Interrupt Timeline

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 tablecontains 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.