I'm too lazy to check to confirm this, but I strongly suspect DMA requests don't get serviced while the machine is halted. If so, you could use the following code fragment to avoid that: 0400 12737 MOV #400, @#174400 400 174400 777 BR . Start at 0400, let it run for a couple of seconds, then halt the machine and look at 01000 and up. Noel