Pengantar Organisasi Komputer

1
IKI20210Pengantar Organisasi KomputerKuliah No.
19 I/O, Interupsi
Sumber1. Hamacher. Computer Organization,
ed-4.2. Materi kuliah CS61C/2000 CS152/1997,
UCB.
20 November 2002 Bobby Nazief (nazief_at_cs.ui.ac.id)
Johny Moningka (moningka_at_cs.ui.ac.id) bahan
kuliah http//www.cs.ui.ac.id/iki20210/
2
What is the alternative to polling?

• Wasteful to have processor spend most of its time
  spin-waiting for I/O to be ready
• Wish we could have an unplanned procedure call
  that would be invoked only when I/O device is
  ready
• Solution use interrupt mechanism to help I/O.
  Interrupt program when I/O ready, return when
  done with data transfer

3
I/O Interrupt

• An I/O interrupt is like a subroutine call
  except
• An I/O interrupt is asynchronous
• More information needs to be conveyed
• An I/O interrupt is asynchronous with respect to
  instruction execution
• I/O interrupt is not associated with any
  instruction, but it can happen in the middle of
  any given instruction
• I/O interrupt does not prevent any instruction
  from completion

4
Interrupt Driven Data Transfer
5
Instruction Set Support for I/O Interrupt

• Save the PC for return
• AVR uses the stacks
• Where go when interrupt occurs?
• AVR defines
• Locations 0x000 0x002 for external interrupts
• Locations 0x003 0x00C for internal interrupts
• Determine cause of interrupt?
• AVR uses vectored interrupt, which associates the
  location of the interrupt service routine (see
  above) with the device that causes it

6
Benefit of Interrupt-Driven I/O

• 400 clock cycle overhead for each transfer,
  including interrupt. Find the of processor
  consumed if the hard disk is only active 5 of
  the time.
• Interrupt rate polling rate
• Disk Interrupts/sec 8 MB/s /16B 500K
  interrupts/sec
• Disk Transfer Clocks/sec 500K 400
  200,000,000 clocks/sec
• Processor for during transfer 250106/500106
  40
• Disk active 5 ? 5 40 ? 2 busy
• ?Determined by disks activity, whereas in
  Polling-driven I/O the Processor will be busy
  polling 40 of the time even if the disk is not
  active

7
Multiple Devices/Interrupts

• Which I/O device caused exception?
• Needs to convey the identity of the device
  generating the interrupt
• Can avoid interrupts during the interrupt
  routine?
• In general, interrupts are disabled whenever one
  is being serviced interrupts will be enabled
  after the service is completed
• What if more important interrupt occurs while
  servicing this interrupt?
• Who keeps track of status of all the devices,
  handle errors, know where to put/supply the I/O
  data?
• In general, these is one of the tasks of
  Operating System

8
Device Identification
Device 1
Device N
Device 2
INTR1
INTR2
INTRn
CPU
wired-OR

• The Interrupting Device may provide its identity
  through
• Interrupt-Request (IRQ) bit in its Status
  Register, which will be evaluated one-by-one by
  the processor (polling)
• Sending special code the the processor over the
  bus (vectored interrupt)

9
Prioritized Interrupt
Device 1
Device N
Device 2
INTR1
INTA1
CPU

• An Interrupt Service Routine may be interrupted
  by other, higher-priority interrupt

10
Daisy Chain Scheme
Device 1 Highest Priority
Device N Lowest Priority
Device 2
INTA
Release
CPU
INTR
wired-OR

• Advantage simple
• Disadvantages
• Cannot assure fairness A low-priority
  device may be locked out indefinitely

11
Exceptions

• Interrupt is only a subset of Exception
• Exception signal marking that something out of
  the ordinary has happened and needs to be
  handled
• Interrupt asynchronous exception
• Unrelated with instruction being executed
• Trap synchronous exception
• Related with instruction being executed
• To recover from errors Illegal Instruction,
  Divide By Zero,
• To debug a program
• To provide privilege (for Operating System)

12
4 Responsibilities leading to OS

• The I/O system is shared by multiple programs
  using the processor
• Low-level control of I/O device is complex
  because requires managing a set of concurrent
  events and because requirements for correct
  device control are often very detailed
• I/O systems often use interrupts to communicate
  information about I/O operations
• Would like I/O services for all user programs
  under safe control

13
4 Functions OS must provide

• OS guarantees that users program accesses only
  the portions of I/O device to which user has
  rights (e.g., file access)
• OS provides abstractions for accessing devices by
  supplying routines that handle low-level device
  operations
• OS handles the exceptions generated by I/O
  devices (and arithmetic exceptions generated by a
  program)
• OS tries to provide equitable access to the
  shared I/O resources, as well as schedule
  accesses in order to enhance system performance

14
OS Interrupt Services

• OSINIT Set interrupt vectors Time-slice clock
  ? SCHEDULER Trap ? OSSERVICES VDT interrupts
  ? IODATA
• OSSERVICES Examine stack to determine requested
  operation Call appropriate routine
• SCHEDULER Save current context Select a
  runnable process Restore saved context of new
  process Push new values for PS and PC on
  stack Return from interrupt

15
I/O ROUTINES DEVICE DRIVER

• IOINIT Set process status to Blocked Initialize
  memory buffer address pointer Call device
  driver to initialize device enable interrupts
  in the device interface (VDTINIT) Return from
  subroutine
• IODATA Poll devices to determine source of
  interrupt Call appropriate device driver
  (VDTDATA) If END 1, then set process status
  to Runnable Return from interrupt
• VDTINIT Initialize device interface Enable
  interrupts Return from subroutine
• VDTDATA Check device status If ready, then
  transfer character If character CR, then set
  END 1 else set END 0 Return from
  subroutine

16
Things to Remember

• I/O gives computers their 5 senses
• I/O speed range is million to one
• Processor speed means must synchronize with I/O
  devices before use
• Polling works, but expensive
• processor repeatedly queries devices
• Interrupts works, more complex
• devices causes an exception, causing OS to run
  and deal with the device
• I/O control leads to Operating Systems