiple programs in main computer memory at the same time
    • Programs compete for the CPU
  • Process management
    • Keeping track of information of active processes
      • Progress, intermediate states, etc.
    • Process
      • Dynamic representation of a program during execution
  • CPU scheduling determines which process in memory is given access to the CPU

Memory Management

  • OS performs the following tasks:
    • Track where and how a program resides in memory
    • Convert logical program addresses into actual memory addresses
  • Keeps track of secondary computer memory in two ways
    1. Maintains a table indicating which blocks of computer memory are free
    2. For each directory, it maintains a table that records information about the files in the directory

Single Contiguous Memory Management

  • A program is loaded into one continuous area of memory
  • Only one program other than the OS can be processed at one time
  • Logical address --> an integer value relative to the starting point of the program
    • To get physical addresses, we add a logical address to the starting address of the program in physical memory
  • Simple to implement and manage
  • Wastes memory space and CPU time

Partition Memory Management

Address resolution in partition memory management

  • Multiple programs allowed in memory
    • OS must make sure they don't access each other's memory space
  • Programs are loaded into those partitions
  • Base register
    • Holds beginning address of the current partition
  • Bounds register
    • Holds the length of the current partition
    • Helps ensure memory addresses are within bounds

Fixed-partition technique

  • Memory is divided into a specific number of partitions
  • Partition size is fixed when the OS initially boots
  • Jobs are loaded into a partition large enough to hold it
  • OS keeps a record of addresses at which each partition begins and its length

Dynamic-partition technique

  • Memory is divided into partitions as needed
  • Each partition is unique to a program
  • Main memory can be viewed as one large empty partition

Paged Memory Management

Paged memory management

  • Main memory is divided into fixed-size blocks called frames
  • Processes are divided pages and in frames when loaded
    • Pages made me scattered around or out of order
    • Pages can be mixed among other pages from separate processes
  • OS maintains a page-map table (PMT) to keep track of each process
  • Processes no longer need to be stored contiguously in memory
  • Demand paging
    • Extension to paged memory
    • Pages are brought into memory only when referenced (on demand)
    • Page swap
      • Bringing in one page from secondary memory, possibly causing another to be removed

Process Management

  • Processes move through specific states in a system
    • Process life cycle
    • New
      • Process created
    • Ready
      • No barriers to its execution
      • Waiting for its chance to use the CPU
    • Running
      • Currently being executed by the CPU
      • Instructions being processed in the fetch-execution cycle
    • Waiting
      • Waiting for resources (other than the CPU)
      • E.g. waiting for user input
    • Terminated
      • Execution is completed
      • No longer active
  • Process control block
    • Data structure used by OS to manage information about a process

CPU Scheduling

  • Act of determining which process in the ready state should move to the running state
  • Nonpreemptive scheduling
    • Occurs when currently executing process gives up the CPU voluntarily
    • Decisions made when a process switches from running --> waiting or when a program terminates
  • Preemptive scheduling
    • Occurs when the OS decides to favour another process, preempting the currently executing process

Computer Science