I/O Layers

• User Interface : OS I/O procedures
  Access to virtual devices (such as files on Unix, streams on DEC10 and datasets on IBM - device independent
  Operations supported : read, write, open, close, seek etc
  Synchronous or asynchronous
• Device Independent Layer : Allocation of dedicated devices
  Protection : user access validation
  Naming and switching : map a virtual device to a real device
  Validate request against operations perform able on device
  Buffering and Spooling
  Error Reporting
• Device Handler : Access device registers
  Initiate operations
  Schedule requests
  Handle errors
  Perform character translation if needed
• Interrupt Handler : Process each interrupt,
  On the completion of a transfer, signal device handler