Computer System Structures

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Computer System Structures
disk
disk
printer
tape
tape
disk controller
printer controller
tape-drive controller
CPU
System bus
memory controller
memory
CPU
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Interrupts

• Signal sent to the CPU by hardware or software
• Hardware triggers an interrupt when an event
  occurs
• Software triggers an interrupt when a system call
  is performed
• This is often called a trap

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How Interrupts Work

• When interrupt occurs, the current program stops
  execution
• The interrupt service routine is executed
• When routine is finished, the current program
  resumes
• This is much like a function call except it is
  done by the hardware

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Interrupt Service Routine

• Each device does something different when
  interrupted
• Each device must have a different interrupt
  service routine
• A table of interrupt service routines is kept by
  the hardware. This table is called the interrupt
  vector and is indexed by the interrupt number

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Interrupt Occurs

• Current program stops
• Using the interrupt number, the starting address
  of the ISR is found in the IV by indexing with
  the interrupt number

• The ISR executes
• When ISR completes, the current program continues
  execution

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Interrupts Priorities

• An interrupt may occur when and ISR is executing.
• Each interrupt has a priority, and the highest
  priority interrupt continues
• If an interrupt interrupts another ISR,
  information must be placed on a stack so
  execution can continue when ISR finishes

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Interrupts Enable and Disable

• If disabled, the current ISR cannot be
  interrupted
• If enabled, the current ISR is interrupted if the
  interrupt has a higher priority than the current
  interrupt

8
Modern Operating Systems

• Interrupt driven
• If there is nothing to do, the OS waits
• When an event occurs, the OS is interrupted and
  the event is handled

• When a system call is done, a trap occurs
• The proper system routine is executed
• Control is then returned to the routine that
  called the system routine

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I/O Structure

• When a routine needs I/O, a system call is done
• The OS consults the device status table to get
  the devices types, address, and state
• The device status table is updated to indicate
  the I/O needed
• When the I/O is completed, the OS is interrupted
  and control returns to the user

10
DMA Transfer

• Some devices maintain a buffer where the data is
  kept
• The OS must copy the buffer when complete or I/O
  cant continue
• It would be better if the device could put the
  data directly into memory

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DMA Transfer (cont.)
Memory
CPU
I/O devices
I/O instructions
12
DMA Transfer (cont.)

• Data is placed directly in memory
• This is done using cycle stealing
• The DMA controller steals memory cycles from the
  CPU
• This can slow down the CPU because it may have to
  wait to get data from memory

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Storage Structure

• Registers - associated with CPU
• Cache - intermediate between CPU and memory
• Main memory - where CPU obtains data
• Secondary storage - permanent storage capacity
• Disks - large capacity
• Tapes - larger capacity
• CDs - only readable

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Disk
Sector
Track
Platter
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Hardware Protection

• Dual-Mode operation
• I/O protection
• Memory protection
• CPU protection

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Dual-Mode Operation

• Provides hardware support for two modes
• User mode - execution is done in behalf of a user
• Monitor mode (supervisor mode or system mode) -
  execution is done in behalf of the OS
• This insures that programs cant do things they
  shouldnt

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Dual-Mode Operation (cont.)

• Mode bit indicates which mode CPU is in
• When an interrupt or trap occurs, hardware
  switches to monitor mode
• When interrupt or trap is finished, hardware
  switches to user mode
• Privileged instructions can only be issued in
  monitor mode

18
I/O Protection

• All I/O instructions are privileged instructions
• This insures that a user cant execute
  instructions that could mess up the machine

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Memory Protection

• OS must keep users within their own memory
• This is done using two registers
• Base register - start of the users memory
• Limit register - size of the users memory
• A checks is done by the hardware that insures
  that every memory access is inside the users
  memory space

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CPU Protection

• Timer - interrupts CPU after a specified period
  to insure the OS maintains control
• Timer is decremented every clock tick
• When timer reaches zero, an interrupt occurs
• Timer is used for time sharing
• Timer is used to compute the current time
• Load-timer is a privileged instruction