Bits 12 and 1 of the  interrupt registers  are assigned to disk interrupts.

Bit 12, DSKSYN, indicates that the  sync register  matches disk data. this
bit generates a  level 5 interrupt .

Bit 1, DSKBLK, indicates "disk block finished." It is used to indicate
that the specified disk DMA task that you have requested has been
completed. This bit generates a  level 1 interrupt .

More information about disk data transfer and  interrupts  may be found
in Chapter 8, "Interface Hardware."