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This chapter explains briefly what an operating system is and what it does. It then introduces a specific operating system, DOS Plus, and describes some of its main features. Finally it shows the relationship between DOS Plus and the BBC Master.
An operating system gives you control of the computer without having to learn complex sets of instructions. It enables you to manipulate files easily. It accepts characters from the keyboard and displays them on the screen. It controls the organisation and cataloguing of files on a disc and it gives you a flexible means of copying, rearranging or even erasing files. The operating system is a powerful tool that enables you to concentrate on the task you want to do, without worrying about how it is done or understanding the internal working of a computer.
There are many different operating systems each with their own particular features and accepting a specific range of instructions. But they all give you more or less the same facilities and they're used in much the same way. You instruct the computer to perform a task by giving it a command or a series of commands.
Some operating systems have become widely known and widely accepted because they are issued in different versions to suit a variety of computers. This is obviously a great advantage. It means that once you're familiar with a set of commands, you can use them on a number of different computers and achieve identical results. It also means that you don't have to worry about differences in hardware; the operating system will reliably and consistently interpret your commands and behave in a predictable manner.
However what matters to most users is not the operating system so much as the quality and variety of the applications software available. That is, the software that performs specific tasks: word processing, stock control, accounting and so on. The principle benefit of using a popular operating system is that a number of major software companies will have developed applications for it. This means that the applications are probably well designed, that you have a wide choice and that you can use the same application on a variety of computer systems.
Two operating systems have achieved particularly wide acceptance. The first is CP/M, developed by Digital Research Inc. and the second is DOS (Disc Operating System) introduced by IBM for the IBM Personal Computer. Because these two operating systems were adopted by a number of computer manufacturers, a wide range of applications has been developed and marketed that will run under versions of CP/M or DOS.
The availability of the applications led in turn to the development by Digital Research of a combined operating system, DOS Plus, that effectively offered the facilities of both DOS and CP/M. With one operating system, you can run either the applications that were originally developed to run under DOS or those that were developed to run under CP/M.
The DOS Plus Operating System allows you to select from an increasing range of commercial software applications to run on your BBC Master. For instance you have access to a number of sophisticated DOS and CP/M based application programs that were originally written for the IBM and other personal computers. However there can be compatibility problems and before buying a particular application you should check that it will function correctly on your BBC Master.
When the BBC Master is running a DOS or CP/M based application parts of its own operating system are disabled and replaced by appropriate parts of CP/M or DOS. This means that the Master no longer responds to the BBC range of commands but it behaves instead as a CP/M or DOS based personal computer.
However the facilities of DOS Plus have been extended to allow you to take advantage of many of the Master's facilities. For example you can use the mode and colour facilities and these are covered in later chapters of this manual.
The original CP/M and DOS operating systems have many features in common, and some fundamental differences. DOS Plus, by virtually combining the facilities of both, exploits the common features and enables you to ignore some of the fundamental differences. For example, CP/M and DOS store data in different formats – and one operating system will not normally read from or write to a disc formatted for the other. However with DOS Plus you need not concern yourself about the different formats. The common features are described in the next section.
With DOS Plus the physical drives are identified by the letters A and B. If, in addition, you have a hard disc, this is identified as drive C. DOS Plus treats the whole of the disc surface as a single drive, so drive A reads and writes files on both sides of one disc and drive B reads and writes files on both sides of another disc.
The DOS Plus prompt is normally
A>
which indicates that A is the currently selected drive. In common with most operating systems DOS Plus has a number of default settings. A default is a value or choice that will automatically be made unless you specify something diferent. Therefore when you load DOS Plus it will automatically default to drive A, or drive C if you have a hard disc. Changing drives is covered in Chapter 2.
Communication with DOS Plus is through a set of instructions called commands. These commands are really just a set of program names. You enter the name of the command and the appropriate parameters, and the program is executed.
For example if you want to erase a file which you have called TEMP.TXT, you could issue the command
A>ERASE TEMP.TXT RETURN
where ERASE is one of the commands that you can use to delete a file.
Generally speaking, filenames can be any combination of up to 8 letters and/or numbers. So you could have files called FRED, FRED1, ACCOUNTS or MAILSHOT. The filename can be followed by a full stop and an optional extension of up to 3 characters. The extension is often used to indicate file types, say BASIC files, text files, GEM files, COBOL files and so on. Therefore the previous files could be named, FRED.GEM, FRED1.BAS, ACCOUNTS.COB, MAILSHOT.TXT
Chapter 2 explains in greater detail how you can use these file identifiers.
The erase command used in an earlier example was shown entered in capital letters but it could have been entered as
A>ErAse TemP.txT RETURN
because DOS Plus ignores the distinction between capital and lower case letters.
Files with names such as TXT, T.T, TEM.TX and Temp.tex will all be correctly identified as different files. But remember that temp.txt and TEMP.TXT will be identified as the same file.
Although DOS Plus ignores the case of letters you do have to be very precise about spaces (and other punctuation characters) because these are interpreted as separators between items. For example:
A>ERASE te mp.txt RETURN
would give you an error message because the space after te would be interpreted as the end of the filename and DOS Plus won't find a file named te. Of course if you do happen to have named a file with the abbreviation te DOS Plus would act on the command and erase it.
To summarise, a filename can be up to 8 characters with an optional 3 character extension that usually indicates the file type. But to specify a file completely you should also identify where it is. In other words you should indicate which drive holds the disc containing the wanted file.
Therefore to specify the file TEMP.TXT you should really define it as A:TEMP.TXT, where the A: indicates that the file is on drive A. In practice if your selected drive is A (confirmed by the prompt A>) you can omit the A: from the filename and DOS Plus will automatically look for the file on drive A. However if TEMP.TXT was in drive B you would need to specify B:TEMP.TXT or DOS Plus would fail to find the file.
Extending this idea a little further, you can use a single command to perform several operations. For example
A>COPY B:temp.txt A:final.txt RETURN
indicates that you want DOS Plus to copy TEMP.TXT from drive B to drive A and name it FINAL.TXT. Remember that the spaces mark where one file specification finishes and the next one starts. If you put an extra space after B: or A: the command will fail to produce the desired effect.
In this manual we have basically to set out the examples of commands in the form that you would probably use. That is we've used lower case letters for filenames and we've left out anything such as drive letters that would be assumed by default. To ensure there is no ambiguity, we have included the prompt A> in each command, although you would not type that in. You might in fact be working with drive C (that is with C> as the prompt).
Anything you type in is shown in a different typeface on a separate line, like this
A>ERASE fred RETURN
The RETURN symbol means press the key on the keyboard marked RETURN.
For clarity we have shown commands in upper case and filenames in lower case as in the above example, but you would probably enter
A>erase temp.txt RETURN
The next chapter covers loading and starting to use DOS Plus and it discusses the concept or default drives in more detail.