Upgrade your BBC with Microwave's alternative to Acorn's disk interface, says Igor Thomas

How much longer are you going to put up with 'Searching', 'Data? Rewind tape', and 'Record then Return' on your BBC micro? If dispensing with the reliable, but slow cassette filing system is worth £110 to you, then it's time to pick up thy cassettes and walk to the nearest Microware dealer.

The MDDFS is supplied as a complete floppy-disk upgrade kit comprising a full set of integrated circuits, a small plug-in circuit board and an Eprom containing the Disk Operating System program. For more or less the same price as an official Acorn Computers upgrade the MDDFS also supports true double-density operation, which the Acorn interface does not.

If you use diskettes with double-density format, the storage capacity of any standard BBC disk can be increased by an impressive 60%. Standard 100K and 200K BBC disk drives will then be capable of 160K and 320K respectively, provided the actual drive units have no integral objection to working with double-density information.

Microware is not the only company to offer a double density interface, although its design approach is the most sensible one available. Instead of inventing yet another new disk format, with all the perils such inventiveness entails, MDDFS has borrowed from the official Acorn DFS design. DFS is admittedly not the best filing system invented but it is widely used, and by adopting Acorn's standard disk format, MDDFS achieves 100% compatibility with all official BBC disks.

The accompanying manual is well written, although a little idiosyncratic in places. It contains not only a good overall reference guide to the additional commands required by the Disk Filing System, but also provides complete installation instructions for the interface kit.

Fitting MDDFS to my issue 4 circuit board was one of the most pleasant BBC upgrades I've made. As I already have an official Acorn interface and 80-track drives, all that was required was for two integrated circuit chips to be removed from their sockets, and the corresponding Microware parts to be installed. Apart from the little circuit board looking slightly unsure about its new home, once I had double-checked the circuit link settings and altered the necessary DIP switch settings inside the keyboard, my new double-density BBC powered-up with a reassuring beep and has worked unfailingly since.

For those of you who have never known the joys (and pains) of using BBC disks, I must digress briefly. In theory all BBC micros are the same. In theory any disk upgrade components you require need only be plugged in. In theory the streets of London are paved with gold.

In real life, there are actually five different and widely sold versions (issues) of the BBC Micro's printer circuit board, thoughout which there are at least three major variations in the floppy disk electronics.

Unfortunately BBC issues 1, 2 and 3 have an errant circuit track which must be repaired before a disk interface can be fitted. Issue 7 BBC's have some of the smaller disk interface components soldered in place during manufacture, which prevents the whole circuit from being easily upgraded. Issues other than 7 need to have the keyboard DIP switch installed. The manual covers how to install the kit into issues 1 through 4, but does not cover issue 7.

All the standard Acorn disk commands work as they should with subtle enhancements here and there. For example, a format program is built into the MDDFS ROM which is impressively sophisticated, and very swift at work. Also, the *CAT command displays a handy disk number from 000 to 999 which can be specified only during formatting.

Both single and double density disks may be intermixed on different drives, and the drives may be either 35, 40 or 80 tracks. A unique feature of the format program includes the ability to create proper 40 track disks on 80 track drives. A final enhancement of note is an improved *DRIVE command which remembers which of the four possible drives you've been using, even after you press BREAK.

My only real disappointment with MDDFS came when I tried to run some Acorn software, such as Planetoid and the Verify program, which produced polite messages of non-compliance. Dismay grew stronger when I tried to use some of the more sophisticated ROMs available for the BBC, notably Computer Concepts' Disc Doctor, and Torch's CPN & Z80 second processor system, which also failed to operate.

The shortcoming here is that all these programs need to use OSWORD 7F calls which are rejected by the reviewed version of the MDDFS ROM. Although the manual refers to four versions of MDDFS, we had only the earliest version (090). Only later versions can cope with OSWORD 7F requests.

Even with the lack of full OSWORD compatibility on the review system, the MDDFS represents a very good investment. Its advanced technical specification surpasses most other systems on the market.