Digital Technology and Computer FundamentalsChapter 6Computer Organization II
ObjectivesAt the end of this chapter, you should be able to:describe the characteristics of different input and output devices;explain why I/O interfaces are required in a computer system;
Objectives (Contd)describe the three different methods for interfacing I/O; andexplain the advantage and disadvantages for each of the I/O interface methods.
ReferencesM. Marris Mano, "Computer System Architecture," third edition, Prentice Hall.G. Shelly, T. Cashman, G. Waggoner, W. Waggoner, Discovering Computers 98, A link to the Future, International Thomson, 1998.
Input / Output DevicesProvide communication for the CPU and the outside worldKeyboardMousePointing DevicesTerminalVisual Display UnitPrintersOthers
KeyboardMost commonly used Keys are micro-switches.Alphabetic keys.Numeric keypad.Arrow keys or cursor control keysFunction keys
MouseA pointing device.Controls the movement of an on-screen symbol called the pointer.A ball or light reflection on the bottom to sense its movement.Electronic circuits translate to electrical signals that are sent to the computer to direct the pointer.
TrackballSimilar to a mouseThe ball is on top of the device.
Touch padA flat surface usually attached in a notebook computer.Controls the movement of the pointer by sensing the motion of a finger on its exterior.
PenPen input devices accept input with hand written characters.Usually a special software must be required for this type of input.Can also be used as a mouse
Touch screenAllows users to touch areas of the screen to enter data.The action of screen-touching is equivalent to the movement and clicking the mouse.Two types of touch screens:combined with the monitorattached in front of the monitor
Image scannerElectronically captures an entire page and converts the document into digital format.Special software required to capture images.To convert the image to text, Optical Character Recognition (OCR) software is needed.
Optical recognition devicesUsing a light source to read optical codes.For example, a bar code reader reads the standard Universal Product Code (UPC) and converts them into digital format.
MicrophoneAccepts voice inputVoice recognition software is required to convert into digital format.For voice to text conversion, familiarization is need for the conversion software to recognize the characteristics of the voice of the user.
TerminalConsists of a keyboard and a screen.Dumb terminals have no independent processing capability.Intelligent terminals have built-in processing capabilities.Special purpose terminals uniquely designed for use in a particular industry.
Visual Display Unit (VDU)A visual output device.A monitor looks like a television and consists of a cathode ray tube (CRT) display screen.An LCD display is a thin display screen, and most often used in notebook computers.
PrintersUsed to produce hard copies.Two categories: impact and non-impact printers.Impact printers: striking a ribbon to deposit ink on the paper.Non-impact printers: use some non-mechanical technique for printing.
Dot-matrix printersConsists of a printer head with a vertical column of 9 or 24 needles.The needles are selectively energized and forced against the ribbon in proper combinations, to form the desired character image on the paper.Major advantage: capability to print multiple copies simultaneously.
Laser printersBased on the principle of photocopying machines.A laser beam produces mirrored images on a photo-sensitize selenium drum.Toner gets stuck to the drum by an electrostatic process, then transferred to the paper.
Laser printers (Contd)Finally, The inked image on the paper is fused by heat.Provide very good quality printing at a moderately fast rate.
Ink-jet printersUsing the dot-matrix approach with droplets shot at the paper to form the desired character.Quiet in operation with good print quality.
PlottersUsed to produce high-quality line drawings.Usually, the outputs of plotters are large size posters, graphics, etc.
Data projectorsIt takes the image that appears on a computer screen and project it.It is a very effective method for presentation.
I/O InterfaceUsually, I/O devices are not connected directly to the system bus in the computer system.An interface module for each device is needed.
Need of interface moduleI/O Operations are different from CPU and memory operations .Operations of different peripheral devices are different.Data format and word lengths are different.Conversion of signal values for each device may be required.
Need of interface module (Contd)The data transfer rate of these devices are much slower than that of the memory or CPU.Direct connection to the system bus is impractical.
SolutionsSpecial hardware components are required to supervise and synchronize the input and output data transfer.These components between the CPU and the peripheral devices are called interface.
I/O Bus and Interface ModulesEach peripheral device connects to a separate interface module All interfaces are attached to the I/O buses.
Interface ModuleEach interface decodes the address and control signals received from I/O bus;interprets and provides signals for the peripheral controller;synchronizes the data flow and supervises the transfer.
Accessing I/OCPU places an address on the address bus.The addressed interface detects it and activates the path. Other devices are disabled.CPU provides an I/O command in the control lines.The interface selected responds and proceeds to execute it.
I/O Bus and Memory BusCPU communicates with both the I/O and the memory.Memory bus contains data, address, and read/write control lines.Need to separate them.
Distinguish between I/O bus and Memory busThree different ways:Two separate buses are used, one for memory and the other for I/O.Common data bus and common address bus with separate control buses for memory and I/O.All buses are common for memory and I/O.
Separate buses for memory and I/OTwo independent sets of buses, one for memory, other for I/O.A separate I/O processor (IOP) is provided in addition to the CPU.Memory communicates with both CPU and IOP through memory bus.
Separate buses for memory and I/O (Contd)IOP also communicates with the I/O devices through a separate I/O bus with its own address, data and control lines.The purpose of the IOP is to provide an independent pathway for the transfer of information between external devices and internal memory.
Isolated I/OOne common bus to transfer data and address information between memory or I/O and the CPU.Separate read and write lines are used to distinguish the memory and I/O operations.
Isolated I/O (Contd)CPU places the address information on the address bus.It then enables one of the two possible read/write lines (memory R/W or I/O R/W) to specify the operation.Specified device will be activated.
Isolated I/O (Contd)Distinct input and output instructions are available.Each is associated with the address of an interface because of the small number of I/O devices.Thus, the program size reduced and decoding speed increased.
Isolated I/O (Contd)Extra cost for the separate sets of read/write buses.
Memory-mapped I/OOnly one set of read and write signals and common address bus for both memory and I/OThe CPU treats an interface as being part of the memory.
Memory-mapped I/O (Contd)Each interface is assigned an address same as the one in the physical memory space. The physical memory space cannot be used.Therefore, the total number of memory address range is reduced.
Memory-mapped I/O (Contd)Usually, a segment of memory address space is reserved for interface.References to these addresses will be ignored by the memory and responded by the specific I/O interface.
Memory-mapped I/O (Contd)No specific input or output instructionsAll access to the I/O can be made used of the large amount of memory-reference instructions.Increases the flexibility for the programmer to design programs.