Contact Information

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Contact Information

• Office 225 Neville Hall
• Office Hours Monday and Wednesday 1200-100 and
  by appointment.
• E-Mail dickens_at_umcs.maine.edu
• Phone [phone]
• Textbook Operating System Concepts, 7th edition,
  Silberschatz, Galvin, and Gagne.
• URL http//umcs.maine.edu/dickens/COS431/COS431.
  html

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Components of a Computer System

• A computer system consists of
• hardware
• system programs
• application programs

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System Components

• Hardware
• Physical Devices
• Memory, CPU, I/O devices.
• Micro-architecture Executes machine-level
  instructions. Hardware or firmware (microcode).
• Machine language Hardware and instructions
  visible to an assembly language program.

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Components of a Computer System

• Operating System
• Hides complexity of hardware and provides
  simplified set of instructions (API, Application
  Program Interface).

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Components of a Computer System

• System programs
• Generally execute in user mode. (Why?)
• Command interpreter (shell), compilers, editors,
  ..

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So What is an Operating System?

• It is an extended machine
• Hides the messy details.
• Presents user with a virtual machine, easier to
  use
• Dont want to program I/O devices, handle
  interrupts, etc.
• It is a resource manager (and scheduler)
• Each program gets time with the resource
• E.g., CPU, I/O devices, memory
• Each program gets space on the resource
• E.g., Disk space.

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So What is an Operating System?

• Control Program
• Manages the execution of user applications to
  prevent errors and improper use of system.
• Program A should not be able to access memory of
  an unaffiliated program B.
• No one user should be able to starve OS and/or
  other users of system resources (e.g., CPU).
• No user should be able to trash I/O devices
  (deliberately or by error).

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Computer Hardware Review
Monitor
Bus

• Components of a simple personal computer

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• Special registers
• Program counter.
• Stack pointer.
• Process Status Word (PSW).

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• CPU and other devices can run concurrently.
• When device needs attention, it issues an
  interrupt.
• Character transferred from keyboard to system
  buffer.
• Disk read/write completed.
• Page fault.
• User programs generate interrupts when making
  system calls.
• Trap instruction.
• Generally termed software interrupt.

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Interrupts

• Operating systems are interrupt-driven.
• Perform action, such as executing a process, then
  wait until receives an interrupt.
• Makes developing an operating system a very
  complex task.
• When an interrupt is generated the CPU stops what
  it is doing and execution is transferred to the
  appropriate interrupt handler.
• Addresses of interrupt handlers are found in the
  interrupt vector.
• Table of addresses
• Each device has a particular offset within the
  table.

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Interrupts

• The state of the interrupted computation must be
  saved before interrupt can be processed.
• This state is restored when the OS returns from
  the interrupt handler.
• Termed a context switch.

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I/O Devices

• Each type of device has its own controller that
  accepts and carries out commands from the
  operating system.
• Because each controller is different,
  manufacturer provides device driver for each
  operating system it supports.
• OS talks to device driver.

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I/O Operation

• Program makes I/O request (e.g., read/write
  to/from a file).
• Device driver loads appropriate controller
  registers.
• Controller reads registers and determines the
  operation requested.
• Starts data transfer from the device to its local
  buffer.
• When completed, generates an interrupt.
• State of currently executing program is saved.
• Interrupt handled by the device driver.
• Device driver returns control to the OS.
• OS restarts suspended program.

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(a)
(b)

• (a) Steps in starting an I/O device and getting
  interrupt
• (b) How the CPU is interrupted

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Three I/O Methods

• Synchronous
• Asynchronous
• DMA (Direct Memory Access).

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Three I/O Methods

• Synchronous
• After I/O starts, control returns to user program
  only upon I/O completion.
• Wait instruction idles the CPU until the next
  interrupt
• Wait loop (contention for memory access).
• Generally a bad idea.

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I/O Methods

• Asynchronous
• After I/O starts, control returns to user program
  without waiting for I/O completion.
• I/O completion signaled by an interrupt.
• More complicated than synchronous I/O.

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Two of the I/O Methods
Synchronous
Asynchronous
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I/O Methods

• DMA
• Used for high-speed I/O devices able to transmit
  information at close to memory speeds.
• Device controller transfers blocks of data from
  buffer storage directly to main memory without
  CPU intervention.
• Only one interrupt is generated per block, rather
  than the one interrupt per byte.

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I/O Methods

• Asynchronous I/O requires the ability to handle
  multiple I/O commands concurrently.
• Uses device status table.
• Queue waiting requests.

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Device-Status Table