Input-output and Communication Prof. Sin-Min Lee Department of Computer Science

Two mechanisms to support communication between OS and hardware: Interrupts and exceptions: Device Interrupt – hardware -> OS Device Registers – OS -> hardware Each device is assigned an interrupt number (for errors, completed tasks) – sets a bit in controller, causes CPU to stop

Interrupt

The memory address in the above table (column 3) is the location of the handler routine that will be executed as a result of the interrupt. Location of table hardwired in machine, initialized by OS

Direct-memory access Often used for block access devices that move large chunks of data, e.g., disk, graphics device I/O Device has smarts built into its controller. The DMA hardware allows device to directly read and write memory without processor intervention. The processor sets up the DMA device controller What operation to perform What locations on the I/O device to access (in the case of a disk access, the track and cylinder of data block) What locations in memory to access When done, the DMA device will interrupt the processor A status register indicates the status of the request once completed or terminated

DMA causes havoc with caches Can invalidate cache lines that are involved Can update the cache Possibly flush cache whenever DMA occurs DMA and virtual memory DMA device could use Physical address of where in memory to place/read data, but the transfer must then be broken into frame sized chunks. DMA device could use Virtual address of where in memory to place/read data, but then it requires the physical mappings of each page to send.

I/O Interfaces - Example of I/O Interface –Receives control information from CPU –A bit in this register determine the operating mode of the device Control register –Contains bit to indicate status condition and record any error during transfer Status register –Communicate with CPU via bidirectional bus Interface registers

I/O Interfaces - Example of I/O Interface Data bus select the interface units through chip select input (CS) and two register select (RS0) and (RS1) CPU and I/O devices are likely to have different clock rate that are not synchronized A control signal is needed to –Indicate time the data is being transmitted –Indicate the window of time when the address is valid

I/O Interfaces - Example of I/O Interface Two methods to perform this timing between CPU and devices –Strobing –Handshaking

I/O Interfaces - Strobing

I/O Interfaces - Handshaking