[Coco] some thoughts about reading coco disks on a PC

[Robert Gault]

Torsten Dittel wrote:

><snip>
> 
> Tracks
> ======
> 
> Bytes		| Contents
> ----------------+--------------
> 0-31		| System controls
> 32-6115		| Sectors
> 6166-6249	| System controls (might vary because of speed variations)
> 
> and for each
> 
> Sector
> ======
> 
> Bytes		| Contents
> ----------------+--------------
> 0-55		| System controls
> 56-311		| Data
> 312-337		| System controls (might vary because of speed variations)
> 
> with the control bytes:
> 

The recommended values as given by Western Digital for the standard MFM 
(IBM System 34) format WD1793 controller are similar but different from 
those used by Tandy.

			bytes	value
lead-in of              80	$4E
			12	00
			3	$F6 (writes C2)
			1	$FC (Index Mark)
Track starts
			50	$4E
> 0-7	$00		12	00
> 8-10	$F5		3	$F5 (writes A1)
> 11	$FE		1	$FE
> 12	track number	1	track number
> 13	$00		1	side (0 or 1)
> 14	sector number	1	sector number
> 15	$01		1	sector length 0=126 1=256 2=512 3=1024
> 16-17	CRC		1	$F7 (2 CRC written)
> 18-39	$4E		22	$4E
> 40-51	$00		12	00
> 52-54	$F5		3	$F5 (writes A1)
> 55	$FB		1	$FB (data address mark)
			256	data (length depends on sector length)
> 312-313	CRC	1	$F7  
> 314-337	$4E	54	$4E
end of disk		~598	$4E until controller interrupts out

> 
> What's so weird about this format? Why can't the PC-controller read that
> one only on some disks? Any other documntation available on this? I
> forgot all this stuff... :-(

Maybe the PC controller does not like the missing disk header. Maybe it 
is the slightly different number of system control bytes. One way to 
find out is to write your own DSKINI routine for Disk Basic using the 
recommended format. If there aren't any problems with such disks on the 
PC, then you will know what was weird.

> 
> Regards,
> Torsten
> 
>