[Coco] Oddball disk format question

[Robert Gault]

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 

>>$42F7.

> 

> 

> 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.