I/O Devices and disk scheduling - Csbdu.in

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• Human readable: Suitable for communicating with the computer user. Eg.Display, Keyboard, and perhaps other devices such as mouse • Machine Readable: Suitable for communication with electronic equipment Eg. disk ,tape drives, sensors, controllers, and actuators • Communication: Suitable for communication with remote devices. Eg. Digital line drivers and modems. There are great differences across these classes are • Data rate : There may be differences in data transfer rate • Application: The use of the device on the software and policies in the operating system and supporting utilities. For eg. A disk used for files requires the support of file management. • Complexity of control: A printer requires a relatively simple control interface. A disk is much more complex. • Unit of transfer : Data may transferred as a stream of bytes or characters(eg.terminal I/O) or in larger blocks(e.g disk I/O) • Data representation : Different data encoding schemes are used by different device, including differences in character code and parity conventions • Error conditions : The nature errors, the way in which they are reported, their consequences, and the available range of responses differ widely from one device to another 4.5 Organization of the I/0 functions Programmed I/O: The processor issues and I/O command, on behalf of a process, to and I/O module; that process then busy waits for the operation to be completed before proceeding. Interrupt-driven I/O: The processor issues an I/O command on behalf of a process. After that the processor continues to execute subsequent instructions and is interrupted by I/O module when the latter has completed its work. The subsequent instructions may be in the same process, if it is not necessary for that process to wait for the completion of the I/O.
Direct memory access: A DMA module controls the exchange of data between main memory and an I/O module. 4.5.1 The Evolution of the I/O Function The evolutionary steps of I/O are 1. The processor controls a peripheral device. This is seen in simple microprocessor- controlled device. 2. A controller or I/O module is added. The processor uses programmed I/O without interrupts. 3. The same configuration as step 2 is used, but now interrupts are employed. The processor need not spend time waiting for an I/O operation to be performed, thus increasing efficiency 4. The I/O module is given direct control of memory view DMA. I t can now more a block of data to or from memory without involving the processor, except at the beginning and end of the transfer 5. The I/O module is enhanced to become a separate processor, with a specialized instruction. The CPU directs the I/O processor to execute program in main memory. The I/O processor to execute theses instructions without processor intervention. 6. The I/O module has a local memory is its own and is, in fact, a computer in its own right. With this architecture, a large set of I/O devices can be controlled with minimal processor involvement. A common use for such architecture has been to control communications with interactive terminals. The I/O processor takes care most of the tasks involved in controlling the terminals. For all of the modules described in steps 4 through steps 6, the term direct memory access is appropriate, because all of these types involve direct control of main memory by the I/O module. Also, the I/O module in step 5 often referred to as an I/O channel and that in step 6 as an I/O processor 4.5.2 Direct Memory Access The DMA module controls the exchange of data between main memory and an I/O module. It needs system bus to do this. The DMA module must use the bus only
Page 1 and 2: Structure 4.0 Introduction 4.1 Obje
Page 3 and 4: Long-term Scheduling Long-term sche
Page 5 and 6: Predictability: A given job should
Page 7 and 8: decreasing level of service. FCFS p
Page 9: Where Sn+1 =1/n ΣTi Ti = processor
Page 13 and 14: transfer is complete, the DMA modul
Page 15 and 16: storage device. Allocation of secon
Page 17 and 18: Double Buffer An improvement over s
Page 19 and 20: N = number of bytes on a track R= r
Page 21 and 22: processed, new requests must be add
Page 23 and 24: stack. If the block was already in

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