OS generally has two parts.
  Standard library functions that interact with the hardware.
    Used directly or indirectly by the user applications or task. 

    ROM-BIOS interrupts.

    System level DLLs or Kernel calls(modern OSes)

  OS process itself, a watchdog program that:
     Configures the system and controls the user tasks.

       Time sharing of tasks and priorities.

       Access control of resources.

General history line of OS.
  Programmer hard-wired the machine for the needed task, programmer is OS.

  Automating program and data loading "reading" into system.
    Overwrites loader in process.
 
    This technique even used for IBM pc.
      Wordperfect 5.1 overwrote OS, providing its own I/O routines.
      MS-Dos reloaded when Wordperfect terminated.

  Loader OS stored in permanent memory, called a monitor.  
    Monitor's activity suspended until task finishes.
  
    Precursor to kernel introduced - set of permanent tasks to be used by 
    any tasks loaded.
   
    User allowed to manually pause task and reactivate monitor. 

    Monitors were extended to actively "monitor" the task running to guard 
      against endless loops.  Simple form of task switching.

  As systems got faster, I/O became a bottleneck.
    Secondary systems connected to buffer and handle speed differences. 

  Monitor has to actively coordinate tasks and I/O system activities.
    Beginning of multitasking.

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