I/O STRUCTURE 0 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. o After I/O starts,control returns to user program without waiting for I/O completion. 0 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.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