1. Chapter 5 Input/Output 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 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 device’s 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.

6. 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.

8. 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

10. 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.

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

13. Principles of I/O Software Goals 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 Buffering
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. Writing a string to the printer using programmed I/O

18. 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

19. I/O Using DMA Printing a string using DMA
code executed when the print system call is made
interrupt service procedure

20. I/O Software Layers I/O Software in four layers: Interrupt handlers
Device drivers
Device-independent operating system software
User-level I/O software

21. Layers of the I/O Software System
I/O Software Layers
Layers of the I/O Software System

22. 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

24. 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

25. Device Drivers Logical position of device drivers is shown here
Communications between drivers and device controllers goes over the bus

26. 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

27. Device-Independent I/O Software
(a) Without a standard driver interface – a lot of new programming effort
(b) With a standard driver interface

28. 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.

29. 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

30. Device-Independent I/O Software
Networking may involve many copies

31. User-Space I/O Software
Layers of the I/O system and the main functions of each layer

32. 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

33. Disks Disk Hardware
Disk parameters for the original IBM PC floppy disk and a Western Digital WD hard disk

34. Disk Hardware Physical geometry of a disk with two zones
A possible virtual geometry for this disk

35. Disk Hardware Raid levels 0 through 2
Backup and parity drives are shaded

36. Disk Hardware Raid levels 3 through 5
Backup and parity drives are shaded

37. Disk Hardware
Recording structure of a CD or CD-ROM

38. Logical data layout on a CD-ROM
Disk Hardware
Logical data layout on a CD-ROM

39. 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

40. A double sided, dual layer DVD disk
Disk Hardware
A double sided, dual layer DVD disk

41. Disk Formatting
A disk sector

42. An illustration of cylinder skew
Disk Formatting
An illustration of cylinder skew

43. Disk Formatting No interleaving Single interleaving
Double interleaving

44. 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

45. Disk Arm Scheduling Algorithms
Initial
position
Pending
requests
Shortest Seek First (SSF) disk scheduling algorithm

46. Disk Arm Scheduling Algorithms
The elevator algorithm for scheduling disk requests

47. 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

48. Analysis of the influence of crashes on stable writes
Stable Storage
Analysis of the influence of crashes on stable writes

49. Clocks Clock Hardware
A programmable clock

50. Three ways to maintain the time of day
Clock Software (1)
Three ways to maintain the time of day

51. Simulating multiple timers with a single clock
Clock Software (2)
Simulating multiple timers with a single clock

52. 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

53. Character Oriented Terminals RS-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

54. Input Software (1) Central buffer pool
Dedicated buffer for each terminal

55. Characters handled specially in canonical mode
Input Software (2)
Characters handled specially in canonical mode

56. 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

57. Display Hardware (1) Memory-mapped displays
Parallel port
Memory-mapped displays
driver writes directly into display's video RAM

58. Display Hardware (2) A video RAM image Corresponding screen
simple monochrome display
character mode
Corresponding screen
the xs are attribute bytes

59. 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

60. Output Software for Windows (1)
Sample window located at (200,100) on XGA display

61. Output Software for Windows (2)
Skeleton of a Windows main program (part 1)

62. Output Software for Windows (3)
Skeleton of a Windows main program (part 2)

63. Output Software for Windows (4)
An example rectangle drawn using Rectangle

64. Output Software for Windows (5)
Copying bitmaps using BitBlt.
before
after

65. Output Software for Windows (6)
Examples of character outlines at different point sizes

66. Network Terminals X Windows (1)
Clients and servers in the M.I.T. X Window System

67. Skeleton of an X Windows application program

68. The SLIM Network Terminal (1)
The architecture of the SLIM terminal system

69. The SLIM Network Terminal (2)
Messages used in the SLIM protocol from the server to the terminals

70. Power consumption of various parts of a laptop computer
Power Management (1)
Power consumption of various parts of a laptop computer

71. Power management (2)
The use of zones for backlighting the display

72. Power Management (3) Running at full clock speed
Cutting voltage by two
cuts clock speed by two,
cuts power by four

73. Power Management (4) Telling the programs to use less energy Examples
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