CS220 Computer Organization

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CS220 Computer Organization

• Chapter 13 Input and Output

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CS220 Computer OrganizationInput and Output

• A brief history of User Interfaces (not from
  text)
• Wires and plugs ENIAC rebuilt the computer
  by reconnecting the components
• Batch mode punched cards, operator
  controlling of jobs
• Input punching cards on a typewriter like
  machine
• Output printout given back to you by the
  operator
• Timesharing late 1960s One CPU is shared
  among multiple users I/O slow so user had
  illusion of control of the machine
• Input TTY (type in program), keyboards came
  later
• Output TTY (results typed out), VDT came later
  (video display terminal)
• GUI early 1970s Xerox Corp. Used GUI
  internally
• Steve Jobs visited Xerox and took the ideas of
  Xerox and a number of Xerox employees and
  introduced the Apple Macintosh in 1984
  considered the first commercially available GUI
  machine

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CS220 Computer OrganizationInput and Output

• Input and Output
• Input and output devices vary in purpose, cost,
  and speed
• Can categorize I/O devices by function
• User Interface Devices (keyboard, printer,
  mouse, video, scanner)
• Mass Storage Devices (disk, magnetic tape drive,
  CD ROM drive)
• Outside Communications (networks and gateways,
  modems)
• Or by Input versus Output
• Input (keyboard, mouse, scanner, CD ROM drive)
• Output (video, printer)
• Both (disk, magnetic tape drive, network, modem,
  CD R/W)

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CS220 Computer OrganizationInput and Output

• The Keyboard (input)
• Simple input device, low data rate (as fast as
  you can type)
• When a key is pressed on the keyboard, an ASCII
  character is sent to the processor.
• Each key is said to be mapped to a particular
  character. Mappings vary from keyboard to
  keyboard.
• Each bit of the ASCII character are sent to the
  processor serially, over a single wire.
• Baud rate the rate at which the bits are sent
  to the processor (in bits per second)
• The keyboard typically appends a start and stop
  bit to the ASCII character to indicate to the
  processor that it is sending it a character and
  when it is done sending that character.

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CS220 Computer OrganizationInput and Output

• The Display (output)
• Video Display Terminal (glass teletype)
  mimics the motions of a typewriter or teletype.
• Attach a keyboard to a video display and you
  have an ASCII Terminal
• Cursor (flashing box, underscore)
• lines scrolled up as they were typed and a
  carriage return was hit
• when a key is hit on the keyboard, it appears on
  the terminal
• Enhanced or Smart Terminals
• A controller receives the character from the
  processor and updates the screen appropriately
• Capable of manipulating pixels instead of
  printing at predetermined positions (known as
  bit-mapped displays)

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CS220 Computer OrganizationInput and Output

• The Rigid Disk (extended or secondary memory)
• Input and Output
• Differs from main memory in that it
  reads/writes large chunks of data at one time in
  blocks.
• Blocks are used because the time to read and
  write to secondary memory is very long compare
  with that of main memory. Also because space is
  not addressed, but requires identifying tags to
  be stored along with the data.
• Physical characteristics
• One or more platters
• Two arms with one or more read/write heads per
  platter to read each surface
• Platters are connected by a spindle and all
  rotate together around the axis of the spindle.
• Platters are sealed to keep the platters clean.

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CS220 Computer OrganizationInput and Output

• The Rigid Disk (extended or secondary memory)
• Physical characteristics (continued)
• Data on platters are organized into tracks
  (concentric circle)
• Each track is broken into sectors
• Each sector in the same position on each track
  of a platter is collectively called a cylinder
• Reading/Writing Data
• Given an address, the disk must seek for the
  proper position in a cylinder on the appropriate
  platter.
• The platter then must rotate until the proper
  sector is positioned at its beginning, under a
  read/write head and reading/writing must occur at
  the precise moment that the proper sector is
  rotating under that head
• The processor MUST provide the bytes to write
  at the proper time and at the proper rate for the
  data to write to the patter correctly. When
  reading, the processor MUST be ready to accept
  the data at the speed that the disk provides it.

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CS220 Computer OrganizationInput and Output

• The Network (input and output)
• A computer can send or receive blocks of
  information instead of single bits.
• Network transfer rates vary depending on
  bandwidth. Bandwidth represents the number of
  bits per second that can be transferred.
• LAN (local area network) versus WAN (wide area
  network)
• A combination of hardware and software
  implements the functionality of a network

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CS220 Computer OrganizationInput and Output

• Processor-I/O Interface
• Basic interface involves the processor sending
  and receiving bits from I/O devices.
• The processor can process input and output
  instructions much faster than the I/O devices can
  supply or accept them. Of primary interest in
  the interface is how to handle this difference.
• Simple case
• The processor must know the status of each
  device ready or busy

Processor
Memory
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CS220 Computer OrganizationInput and Output

• Processor-I/O Interface
• Implementation of status is as easy as using
  one bit for the processor to look at to determine
  if a device is ready or if it is busy.
• Input/Output Programming
• Some architectures have special instructions
  for I/O device communication.
• Another way is to designate a section of memory
  is set aside for memory-mapped I/O.
• Memory-mapped I/O allows the processor to
  access I/O without the need for new instructions.
  Reading from or writing to these special memory
  locations is all that is needed to receive input
  or provide output.

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CS220 Computer OrganizationInput and Output

• Processor-I/O Interface - Input/Output
  Programming
• Actual memory locations in the mapped memory
  are not used, but the data and device status must
  be stored somewhere.
• This can be done within the device itself, or,
  more typically by special hardware called a
  controller.

Processor
Display_Status
Display_Data
Memory
Keyboard_Status
Keyboard_Data
memory-mapped
mov EAX, dword ptr Keyboard_Data mov dword ptr
Display_Data, EAX
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CS220 Computer OrganizationInput and Output

• Processor-I/O Interface
• Code to implement I/O functions must take the
  status of the device into account
• Lets assume that a status with the most
  significant byte of 1 as ready and zero as busy.

keyboard_wait test Keyboard_Status,
80000000h jz keyboard_wait mov EAX,
dword ptr Keyboard_Data
display_wait test Display_Status,
80000000h jz display_wait mov dword ptr
Display_Data, EAX
If no characters are being typed, the keyboard is
in a busy state. When a key is hit on the
keyboard, the controller gets the ASCII character
and places it in Keyboard_Data. It then changes
the Keyboard_Status to zero to indicate that it
is ready. The wait loop is then exited and the
processor can move the character to its regester.
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CS220 Computer OrganizationInput and Output

• Processor-I/O Interface
• In a simple system, the wait loops and
  memory-mapped I/O are a good solution. It is
  unrealistic outside of this simple system.
• Current systems have many I/O devices and are
  expected to run more than one program,
  concurrently (multiprogramming).
• In a more complex system, these programmed I/O
  solutions have several problems
• Cant waste processor resources sitting and
  waiting for I/O devices, which are typically much
  slower than the processor
• Input devices might flip to the ready state
  while the processor is working on executing
  another program
• If more than one program is running in the
  multiprogrammed environment, if both need to use
  an I/O device, there is nothing to keep track of
  which program gets to use it

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CS220 Computer OrganizationInput and Output

• Processor-I/O Interface
• Solution to these problems is to let the
  operating system handle it
• Track what I/O device is available to each
  program
• Ensure that programs do not interfere with each
  other
• Provides methods for programs to request I/O
  operations
• The operating system may still use the wait
  loops, but maintains them as part of the
  operating system. The architecture may restrict
  I/O instructions or access to mapped-memory to
  the operating system and not allow access by
  individual programs.
• It is then the operating systems job to ensure
  that the I/O devices are monitored and used
  efficiently and correctly. For example, the
  Keyboard_Status must be polled often enough such
  that a Keyboard_Data gets transmitted to the
  processor before the next keystroke overwrites it
  and data is lost.

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CS220 Computer OrganizationInput and Output

• Processor-I/O Interface
• DMA (Direct Memory Access)
• To move a large amount of data at one time, for
  example in mass storage devices or for network
  transfers, programmed I/O would tie up the
  processor for an extended period of time.
• A controller for these types of devices can be
  expanded to move data from the storage device to
  memory, with only direction from the processor to
  do so.
• The processor, after providing the necessary
  direction, is free to do other things.
• The controllers for these devices become simple
  computers executing a program that does DMA.

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CS220 Computer OrganizationInput and Output

• Processor-I/O Interface
• DMA (Direct Memory Access)
• The processor simple tells the controller the
  details of the block transfer (Source,
  Destination, How much).
• The controller then does the transfer
  independent of the processor.

Processor
DMA Controller
Disk
Memory
memory-mapped