I installed the “RetroClinic DualOS” for the BBC Master 128k today.
The DualOS Master Board from RetroClinic replaces the original 1Mbit ROM on the BBC Master 128. It contains 2 versions of the Master MOS, the original 3.20 that most machines come with, and the later MOS 3.50, with all the later versions of the utilities.
A switchable MOS gives the best of both worlds for compatibility. As was the case with the B+, and the Master when it first came out, because of changes to the OS code, software that accesses the operating system directly did not like the changes, and either ran badly, or crashed completely. This is still the case with the MOS 3.50. If you use only the later version, some of the games and utilities you used to enjoy and use will not work any more. Having both in your machine, available at the flick of a switch, gives you the most flexibility.
Both versions of the MOS have been correctly patched for the Y2K bug, so the year displays correctly from 1980 to 2079 – essential if some software was not written to be Y2K compliant, and you need to back the year off to make it run. Also, the DFS in MOS 3.50 has been patched to correct several bugs, and allow double density formatting under the DFS with the correct system calls. Add to that, the ADFSs on both versions of the operating system have been patched to use the Compact Flash Hard Drive.
You can find out more about the DualMOS at http://www.retroclinic.com/acorn/acorn.htm