68k Macs have some magic where you can do a DWORD read of the 5380 SCSI controller and it'll freeze the processor via DTACK until 4 bytes come in over the SCSI bus. That's the situation where no DTACK is fatal. If it's just causing a wait state, you can m_maincpu->adjust_icount(cycles) to add more cycles to the access. (the # of cycles must be *negative* to have a wait state, a positive number makes the CPU effectively speed up on access instead ;-)