Input/Output (I/O) Subsystem in Operating Systems - Prof. Prasad Kulkarni, Study notes of Operating Systems

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    n Terms and definitions n Basic concepts 2

            n I/O subsystem l component of the OS that implements the application – I/O device interface n Port l connection point for an I/O device with the computer l consists of data-in, data-out, status, and control registers n Bus l set of wires with protocol for what can be transferred on the bus l PCI, PCI-X, PCIe, SCSI, HyperTransport, etc. n Controller l circuit to operate a port, a bus, or a device l each device controller provides registers for the CPU to send data and controls signals to that device 3

           
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             n Polling l during I/O, the computer does nothing but constantly monitor the status register l no other way to determine device status l inefficient, wastes CPU resources n Interrupts l device asynchronously notifies the CPU on status changes l I/O devices produce hardware interrupt, software interrupt is a synchronous event in software l save current execution state (context switch) and begin execution of an interrupt service routine (interrupt handler) l CPU typically provides 1-2 pins for indicating interrupts l interrupts have priorities, may be maskable l efficient 5

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            "#$ n Direct Memory Access l used to avoid programmed I/O for large data movement l DMA controller allows direct data transfers between memory and devices (disk controllers, graphics controllers, network cards, etc.) l CPU initiates the DMA transfer l bypasses CPU to transfer data directly between I/O device and memory l CPU free to do other tasks 7

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% n Block device l stores information in fixed sized blocks l each block has its own address l generally use buffered input and output routines l e.g., disks n Character device l delivers or accepts a stream of characters l individual characters are not addressable l often use unbuffered input and output routines l e.g., keyboards, printers, network cards