David Spencer looks at a revised operating system for the Master 128.

Product: Alternative Master MOS Supplier: Acorn Computers Ltd., Fulbourn Road, Cherry Hinton, Cambridge CB1 4JN. Tel. (0223) 245200 Price: 44.85 (inc. VAT) 42.71 to Beebug members

Just as many were saying that Acorn only cared about the Archimedes and beyond, the company has bitten back by releasing a revised operating system for the Master 128. The alternative ROM, as Acorn refers to the new MOS, is a plug in replacement for the standard ROM, and updates all the system software with the exception of the ANSI terminal emulator Terminal.

Installing the alternative ROM is little different to installing any other ROM inside the Master. Having removed the top cover of the computer, you have to identify and remove the system ROM from its socket. On some early systems, this is attached to a small carrier board, and in a very few it is soldered directly to the board. In this latter case, the machine must be taken to an Acorn dealer who will unsolder the ROM and install a socket in its place. However it's done, once the old ROM has been removed, the new one simply plugs in, and that is the installation operation over.

There are far too many changes and new features incorporated in the alternative ROM to list them all here. Most are either minor, such as modifying *SHOW to display all the function keys if no argument is given, or fix bugs in the original MOS, such as the infamous CLOSE #0 bug in the DFS (see last month's Beebug). However, there are a number of major changes and additions.

Perhaps the area of most change is ADFS. For one thing, this has speeded up by a factor of at least two when using floppies. Furthermore, the Format, Verify and Backup utilities are now included in the ROM, meaning that you can throw away the dreaded Master utility disc. *COMPACT within ADFS has also been simplified. You no longer need to specify an area of memory that can be used, and the call does not corrupt the user workspace. DFS has been modified to speed up the OSGBPB call and to allow files longer than 64K to be saved.

Basic has been modified by using improved floating point arithmetic and trigonometry routines. These can result in appreciable speed increases in programs that perform a lot of floating point computation.

View now features a built-in Epson printer driver (as on the Master Compact) and also allows the justification, formatting and insert/overtype status to be configured in CMOS RAM. Edit has been extended to allow files to use either carriage returns or line feeds as terminators, and a new key command has been added to switch between these. The annoying feature that Return had to be pressed after entering the mode number for the change mode command has been removed, and a print "As Is" option has been added to bypass the normal formatting commands that are applied when text is printed. Finally for Edit, an "End of file replace' option has been added to stop search and replace from prompting for each change. This is particularly useful when specifying a complex matching template, as you can try it on the first few occurrences, and then apply it globally to the rest of the file when you have checked that it is correct.

Another major change is the addition of support for 8-bit keyboard codes. Basically, this allows international characters to be entered from the keyboard. However, the alternative ROM does not, unlike the Master Compact, provide this directly. (The Compact has a Code key which allows international characters to be entered.) Instead, it offers a set of 'hooks' which attach to another ROM, called the International ROM. Acorn will make this ROM available if it is needed.

Finally, all the known bugs in the standard MOS ROM have been fixed.

The Relocator is one feature new to the alternative ROM which will only be of benefit to owners of 6502 second processors (or the Master Turbo board). To understand its purpose, and how it works, you need to look at the way in which the memory map of the Master differs from that of the second processor.

In the Master, any 16K language ROM occupies the memory between addresses &8000 and &BFFF, with the memory from &C000 to &FFFF being used for the operating system. On a second processor, the operating system is only 2K long, and occupies memory from &F800 to &FFFF. Hence, if a ROM is copied to the second processor at its address of &8000, the 14K between &C000 and &B7FF is wasted. What is needed is to move the ROM image up to start at address &B800. This will close the gap, and allow the extra memory below the image to be used. However, the situation is not as simple as this, as instructions in the ROM image that refer to absolute addresses within that image must be changed. This is the function of the Relocator, which uses a 'bit-map' to determine for each byte in the ROM image whether it needs to be altered or not, and by how much. All this is done automatically as the image is transferred to the second processor.

The alternative ROM uses the Relocator to move-up Basic and Edit (eliminating the need for HiBasic and HiEdit), but not Viewsheet. View includes its own relocator which performs a similar function.

Acorn have decided not to include the alternative ROM as standard in new Masters. The reason, they say, is that while the new ROM is highly compatible, there will be some existing programs that don't work with it. In particular, the new 8-bit key handling may confuse some programs, and any program which directly accesses routines within the ROM will almost certainly fail. I feel that Acorn have made a wise decision, considering the amount of software already available for the Master, but I don't think that you should be put off by incompatibility - in practice most programs will work.

The alternative ROM is accompanied by a 26-page A5 user guide, split into four sections. The first of these is an introduction including fitting instructions, while the second and third sections describe the changes and new features from the user's point of view, and a technical view point respectively. The final section explains the workings of the Relocator, and shows how you can write self-relocating ROM images. The listing of a program to do this for you automatically is also included.

The documentation is designed to be used in conjunction with the two-part Master Series Reference Manual, and is totally adequate for describing the alternative ROM.

Acorn should have released the alternative ROM a year ago - a move that would not have been impossible, as a number of people, myself included, had test versions as long as two and a half years ago. It is a fact of progress that a large number of the more "active' Master users, who would have probably been the first to buy this product, have now shelved their machines and moved onto pastures new, such as the Archimedes. However, having said that, I feel that Acorn have done themselves no harm releasing the new ROM now, and they may well have caught out a few of their critics.

Whether you actually need the alternative ROM or not depends a lot on what you use your Master for. In some cases you may not benefit at all, and may even suffer if you spend most of the time using one of the very few packages that is incompatible with the alternative ROM. On the other hand, I would say that both second processor owners and regular ADFS users should put their order in the post today as they will benefit highly from the new features.