Styrning av in/ut-matning. Input/output devices [in/ut-enheter] provide a means for people to make use of a computer. Datorsystem.

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1 Fö 4 In/Ut In/ut-enheter Styrning av in/ut-matning In/ut-kanalenheter Zebo Peng, IDA, LiTH 1 Input/Output Devices Input/output devices [in/ut-enheter] provide a means for people to make use of a computer. Datorsystem Inenhet Dator Utenhet Sekundärminne There are two major types of I/O devices: 0 Interactive devices, e.g., a terminal. 0 Indirect devices, e.g., a printer. Secondary memories can also be considered as I/O devices. Zebo Peng, IDA, LiTH 2 1

2 Characteristics of I/O Since I/O devices run at much slower speed than the CPU, special technical must be used to control them to avoid that the CPU has to wait for them all the time. Some I/O devices function as an interface between a computer system and other physical systems. Such interface usually consists of A/D and D/A converters. Temperature sensor A/D Datorsystem Dator D/A Heater current Sekundärminne Zebo Peng, IDA, LiTH 3 Input Device Examples Device Keyboard [tangentbord] Features Like a typewriter Advantages Efficient for inputting text Disadvantages Relatively slow, speed depends on operator Terminal [terminal] I/O together As above. Can also have intelligent As above Mouse [mus] Move around on the desk Efficient for iconbased input, and menu selection Needs much software support Joystick [joystick] Used for games and control As above and relatively fast Needs much software support Zebo Peng, IDA, LiTH 4 2

3 Input Device Ex (Cont d) Device Features Advantages Disadvantages Light pen [ljuspenna] Point at screen Easy to use Needs much software to make it versatile Graphics tablet Graphics input Input picture and freehand sketch Very slow Scanner [scanner] Copy pictures Fast input of graphics Bit-mapped graphics only Voice input [talat ljud] User friendly No hands needed Limited vocabulary, speech recognition software needed Zebo Peng, IDA, LiTH 5 Output Device Examples Device Features Advantages Disadvan. Speed Display Screen [bildskärm] Most versatile, both text and graphics No waste of paper etc. No hard copy Terminal [terminal] As above, I/O together As above. Can have intelligent As above Line printer [radskrivare] Impact printer, Very fast. Can cope with high volume Large versions are very noisy up to 8000 cps Dot matrix printer [matrisskrivare] Versatile text and graphics Inexpensive low quality and speed up to 300 cps Zebo Peng, IDA, LiTH 6 3

4 Output Device Examples (Cont d) Device Features Advantages Disadvan. Speed Laser printer [laserskrivare] High quality text and graphics Very fast, high volume (used to be) relatively expensive A page per second Plotter High quality graphics large graphics output possible Large machine, expensive Pen up to 1 meter/s Voice output Natural for certain applications Don t need to use eyes (used to be) Limited range of sounds Normal speech A plotter Zebo Peng, IDA, LiTH 7 Typical I/O Data Rates Zebo Peng, IDA, LiTH 8 4

5 Input/Output Module An I/O module is responsible for the control of one or several devices, and for the exchange of data between the devices and the main memory and/or CPU registers. CPU MM Zebo Peng, IDA, LiTH 9 Control and timing CPU communication Device communication Functions of an I/O Module Data buffering Error detection CPU MM Zebo Peng, IDA, LiTH 10 5

6 Control of I/O Devices 1. Programmed I/O [programstyrning] 0 The input and output are controlled by I/O instructions, for example, READ and WRITE. 0 The instructions specify: the I/O operation to perform; the device by giving its address (its ID number). 0 The CPU will wait for the I/O operation to be finished before it executes the next instruction. 0 Since the I/O devices are usually very slow, the CPU has to wait instead of doing useful work. This is therefore an inefficient method. Zebo Peng, IDA, LiTH 11 Execution flow of a WRITE instruction: Programmed I/O Example Select I/O device Send data to the device interface Check device status no Ready? yes Next instruction Zebo Peng, IDA, LiTH 12 6

7 Control of I/O Devices (Cont d) Interrupt-driven I/O [avbrottsstyrning] 0 After the CPU sends an initialization signal to an I/O device, it continues with the execution of the program. 0 When the I/O device is ready or wants to get the attention of the CPU, it sends an interrupt signal [avbrottssignal, brytsignal] to the CPU. 0 When the CPU receives an interrupt signal, it will first finish the execution of the current instruction and then execute an interrupt service routine (ISR). 0 The interrupt mechanism is used to free the CPU from having to check periodically on external devices to see it they are in need of any attention. Zebo Peng, IDA, LiTH 13 Interrupt Service Routine (ISR) Save all status information which is needed to resume execution of the current sequence of instructions. Deal with the interrupt, for example, by reading data from the input device. Restore the saved status information and resumes execution of the interrupted program. interrupt routine current sequence original sequence resumed interrupt Zebo Peng, IDA, LiTH 14 7

8 Instruction Cycle w. Interrupts Start Fetch next instruction Fetch cycle Stop Interrupts disabled Execute instruction Execute cycle Interrupts enabled Check for and process interrupt Interrupt cycle Zebo Peng, IDA, LiTH 15 Multiple Interrupts A new interrupt occurs while the current interrupt is being processed. 0 Disabled interrupt approach: CPU ignores new interrupt signal while processing an interrupt. 0 Priority-based approach: An interrupt of higher priority will interrupt the processing of a lower-priority interrupt. interrupt routine current sequence interrupt original sequence resumed Zebo Peng, IDA, LiTH 16 8

9 Multiple Interrupts A new interrupt occurs while the current interrupt is being processed. 0 Disabled interrupt approach: CPU ignores new interrupt signals while processing an interrupt. 0 Priority-based approach: An interrupt of higher priority will interrupt the processing of a lower-priority interrupt ISR Higher-priority interrupt ISR Lower-priority interrupt current sequence interrupt original sequence resumed Zebo Peng, IDA, LiTH 17 Control of I/O Devices (Cont d) Direct Memory Access (DMA) [direkt minnesaccess]: 0 Allow the transfer of a whole block of data from an I/O device directly to the memory without going through the CPU. CPU Main Memory I/O device control info stop finish A N DMA A DMA has essentially the same function as the data transfer capabilities of the CPU. Zebo Peng, IDA, LiTH 18 9

10 I/O Channels and Processor I/O Channels [In/Ut-Kanalenheter]: 0 Consisting of a special-purpose processor which executes I/O instructions stored in the main memory. 0 The CPU specifies a sequence of I/O activities to be performed by the I/O processor, and is only interrupted when the entire sequence has been performed. I/O Processor [In/Ut-Processenheter]: 0 Consisting of a processor and its local memory, thus it is itself a computer. 0 It is usually used to control communication with interactive terminals. Zebo Peng, IDA, LiTH 19 I/O Operation Summary The data exchange of a computer system with the outside world is provided by the I/O devices. Since computers deal with many different types of users and interface also with many different systems, there is a large variety of I/O devices. We need also a wide variety techniques to control the I/O devices, due to the large differences between various methods of I/O operations. The I/O function is usually the most unreliable part of a computer system, and we need techniques for error detection and recovery. Zebo Peng, IDA, LiTH 20 10