I/O Structure

After I/O starts, control returns to user program only upon I/O completion.

Wait instruction idles the CPU until the next interrupt

Wait loop (contention for memory access).

At most one I/O request is outstanding at a time, no simultaneous I/O processing. After I/O starts, control returns to user program without waiting for I/O completion.

System call – request to the operating system to allow user to wait for I/O completion.

Device-status table contains entry for each I/O device indicating its type, address, and state.

Operating system indexes into I/O device table to determine device status and to modify table

entry to include interrupt.

Two I/O Methods

           Synchronous                                  Asynchronous

Device-Status Table

Direct Memory Access Structure Used for high-speed I/O devices able to transmit information at close to memory speeds. Device controller transfers blocks of data from buffer storage directly to main memory without CPU intervention. Only on interrupt is generated per block, rather than the one interrupt per byte.