Many avoided the issues with the BDOS and simply went through the CBIOS vector. That's why it was important that your "cold boot" jump at 0000 point to the proper entry in the BIOS jump vector list and not to the cold boot routine directly. A lot programs used the cold boot jump at 0000 to find the BIOS jump list--just take the address in the jump instruction and set the low byte to 00. Timer as well as console interrupt I/O was pretty much necessary for MP/M functioning (interrupt-driven disk I/O was *strongly* suggested) and recommended for CP/M 3.0, as was bank-switching. I did a CP/M 2.2 implementation using interrupt-driven I/O for everything but the memory-mapped display. It worked pretty well, but the effort was wasted for the disk I/O--there was nothing to do while you were waiting for the operation to complete. On the other hand, it did make writing an MP/M XIOS much simpler. The problem with IOBYTE is that it was defined in terms of console, reader, punch and list. Well, most systems didn't have a "reader" or "punch". This made what to do with those items a matter of implementation. BDOS Function 3 is defined as "Reader Input" and 4 as "Punch Output". By the time that 2.2 was offered, most CP/M systems were CRT oriented with a printer and perhaps a serial port used to connect to a modem. Instead of acknowledging that as the model configuration, DRI stubbornly stayed with TTY and paper tape as their model of character I/O. Cheers, Chuck