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Module 12: I/O Systems.• I/O hardwared.• Application I/O Interface.• Kernel I/O Subsystem.• Transforming I/O Requests to Hardware Operations.• Performance. 12.1 Silberschatz and Galvin 1999 I/O Hardware.• Incredible variety of I/O devices.• Common concepts. – Port. – Bus (daisy chain or shared direct access). – Controller (host adapter).• I/O instructions control devices.• Devices have addresses, used by. – Direct I/O instructions. – Memory-mapped I/O. 12.2 Silberschatz and Galvin 1999 Polling.• Determines state of device. – command-ready. – busy. – error• Busy-wait cycle to wait for I/O from device. 12.3 Silberschatz and Galvin 1999 Interrupts.• CPU Interrupt request line triggered by I/O device.• Interrupt handler receives interrupts.• Maskable to ignore or delay some interrupts.• Interrupt vector to dispatch interrupt to correct handler. – Based on priority. – Some unmaskable.• Interrupt mechanism also used for exceptions. 12.4 Silberschatz and Galvin 1999 Interrupt-drive I/O Cycle. 12.5 Silberschatz and Galvin 1999 Direct Memory Access.• Used to avoid programmed I/O for large data movement.• Requires DMA controller.• Bypasses CPU to transfer data directly between I/O device and. memory. 12.6 Silberschatz and Galvin 1999 Six step process to perform DMA transfer. 12.7 Silberschatz and Galvin 1999 Application I/O Interface.• I/O system calls encapsulate device behaviors in generic classes.• Device-driver layer hides differences among I/O controllers from. kernel.• Devices vary in many dimensions. – Character-stream or block. – Sequential or random-access. – Sharable or dedicated. – Speed of operation. – read-write, read only, or write only. 12.8 Silberschatz and Galvin 1999 Block and Character Devices.• Block devices include disk drives. – Commands include read, write, seek. – Raw I/O or file-system access. – Memory-mapped file access possible.• Character devices include keyboards, mice, serial ports. – Commands include get, put. – Libraries layered on top allow line editing. 12.9 Silberschatz and Galvin 1999 Network Devices.• Varying enough from block and character to have own interface.• Unix and Windows/NT include socket interface. – Separates network protocol from network operation. – Includes select functionality.• Approaches vary widely (pipes, FIFOs, streams, queues,. mailboxes). 12.10 Silberschatz and Galvin 1999 Clocks and Timers.• Provide current time, elapsed time, timer.• if programmable interval time used for timings, periodic interrupts.• ioctl (on UNIX) covers odd aspects of I/O such as clocks and. timers. 12.11 Silberschatz and Galvin 1999 Blocking and Nonblocking I/O.• Blocking - process suspended until I/O completed. – Easy to use and understand. – Insufficient for some needs.• Nonblocking - I/O call returns as much as available. – User interface, data copy (buffered I/O). – Implemented via multi-threading. – Returns quickly with count of bytes read or written.• Asynchronous - process runs while I/O executes. – Difficult to use. –
