The disk controller can issue three kinds of interrupts:

   *   dsksync  (level 5,  intreq  bit 12) -- input stream matches the
                                             DSKSYNC register.
   *   dskblk  (level 1,  intreq  bit 1) -- disk dma has completed.
   *   index  (level 6, 8520 flag pin) -- index sensor triggered.

Interrupts are explained further in the section
 length, direction, dma enable .

See Chapter 7, "System Control Hardware," for more information about
  interrupts .

See  appendix f  for more information on the 8520.