[Coco] Oddball disk format question

Robert Gault robert.gault at worldnet.att.net
Mon Nov 22 16:50:01 EST 2004

tim lindner wrote:
> Robert Gault <robert.gault at worldnet.att.net> wrote:
>>Does anyone know how it might be possible to create a track where the 
>>track ID number does not match the disk location, the sectors have 
>>values 247 & 248, and there is data on sector 248?
> This is easy, so I must be misunderstanding your qeustion.
> Here is how I would answer the above question: "Step to the track you want, then
> write the track with the data stream you need." If the data stream contains the
> "wrong" track numbers then so be it.
> Since the above is obvious, I must be missing something.

So far, so good if and only if the data does not contain $F5, $F6, or 
$F7 which are not permitted byt the 1773, 1793 or 2793 in MFM mode. $F5 
writes $A1, $F6 writes $C2, and $F7 writes CRCs.

>>I have determined the following. The track can be created by stepping to 
>>the desired location (T35) and issuing a Write Track command with an 
>>incorrect track value.
>>The Coco controller will accept a sector value of 
>>248 even though it is not "legal. A sector with a value of 247 = $F7 
>>(which is a reserved Western Digital command) can coincidentally be 
>>created if the track ID is 164 when a crc is requested in the "wrong" 
>>place. $F5, $F5, $F5, $FE, $A4, $F7 will create $A1, $A1, $A1, $FE, 
>>$A4(track) ,$42(side), $F7(sector) because the crc of $A1A1A1FEA4 is 
> Tricky. Does it then write two of its own CRC characters, or does it issue
> another $F7?
No more CRCs unless you enter another $F7. The track in question did not 
seem to do this.

>>It is not possible to place the desired data on sector 247 during 
>>a format because it contains values in the range $F5-$FE.
> Why not? Becuase you don't finish the sector ID field properly? Does the WD179x
> get out of sync when you write a data field after the malformed sector ID field?
> During a 'Format track' shouldn't this data stream work:
> F5 F5 F5 FE A4 F7 01 xx xx (20 x 4E) (11 x 00) F5 F5 F5 FB (256 x data) F7
>                   ^   \ /
>                   |    + Proper CRC bytes

You could do this but it is not relevant. See details above.

>                   + Sector size
>>I can't 
>>access the sector with a disk zap program because of the mismatch 
>>between disk and track IDs.
> You need a better disk zap program. One that turnes off the track checking in
> the read/write sector commands.

Possibly, I can try that. By the way Anadisk 2.07 can't read the sectors 
on such a disk (although it can do a Track Read as can the Coco) because 
with 17 sectors of 248, Anadisk never sees the sector 247. Might also be 
do to the screwy side value of $42.

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