booting from RQDX3

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. While the PDP-11 is busy chewing on that, the 2901 sees what has happened and copies sector zero from the floppy to address zero via DMA, then it changes the contents of 173000 to "CLR PC", which causes the PDP-11 to execute the boot code. So just because you don't see ROMS doesn't meant the board can't boot. -- Jonathan Engdahl Rockwell Automation Principal Research Engineer 24800 Tungsten Road Advanced Technology Euclid, OH 44117 USA Euclid Labs http://users.safeaccess.com/engdahl