And Elk Will Speak To PC

By Derek Hilton

Originally published in EUG #08

Thanks to JD for the pointer to a source of DOSCopy. So far I've only had the BBC PD Summary Catalogue, with a note from Alan Blundell advising that as far as he knows this only works on a BBC Master! However, in the meantime I wrote to Western Digital who, whilst pointing out that this was a discontinued design, were good enough to supply a data sheet for the WD1770. After a study of this, the EFORM program and some experiements, I had managed to read sectors from (but not yet tried to write to) ADFS, DFS and MSDOS disks and therefore am hopeful of being able to modify DOSCopy if this proves necessary.

I was at first worried about timing, since the WD1770 data rata is very close to the time (by cycle count at the 1 Mhz RAM clock rate) for the tightest test/read loop, but was then confused by the MSDOS sector layout. Acorn ADFS "L" formal fills side 1, tracks 0-79 then side 2 track 0. To save head movement however, MSDOS follows side 1, sector 9 with side 2 sector 1 of the same track (or "Cylinder").

If anyone is interested, apart from WD1770 itself at:

&FCC4   Command reg (Write) or Flags (Read)
&FCC5   Track reg
&FCC6   Sector reg
&FCC7   Data reg

There is a latch of some kind in the Plus 3 at &FCC0, of which at least 4 bits switch one or more outputs when added to &20 and stored in FCC0.

Bit/Val   Selects

 0 &01    Drive 0
 1 &02    Drive 1
 2 &04    Side 2
 3 &08    Single-density
   &00    Double-density
          (connects to WD1770's active low DDEN or Double DENsity input)
 4 &10    No function yet found
 5 &20    When bit 5 is set on write, bits 0-3 clear their latches.
Storing &00 in &FCC0 apparently clears all latches.

Derek Hilton

As a professional idiot, I didn't understand a word of that. Any comments from people who did?