Operating System Structure

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Operating System Structure

• A key concept of operating systems is
  multiprogramming.
• Goal of multiprogramming is to efficiently
  utilize all of the computing resources.
• When a job issues an I/O request (e.g., open a
  file, read data from a file), it cannot continue
  until the request is fulfilled.
• The CPU then becomes idle (the job is blocked on
  the request).

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Operating System Structure

• Basic idea of multiprogramming
• Keep multiple jobs in memory.
• When one job blocks on I/O (or other events), the
  operating system
• Starts the I/O operation.
• Switches to another job that is ready to execute.
• Now the CPU and I/O device are executing in
  parallel.
• When the I/O device has completed request, it
  generates an interrupt to inform the CPU.
• Virtually all general purpose computers support
  multiprogramming.

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• First developed for batch systems in the 60s.
• Go to the computer center and give them your
  program (stored on punch cards).
• The computer operator batched several jobs
  together and loaded them into the computer.
• Come back at 500 to get the results of your
  program.
• Batch systems are non-interactive.

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Time-Sharing Systems (Multitasking)

• Logical extension of multiprogramming termed
  multitasking.
• Quite often sitting at terminal using a command
  line interface to interact with computer.
• Types in commands from keyboard.
• A system program called a shell reads command
  from the command line and makes OS system calls
  to carry out commands.
• OS switches between users programs very quickly,
  generally in round-robin fashion.

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User 2
Shared Computer
User1
User 3
User 4
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User 2
Shared Computer
User 3
User1
User 4
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User 2
Shared Computer
User 3
User1
User 4
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User 2
Shared Computer
User 3
User1
User 4
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User 2
Shared Computer
User 3
User1
User 4
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Time-Sharing Systems (Multitasking)

• Switching between users is very fast.
• Goal is to give the illusion that each user has
  own machine.
• Think about using a text editor, or getting a
  directory listing.
• Response time is a priority.

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Protection of System Resources

• I/O Devices
• Memory
• CPU
• Files
• Operating System

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Protection of System Resources

• Based on dual-mode execution
• kernel mode and user mode.
• Privileged instructions can be issued only in
  kernel mode.
• Mode bit in Process Status Word (PSW), checked on
  every instruction.

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User process executing
Mode Bit1
Continue Execution
System Call
Trap, mode bit 0
Return. Mode bit 1
Execute System Call
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Protection of I/O Devices

• All I/O instructions are privileged instructions.
• Only accessed through system calls.

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Memory Protection

• Must provide memory protection for the interrupt
  vector, interrupt service routines, and other
  applications address space.
• Two registers that determine the range of legal
  addresses a program may access
• Base register holds the smallest legal physical
  memory address.
• Limit register contains the size of the range
• Memory outside the defined range is protected.

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Use of A Base and Limit Register
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Hardware Address Protection
limit
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CPU (and OS) Protection

• Keep user from monopolizing CPU.
• Ensure OS regains control of CPU.

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CPU Protection

• Timer interrupts computer after specified
  period to ensure operating system maintains
  control.
• Timer is decremented every clock tick.
• When timer reaches the value 0, an interrupt
  occurs.
• Timer commonly used to implement time sharing.

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Privileged Instructions

• Load base and limit registers?

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Privileged Instructions

• Load base and limit registers?
• Set the system timer?

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Privileged Instructions

• Load base and limit registers?
• Set the system timer?
• Read the system clock?

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Privileged Instructions

• Set the system timer?
• Read the system clock?
• Load base and limit registers?
• Open a file?

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Privileged Instructions

• Load base and limit registers?
• Set the system timer?
• Read the system clock?
• Open a file?
• Compile a program and create executable?

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Privileged Instructions

• Load base and limit registers?
• Set the system timer?
• Read the system clock?
• Open a file?
• Compile a program and create executable?
• Enable/disable interrupts?