How compatible were "MS-DOS Compatible" machines?

Short answer--yes. It was called an "OEM Kit". Basically, you got the binaries and instructions on how and what I/O drivers to write and you linked the whole mess together to form two files, IO.SYS and MSDOS.SYS, though you weren't confined to those names. Many utilities, such as MODE and FORMAT were provided in skeleton form with the vendor filling out the details. This lead to a some interesting variations on MODE. For example, the Grid MS-DOS version of MODE allows one to control the power status of the peripherals and a whole bunch of other things. I may even still have the OEM documentation wandering around somewhere, printed up on fading tractor-feed paper. A key difference between the two OEM arrangements was that you agreed to serialize the copies of DRI products that were sold, while Microsoft didn't require that. I think that MS simply charged you on the basis of the number of systems shipped--I'd have to go back to my files to make sure. When you get that "Synchronization error" message from MOVCPM, it means that MOVCPM has compared the serial number of the copy of CP/M that it contains with the one of the system on which it's running and the two don't match. Easy to patch out, however--but beware if you "fill out" your CP/M disk with another's files. Cheers, Chuck