Operating Systems Lecture 1

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Operating SystemsLecture 1

• Crucial hardware concepts review
• M. Naghibzadeh
• Reference M. Naghibzadeh, Operating System
  Concepts and Techniques, iUniverse publisher,
  2005.
• To order www.iUniverse.com, www.barnesandnoble.
  com, or www.amazon.com

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Introduction

• A computer is a device which runs programs.
• A program is a set of instructions that is
  prepared to do a specific job.
• The operating system is an special program which
  sits on top of the computer hardware. It acts as
  an interface between the running programs and the
  computer hardware to
• Provide basic software modules to be used by
  programs
• Facilitate efficient use of computer resources
• Prevent running programs interference

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Computer Hardware Organization

• Within every computer there are many hardware
  devices.
• Control unit,
• Arithmetic-logic unit,
• Cache memory,
• And main memory are the basic devices.
• Other devices are usually Input/Output devices.
• Figure 1 shows essential logical interconnctions
  of the devices

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Essential logical interconnctions of a computers
devices

• Figure 1 Computer organization model

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Physical interconnctions of a computers devices

• To reduce internal interconnection wiring, buses
  are used to connect devices.
• A bus is similar to a highway via which different
  parts of a city are connected.
• There are at least three busses, address bus,
  data bus and control bus, in every computer.
• Some computer devices are not directly connected
  to the computers internal bus.
• In a 32-bit computer, for example, the address
  bus and data bus are each 32 bits wide.
• There are mechanisms for possessing the address
  and data buses, otherwise data and addresses from
  different devices could become mixed up.
• A model of the bus-based computer organization is
  shown in Figure 2.

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Essential Physical interconnctions of a
computers devices

• Figure 2 A bus-based Computer organization model

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The Fetch-Execute Cycle

• The computer reads an instruction from the main
  memory (RAM) and moves it to a temporary place
  called the Instruction Register (IR) in the
  Central Processing Unit (CPU).
• It then finds out what has to be done with
  respect to this instruction and performs the
  actions.
• It repeats this cycle of moving an instruction
  from the main memory to the CPU and executing it
  over and over again.
• This is called Fetch-Execute Cycle

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The Details of Fetch-Execute Cycle

• Figure 3 A simplified functional model of a
  computer

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Program Counter (PC) update

• It is important to understand how the PC is
  filled.
• The PC is automatically filled with an initial
  value when the computer is started or restarted.
• When a new program is to be executed, the
  operating system will fill the PC with the first
  executable instruction of the program.
• Otherwise, it is updated as shown in the figure
  3.
• When an instruction is executed, the next
  instruction to be fetched is most likely the one
  immediately following this instruction. However,
  this is not always the case. Look at the piece of
  program below that is written in a pseudo code
  format.
• 1. If A gt B
• 2. then print A
• 3. else print B
• 4. endif
• Most of the time, the PC update during the fetch
  cycle suffices.

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Interrupt

• Interrupt is a signal that is sent to the CPU to
  capture its attention. It may be issued by
  different sources, like
• By a fault detecting hardware that is trying to
  informs the CPU of a circuitry malfunction
• By a monitoring circuit within the ALU that
  controls the acceptability of the data size for
  the operation being performed and, consequently,
  the operation result size
• By a user that may press a special key like
  break at any time
• By an slave processor that wants to inform the
  CPU of the completion of the assigned task

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Interrupt masking

• Some interrupts are maskable by user programs.
• Masked interrupts are ignored by the CPU. This
  is analogous to ignoring a door knock.
• Some interrupts are not user-maskable. It is not
  reasonable to let the computer continue in the
  presence of a hardware failure.
• However, all interrupts are maskable within the
  kernel of an operating system.

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Interrupt Categorization

• Interrupts are categorized into few classes.
• Different classes of interrupts require.
  different handling consideration.
• Each category of interrupts is represented by a
  bit in interrupt vector within the CPU.
• Interrupt categories have priorities.
• When handling a higher priority interrupt
  handling any lower priority interrupt is
  postponed.

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Program execution suspension

• A non-masked interrupt causes the suspension of
  the currently executing program.
• It is usually possible to resume the execution of
  the program after the interrupt is handled.
• Interrupt signals reach the CPU and are stored in
  the interrupt vector. The interrupt(s) is ignored
  until the currently executing machine instruction
  is completed.
• There are very few exceptions. One example might
  be the move instruction in some computers that
  is suppose to move a big chunk, say 256 bytes, of
  data from one location of main memory to another.
  
• The CPU always looks for interrupt signals just
  before fetching a new instruction to execute. If
  there are any interrupt signals, the system
  handles them in the order of priorities and
  urgencies.

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Fetch-Execute Cycle with interrupt

• Figure 4 A simplified functional model of a
  computer with interrupt

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Privileged instructions

• Privileged instructions are those which are not
  usable by assembly application programmers.
• Privileged instructions are executable only
  within the operating system kernel
• Other parts of the OS and application programmers
  may execute privileged instruction by special
  means that are provided by the operating system
• Disable interrupt which disables all interrupts
  is a sample privileged instruction.

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CPU speed

• CPU speed is usually measured by its clock cycle
  which is around Giga hertz for PCs.
• A clock cycle is usually the time that takes to
  move one data (instruction, or address) from one
  internal register to another.
• A machine instruction execution takes around 10
  clock cycles.
• There are other units of measuring a CPU speed,
  like Million instruction per second (MIPS) and
  Million Floating point instructions per second
  (MFLOPS).

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Summary

• The following subjects were discussed
• Computer, Program, and operating system
• Logical Computer organization model
• Physical Computer organization model
• Functional Computer organization model and the
  Fetch-Execute Cycle
• Interrupt, Interrupt masking, Interrupt
  categorization, and interrupt handling.
• CPU speed.

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Find out

• Your computers instruction categories based on
  their length.
• The benefits of adjusting the PC register during
  the fetch cycle even though we have to readjust
  it during the execute cycle for some
  instructions?
• You have just bought a new personal computer. No
  operating system is installed, what software does
  it have and for what purposes?
• If your computer is a 32-bit one, what is the
  maximum memory size that the CPU can directly
  address (without using a base register)?
• Your computers speed in terms of MIPS.
• Your computers interrupt classes
• Your computers interruptable instructions
• Your computers privileged instructions

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• Thank you!