The advanced disc filing system on the Plus 3 sets PAGE to &1D00. This makes life complicated as you have to load large programs high up in memory, download and then RUN them. The Plus 3 user guide is very comprehensive and gives instructions on how to do that. However, it is much more practical to have PAGE set at &E00 but still be able to run the disc system. This can be achieved by having RAM onboard the disc interface.

This is exactly what the ACP Plus 4 has. The benefits are many including the easy transfer of cassette-based games and no need for downloaders. The Plus 4 is also completely compatible with the standard Acorn DFS as used on most BBC Micros. Any software stored on a DFS disc can be read by the Plus 4.

Whether or not the software itself will run is another matter entirely but at least you can load the programs in. No more converting from one format to another. Just take any DFS disc and try it on the Elk with a Plus 4 fitted. If the program is in BASIC and uses little or no Mode 7 text, then it should be compatible. As part of my job I have to try all submissions to A&B on the Electron and, up until now, it meant converting DFS 5.25" disk software to ADFS 3.5". A long and laborious operation has now been eliminated thanks to the Plus 4.

The DFS itself has all the commands you would expect such as *COPY, *COMPACT, *DELETE, etc. Unlike some early versions of the BBC DFS it has built-in *FORMAT and *VERIFY commands. Also included is the *FREE command that displays the amount of room used on the disc and how much is left free. Another bonus is the *ROMS utility which lists all the ROMs which are present, showing what they are and in which socket they have been put.

Some nice touches have been included in the software on ROM which enables formatting and verifying of more than just one drive at a time. The *DRIVE command will not only set the current disc drive in use, but will allow double stepping on an 80 track drive to that 40 track discs can be read. These added features make the Plus 4 very user-friendly. The design of the interface is very pleasing to me because it allows easy disconnection of the disc drive cable. As I only have one 5.25" drive and have to share it among three different computers I really appreciate easy plugging and unplugging.

Once inserted into the rear socket of the Plus 1 the Plus 4 stands at the same height as a ROM cartridge (half the height of the Cumana interface). It looks neat and tidy with just one cable running from the back to the disc drive. Incidentally, the Plus 4 only works with disc drives which have their own mains power supply. It feels solid in the slot and doesn't wobble at all.

Inside the interface itself is a spare ROM socket into which you can plug any Electron compatible ROM. I tried using the Acorn ADFS but this refused to work with the Plus 4. The accompanying user guide is very well written and put together in an easy-to-read, easy-to-refer to way. There are contents and full index pages so you can quickly find the information you need. Subjects are dealt with in a logical orderly manner. BASIC disc theory is dealt with first, progressing on to deeper subjects such as random access files and how to use the DFS in machine code (mind boggling stuff).

Matthew Fifield