four fundamental os concepts

The Operating System is a:

• Referee

  • Manage protection, isolation, and sharing of resources
  • Resource allocation and communication

• Illusionist

  • Provide clean, easy-to-use abstractions of physical resources
  • Infinite memory, dedicated machine
  • Higher level objects: files, users, messages
  • Masking limitations, visualisation
  • fight complexity: remove software/hardware quirks
  • help the programmer: optimise for convenience, utilisation, reliability, ...

• Glues common services

  • Storage, Window System, Networking
  • Sharing, Authorisation
  • Look and feel

• An OS abstracts underlying hardware to help tame complexity

  • Processor :: Thread
  • Memory :: Address Space
  • Disks, SSDs, ... :: File System
  • Networks :: Sockets
  • Machines :: Processes

1. Thread: Execution Context

• Single unique execution context
• A thread is executing on a processor (core) when it is resident in the processor registers
  • Resident here refers to the fact that registers hold the context (program counter, registers, execution flags, stack, memory state) of a thread
• A thread is not executing (suspended) when its state is not loaded (resident) into the processor
• If there is a single processor (core). How do we provide the illusion of multiple processors? Multiplex in time
• all threads share non-CPU resources
• Threads are virtual cores
  • If the thread is running it is on the physical core otherwise it is saved in a chunk of memory (Thread Control Block)
• How does (Context) Switching occur ?
  • The OS ran
  • Saved context of thread 1 in Thread Control Block (memory)
  • Loaded context of thread 2 from TCB and jumped to PC
  • Triggered?: Timer, Voluntary yield, I/O, ...
• The time taken for context switching must be minimised
• Threads encapsulate concurrency

2. Address Space

• the set of accessible addresses and the state associated with them
• What happens when we attempt to read/write to an address?
  • perhaps acts like regular memory
  • perhaps ignores writes
  • perhaps causes I/O operation
  • perhaps causes exception (fault)
  • communicates with another program
  • ...
• A simple implementation of multiplexing has no protection !!:
  • Must protect itself from user programs
    • Reliability: compromising the operating system generally causes it to crash
    • Security: limits the scope of what threads can do
    • Privacy: limits each thread to the data it is permitted to access
    • Fairness: each thread should be limited to its appropriate share of the system resources
  • Must protect user programs from each other
    • Prevent threads owned by one user from impacting threads owned by another user
• Address Spaces encapsulate protection

3. Process: an instance of a running program

• Execution environment with restricted rights
  • protected address space with one or more threads
  • owns memory (address space)
  • owns file descriptors, file system context,
  • encapsulate one or more threads sharing process resources
• a programs executes as a process
• Why?
  • Reliability: bugs can only overwrite memory of process they are in
  • Security: A malicious or compromised process can't read or write other process' data
• Fundamental tradeoff:
  • communication easier within a process
  • communication harder between processes
• In a modern OS, anything that runs outside of the kernel runs in a process

4. Dual mode operation / Protection

• Hardware provides at least 2 modes:
  • Kernel (or supervisor) mode
  • User mode
• Certain operations are prohibited when running in user mode
• carefully controlled transitions between user mode and kernel mode
• How do we transfer/switch??
  • syscall:
    • process requests a system service
    • like a function call but outside the process
    • does not have the address of the system function to call
  • interrupt:
    • external asynchronous event triggers a context switch
    • independent of user process
  • trap or execution:
    • internal asynchronous event in process triggers context switch
• Processes executes in user mode
  • to perform a privileged action, processes must request services from the OS kernel
  • carefully controlled transition from user to kernel mode
• kernel executes in kernel mode
  • performs privileged action to support running processes