Yellow Pig's BBC Computer Pages

The BBC Master 512

About the 512 | Outline | Bibliography | Problems | Extra Info | Software | Photos | Index

Outline of the Master 512 System

The BBC Master 512 was intended to be a computer that could be operated as a BBC Computer and also as a PC-Compatible. The main items that make this work are, apart from the BBC Computer itself, a co-processor board based around the 80186 chip and four disks of software. (You also need at least one double-sided, 80-track floppy disk drive, and in practice you need either two such drives, or one plus a hard disk.)

The co-processor board could be sold ready-installed in the BBC Master (and in this situation the name "BBC Master 512" was at least slightly approriate). Alternatively it was sold separately for installation inside the Master's case. This way the co-processor is attached to the "Internal Tube".

Some manufacturers produced an external co-processor adapter box, and the 512 board could be installed in one of these. By doing this it could be used with the BBC Model B or B+, or it could be attached to the "External Tube" on a BBC Master. (The host must have a WD1770 double-density disk controller, and it needs to have ADFS installed in order for DOS-Plus to boot up. It won't work with the original Model B's 8271-based disk interface.)

With the co-processor switched off (or on a Master after *CONFIGURE NOTUBE) the machine is exactly a BBC Computer.

When the co-processor is switched on it runs (approximately) as a PC-compatible. The hardware is quite different from a true PC, so total compatibility was perhaps never going to be possible. However, given the complexity of the task, a surprisingly good attempt was made.

The aim was for the Master 512 to be compatible with the original PC or XT (based around Intel's 8088 chip) or similar compatibles based around the 8086. Of the early display adapters available for the PC, the Master 512's display is designed to be compatible with the "Color Graphics Adapter" (or CGA). The software provided is meant to be compatible with MS-DOS Version 2.

In a Little More Detail


Software in the Co-processor ROM

The ROM enables DOS-Plus to get started, but also provides the 80186 Monitor. To enter this press <Break> (without the <Ctrl> key), and you will get a "*" prompt. Here you can enter BBC MOS commands, just as you would following "*" on the standard BBC Computer. The Monitor also has a few commands of its own, though mainly designed for debugging the system.

For most users the only practical use of the monitor is to enable you to enter *CONFIGURE NOTUBE on a Master, and thus switch the co-processor off to return to a native BBC Computer.


This is the software that most users will want to have running most of the time. To boot DOS-Plus simply have Disk 1 in the first floppy drive, or have it already installed on a hard disk, and press <Ctrl-Break> (or just switch on).

DOS-Plus was an operating system produced by Digital Research Inc, and was designed to be compatible with both MS-DOS and CP/M-86. (CP/M-86 was the earlier operating system that Digital Research had designed for the PC, building on its design of CP/M – already a standard for 8-bit business computers. If history had unfolded differently, CP/M-86 might have become the standard operating system for PCs. In the event, as we all know, Microsoft's system was used instead, and hardly any programs were ever written for CP/M-86.)

Master 512 DOS-Plus aims to be compatible with MS-DOS Vn 2, and on the whole it is. It comes with a different set of support utilities from MS-DOS, but generally they allow you to do the same sorts of things. All the file manipulating activities possible with MS-DOS can also be done with these – they just have a different command-line syntax sometimes – and there are a few extras allowing files to be written to or read from a BBC format disk, for instance. The utilities are well documented in Chris Snee's User Guide.

The system also allows some extra floppy disk formats over and above those catered for by MS-DOS. The latter uses 5¼" floppy disks in 360kb format, or (in version 3.2 onwards) 3½" disks in 720kb format. (These are the "double density" (MFM) formats, before the use of "high density" became common.) Master 512 DOS-Plus can cope with both of these formats on either size of floppy disk. It can handle CP/M disks too and also has two formats of its own – a 640kb format which is a kind of hybrid of DOS and ADFS and is used to boot the system from floppy disk, and an 800kb format that is both bigger and faster than any of the others.

DOS-Plus was first released for the Master 512 as Version 1.2. A later version was Vn 2.1 (a free upgrade for those who had the earlier one). This had some of the bugs in the earlier release sorted out, and included a couple of extra utilities. There was also an intermediate version usually referred to as Version 1.2a. This was the same as Vn 1.2 with a slight modification to allow a larger memory, and with one seriously bug-ridden utility omitted.

All of these versions of DOS-Plus are designed to be compatible with the same version (Vn 2) of MS-DOS. There are, though, some minor problems which, together with the hardware differences, mean that some DOS programs will run on a true PC under MS-DOS Vn 2 (or earlier) but will not run properly on the Master 512.

There was never any attempt to release an operating system for the machine compatible with any later version of MS-DOS, and because of the hardware differences MS-DOS itself cannot be used.


GEM is a Graphical User Interface (GUI) designed for the PC by Digital Research, and supplied for the Master 512. After booting DOS-Plus, just insert Disk 2 in the current floppy drive and enter GEM at the DOS command line (or simply press f0). GEM can also be installed on the hard disk.

The way it works is quite intuitive, and pretty obvious to anyone used to more modern computers.

GEM runs by default on the Master 512 in monochrome mode. There is an alternative (and in my view improved) monochrome screen driver supplied in addition, and GEM can also be run in 4-colour mode. See my GEM Configuration document for details of how to use either of these.

The GEM supplied with the Master 512 is Version 2. It is a special version for the machine, slightly improved over the standard one because it will use the 256 (pixel) lines possible on a BBC Micro screen rather than the 200 lines standard on a PC with CGA. It comes with an integral mouse driver.

Master 512 GEM comes with two programs supplied, GEM Write and GEM Paint, in addition to the Desktop and Output utility. Digital Research produced a number of other programs to go along with them and those can be used on the machine too.

A few other packages were released to run under GEM, notably Timeworks, the DTP package from GST. (Some of these may require GEM Version 3. This can be installed on the Master 512.)

GEM was, of course, eventually eclipsed by Windows. No version of Windows will run on the Master 512.



The Master 512 uses the 80186 processor. Although not a commonly used processor in PCs, it is totally compatible with the 8088 (as used in the original PC and XT) and the 8086 (as used in many early PC-compatibles). It is also rather quicker. Not only do many of the instructions take fewer clock cycles, but it runs at 10MHz as against the rather eccentric 4.77MHz used in the original PC or the 6MHz or 8MHz common in many 8086 machines.


The other main constituent of the co-processor board is the RAM memory, and as the name indicates, there is 512kb of it.

This is actually a little bit misleading when compared with genuine PCs. Memory size quoted for PCs does not include the memory in the screen adapter. In the Master 512, however, 64kb (out of the 512kb) is used in place of the screen adapter memory. In addition, because it includes all the CP/M-86 code, DOS-Plus takes up a little over 25kb more memory than does MS-DOS Vn 2. This means that in effect the Master 512 has about 90kb less memory than you would expect. It is equivalent to a PC with about 420kb of RAM.

Compared with the earliest PCs, which came with a total of 16kb to 64kb of RAM, this seemed plenty. But very quickly PC-compatibles started being produced with far more RAM, and before long the maximum of 640kb became the standard.

Quite a lot of programs were released which required this maximum of 640kb or near it, and these will not run on the Master 512.

It was possible with very little modification to extend the memory of the Master 512 to 1024kb (or 1Mb), and a couple of companies offered this extension. Using this the machine has as much memory as any 8086-based PC, and there are some programs that can use the "Upper Memory Area" (above the screen memory) too.



The Master 512 came with a mouse that plugs into the BBC's User Port.

The supplied GEM software includes a mouse driver (as part of the screen driver file) and so GEM can use the mouse.

No other mouse driver was provided with the Master 512 and, because the connection is totally different from any PC, no standard mouse driver will work with it. It is possible to write a mouse driver for the Master 512, and one is available here.

Keyboard Annotation

Provided with the machine was a key correspondence chart for the Master's keypad (with a reminder that the BBC's "COPY" key works like the PC's "Alt"). Also there is a function-key strip for use with GEM Write. These easily get lost, so I have made them available here and they can be printed off.

In Summary

In some ways the Master 512 was an improvement on the genuine PC. It ran faster, both because the 80186 chip was faster than the 8088 or 8086 (in the clock rate and also in the number of cycles taken for each instruction), and also because the 6502 in the BBC Micro host could handle some of the processing. It could get more data on a single floppy disk. It could run CP/M programs as well as those written for MS-DOS. It could display more lines on the screen.

It was also, in the mid-1980's, quite a bit cheaper.

However, none of these advantages really outweighed the problems of not-quite-compatibility and lack of upgradability.

About the 512 | Outline | Bibliography | Problems | Extra Info | Software | Photos | Index