> Writing a BIOS for a new machine in a single afternoon. Those were the days...

This was not a BIOS in the sense of the PC ROM-BIOS, it was an OS module containing device drivers - the equivalent in PC-DOS is IBMIO.COM, in MS-DOS it is IO.SYS.

Of course, writing it in an afternoon was still impressive, but it was a lot smaller than even the IBM PC ROM-BIOS; the MVP was essentially just two device drivers, one for the floppy drive, the other for either the screen/keyboard or for the serial port (if you used an attached serial terminal for IO). Probably IMSAI already had code for those drivers and they were just taking that existing code and interfacing it with the CP/M BIOS API