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Master 512 Forum by Robin Burton
Beebug Vol. 8 No. 10 April 1990

After the last two Forums which were pretty easy going, it's time to move on to more pithy matters.

The next three Forums are the direct result of reader's letters. By the way, I haven't said it for quite a while, but if there's something you'd like to see in the Forum, or a view you wish to air, write to me and say so!

The first topic is for the benefit of those who know little or nothing about the 512, while next month's Forum is for those of you who are fairly experienced and a bit adventurous too. Read on if you want to know more.


The first letter was from Graham Lowe, who says he reads the Forum every month even though he doesn't own a 512. This of course is very gratifying, but the main point of his letter is that while he finds many topics fascinating (his word, not mine), they would be even more so if he knew more about the 512. Specifically, 'what exactly is it and what does it do'. The other point he makes is that he might well be tempted to buy a 512 if he knew more about it. Could I therefore spare a bit of space in the Forum to explain?

Although many 512 users will know most of this, I thought Graham's letter made a reasonable request. It's easy to overlook if you're not in this category, but there may be many BBC micro users who know of the 512 but who, if they have no dealings with PCs in everyday life, aren't likely to know much else. In that situation they're not likely to buy a 512, even secondhand, because they can't appreciate the machine's merits and capabilities.

Perhaps you knew and had used CP/M and 86 series PC machines when you purchased your 512. Maybe you found a dealer who was keen to make a sale and was prepared to demonstrate. Well, of course it's easy then, but Graham's letter made me consider the fact that not everyone automatically falls into the first category, while these days it's unlikely that anyone falls into the second. As I said, I think it's a perfectly reasonable request, so here goes.

This month's Forum is aimed at those who know little or nothing about the history of DOS and PCs. It is, I hope, entertaining for all readers, but above all I hope it's a reasonable insight into why the 512 exists and what it provides for those who may perhaps yet become 512 users.


We all know that home micros come in many shapes and sizes and from various manufacturers. In the main, apart from particular cases like the BBC and the Z88 portable, most of these machines have little or nothing in common in terms of operation, facilities, commands, disc formats, program code and so on. Of course all these machines work in broadly similar ways, but nothing is directly compatible with anything else.

For home micros this is acceptable, as virtually every user is isolated and, apart from BBC micros, the majority of machines are used to play games, but it's a far from ideal situation in business.

Digital Research recognised this problem very early on in the micro story which began in the 1970s. They therefore designed an operating system to overcome this limit to growth. During that decade, as processor power began to grow to useful levels, Digital Research virtually cornered the business micro operating system market with something called CP/M (CP/M by the way stands for 'Control Program for Microcomputers' – how's that for imagination?).

CP/M was 'designed from the ground up' with the objective of becoming the 'de-facto' standard business operating system for microcomputers (they weren't called PCs in those days). To achieve this, Digital knew that a revolutionary modular approach to system design was required, especially if the system was to be capable of growth as well as running on almost all machines without throwing everything away and starting again each time.

The crux of the problem was that, although CP/M would run in an 8 bit Zilog Z80 or Intel 8080 based machine (because these were the two processors of the day) various parts of the hardware in different manufacturer's micros (e.g. the discs, the filing system controller, the screen and keyboard) were still different, just like home micros are now.

However, the deliberate modular design of CP/M meant that only one part of the operating system needed changing to allow it to run on a new manufacturer's hardware, specifically that part concerned with basic input/output functions. This module is called the BIOS (Basic Input Output System – more imagination at work here). In fact to make it as easy as possible for manufacturers, Digital supplied their operating system to hardware manufacturers in a sort of kit form.

The parts of the BIOS which control the peripherals were therefore customised by the manufacturer to suit the particular hardware devices employed, while the rest of the operating system remained fixed. Each manufacturer provided only the peripheral handling code for the BIOS and could then very simply generate a complete new CP/M operating system with the tools supplied.


Software houses were naturally quick to see the opportunities this approach offered. It meant they could write an application for any type of CP/M machine and it would work in virtually all of them immediately. This was because the application's interface to the operating system no longer changed with the hardware, only the input/output part of the operating system which drove peripherals did. Of course applications (should) only communicate with the operating system by legal calls, and if they did compatibility was virtually guaranteed.

The result was that software wasn't tied to specific makes of hardware, only to an operating system which could be customised to suit almost any machine of the day. The converse was that hardware manufacturers felt obliged to support CP/M if their machines were to be capable of running this ever growing pool of business software.

Business users were keen on the idea too. Now they weren't tied to a single type of machine or to a single hardware or software supplier. Any CP/M machine could run any CP/M program in the same way with the same user interface regardless of who manufactured the micro and who wrote the software. The personal business computer (as opposed to the anonymous mainframe which 'belonged' to no individual) was created, or perhaps more accurately it was inevitable.

At the beginning of the eighties 16 bit processors appeared commercially, which allowed more memory to be provided on 'small' micros at less cost (8 bit processors, like the BBC's 6502 or the 80 series can only address 64K bytes, while 16 bit processors can address up to 1Mbyte). For these new processors a new operating system was needed of course, and with the huge success of CP/M proving that the design was absolutely correct it was repeated for these new processors.

In the event the new 'standard' operating system has turned out to be DOS rather than CP/M(86). Essentially, after the brilliance of CP/M, Digital sat on its laurels a bit too long and missed the opportunity of a single-handed encore with the new processors. Even so, if you investigate a little you'll find that all versions of DOS, including MS-DOS and PC-DOS are direct descendants of CP/M with precisely the same internal design concepts as the original, even to the point where many operating system calls are exactly the same.


IBM was keen to get into this now rapidly expanding market and so with the arrival of 16 bit processors they produced the IBM Personal Computer, but instead of selling hardware they marketed a concept – the PC. They originally intended to offer a choice of at least two operating systems with their new PCs, but as Digital were a bit slow this time it didn't happen. Instead only one, DOS, produced by a then small, almost unknown software house called the Microsoft Corporation was offered, though in a modified form to suit IBM hardware. This was, and still is, PC-DOS.

Other manufacturers began to produce PCs, also using DOS, this time the original Microsoft offering, so a second version of DOS was assured. This one, with the by now expected flair, is called MS-DOS. You can work the name out for yourself.

Right up to the present, IBM uses PC-DOS and all PC clones (everything except IBM machines) use MS-DOS. In fact apart from the hardware there's almost no difference between them. Whenever a new version of one appears the other follows almost at once, so they keep facilities in step and even use the same version numbers. Digital Research, though late into this battle, also produced their own version of DOS, which they called DOS Plus, though the current version is now called DR-DOS.

Just like the original design of CP/M, all versions of DOS operate in the same way in all machines as far as both the user and the application is concerned. The BIOS is customised to suit the hardware, while the rest of the operating system and its facilities remains fixed. If code is legally written it will run in any DOS machine.

This more or less brings us up to date. DOS continues to develop, but more slowly now than in its first few years, and over the (nearly) ten years of its existence as the standard PC operating system, literally tens if not hundreds of thousands of programs have been written covering just about every conceivable application.


So where does the 512 fit in? Simple – it allows BBC users to run a version of DOS which, apart from some difficulties with programs that aren't strictly legal (caused by unavoidable differences like the two processor configuration) extends the same facilities to Acorn users as are enjoyed by PC users, without the need for the considerable expense of a 'real' PC.

Remember that when the 512 appeared Amstrad hadn't entered the market, so the 'cheap' £500 PC didn't exist. In the early 80s even the most meagre PC would cost upwards of £3000. Things have changed somewhat in the last few years, but even now a PC to match the 512's performance will cost around £1000 and quite possibly more unless you import it from Taiwan yourself. At its original price the 512 was a snip. Now second-hand prices seem to be increasing, reflecting the almost cult following the 512 has developed. That said you can still buy a 512 for less than (just) the price of a second-hand model B, but look at what the 512 can do.

It's true a few business machines don't use DOS even now, the Apple Macintosh for example, and in larger systems UNIX tends to be used. Even so, it's almost certain that, worldwide, PCs using DOS outnumber all other types of computer put together. I don't know how many PCs are estimated to be in use, but it's certainly tens of millions.

This explains the attraction of the 512. For a modest outlay the BBC micro owner can tap into the biggest pool of software the world has ever seen, with enough memory and enough processor power to tackle jobs that the BBC micro, excellent though it is, can't even think about (and you don't even have to give up your BBC micro to do it).

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