Year Of The Disc Feature

Discs for beginners

Volume 2

Number 01

January 1984

All the facts you need to get you started

JIM NOTMAN answers your basic questions

RECENTLY there has been a great increase in interest in discs and disc drives for the BBC Micro. Many owners feel that once they have mastered the cassette filing system they need to move on to something better.

Why use discs?

The most impressive feature of using any disc system is its speed. A program which could take many minutes to load from tape takes only a few seconds from disc.

Even if it is the last program that was put onto the disc it is found very quickly. Contrast that with a tape with several programs on it. It may take a considerable time to just find the program.

Disc systems allow the more flexible use of files, like relative (random or direct) access files.

Tape files need to read from the start. Disc files can be read from or written to any position you like. Just as with a long playing record, you don't need to start from the beginning, but from the track you require.

Disc systems load and save with far fewer errors than on the cassette system.

Converting to discs may increase your use of the micro. It's rather offputting waiting 10 minutes to load some software from tape when you have only half an hour to spare.

Are there disadvantages?

Cost is the main one. About £300 for the simplest system, though by keeping your eyes open (looking through The Micro User!) you may pay considerably less.

The disc system reserves 2 3/4k of memory for its own use. In a cassette-only system, the memory from 3584 (&E00) is available for program use. In the Acorn disc system only the memory from 6400 (&1900) is free.

Discs need to be looked after more carefully than cassettes as they are more vulnerable to damage. A fingerprint or a piece of cigarette ash could be all it takes to ruin a disc.

Many software companies have been slow to issue their programs on disc. The excuse has been that there are not enough disc users to make it viable.

This is no longer true, as a large number of BBC Micro owners are converting to disc.

What do I need to get started?

Disc interface.

Disc Filing System (DFS). This may come with the interface.

Floppy disc drives.

Connecting leads.

Utilities disc.

Blank discs.

Why do you need an interface?

The interface can be thought of as an electronic translator which allows communication between the BBC Micro and disc drives. While most of the components cost only a few pence each, the floppy disc controller chip costs more than £40.

The Acorn interface includes the DFS, which is plugged into one of the sideways ROM sockets. It does not, however, come with a manual. The "official" BBC manual is only issued with BBC disc drives.

The standard interface is single density. This refers to how closely the information is packed on the disc.

At least two firms are working on a double density system (requiring a different floppy disc controller) which can put twice as much information onto a disc.

These systems need to have their own special DFS to instruct the different disc controller. (This is not the same as having 40 or 80 track drives.)

Why do you need a filing system?

Whenever the BBC Micro is switched off any programs or data that were in RAM are lost. If you want to keep any of this information it must be recorded in some way. File systems allow you to do this.

Since each method of saving the information is different, each must have its own filing system.

The DFS interprets any filing commands you give and carries them out without you having to know how it does it.

The operating system of the BBC Micro is arranged so that commands are not changed with filing system changes. Any command associated with filing is directed to the correct filing system.

Changing the file system is very easy:

For the DFS type in *DISC (or *DISK).

For the tape system *TAPE (for 1200 baud), *TAPE3 (for 300 baud).

For the Econet system *NET.

Will I have to learn a lot of new commands?

All the commands that you learned for the cassette filing system will work equally well with the DFS.

In addition there are extra commands just for the DFS.

*HELP (Return) gives a list of the paged ROMs that are in the BBC Micro.

*HELP DFS (Return) lists the extra DFS commands, as illustrated in Figure I.

DFS 0.90
ACCESS <afsp> (L)
BACKUP <src drv> <dest drv>
COMPACT (<drv>)
COPY <src drv> <dest drv> <afsp>
DELETE <fsp>
DESTROY <afsp> DIR (<dir>)
DRIVE (<drv>)
LIB (<dir>)
RENAME <old fsp> <new fsp>
TITLE <title>
WIPE <afsp>
Figure I

Most of the commands manipulate program or data files that already exist.

Many are self explanatory like *RENAME for changing a files name, *COPY and *DELETE. *TITLE allows the disc itself to be given a name.

*DESTROY seems somewhat over dramatic, but is able to delete a number of files at the same time.

*INFO gives information about the file like its load address and where the file is physically located on the disc.

As well as the main DFS commands there are a number of disc utilities. The commands are given by *HELP UTILS, illustrated in Figure II.

DFS 0.90 BUILD <fsp>
DUMP <fsp>
LIST <fsp>
TYPE <fsp>
Figure II

DUMP gives the hexadecimal value and Ascii equivalent of each byte of a named file. This can be helpful if you like to see what is being put onto the disc.

Discs can be made far easier to use than tape. The DFS will look for a file called !BOOT when the Shift key is held down together with the Break key.

This allows programs to be loaded and run that is, if you've given the disc the correct instructions!

Are there other DFSs?

There are a number of alternatives to the official Acorn 0.90 DFS. Watford Electronics and Amcom both produce their own.

One of the main problems with these others is software compatibility. This is especially true of software packages which employ software protection.

The main advice here is to ask the dealer whether the programs you buy are compatible with the DFS you have.

Why bother with alternative DFSs?

They usually have a number of features in them which are lacking in the Acorn version, some of which are very useful.

For example, with the Acorn system only 31 different files can be held on one side of a disc. Several different DFSs increase this limit to about 60.

Can I copy my programs from tape to disc?

With Basic programs this is very easy. Select the tape system with *TAPE, LOAD the Basic program, select the disc system with *DISC then SAVE the Basic program.

If this program is now too long because of the memory space the DFS has reserved for itself, move the program down in memory using the following:

1 *KEY 0 *TAPE¦M N%=PAGE-&E00 : FOR L%=PAGE TO TOP STEP 4 :!(L%-N%)»!L% : NEXT : PAGE=&E00¦M OLD¦M

Machine code programs are a little more awkward. They must be put into exactly the same memory locations they would have occupied in the cassette system if they are to work correctly.

First switch to the tape system with *TAPE. Now type in *OPT1,2.

Looking at the program with *CAT will give you the load address of the program, the execution address (where a machine code program is run from) and the length of the program.

Once these details are known the program may be loaded into memory above the area the DFS reserves, then moved to the memory locations it would have occupied with the tape system.

All that remains is to CALL the execution address.

Further details on transferring cassette programs to disc are given in the August issue of Micro User.

Not all programs can be copied easily, because many companies are now using some form of software protection.

I've heard of hard discs and floppy discs. What's the difference?

Nearly all discs used with the BBC are floppy discs, so called because they are thin flexible discs of plastic coated with a magnetic recording emulsion.

Hard discs are much more sophisticated. A unit consists of a number of magnetic coated metal discs, mounted in a sealed unit and turned at high speed.

The reading and recording heads "float" a very small distance above the disc. Even very small particles of dust, human hairs or tobacco smoke may damage the discs if they come between the heads and the disc surface. Hence the sealed unit.

They are more expensive than floppies but can hold much more information, several megabytes rather than the hundreds of kilobytes the floppies have to offer.

A number of companies are working on hard discs for the BBC, though in several cases the software seems rudimentary. Hopefully this should be resolved shortly.

What sorts of floppy disc drives can you get?

It is possible to use the BBC Micro with 8in floppy discs, a size associated more with larger or older computers.

More recently 3in micro floppies have started to appear, though there is more than one type of these. They are very compact and should prove popular, but at the moment there is little software written on them.

The commonest drives use the 5 1/4in mini floppy disc. Whatever the size of drive, the information is written on it in circular tracks.

There are two main groups of disc drives, those having 40 tracks and those with 80.

The 80 track machines are more expensive because their tracks are much narrower and therefore must be made more accurately. A 40 track disc can store 100k of information on one side, an 80 track disc 200k.

Some drives use only one side of the disc and are known as single sided. Others which use both sides are called double sided.

Then you may have a single drive or two drives in the same box. These are known as dual drives.

There is also a choice of having either a power supply from the BBC itself or having a separate supply in the drive unit. As there are a number of combinations the choice becomes bewildering.

Which disc drive should I get?

For small business use you really need to use an 80 track system with dual drives, preferably double sided.

For home use the main consideration will probably be cost. 40 track drives are perfectly adequate. Even the least expensive, a single sided, single 40 track drive is better than not having discs at all.

If you can afford dual drives you'll find them more useful, especially when using data files from different discs and file copying.

I prefer drives which have their own separate power supply as this leaves the power point on the BBC for other uses.

Do I need a utility disc?

This will depend on which DFS you buy. To be able to use a new, blank disc it must be formatted.

This involves putting a number of magnetic marks on the disc so that the floppy disc controller knows where it can place or read data.

Some of the alternative DFSs have a format command included in the DFS ROM - unlike the Acorn system where the format program is on a utility disc.

The other useful utility needed is VERIFY. This looks through the disc checking that it can be correctly read.

Apart from the basic utility disc, you may want some other utilities.

One that is particularly useful is a package which transfers tape programs onto a disc. Even many programs which are software protected can be transferred, though not all.