Putting a ROM on the disk controller at 173000 is the usual way of doing it, but here's a slick alternative that is used on the DSD 4140 floppy controller: There is no ROM onboard other than the microcode for the AMD 2901 (one 4-bit slice!), but there is a single register mapped onto the Qbus at 173000. If the PDP-11 jumps there, the controller gives back a jump-to-self opcode.