The AP4, from Advanced Computer Products, is yet another disc system for the Electron. There are already three, the Acorn Plus 3, the Cumana Disk Interface and the Solidisk one, so why bring out another?

Well, each has its own advantages and disadvantages. Each works in a different way and none are compatible with each other. This means, for instance, that if you have one disc system you can't swap discs with a friend who has a different system - without a lot of hassle, that is.

ACP's offering is an Acorn-style creme coloured interface that plugs into one of the Acorn Plus 1's ROM cartridge sockets. It's the same height and depth as an ordinary cartridge but about twice the width, making it quite a neat unit since most of it disappears into the cartridge slot.

This is more important than you might think because unfortunately, once you start to expand your Electron, you'll find it can take up quite a large amount of desk space. Believe me, that old joke about the Electron becoming so big that it's falling off the back of the table is true.

At the rear of the AP4 is a standard socket to take a disc drive, identical to the one on a BBC Micro. You can use 40 or 80 track 3.5" or 5.25" drives, double or single sided, provided they have their own power supply, so there is plenty of choice.

On opening up the AP4 you'll find four sockets, three are filled and one empty.

A WD 1770 disc controller is fitted in the first. It's quite popular since it is relatively cheap and can be used in single or double density mode, which crams more on a disc. This is the same as used in the Acorn Plus 3, the BBC B+ and the new Master 128 series.

Next comes a standard ROM socket. ACP will tell you it's for ADT, their Advanced Disc Toolkit ROM, but it can be used with any available Electron ROMs.

The third socket is fitted with ACP's 1770 DFS. This is virtually identical to the DFS used in the BBC B+ and Master and is designed to be as compatible as possible with the old Intel 8271 disc controller and DFS used in the ordinary BBC Micro. This means that the disc controller is restricted to single density mode and the DFS restricts the number of files on a disc to 31. Directory names are single character only and filenames are up to seven characters.

The advantage of this compatibility is that it is possible to save a program to disc on your Electron, put the disc into a drive connected to a BBC Micro and load it straight in, and vice versa. The discs used are identical so there's no problems with swapping unprotected software.

You'll find a full review of ACP's 1770 DFS in the February 1986 issue of Electron User.

The last socket in the AP4 is fitted with a 6264 8k static RAM chip. This is used exclusively by the DFS and isn't an addition to the ordinary RAM available to Basic.

Although it doesn't provide you with any extra memory the important advantage of this system is that you don't lose any. All disc filing systems require some workspace in which to operate. For instance, the ADFS in the ACORN PLUS 3/AP3 requires about 4k which is grabbed from the free memory available to BASIC.

This can pose a problem when running programs in Modes 0, 1 and 2 as it is all too easy to run out of memory.

With ACP's AP4 fitted PAGE stays fixed at &E00 so you've got exactly the same free memory as before. Those long programs that ran from tape will run from disc without any modification and without the need for downloaders.

An added bonus is that if you can beg, steal or borrow the ADFS ROM out of an Acorn Plus 3 or AP3 then you've got both ADFS and DFS, enabling you to access both discs designed for the Acorn Plus 3 and BBC discs. PAGE is set to &1D00 by the ADFS, but you can reset it to &E00 and use the DFS instead.

The AP4 has been in use in the office for some time and has performed perfectly. I can't fault it.

Weighing up the advantages and disadvantages of all four disc systems currently available for the Electron, I think ACP's AP4 comes out on top. I can recommend it to anyone contemplating upgrading to disc.