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.