InputOutput

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
2
I/O Software Layers

• Each layer has well-defined interface to the
  adjacent layers

3
Interrupt Handlers

• Steps must be performed in software when an I/O
  interrupt occurs
• Save registers
• Set up context for interrupt service procedure
• Set up stack for interrupt service procedure
• Acknowledge interrupt controller
• Copy registers from where saved to the process
  table
• Run service procedure
• Choose the process to run next
• Set up MMU context for process to run next
• Load new process' registers
• Start running the new process

4
Device Drivers

• Device-specific code required for controlling a
  device.
• A specific driver is required for a mouse,
  joystick, etc.
• Typically written by the device manufacturer and
  delivered with the device.
• Device drivers are OS-specific. Typically, a
  different one is needed for each operating
  system.
• For modern operating systems, device drivers are
  dynamically loaded into the system during
  execution.

5
Device Drivers (cont)

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

6
Functions ofA Device Driver

• For each I/O request on a device, the device
  driver will
• Initialize the device
• check for valid input parameters
• for disk drivers, convert linear block number to
  head, track, sector and cylinder.
• Check status of device
• If the device is busy, add request to a queue
• If the device is idle, turn the device on if off
  and handle the request
• Issue commands to the device
• Determine the command sequence
• Write the command to the device controllers
  registers
• Block if the device operation is long and wake up
  with an interrupt or, continue
• Check for error
• Pass data or error message to device-independent
  software layer above.

7
Device Independent I/O Software

• Basic function is to perform the I/O functions
  that are common to all devices and to provide a
  uniform interface to the user-level software.
• Specific functions include
• Uniform interfacing for device drivers
• Buffering
• Error reporting
• Allocating and releasing dedicated devices
• Providing a device-independent block size

8
System without a standard driver interface

• Each device driver has a different interface to
  the system requiring a lot of programming for
  each new driver.
• Driver functions that the OS can call will be
  different for each device.
• The OS functions the driver needs will be
  different for each device.

9
System with a standard driver interface

• All drivers have the same interface.
• All drivers whose interface conforms to the
  standard is pluggable.
• Device driver programmers need to know in advance
  what functions they may provide and what OS
  functions they may call.
• Symbolic device names are mapped to the proper
  drivers.
• Protection of access to devices by users is
  enforced.

10
Buffering

• Unbuffered input
• - The user process is blocked/unblocked with
  each character read.
• - Inefficient
• Buffering in user space
• - User process is interrupted only after the
  buffer is filled up
• - More efficient than (a) but will cause
  problems if the buffer is paged out when the
  character arrives

11
Buffering

• (c) Buffering in the kernel followed by copying
  to user space
• - More efficient than (a) and (b)
• - Minimizes the risk of the user page being
  paged out when the copying is performed.
• - Will cause problems if the kernel buffer is
  full and ready to be copied but the user buffer
  is still being paged in for copying.
• (d) Double buffering in the kernel
• - Solves the problem with (c)

12
Error Reporting

• Handle errors that cannot be handled by device
  drivers
• Programmer errors
• Writing to an input device (keyboard)
• Reading from an output device (monitor)
• Actual I/O errors
• Damaged disk block
• Error handling
• Display a dialog box to the user
• Retry, ignore or abort (kill the process)

13
Allocating and Releasing Dedicated Devices

• Examples are CD-ROM, scanner
• Two ways to allocate and release dedicated
  devices
• Require processes to perform an open command
  (allocate). If the open fails, the device is in
  use and the process has to try again. A close
  command releases the device.
• Have a special mechanism for requesting and
  releasing the device
• When a user process tries to acquire a device
  that is held by another process, the user process
  is blocked and placed in a queue. When the
  device becomes available, the first process in
  the queue is allowed to acquire the device and
  continue exectuion.

14
Device-Independent Block Size

• Hide the fact that different devices deliver
  different size inputs or receive different size
  outputs
• disks may have different sector sizes
• provide a uniform block size to the higher layers
  by treating several sectors as a single block
  size.
• Modems may deliver data one character at a time
• Provide a uniform block size to higher layers by
  buffering the input to a uniform block size
  before delivering to higher layers.

15
User-Space I/O Software

• Most I/O software is in the kernel. A small
  portion are user-level, library procedures.
• Categories of user-level I/O software
• Library procedures
• Make I/O system calls
• count write( fd, buffer, nbytes)
• Formatting of I/O
• Printf( The square of 3d is 6d\n, i, iI )
• Spooling system
• Printer
• A way to deal with dedicated devices

16
Layers of the I/O System Summary
17
Disks

• Magnetic disks
• Reads and writes equally fast
• Ideal secondary storage (paging, file systems,
  etc.)
• eg. hard disks, floppy disks
• Optical disks
• For distribution of programs, data and movies
• CD-ROMs
• DVDs

18
Disks parameters floppy and hard disks
19
Physical vs. Virtual Disk Geometry
cylinder
tracks
sectors

• Physical geometry of a disk with two zones
• Modern disks are divided into zones with more
  zones in the outer section than in the inner ones
• A possible virtual geometry for this disk
• This is presented to the OS.
• The controller remaps the request made using the
  virtual geometry to the real cylinder, head and
  sector in the physical geometry

20
Low Level Disk Formatting
512 bytes

• Low-level formatting of a disk sector done by
  software
• Preamble
• contains certain bit patterns that allow hardware
  to recognize the start of a sector
• Contains cylinder, track and sector numbers along
  with other information
• Data
• To contain the actual data the will be copied to
  the disk
• ECC (error correction code)
• Used to recover from read errors
• Size and content vary from manufacturer to
  manufacturer
• Spare sectors are allocated to replace defective
  sectors.
• Results in reduced disk capacity of about 20
• Uses cylinder skew

21
Cylinder Skew

• The position of sector 0 in each track is offset
  from the previous track
• Allows for reads of multiple tracks in one
  continuous operation without losing data.
• The amount of cylinder skew depends on the disk
  geometry

22
Interleaving

• Problem
• A controller with one-sector size buffer is ask
  to read two sectors from the disk. The
  controller needs to read the first sector from
  the disk, perform error checking and transfer the
  data to main memory. While all of this is taking
  place, the second sector moves past the disk arm
  preventing the controller from reading the
  sector. Now the controller will have to wait for
  the disk to complete its rotation for the second
  sector.
• Solution
• Use interleaving

23
Interleaving

• Interleaving gives the controller time between
  consecutive sector reads to process a sector
  read.
• (a) No interleaving
• problem
• (b) Single interleaving
• May not be sufficient if the time to process a
  sector by the controller is slow.
• (c) Double interleaving
• Gives the controller more time

24
Disk Arm Scheduling

• Time required to read or write a disk block is
  determined by 3 factors
• Seek time
• Time to move the arm to the correct cylinder
• Rotational delay
• Time for the correct sector to rotate under the
  head
• Actual transfer time
• Time to transfer data to the controller
• Seek time dominates
• It is the longest
• Reducing seek mean time can improve system
  performance substantially.

25
Disk Arm Scheduling Algorithms

• Requires that disk drivers maintain a table,
  indexed by cylinder number, where all the pending
  requests for each cylinder are chained together
  in a linked list headed by the table entries.
• FCFS
• first-come, first served
• SSF
• shortest seek first
• Elevator
• the same algorithm used in building elevators

26
First-Come First Served

• Requires disk arm motions of
• 11?1 10 16?34 18
• 1?36 35 34?9 25
• 36?16 20 9?12 3 Total of 111 cylinders

27
Shortest Seek First

• Requires disk arm motions of
• 11?12 1 16?1 15
• 12?9 3 1?34 33
• 9?16 7 34?36 2 Total of 61 cylinders

28
Elevator Algorithm

• Requires disk arm motions of
• 11?12 1 34?36 2
• 12?16 4 36?9 27
• 16?34 18 9?1 8 Total of 60 cylinders

29
Disk Read Optimization

• Disk scheduling algorithm
• See previous discussion
• Caching
• The controllers cache may be used to hold
  blocks that have not been requested but were
  convenient to read because they happen to pass
  under the head as a side effect of some other read

30
Disk Errors

• Bad sector
• Manufacturer defect
• Bad sectors in a new disk caused by the
  manufacturing process
• Use spare sector to replace
• Transient
• Dust
• Simply re-read
• A sector that failed
• Use spare sector to replace
• Mechanical disk arm
• A seek operation ends at the wrong cylinder
• The disk controller recalibrates the arm.

31
Replacing a bad sector

• (a) A disk track with a bad sector
• (b) Remap one of the spares as sector 7, or
• (c) Shift all the sectors to bypass the bad one

32
Clocks

• Also called timers
• Some functions
• Maintains the time of day
• Prevent one process from monopolizing the CPU

33
Clock Hardware

• The value in the holding register is loaded to
  the counter
• The crystal oscillator generates a signal of very
  high frequency
• This signal is multiplied by a small integer to
  get frequencies up to 1000 MHz or more resulting
  in the synchronization signal used by the
  computer
• The synchronization signal is fed into the
  counter causing it to decrement with each pulse
  from the crystal until it counts down to zero.
• When the counter gets to zero, a CPU interrupt
  occurs then the cycle is started all over again.

34
The Clock Software Clock Driver

• Functions of the clock driver
• Maintain the time of day
• Requires incrementing a time-of-day counter at
  each clock tick
• Prevent process from running longer that they are
  allowed to
• When a process is allowed to run, the scheduler
  loads a counter with that process quantum in
  clock ticks
• At every clock interrupt, the clock driver
  decrements the quantum counter by 1.
• When the counter gets to zero, the clock driver
  calls the scheduler to run another process.
• Accounting for CPU usage
• Keep track of how much CPU time a process has had.

35
Clock Driver (cont.)

• Functions of the clock driver
• Handling alarm system call of user processes
• When a timer goes off, the clock driver sends an
  alarm to the user process
• Useful in networking.
• When a packet is sent out and an acknowledgement
  is not received, the packet has to be
  retransmitted.
• Provide watchdog timers for parts of the system
  itself
• When a timer goes off, the clock driver calls a
  procedure supplied by the user process.
• When a floppy disk motor is started, a timer goes
  off after a sufficient time interval to notify
  the user process that the disk is ready to read
  or write.
• Do profiling, monitoring and statistics gathering
• The OS builds up a histogram of a processes
  program counter to see where it is spending its
  time

36
Character-Oriented Terminals

• Terminal
• A generic term to denote a keyboard and an
  attached display
• Standalone terminals with RS-232 interfaces used
  on mainframes
• Includes the keyboard and monitor
• PCs with graphical user interface
• Network terminals

37
Standalone RS-232 Terminal Hardware

• Text-only terminals
• 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
• The bits are buffered in the interface card. The
  UART then shifts out the character one bit at a
  time over the serial line.
• Windows uses COM1 and COM2 ports to link to
  serial lines
• All modems use this interface

38
Input Software

• Keyboard driver
• Collect input from the user and pass it to the
  user programs
• On every key action, the keyboard driver extracts
  the character typed by reading an I/O port.
• Two implementation paradigm for the driver
• Raw mode (noncanonical)
• accept the input and pass it, unmodified, to the
  user process.
• Eg. Dste corrected to Date will get passed as
• Dste???ateCR
• Character-oriented
• Cooked mode (canonical)
• Sends only the corrected input. The driver
  handles all intraline editing and just delivers
  the corrected lines
• Line-oriented

39
Buffering Requirements

• Raw mode
• Allow the user to type ahead even before the
  programs have asked for the input
• Cooked mode
• Characters must be stored until the entire line
  has been accumulated in case the user decide to
  erase part of the line.

40
Buffering Technique- Central Buffer Pool

• The keyboard driver maintains a central pool of
  buffers, each buffer holding 10 characters.
• Associated with each terminal is a data structure
  which contains a pointer to the chain of buffers
  for input collected from that terminal.
• As more characters are typed, more buffers are
  acquired and attached to the chain.
• When characters are passed to the user program,
  the buffers are released and put back on the pool.

41
Buffering Technique- Dedicated Buffer

• Use the terminal data structure to buffer the
  characters typed directly.
• Allocate something like 200 characters per
  terminal

42
Output Software

• Displays characters to the monitor
• Can use the same buffering techniques as the
  input software.
• When a program writes to the terminal, the
  characters are first copied to the buffer. When
  the output from the buffer to the monitor starts,
  the driver goes to sleep.

43
PCs with Graphical User Interface

• PCs can use character-based interfaces
• MS-DOS is character-based
• PCs also use graphical user interfaces (GUIs)
• Essential elements of a GUI (WIMP
• Windows
• Rectangular boxes in the screen to run users
  programs
• Icon
• Small symbols that can be clicked to cause some
  action to happen
• Menus
• List of action from which one can be chosen
• Pointing device
• Used to move the cursor around the screen to
  select items

44
PCs Input and Output Devices

• Keyboard
• Similar in concept to the RS-232 input device
• Mouse
• Sends a message to the computer that contains
  three items ?x, ?y, button
• ?x, ?y change in y and x positions since last
  message
• Button status of the buttons
• Display hardware
• Vector graphics
• Devices that accept and carry out commands such
  as draw points, lines, geometric figures and text
• plotters
• Raster graphics (bitmap graphics)
• Devices that represent the output area as a
  rectangular grid of oints called pixels
• Pixels has some grayscale value or color
• Implemented by a graphics adapter as
  memory-mapped displays

45
Graphics Adapter
Serial, parallel or USB port

• Memory-mapped displays where the video driver
  writes directly into display's video RAM
• Video Ram
• Forms part of the computers address space
• Video Controller
• Pulls characters or bits out of the video RAM and
  generates the video signal to drive the monitor

46
Video Ram

• Part of the computers address space, it is
  addressable by the CPU the same way as the rest
  of memory.
• Monochrome starting address B0000000
• Color starting address B00080000
• Stores the screen image character mode or bit
  mode
• Character mode a byte or two bytes of video RAM
  contains one character to be displayed
• Bit mode each pixel on the screen is represented
  with 1 bit for black to 24 or more bits for high
  quality color display in the video RAM

47
Video RAM and the Monitor

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

48
Output Software for Windows

• The basic item on the display screen is the
  window created by a Windows program.
• Sample window located at (200,100) on XGA display

49
A Windows Program

• Message oriented
• User actions involving the keyboard or mouse are
  captured by Windows and converted into messages
  to the program owning the window being addressed.
• Each program has a message queue to which the
  messages relating to all its windows are sent
• The programs behavior is driven by the incoming
  messages.

50
Network Terminals

• Used to connect a remote user to a computer over
  a network
• Types of network terminals
• a fat terminal
• Contains a CPU, memory, keyboard and terminal
  that can run complex protocols to compress large
  amounts of data over the network
• X Window System
• a SLIM terminal
• Simple, basically displaying pixels and not doing
  much thinking
• cheap

51
X Windows System

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

52
The SLIM Network Terminal

• The architecture of the SLIM terminal system

53
The SLIM Network Terminal (2)

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