Reviving elderly BBC Micros

It is single-density. The original BBC micro came in Model A and Model B versions. Both usd the smae PCB, the 'A' had fewer ICs fitted. Often the empty spaces in an 'A' were fitted with DIL sockets, which cost almost as much as the missing ICs. Go figure... Anyway, neither had a disk controler as standard. It was an option, a handful of ICs to plug into the main PCB. The actual disk controller chip was an Intel 8271 (no, I do not mean 8272), which was single density only. The same IC had been used on the earlier 'System' machines made on multiple Eurocards. For various reaoons, later models used a 1770 disk controller. This could be fitted to the Model A/Model B in the form of a daughterboard that went in the 8271 socket. The B+ and Master (and machines baded on them, like the ABC and ACW) all have a 1770. This supports single and double density. The firmware (known as 'DFS' -- Disk Filing System) is different for the 8271 and 1770 controllrs. IIRC there is a routine in the 1770 DFS to accept 8271 commands and convert them to 1770 commands so that copy-protected software that did odd things to the disk controller, would still run. I seem to rememebr that ADFS (Advvanced Disk FIling System) needs a 1770. I also seem to rememebr that ADFS will do MFM, -lain DFS -- even the 1770 version -- is FM (single density) only. Beebs support 40 and 80- cylidner drives. DFS treats all disks as single-sided, if you have a double-head drive, the 2 sifes are different disks to the system, ADFS also supports 40 and 80 cylidner drives, I can't remembr how it handles 2 sides. -tony