chapter11 - Categories of I/O Devices External devices that...

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1 I/O Management and Disk Scheduling Chapter 11 Categories of I/O Devices External devices that engage in I/O with computer systems can be grouped into three categories: Human readable Used to communicate with the computer user Printers, terminals, video display, keyboard, mouse Machine readable Used to communicate with electronic equipment disk drives, USB keys, sensors, controllers Communication Used to communicate with remote devices Modems, digital line drivers Differences in I/O Devices Data rate May be differences of several orders of magnitude between the data transfer rates Application The use to which a device is put has an influence on the software Complexity of control The effort on OS is filtered by the complexity of the I/O module that controls the device Unit of transfer Data may be transferred as a stream of bytes for a terminal or in larger blocks for a disk Data representation Different data encoding schemes are used by different devices Error conditions Devices respond to errors differently Three techniques for performing I/O are: Programmed I/O the processor issues an I/O command on behalf of a process to an 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 if non-blocking – processor continues to execute instructions from the process that issued the I/O command if blocking – the next instruction the processor executes is from the OS, which will put the current process in a blocked state and schedule another process Direct Memory Access (DMA) a DMA module controls the exchange of data between main memory and an I/O module Organization of the I/O Function Techniques for Performing I/O Programmed I/O Interrupt-driven I/O Direct Memory Access
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2 Programmed I/O I/O module performs the action, not the processor Sets appropriate bits in the I/O status register No interrupts occur Processor checks status until operation is complete Process is busy-waiting for the operation to complete Interrupt-Driven I/O I/O command is issued Processor continues executing other instructions Processor is interrupted when I/O module ready to exchange data No needless waiting Consumes a lot of processor time because every word read or written passes through the processor Direct Memory Access Processor delegates I/O operation to the DMA module DMA module transfers a block of data directly to or from memory Processor interrupted only after entire block has been transferred The processor is only involved at the beginning and end of the transfer DMA Alternative Configurations Operating System Design Issues Efficiency Most I/O devices extremely slow compared to main memory Use of multiprogramming allows for some processes to be waiting on I/O while another
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chapter11 - Categories of I/O Devices External devices that...

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