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

• 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

• 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
  • 
  • 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