MultiOS - Switchable 4 way MOS 3.50, 3.20, 2.00 and 1.20 for the BBC Master 128

WITH IDE COMPATIBLE ADFSs

If you require a MultiOS with SCSI Compatible ADFSs, please see my other listings

This is a buy-it-now sale for a MultiOS kit from RetroClinic, for the BBC Master 128.

The MultiOS is a small PCB which replaces the MOS (Master Operating System) chip inside your BBC Master. It contains 4 operating systems to choose from:

The kit also comes with a rotary switch that selects which OS you wish to use. No drilling is required to fit the switch, as it fits snugly into the cutout for the modem cable, as shown in the photos.

A switchable MOS for your Master gives you effectively 3 machines in one. Not only do you have both versions of the OS for the Master 128, you can also select it to emulate a Model B or B+. Some early software was written before the Master came out, so wasn't designed to take advantage of the newer features. Sometimes these programs used functions that the new Master OS didn't like, and these programs can misbehave, with screen or graphics issues, or even not run at all. With the machine in B or B+ mode, you're running the original OS that came with the early machines, albeit patched slightly to run on the Master, so these programs feel more at home, and are more likely to run correctly. It doesn't guarantee to make every single program run, as the original Model B, B+ and Master 128 all have slightly different CPUs, that are mostly compatible, but not quite. However, most stuff that runs on a Model B will now run on your Master 128 fitted with our MultiOS system.

The ADFS filing system in the two MOS banks, are patched to use IDE devices, such as the RetroClinic DataCentre, and other IDE based hard drive systems that run off the 1MHz bus. If you have an original Winchester drive, or one of the newer BeebSCSI devices, please see my other MultiOS listing, that is compatible with SCSI devices. Either type can be used with ADFS floppy disks.

If you don't use a Winchester style hard drive with ADFS, then either version of the MultiOS will be fine for you to use.

The Master OSs are also patched to fix the Y2K bug, so years are displayed correctly in compatible software, and when using *TIME

Here is a little more technical information, about what is contained in each of the 4 ROM banks.


OS 3.50 - The updated last OS for the Master 128

It's this configuration that's you'd use the most. It contains the last images and ROMs for the Master 128 released by Acorn. There are later OSs, including 4.00, 5.00 and 5.10, but these are for the ET and Compact, so are not suitable for use in a 128. The MOS 3.50 also solves some of the problems of games and other software not behaving under OS 3.20.

One other feature of this version of OS is that it will automatically fill the CMOS configuration with default data, instead of all zeros, like MOS 3.20 did. Doing an <R> power on resets the CMOS under 3.50 to a set of sensible values, but you may want to reconfigure the filing system back to DFS by typing:

*CONFIGURE FILE 9


OS 3.20 - The standard OS supplied in the Master 128

If you prefer the original MOS, then this is the one to use.

OS 2.00 - The OS from the 64K or 128K BBC Micro B+

This image was put together with these ROMs for the more serious programmer in mind. The memory display when this OS is booted will either display 64K, 96K, or 128K, depending on the way links 18 and 19 are set. If they are both set for sideways RAM (to the left), then it will show 128K, if they are both set for mapping to the ROM sockets (to the right), then it will show 64K, and if each is set different, you will get 96K displayed. The SRAM commands, in ADT, ARM and SRAM are useful for creating your own ROM images.

Shadow screen modes are available in OS 2.00, so you have the extended modes 128 through 135, which use shadow memory rather than main memory, freeing up space for your programs. This is the same behavior as the later Master OSs.


OS 1.20 - The original OS for the 32K BBC Micro Model B

This bank has been designed for the gamer. It provides OS 1.20, the most compatible of OSs for the early games. Games that did not run on OS 3.2 or 3.5, will most likely run when you select OS 1.20. However not all will. The Master has a different CPU to the Model B, with a slightly different instruction set. In the early days, some programmers didn't adhere to the rules when they wrote software, especially games, and used what's known as "illegal instructions" to make their software perform better. The later CPU doesn't like these instructions, as some of them have now been implemented to do different things. One example game of this is ZALAGA. The early version will still crash, but there is a later version of the game available, which will run under OS 3.20 and up.

This bank also has three popular titles from Acornsoft built in. To use any of these, you simply type a * then the name. This is detailed in the leaflet that comes with the system.

It should be noted that under OS 1.20, the 4 banks of sideways RAM, if available, are usable as in OS 2.00, however, the other 32K of shadow memory is not.

The kit comes with full install instructions. Reprints of the User Guides for each of the ROMs are available separately, please get in touch if you would like one or more of those. The board has an additional removable socket, so you can sit the board lower in your machine if required, and it also protects the pins, being easily replaceable if you should bend and break one.

Thanks for looking, any questions, please ask!

** UK POSTAGE BY ROYAL MAIL **

** Worldwide shipping by the eBay Global Shipping Program **

We also supply GOTEK USB drives for the BBC Micro, as well as Raspberry Pi multi-core Second processors in internal and external versions.
As well as 1770 Disk Upgrade kits, 4 way switchable OSs for the Master 128 and 2 way for the B+, we can supply other refurbished BBC Model Bs and Master 128s to your specification.
Please check my other items for sale, or if they've sold out on my listings, please contact me for details.