[Coco] Re: Oddball disk format question

[Robert Gault]

Torsten Dittel wrote:
>>$?? $A1 $A1 CRC CRC $F7(!)
> 
> 
> Wasn't the CRC initialized with a special value? AFAIR it was $CD $B4.
> By the way, the byte $xx after the CRC ($F7 $xx) is written in any case
> (e.g. $F5, $F6 are not changed to sync bytes ($A1, $C1)), I forgot to
> mention that.
> 
> 

The data sheets for the Coco WD controllers says one thing but the 
controllers actually look for an $A1 (or maybe 3 $A1) and then a 
following $F7. At that time a two byte CRC is calculated and includes 
everything from the first $A1 up to the $F7. A 2 byte CRC is written to 
the track and then writing continues with the next byte of data.

So, if there is no $A1 preceding an $F7, the $F7 gets written. I have 
not found anything that can protect an $F5 from being changed to an $A1, 
an $F6 to a $C2, or an $F7 with a preceding $A1 from generating a CRC.

I've been given the Read Track data for a game which has 17 sectors 
numbered 248 and one numbered 247. There is ml data in sector 247 that 
contains $F5, $F6, and $F7.