An operating system for a uniprocessor is some code loaded
into main memory to handle hardware interrupts from devices
and software interrupts (system calls) from user programs.
(The Figure numbers below refer to Tanenbaum and Woodhull's book.)

Why write this code?

  . for the user, this makes the machine more user friendly, i.e.,
    extends the bare hardware into a virtual machine that has easier
    to use instructions (the system calls, Fig 1-16)

  . for the computer, to manage and allocate resources (CPUs, main
    memory, devices like tape drives, disk space, network)

Onion layers view of a computing system

  user application software
  ----------------------------------------------------
  system software (compilers, editors, etc.)
  ----------------------------------------------------
  OS kernel (handles software and hardware interrupts)
  ----------------------------------------------------
  hardware

Categories of hardware and software interrupts

              CPU             memory         files       network

user          raise,          acquire,    open,          socket,
system        lower           release     read,          connect,
calls         priority        memory      write          listen, accept

device        round-robin     first-fit   directories,   packets
independent   scheduling                  files of
(management)                              logical blocks

device        context         MMU         disk           controller
dependent     switching                   controller     card
(driver)                                  commands       commands

interrupt     clock           fault,      disk           controller
handlers                      parity      controller     card

OS history

  . libraries of complicated device drivers
  . resident monitor to read card decks representing user jobs
  . hardware advances
    - multiprogramming: sharing IO and CPU, concurrent execution
    - memory protection
    - kernel mode
    - hardware clock
    - priviledged instructions: IO, halt, modify timer
  . distributed systems

OS design structures

  . monolithic,                          Fig 1-17
  . layered,                             Fig 1-18
  . VM (virtual machine),                Fig 1-19
  . client-server e.g. MINIX and others, Fig 1-20
  . distributed,                         Fig 1-21