Module- 6 I/O Organization and Peripherals

Transcription

1 Module- 6 I/O Organization and Peripherals Motivation: The following are the motivational factors for an I/O organization: The presence of a wide variety of peripherals with various methods of operation The data transfer of peripherals is slower/faster than that of the memory or processor Peripherals use different data formats and word lengths than the computer to which they are attached. Objective: devices. To get knowledge about the I/O systems, techniques used for I/O operations and various peripheral Prerequisite: Detailed functioning of the processor and memory of a computer. Syllabus: Module Content Duration Self Study Time 6.1 Input/Output systems 1 lecture 1 hour 6.2 I/O Modules and 8089 IO processor 1 lecture 1 hour 6.3 Types of data transfer techniques: Programmed I/O 1 lecture 1 hour 6.4 Interrupt driven I/O and DMA 2 lectures 2 hours I/O Organization and Peripherals 167

2 6.5 Peripheral Devices: Introduction to peripheral devices, scanner, plotter, joysticks, touch pad 1 lecture 1 hour Learning I/O systems Techniques for I/O operation I/O modules Peripheral devices Definitions: 1. Input/Output: Input/Output, or I/O, refers to the communication between an information processing system (such as a computer), and the outside world, possibly a human, or another information processing system. Inputs are the signals or data received by the system, and outputs are the signals or data sent from it. 2. I/O module: This interfaces to the system bus or central switch and controls one or more peripheral devices. It contains some intelligence ; that is, it contains logic for performing a communication function between the peripheral and the bus. 3. I/O Channel: An I/O module that takes on most of the detailed processing burden, presenting a highlevel interface to the processor, is usually referred to as an I/O channel or I/O processor. I/O channels are used on mainframes. 4. I/O Controller: An I/O module that is quite primitive and requires detailed control is usually referred to as an I/O controller or device controller. I/O controllers are commonly seen on microcomputers. 5. Peripheral: An external device connected to an I/O module is often referred to as a peripheral device, or simply, a peripheral. I/O Organization and Peripherals 168

3 6. Direct Memory Access (DMA): Direct memory access is a feature of modern computers and microprocessors that allows certain hardware subsystems within the computer to access system memory for reading and/or writing independently of the central processing unit. 7. Programmed I/O: Programmed input/output is a method of transferring data between the CPU and a peripheral such as a network adapter or an ATA storage device. In general, programmed I/O happens when software running on the CPU uses instructions that access I/O address space to perform data transfers to or from an I/O device. 8. Interrupt driven I/O: This is an I/O mechanism in which the processor issues an I/O command to a module and then go on to do some other useful work. The I/O module will then interrupt the processor to request service when it is ready to exchange data with the processor. The processor then resumes its former processing. 9. Cycle stealing: This is referred to the act of forcing the processor to suspend its operation temporarily by the DMA module. Key Definitions: I/O Input Output DMA Direct Memory Access USB _Universal Serial Bus PCI Peripheral Component Interconnect ATA -Advanced Technology Attachment PSW Program Status Word PC Program Counter CRT - Cathode-Ray Tube LCD Liquid Crystal Display I/O Organization and Peripherals 169

4 CD-ROM Compact Disc Read Only Memory WORM Write Once Read Many Theory 6.1 Input/Output systems: In addition to the processor and a set of memory modules, the third key element of a computer system is a set of I/O modules. Each module interfaces to the system bus, and controls one or more peripheral devices. An I/O module is not simply a set of mechanical connectors that wire a device into the system bus. Rather, the I/O module contains some intelligence ; that is, it contains logic for performing a communication function between the peripheral and the bus. An I/O module has two major functions: i) Interface to the processor and memory via the system bus or central switch ii) Interface to one or more peripheral devices by tailored data links I/O operations are accomplished through a wide assortment of external devices that provide a means of exchanging data between the external environment and the computer. An external device attaches to the computer by a link to an I/O module (Fig. 6.1). The link is used to exchange control, status, and data between the I/O module and the external device. Figure 6.1: Generic Model of an I/O Module I/O Organization and Peripherals 170

5 The interface to the I/O module is in the form of control, data, and status signals. Control signals determine the function that the device will perform, such as send data to the I/O module (INPUT or READ), accept data from the I/O module (OUTPUT or WRITE), report status, or perform some control function particular to the device. Data are in the form of a set of bits to be sent to or received from the I/O module. Status signals indicate the state of the device. Examples are READY/NOT-READY to show whether the device is ready for data transfer. The major functions or requirements for an I/O module fall into the following categories: Control and timing Processor communication Device communication Data buffering Error detection 6.2 I/O Modules and 8089 IO Processor I/O Module Structure Figure 6.2 provides a general block diagram of an I/O module. The module connects to the rest of the computer through a set of signal lines (e.g., system bus lines). Data are transferred to and from the module are buffered in one or more data registers. There may also be one or more status registers that provide current status information. A status register may also function as a control register, to accept detailed control information from the processor. I/O Organization and Peripherals 171

6 Figure 6.2: Block diagram of an I/O module The logic within the module interacts with the processor via a set of control lines. The processor uses the control lines to issue commands to the I/O module. Some of the control lines may be used by the I/O module (e.g., for arbitration and status signals). The module must also be able to recognize and generate addresses associated with the devices it controls. Each I/O module has a unique address or, if it controls more than one external device, a unique set of addresses. Finally, the I/O module contains logic specific to the interface with each device that it controls. An I/O module that takes on most of the detailed processing burden, presenting a high-level interface to the processor, is usually referred to as an I/O channel or I/O processor. An I/O module that is quite primitive and requires detailed control is usually referred to as an I/O controller or device controller. I/O controllers are commonly seen on microcomputers, whereas I/O channels are used on mainframes. Three techniques are possible for I/O operations. They are (1) Programmed I/O (2) Interrupt-driven I/O (3) DMA. All these are explained in the following sections IO Processor The Intel 8089 input/output coprocessor was available for use with the 8086/8088 central processor. It used the same programming technique as 8087 for input/output operations, such as transfer of data from memory to a peripheral device, and so reducing the load on the CPU. Because IBM didn't use it in IBM PC design, it did not become well known; later I/O-coprocessors did not keep the x89 designation the way math coprocessors kept the x87 designation. I/O Organization and Peripherals 172

7 Figure 6.3: Pinout for 8089 Processor 6.3 Types of data transfer techniques: Programmed I/O: In this I/O technique, data are exchanged between the processor and the I/O module. The processor executes a program that gives it direct control of the I/O operation, including sensing device status, sending a read or write command, and transferring the data. When the processor issues a command to the I/O module, it must wait until the I/O operation is complete. If the processor is faster than the I/O module, this is wasteful of processor time. When the processor is executing a program and encounters an instruction relating to I/O, it executes that instruction by issuing a command to the appropriate I/O module. With programmed I/O, the I/O module will perform the requested action and then set the appropriate bits in the I/O status register. The I/O module takes no further action to alert the processor. In particular, it does not interrupt the processor. Thus, it is the I/O Organization and Peripherals 173

8 responsibility of the processor periodically to check the status of the I/O module until it finds that the operation is complete. I/O Commands To execute an IO-related instruction, the processor issues an address, specifying the particular I/O module and external device, and an I/O command. There are four types of I/O commands that an I/O module may receive when it is addressed by a processor: Control: Used to activate a peripheral and tell it what to do Test: Used to test various status conditions associated with an I/O module and its peripherals. Read: Causes the I/O module to obtain an item of data from the peripheral and place it in an internal buffer. The processor can then obtain the data item by requesting that the I/O module place it on the data bus. Write: Causes the I/O module to take an item of data (byte or word) from the data bus and subsequently transmit that data item to the peripheral. I/O Instructions With programmed I/O, the instructions are easily mapped into I/O commands, and there is often a simple one-to-one relationship. The form of the instruction depends on the way in which external devices are addressed. Each device is given a unique identifier or address. Each I/O module must interpret the address lines to determine if the command is for itself. When the processor, main memory, and I/O share a common bus, two modes of addressing are possible: memory mapped and isolated. With memory-mapped I/O, there is a single address space for memory locations and I/O devices. The processor treats the status and data registers of I/O modules as memory locations and uses the same machine instructions to access both memory and I/O devices. So, for example, with 10 address lines, a combined total of 2 10 =1024 memory locations and I/O addresses can be supported, in any combination. I/O Organization and Peripherals 174

9 With isolated I/O, the bus may be equipped with memory read and write plus input and output command lines. Now, the command line specifies whether the address refers to a memory location or an I/O device. The full range of addresses may be available for both. Again, with 10 address lines, the system may now support both 1024 memory locations and 1024 I/O addresses. This mode is referred to as isolated I/O because the address space for I/O is isolated from that for memory. A flow chart that depicts the processing of programmed I/O is given below: Figure 6.4: Programmed I/O process 6.4 Interrupt-driven I/O and DMA: Interrupt-driven I/O The problem with programmed I/O is that the processor has to wait a long time for the I/O module of concern to be ready for either reception or transmission of data. This degrades the level of performance of the entire system. With interrupt-driven I/O, the processor issues an I/O command, continues to execute other I/O Organization and Peripherals 175

10 instructions, and is interrupted by the I/O module when the latter has completed its work. The processing of the interrupt-driven I/O is given below. Figure 6.5: Interrupt-driven I/O process The interrupt processing is also depicted by the following flow chart. I/O Organization and Peripherals 176

11 Figure 6.6: Simple Interrupt Processing Design Issues Two design issues arise in implementing interrupt I/O. First, because there will almost invariably be multiple I/O modules, how does the processor determine which device issued the interrupt? And second, if multiple interrupts have occurred, how does the processor decide which one to process? Device identification is considered first. Four general categories of techniques are in common use: Multiple interrupt lines Software poll Daisy chain (hardware poll, vectored) Bus arbitration (vectored) I/O Organization and Peripherals 177

12 Multiple interrupt lines: Here multiple interrupt lines are provided between the processor and the I/O modules. However, it is impractical, even if multiple lines are used, it is unlikely that each line will have multiple I/O modules attached to it Software poll: In this case, when the processor detects an interrupt, it branches to an interrupt-service routine whose job is to poll each I/O module to determine which module caused the interrupt. The poll could be in the form of a separate command line. The disadvantage of the software poll is that it is time consuming. Daisy chain: This technique provides a hardware poll. For example if interrupts are considered, all I/O modules share a common interrupt request line. The interrupt acknowledge line is daisy chained through the modules. When the processor senses an interrupt, it sends out an interrupt acknowledge. This signal propagates through a series of I/O modules until it gets to a requesting module. The requesting module typically responds by placing a word on the data lines. This word is referred to as a vector and is either the address of the I/O module or some other unique identifier. In either case, the processor uses the vector as a pointer to the appropriate device-service routine. This avoids the need to execute a general interrupt-service routine first. This technique is in turn called a vectored interrupt. Bus arbitration: This also makes use of vectored interrupts. With this technique, an I/O module must first gain control of the bus before it can raise the interrupt request line. Thus, only one module can raise the line at a time. When the processor detects the interrupt, it responds on the interrupt acknowledge line. The requesting module then places its vector on the data lines. The above mentioned techniques serve to identify the requesting I/O module. They also provide a way of assigning priorities when more than one device is requesting interrupt service. With multiple lines, the processor just picks the interrupt line with the highest priority. With software polling, the order in which modules are polled determines their priority. Similarly, the order of modules on a daisy chain determines their priority. Finally, bus arbitration can employ a priority scheme (e.g., distributed or centralized). I/O Organization and Peripherals 178

13 6.4.2 DMA (Direct Memory Access) With both programmed and interrupt I/O, the processor is responsible for extracting data from main memory for output and storing data in main memory for input. Thus, both these forms of I/O suffer from two inherent drawbacks: 1. The I/O transfer rate is limited by the speed with which the processor can test and service a device. 2. The processor is tied up in managing an I/O transfer; a number of instructions must be executed for each I/O transfer. The alternative is known as direct memory access DMA). In this mode, the I/O module and main memory exchange data directly, without processor involvement. DMA Function DMA involves an additional module on the system bus. The DMA module (fig.6.6) is capable of mimicking the processor and, indeed, of taking over control of the system from the processor. It needs to do this to transfer data to and from memory over the system bus. For this purpose, the DMA module must use the bus only when the processor does not need it, or it must force the processor to suspend operation temporarily. The latter technique is more common and is referred to as cycle stealing, because the DMA module in effect steals a bus cycle. I/O Organization and Peripherals 179

14 Figure 6.7: DMA block diagram When the processor wishes to read or write a block of data, it issues a command to the DMA module. The processor then continues with other work. It has delegated this I/O operation to the DMA module. The DMA module transfers the entire block of data, one word at a time, directly to or from memory, without going through the processor. When the transfer is complete, the DMA module sends an interrupt signal to the processor. Thus the processor is involved only at the beginning and end of the transfer. This is depicted by the following flow chart. Figure 6.8: DMA process I/O Organization and Peripherals 180

15 Alternate DMA Configurations: The DMA mechanism can be configured in a variety of ways. Three possibilities are discussed here. In the first case, all modules share the same system bus. The DMA module, acting as a surrogate processor, uses programmed I/O to exchange data between memory and an I/O module through the DMA module. This configuration, while it may be inexpensive, is clearly inefficient. As with processor-controlled programmed I/O, each transfer of a word consumes two bus cycles. Figure 6.9: Single-bus, detached DMA In the second configuration the drawback of the first one is overcome by integrating the DMA and I/O functions. By this, the number of required bus cycles can be cut substantially. As fig.6.10 indicates, this means that there is a path between the DMA module and one or more I/O modules that does not include the system bus. The DMA logic may actually be a part of an I/O module, or it may be a separate module that controls one or more I/O modules. This concept can be taken one step further by connecting I/O modules to the DMA module using an I/O bus as shown in fig.6.11 which is the third type of configuration. This reduces the number of I/O interfaces in the DMA module to one and provides for an easily expandable configuration. Figure 6.10: Single bus-integrated DMA-I/O I/O Organization and Peripherals 181

16 Figure 6.11: I/O bus In all of these cases (second and third configurations), the system bus that the DMA module shares with the processor and memory, is used by the DMA module only to exchange data with memory. The exchange of data between the DMA and I/O modules takes place off the system bus. 6.5 Peripheral Devices Introduction Any device connected internally or externally to a computer and used in the transfer of data. A personal computer or workstation processes information and, strictly speaking, that is all the computer does. Data (unprocessed information) must get into the computer, and the processed information must get out. Entering and displaying information is carried out on a wide variety of accessory devices called peripherals, also known as input/output (I/O) devices. Some peripherals, such as keyboards, are only input devices; other peripherals, such as printers, are only output devices; and some are both. Monitor The monitor is the device on which images produced by the computer operator or generated by the program are displayed on a cathode-ray tube (CRT). Electron guns one in a monochrome monitor, three in a color monitor irradiate phosphors on the inside of the vacuum tube, causing them to glow. The flat-panel I/O Organization and Peripherals 182

17 displays on most portable computers, known as liquid-crystal displays (LCDs), use two polarizing filters with liquid crystals between them to produce the image. Keyboard The computer keyboard, based on the typewriter keyboard, contains keys for entering letters, numbers, and punctuation marks, as well as keys to change the meaning of other keys. The function keys perform tasks that vary from program to program. Mouse The mouse is a device that is rolled on the desktop to move the cursor on the screen. A ball on the bottom of the mouse translates the device's movements to sensors within the mouse and then through the connecting port to the computer. There are also mice that substitute optical devices for mechanical balls, and mice that use infrared rather than physical connections. Trackball The trackball is essentially an upside-down mouse, with the ball that is used to move the cursor located on the top rather than on the bottom. Joystick The joystick is a pointing device used principally for games. Light pen The light pen performs the same functions as a mouse or trackball, but it is held up to the screen, where its sensors detect the presence of pixels and send a signal through a cable to the computer. Graphics tablet The graphics, or digitizing, tablet is a pad with electronics beneath the surface which is drawn upon with a pointed device, called a stylus. The shapes drawn appear on the monitor's screen. Disk drives The most common input, or storage, device in personal computers or workstations is a hard disk drive, a stack of magnetized platters on which information is stored by heads generating an electrical current to I/O Organization and Peripherals 183

18 represent either 1 or 0 in the binary number system. The device is called hard because the platters are inflexible, and is called a drive because it spins at 3600 revolutions or more a minute, within a sealed case. Diskettes, made of flexible film like that used in recording tape, are usually stored within a hard shell and are spun by their drives at about 360 revolutions per minute. Data can also be stored and retrieved with light, the light of a laser beam reading a pattern of pits on an optical disk. The most familiar type of optical disk is the CD-ROM (compact disk-read only memory). Another kind of optical disk is the WORM (write once read many times). As a consequence of their greater efficiency and speed, disk drives have quickly replaced tape drives as the primary means of data and program storage. Tape drives are still in use for backup storage, copying the contents of a hard disk as insurance against mechanical failure or human error. Scanner The scanner converts an image of something outside the computer, such as text, a drawing, or a photograph, into a digital image that it sends into the computer for display or further processing. The image is viewed as a graphics image, not a text image, so it can be altered with a graphics program but cannot be edited with a word-processing program, unless the scanner is part of a character-recognition system. To digitize photographs, a scanner may dither the image (put the dots a varying amount of space apart), or use the tagged image file format (TIFF), storing the image in 16 gray values. Some scanners can use standard video cameras to capture images for the computer. Printer The printer puts text or other images produced with a computer onto paper or other surfaces. Printers are either impact or nonimpact devices. Daisy-wheel or thimble printers are so called from the shape of the elements bearing raised images of the characters. Their speed, perhaps 30 characters per second, is now considered unacceptably slow. Dotmatrix printers produce their images by striking a series of wire pins, typically, 9, 18, or 24, through the ribbon in the pattern necessary to form the letter, number, line, or other character. Ink-jet printers carry their ink in a well, where it is turned into a mist by heat or vibration and sprayed through tiny holes to form the pattern of the character on paper. Laser printers are similar to photocopying machines. The quality of laser-printer output is the highest. I/O Organization and Peripherals 184

19 Plotter The plotter is a computer printer for printing vector graphics. In the past, plotters were used in applications such as computer-aided design, though they have generally been replaced with wide-format conventional printers. A plotter gives a hard copy of the output. It draws pictures on paper using a pen. Plotters are used to print designs of ships and machines, plans for buildings and so on. Touchpad A touchpad or trackpad is a pointing device featuring a tactile sensor, a specialized surface that can translate the motion and position of a user's fingers to a relative position on the operating system that is outputted to the screen. Touchpads are a common feature of laptop computers, and are also used as a substitute for a mouse where desk space is scarce. Because they vary in size, they can also be found on personal digital assistants (PDAs) and some portable media players. Wireless touchpads are also available as detached accessories. References: 1. William Stallings, Computer Organization and Architecture: Designing and performance : Prentice- Hall India 2. Carl Hamacher, Zvonko Vranesic and Safwat Zaky Computer Organization, McGraw Hill Objective Questions: 1. The input or output device attached to your computer is also called: a) Electromechanical devices b) Electromagnetic devices c) Peripheral devices d) Electronic devices I/O Organization and Peripherals 185

20 2. When the DMA takes control of the bus system, it communicates directly with the CPU: a) True b) False 3. Which one of the following is the technique for performing I/O operations? a) Programmed I/O b) Interrupt-driven I/O c) DMA d) All of the above 4. Cycle stealing is the technique implemented by which of these components in the system. a) DMA b) Programmed I/O c) Interrupt driven I/O 6. The I/O bus consists of: a) Data lines b) Address lines c) Control lines d) All of the above 6. With , there is a single address space for memory locations and I/O devices. a) memory-mapped I/O b) isolated I/O c) programmed I/O d) interrupt I/O I/O Organization and Peripherals 186

21 7. With , the bus may be equipped with memory read and write plus input and output command lines. a) memory-mapped I/O b) isolated I/O c) programmed I/O d) interrupt I/O 8. An I/O module that takes on most of the detailed processing burden, presenting a high-level interface to the processor, is usually referred to as a) I/O controller b) I/O channel c) I/O system d) None of the above 9. An I/O module that is quite primitive and requires detailed control is usually referred to as a) I/O controller b) I/O channel c) I/O processor d) None of the above 10. Which of the following is an I/O command? a) Test b) Control c) Read d) All of the above I/O Organization and Peripherals 187

22 Answers: 1. a) 2. a) 3. d) 4. a) 6. d) 6. a) 7. b) 8. b) 9. a) 10. d) Short Answer Questions: 1. Mention the group of lines in the system bus. Ans: Address lines, Data lines, Control lines 2. What is bus arbitration? Ans: Bus arbitration is the process of granting access to the bus for a device when more than one device need control of the bus. 3. What is DMA? Ans: DMA (Direct Memory Access) is an I/O technique which involves an additional module on the system bus. The DMA module is capable of mimicking the processor and indeed, of taking over control of the system from the processor. It needs to do this to transfer data to and from memory over the system bus. 4. Why does the DMA gets priority over CPU when both request memory transfer? Ans: DMA gets more priority because it is capable of transferring large volumes of data, consuming less number of bus cycles. 6. List out the types of interrupts. Ans: Internal interrupt, External interrupt, Software interrupt. 6. What is the need for DMA transfer? Ans: To reduce the number of bus cycles by integrating the DMA and I/O functions. I/O Organization and Peripherals 188

23 7. What is memory-mapped I/O? Ans: With this addressing mechanism, there is a single address space for memory locations and I/O devices. The processor treats the status and data registers of I/O modules as memory locations and uses the same machine instructions to access both memory and I/O devices. 8. List down the functions performed by an Input/Output module. Ans: -Control and Timing -Processor communication -Device communication -Data buffering -Error detection Long Answer Questions: 1. Write notes on i) Programmed I/O ii) Interrupt driven I/O Ans: Refer sections 6.3 and Explain DMA in detail? Ans: Refer section Explain I/O Module. Ans: Refer section Explain about 8089 IO processor? Ans: Refer section I/O Organization and Peripherals 189

24 University Questions: 1. Compare & contrast DMA, Programmed I/O & interrupt driven I/O. (Dec. 2010, 10 M; Dec 2012, 10M) 2. Write short notes on DMA (May 2010, 5 M) 3. Write short notes on (any two) (Dec. 2009, 10 M; May 2012, 5M) (i) DMA (ii) Programmed I/O (May 2015 (4 M)) (iii) Interrupt Driven I/O (May 2015 (6 M)) 4. Write short notes on:- (i) DMA (ii) Interrupt driven I/O (May 2011, 10 M) 6. Explain I/O processor and I/O channels with diagram. (Dec M) 6. What do you mean by initialization of DMA controller? How DMA controller works? Explain with suitable diagram. (May 2012, 10M) 7. Write short note on I/O processor. (Dec 2012, 5M) 8. Define I/O module. State the difference between programmable and non programmable devices with suitable examples (May 2013, 10M) 9. What are major requirements for an I/O module? (May 2015, 6 M) 10. Explain DMA based data transfer technique for I/O devices. (May 2015, 7 M) 11. Compare interrupt driven I/O and DMA. (Dec 2015, 10 M) I/O Organization and Peripherals 190