Operating Systems and Computer Organization

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Operating Systems and Computer Organization
Chapter 4
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Operating System and Computer Organization

• Organization of Conventional Computers
• The ALU
• The Control Unit
• The Memory Unit
• I/O Devices
• Interrupts
• Hardware protection

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Conventional Computer Organization
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The ALU

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Memory Data Access
Main Memory
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Program Specification
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Machine Language
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Control Unit
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Control Unit Operation

• Instruction Fetch Phase instruction is retrieved
  from memory
• Instruction Decode Phase instruction is decoded
  and control signals generated
• Instruction Execute Phase ALU op, memory data
  access, or I/O op is executed.

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Control Unit Operation

• PC ltMachine-Start-Addressgt
• IR MemoryPC
• haltFlag CLEAR
• Decode(IR)
• while (haltFlag not SET)
• Execute(IR)
• PC PC InstructionSize
• IR MemoryPC
• Decode(IR)

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How does it all start?

• When the computer is started, the control unit
  branches to a fixed memory location e.g. initial
  PC value hardwired.
• The fixed location is a ROM address that contains
  a small bootstrap loader.
• The bootstrap loader may be comprehensive enough
  to load the nucleus of the OS Otherwise, it
  loads a loader program that does so.
• Once bootstrap phase is done, any program can be
  run by loading it in memory and loading its
  initial address in the PC (fetch-decode-exec
  algorithm)

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Memory Unit

• Main (Primary) Memory holds both program and data
  while they are being executed by CPU.
• The main memory interface consists of the
  registers MAR, MDR, CMD.
• Unit of information accessed depends on the
  width of memory and width of data bus.
• Max. addressing space depends on width of address
  bus.
• One request at a time.
• CPU and I/O devices may contend for memory access.

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I/O devices

• Each I/O device consists of a device controller
  and the physical device itself.
• Devices
• - storage devices for permanent storage (e.g.
  disk, tape)
• - communication devices to transfer data
  from the computer to another machine (e.g.
  keyboard, a terminal display, or a serial port
  to a modem or a network).

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I/O Devices (cont.)

• Device controller hardware that connects the
  device to the computer.
• continuously monitors and controls the operation
  of the device.
• provides an interface to the computer.
• The device communicates with the computer via a
  communication point called a port.
• Since several devices need to be connected to a
  computer, they are connected trough the bus.

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I/O Devices (cont.)

• How does the computer communicate with an I/O
  device?
• device controller has a set of registers Command
  reg., Status reg., Data regs., Address regs, etc.
• Status reg. tells the computer the status of
  device idle, busy, non functional, etc.
• A process can request an operation from device by
  placing a command in devices Command reg.
• Data regs. Are used to exchange data
• Address regs used to indicate address of
  source/destination

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I/O Devices (cont.)

• How does the computer operate on device
  controller registers? How does it identify each
  device?
• 1. Instruction set of the CPU may have special
  instructions to operate on I/O devices. E.g.
• Input DeviceN, DeviceRegisterN, Rn Output
  Rn, DeviceN, DeviceRegisterN
• etc.
• 2. Memory Mapped I/O

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Memory Mapped I/O

• A set of memory addresses are reserved for I/O
  devices. E.g 0000 to 3FF in Intel 8086
• each device is assigned a sub-set of memory
  addresses.
• A memory address is assigned to each register of
  each device controller
• regular CPU instructions are used to interact
  with device controller.

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I/O Devices (cont.)

• How does the CPU know when a device controller
  has completed the requested operation?
• 1. Polling CPU continually check status
  register of device controller
• 2. Interrupt driven I/O device controller sends
  a signal to CPU through the bus.
• bus must support interrupts
• CPU must include an interrupt flag
• CPU instruction set must include instructions to
  test and set/clear interrupt flag.

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Fetch-Execute Cycle with Interrupt

• PC ltMachine-Start-Addressgt
• IR MemoryPC
• haltFlag CLEAR
• Decode(IR)
• while (haltFlag not SET)
• Execute(IR)
• PC PC InstructionSize
• if (InterruptFlag)
• save current PC // (e.g. in system stack)
• PC AddressOfInterruptHandler
• IR MemoryPC
• Decode(IR)

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Hardware Protection

• Dual-Mode Operation
• I/O Protection
• Memory Protection

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Dual-Mode Operation

• Sharing system resources requires that the
  operating system ensures a level of protection to
  the users
• an incorrect program can not cause other programs
  to execute incorrectly
• that a user program cannot have access resource
  for it does not have permission
• some operations should not be performed by user
  programs

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Dual-Mode Operation (Cont.)

• Provide hardware support to differentiate between
  at least two modes of operations
• 1. User mode - execution done on behalf of the
  user
• 2. Supervisor mode (also called monitor mode or
  privileged mode or protected mode or system mode
  etc.) - execution done on behalf of the operating
  system

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Dual-Mode Operation (Cont.)

• Mode bit is added to computer hardware (to CPU)
  to indicate the current mode.
• Supervisor mode (0) user mode (1)
• Instruction set privileged instructions and
  non-privileged instructions - privileged
  instructions can be executed only in supervisor
  mode.
• Operating system can execute any instruction.
• Must insure that a user program can not execute
  privileged instructions directly.

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Going from User Mode to Supervisor Mode

• When a user program need service from the OS
  (e.g. open a file, read, write, allocate memory
  etc.), it makes a system call i.e. a call to one
  of the OS functions.
• OS functions must run in supervisor mode.
• User processes run in user mode.
• computer designers had to come up with some kind
  of approach that would allow a user process to
  miraculously change the CPU mode to supervisor
  and branch to one of these OS functions
  simultaneously. The trap instruction is just the
  ticket.

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Trap Instruction

• A trap instruction is a machine-level instruction
  that
• Switches the CPU mode to supervisor
• Looks up the address of the target function in a
  kernel-space trap table
• Branches to the entry point of the kernel-space
  function using the address from the trap table
• The trick is that the instruction does all three
  of these steps rather than just one or two. And
  trap is the only instruction that sets the CPU
  mode bit to supervisor.

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• The system call is translated into a trap
  instruction
• the trap instruction is a machine level
  instruction that is part of the instruction set
  of the processor
• The trap instruction will do the following
• change mode-bit to supervisor mode
• jump to a trap handler which will determine which
  OS function is being requested etc.

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Dual-Mode Operation (Cont.)

• Processor must provide the following
• A Mode-bit flag
• Instruction set privileged instructions and
  non-privileged instructions
• A trap instruction

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I/O protection

• Instruction to change the mode must be privileged
• I/O operation are privileged instructions
• How can user program perform I/O?
• It makes a system call to OS.
• When a system call is made a trap instruction is
  executed
• sets the mode to supervisor mode
• OS function verifies that parameters are correct
  and legal, executes the request, sets mode to
  user mode, and returns control to user program

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Memory Protection
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Example of Memory Protection
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Protection Hardware