Figured it out - There is one fundamental difference between 16-bit mode of the CPU 8085/88 and the CPU 8086/8087. It needs RAM at FFFF0 in order to start correctly. With the 8086/8087 the Disk-1A puts it ROM at FFC00-FFFFF which boots the system. Since the 8086 has it's vectors at 00000, it never needs "high RAM" again. With the 8085/8088 the system starts in 8085 mode with the boot ROM at 00000. The loader (from disk) tries to write a JMP to FFFF0 before starting the 8088 - this was failing because I didn't have RAM there. Putting my 64k board at Fxxxxx cured the problem and now it boots CP/M-86 reliably. -- dave04a (at) Dave Dunfield dunfield (dot) Firmware development services & tools: www.dunfield.com com Collector of vintage computing equipment: http://www.parse.com/~ddunfield/museum/index.html