Input/Output

1
Input/Output

• Chapter 5

5.1 Principles of I/O hardware 5.2 Principles of
I/O software 5.3 I/O software layers 5.4
Disks 5.5 Clocks 5.6 Character-oriented
terminals 5.7 Graphical user interfaces 5.8
Network terminals 5.9 Power management
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I/O Device

• I/O devices can be divided into two categories
• A block devices is one that stores information in
  fixed-size blocks.
• A character device delivers or accepts a stream
  of characters, without regard to any block
  structure.
• Some devices do not fit in clocks, memory-mapped
  screens.

3
Principles of I/O Hardware

• Some typical device, network, and data base rates

4
Device Controllers

• I/O devices have components
• mechanical component
• electronic component
• The electronic component is the device controller
  or adapter.
• may be able to handle multiple devices
• On PCs, it often takes the form of a printed
  circuit card that can be inserted into an
  expansion slot.
• Controller's tasks
• convert serial bit stream to block of bytes
• perform error correction as necessary
• make available to main memory

5
Memory-Mapped I/O

• Each controller ha a few registers that are used
  for communicating with the CPU. The operating
  system can command the device by writing into
  these registers and learn the devices state by
  reading from these registers.
• Many devices have a data buffer that the
  operating system can read and write. Two
  approaches exist
• Each control register is assigned an I/O port
  number.
• All the control registers are mapped into the
  memory space. This is called memory-mapped I/O.

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Memory-Mapped I/O

• Separate I/O and memory space
• Memory-mapped I/O PDP-11
• Hybrid - Pentium

7
Memory-Mapped I/O

• Advantages of memory-mapped I/O
• An I/O device driver can be written entirely in C
  
• No special protection mechanism is needed to keep
  user process from performing I/O.
• Every instruction that can reference memory can
  also reference control register.
• Disadvantages of memory-mapped I/O
• Caching a device control register would be
  disastrous (not reflect current device status
  change).
• All memory modules and all I/O devices must
  examine all memory references.

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Memory-Mapped I/O

• (a) A single-bus architecture
• (b) A dual-bus memory architecture

9
Direct Memory Access (DMA)

• Direct Memory Access (DMA) is a capability
  provided by some computer bus architectures that
  allows data to be sent directly from an attached
  device (such as a disk drive) to the memory on
  the computer's motherboard.
• DMA operations
• CPU program the DMA controller
• DMA requests transfer to memory
• Data transferred
• The disk controller sends an acknowledgement

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Direct Memory Access (DMA)

• Operation of a DMA transfer

11
Interrupts Revisited

• The interrupt vector is a table holding numbers
  on the address lines specifying devices.
• Precise interrupt
• The PC (Program Counter) is saved in a known
  place.
• All instructions before the one pointed to by the
  PC have fully executed.
• No instruction beyond the one pointed to by the
  PC has been executed.
• The execution state of the instruction pointed to
  by the PC is known.

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Interrupts Revisited

• How interrupts happens. Connections between
  devices and interrupt controller actually use
  interrupt lines on the bus rather than dedicated
  wires

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Principles of I/O SoftwareGoals of I/O Software

• Device independence
• programs can access any I/O device
• without specifying device in advance
• (floppy, hard drive, or CD-ROM)
• Uniform naming
• name of a file or device a string or an integer
• not depending on which machine
• Error handling
• handle as close to the hardware as possible

14
Goals of I/O Software

• Synchronous vs. asynchronous transfers
• blocking transfers vs. interrupt-driven
• Most physical I/O is interrupt-driven.
• Buffering
• data coming off a device cannot be stored in
  final destination
• Sharable vs. dedicated devices
• disks are sharable
• tape drives would not be

15
I/O Execution

• There are three ways that I/O are performed
• Programmed I/O
• Disadvantage tying up the CPU full time until
  all the I/O is done.
• Interrupt-driven I/O
• Interrupts might waste time.
• I/O using DMA
• Slower than CPU

16
Programmed I/O

• Steps in printing a string
• String in the user buffer
• A System call to transfer the string to the
  kernel.
• String printed

17
Programmed I/O

• Writing a string to the printer using programmed
  I/O

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Interrupt-Driven I/O

• Writing a string to the printer using
  interrupt-driven I/O
• Code executed when print system call is made
• Interrupt service procedure

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I/O Using DMA

• Printing a string using DMA
• code executed when the print system call is made
• interrupt service procedure

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I/O Software Layers

• I/O Software in four layers
• Interrupt handlers
• Device drivers
• Device-independent operating system software
• User-level I/O software

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I/O Software Layers

• Layers of the I/O Software System

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Interrupt Handlers

• Interrupt handlers are best hidden
• have driver starting an I/O operation block until
  interrupt notifies of completion
• Interrupt procedure does its task
• then unblocks driver that started it

23
Interrupt Handlers

• Steps must be performed in software after
  interrupt completed
• Save registers not already saved by interrupt
  hardware
• Set up context for interrupt service procedure
• Set up stack for interrupt service procedure
• Acknowledge interrupt controller, reenable
  interrupts
• Copy registers from where saved
• Run service procedure
• Set up MMU context for process to run next
• Load new process' registers
• Start running the new process

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Device Driver

• The device driver is the device-specific code for
  controlling the I/O device attached to a
  computer.
• Current operating systems expect drivers to fun
  in the kernel.
• Operating systems usually classify drivers into
• Block devices
• Character devices

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Device Drivers

• Logical position of device drivers is shown here
• Communications between drivers and device
  controllers goes over the bus

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Device-Independent I/O Software
Uniform interfacing for device drivers
Buffering
Error reporting
Allocating and releasing dedicate devices
Providing a deice-independent block size

• Functions of the device-independent I/O software

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Device-Independent I/O Software

• (a) Without a standard driver interface a lot
  of new programming effort
• (b) With a standard driver interface

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Buffering

• Buffering is a widely-used technique. If data get
  buffered too many times, performance suffers.
• Classes of I/O errors
• Programming errors
• Actual I/O errors
• Some I/O software can be linked with user
  programs.
• Spooling is a way of dealing with dedicated I/O
  devices in a multiprogramming system.
• A spooling directory is used for storing the
  spooling jobs.

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Device-Independent I/O Software

• (a) Unbuffered input
• (b) Buffering in user space
• (c) Buffering in the kernel followed by copying
  to user space
• (d) Double buffering in the kernel

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Device-Independent I/O Software

• Networking may involve many copies

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User-Space I/O Software

• Layers of the I/O system and the main
  functions of each layer

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Disks

• Disks come in a variety of types
• Magnetic disks (hard disks and floppy disks)
• Arrays of disks
• Optical disks
• CD-ROMs
• CD-Recordables
• CD-Rewritables
• DVD

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DisksDisk Hardware

• Disk parameters for the original IBM PC floppy
  disk and a Western Digital WD 18300 hard disk

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Disk Hardware

• Physical geometry of a disk with two zones
• A possible virtual geometry for this disk

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Disk Hardware

• Raid levels 0 through 2
• Backup and parity drives are shaded

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Disk Hardware

• Raid levels 3 through 5
• Backup and parity drives are shaded

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Disk Hardware

• Recording structure of a CD or CD-ROM

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Disk Hardware

• Logical data layout on a CD-ROM

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Disk Hardware

• Cross section of a CD-R disk and laser
• not to scale
• Silver CD-ROM has similar structure
• without dye layer
• with pitted aluminum layer instead of gold

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Disk Hardware

• A double sided, dual layer DVD disk

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Disk Formatting

• A disk sector

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Disk Formatting
An illustration of cylinder skew
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Disk Formatting

• No interleaving
• Single interleaving
• Double interleaving

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Disk Arm Scheduling Algorithms

• Time required to read or write a disk block
  determined by 3 factors
• Seek time
• Rotational delay
• Actual transfer time
• Seek time dominates
• Error checking is done by controllers

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Disk Arm Scheduling Algorithms
Pending requests
Initial position

• Shortest Seek First (SSF) disk scheduling
  algorithm

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Disk Arm Scheduling Algorithms

• The elevator algorithm for scheduling disk
  requests

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Error Handling

• A disk track with a bad sector
• Substituting a spare for the bad sector
• Shifting all the sectors to bypass the bad one

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Stable Storage

• Analysis of the influence of crashes on stable
  writes

49
ClocksClock Hardware

• A programmable clock

50
Clock Software (1)

• Three ways to maintain the time of day

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Clock Software (2)

• Simulating multiple timers with a single clock

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Soft Timers

• A second clock available for timer interrupts
• specified by applications
• no problems if interrupt frequency is low
• Soft timers avoid interrupts
• kernel checks for soft timer expiration before it
  exits to user mode
• how well this works depends on rate of kernel
  entries

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Character Oriented TerminalsRS-232 Terminal
Hardware

• An RS-232 terminal communicates with computer 1
  bit at a time
• Called a serial line bits go out in series, 1
  bit at a time
• Windows uses COM1 and COM2 ports, first to serial
  lines
• Computer and terminal are completely independent

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Input Software (1)

• Central buffer pool
• Dedicated buffer for each terminal

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Input Software (2)

• Characters handled specially in canonical mode

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Output Software

• The ANSI escape sequences
• accepted by terminal driver on output
• ESC is ASCII character (0x1B)
• n,m, and s are optional numeric parameters

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Display Hardware (1)
Parallel port

• Memory-mapped displays
• driver writes directly into display's video RAM

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Display Hardware (2)

• A video RAM image
• simple monochrome display
• character mode
• Corresponding screen
• the xs are attribute bytes

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Input Software

• Keyboard driver delivers a number
• driver converts to characters
• uses a ASCII table
• Exceptions, adaptations needed for other
  languages
• many OS provide for loadable keymaps or code pages

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Output Software for Windows (1)

• Sample window located at (200,100) on XGA display

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Output Software for Windows (2)

• Skeleton of a Windows main program (part 1)

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Output Software for Windows (3)

• Skeleton of a Windows main program (part 2)

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Output Software for Windows (4)

• An example rectangle drawn using Rectangle

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Output Software for Windows (5)

• Copying bitmaps using BitBlt.
• before
• after

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Output Software for Windows (6)

• Examples of character outlines at different point
  sizes

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Network TerminalsX Windows (1)

• Clients and servers in the M.I.T. X Window System

67
X Windows (2)

• Skeleton of an X Windows application program

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The SLIM Network Terminal (1)

• The architecture of the SLIM terminal system

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The SLIM Network Terminal (2)

• Messages used in the SLIM protocol from the
  server to the terminals

70
Power Management (1)

• Power consumption of various parts of a laptop
  computer

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Power management (2)

• The use of zones for backlighting the display

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Power Management (3)

• Running at full clock speed
• Cutting voltage by two
• cuts clock speed by two,
• cuts power by four

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Power Management (4)

• Telling the programs to use less energy
• may mean poorer user experience
• Examples
• change from color output to black and white
• speech recognition reduces vocabulary
• less resolution or detail in an image