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Operating Systems (Processes (Inter-Process Communication (Mechanism that…

- - - - Direct Communication
        
        pipe() is an example of this
        
        each process explicitly names the recipient / sender
        
        link is established between the two.
      - Indirect Communication
        
        Mailbox
        
        Each process is linked to a mailbox
- - - - Low level Hardware signal
        1) Control pin on CPU is set
        2) Error (Divide by Zero)
        3) Software (TRAP)
      - Modern OS design is interrupt driven - most features are implemented with interrupt.
        -Interrupt Handler - Contains algorithm to choose from an interrupt handler vector.
      - Interrupt Handler
        1) save CPU status
        2) Determine INT type with Polling or Vector
        3) Deal with event
        4) Restore CPU status
        5) Return (reload PC, continue)
        *Nested INT possible if interrupts are not disabled.
    - - Programmed I/O (Busy-waiting)
        -CPU requests from I/O and waits until I/O is ready to send data. Data is then sent to CPU which is then sent to Memory.
      - Interrupt driven
        -Similar to above, but CPU will not wait. It will get an INT later when I/O is ready to send. CPU still sends to MEM.
      - Direct Memory Access (Most efficient)
        
        CPU does not wait, once I/O is ready it will send data directly to MEM
  - - - Memory Protection
        -Allow to write to specific job in memory
        -Base register & limit register
      - CPU Protection
        
        protect from infinite loop and mechanisms for OS to regain control.
        -Timer - interrupts every specified duration
      - I/O Protection
        -User requires OS to do I/O on its behalf.
        -Through system calls.
      - SystemCall
        -Unique number #M associated with each sys call.
        -Interrupt handler program is used on #M
        -Pass parameters: Registers, table in memory, stored in stack.
      - Dual-mode Protection strategy
        -Kernal (privilege) / User mode switching.
        -Uses a 'mode bit' called flag
        
        Interrupts will switch to kernel mode and go to OS.
        After the INT is handled, switches back to User mode.
        There is no instruction that switches to kernel mode, HARDWARE only
- - - - Thread Mapping
        Kernel to User Thread
        1) one to one - Waste Kernel space
        2) one to many - cant achieve concurrency
        3) many to many - improve concurrency
      - Thread Pools
        
        Create number of threads at startup & place into pool
        
        Faster to service a request
        
        Limits total number of threads (no overload)