Date   : Sat, 31 Jul 2010 03:39:22 +0200
From   : rick@... (Rick Murray)
Subject: Why did Acorn ADFS only allow 640KB on a floppy?

On 31/07/2010 00:55, Tom Seddon wrote:

> offer(ed) 720K discs, using a normal 1770 FDC. 2 sides, 80 tracks, 18
> sectors per track, 256 bytes per sector. Don't know why this layout
> wasn't more popular, but perhaps there's a good reason.

Does anybody here have the timings (especially GAPx) for the 18 sector 
type format(s)?


For reference, I have a file (no author ID in the file!) which states:
--8<--------
    Double density formats:
    ======================

       Recommended      |      Acorn ADFS      |
         format         |     (master 128)     |
    No. of   Hex value  |  No. of   Hex value  |
    bytes    to write   |  bytes    to write   |  Info
      60        4E      |    60        4E      | Post-index Gap (Gaps 1+2)
   /  12        00      | /  12        00      | Pll lock-up time
   |   3        F5      | |   3        F5      | 3 synchronisation bytes
   |   1        FE      | |   1        FE      | ID address mark
   |   1                | |   1                | Track number
   |   1                | |   1                | Side number
   |   1                | |   1                | Sector number
   |   1                | |   1        01      | Sector length
   |   1        F7      | |   1        F7      | Writes 2 CRC bytes
   |  22        4E      | |  22        4E      | Sector ID / Data Gap 
(Gap 3a)
   |  12        00      | |  12        00      | /Write splice time (Gap 3b)
   |                    | |                    | \Pll lock-up time
   |   3        F5      | |   3        F5      | 3 synchronisation bytes
   |   1        FB      | |   1        FB      | Data address mark
   |   n                | | 256  (5A on format)| Data (number of bytes 
depends
   |                    | |                    | on the sector length)
   |   1        F7      | |   1        F7      | Writes 2 CRC bytes
   \  24        4E      | \  43        4E      | Post Data Gap (Gap 4)
     668 approx 4E      |              4E      | Runout Gap. Write until
                        |                      | next index hole.
--8<--------

It looks like with playing around a little, you can squeeze more onto 
the disc, though I think we'd need to work the numbers to see if it was 
a viable format, or if it was pushing the FDC...


Best wishes,

Rick.

-- 
Rick Murray, eeePC901 & ADSL WiFI'd into it, all ETLAs!
BBC B: DNFS, 2 x 5.25" floppies, EPROM prog, Acorn TTX
E01S FileStore, A3000/A5000/RiscPC/various PCs/blahblah...