Re: LDA C08C,X

[mjmahon@aol.com (Michael J. Mahon)]

In article <Pine.A41.4.44.0302242141050.70864-100000@zivunix.uni-muenster.de>,
Holger Picker <pickerh@uni-muenster.de> writes:

>On 24 Feb 2003, Michael J. Mahon wrote:
>
>> Date: 24 Feb 2003 20:37:39 GMT
>> From: Michael J. Mahon <mjmahon@aol.com>
>> Newsgroups: comp.sys.apple2
>> Subject: Re: LDA C08C,X
>>
>>
>> If the third header nibble is not a $96 (for an address mark) and not an
>> $AD (for a data mark), then we have not correctly recognized _either_
>> header, and we should start over, looking for an address mark.
>>
>> -michael
>>
>> Check out amazing quality 8-bit Apple sound on my
>> Home page:  http://members.aol.com/MJMahon/
>>
>
>I know what you mean, sir, but, I'm afraid, it still does not make sense to
>me. Please correct me. I thought when you check for the data mark you will
>have already successfully decoded an address header (including checksum).
>Chances are zero that you will find another address header with the same
>valid information because it would mean that this had been written twice to
>the disk. Such a situation could only occure with corrupted disks, e.g.
>those which were not formatted correctly or perhaps had some special copy
>protection that makes use of this.

And if you have already found a $D5 and an $AA, and then fail to find the $AD
that you expect, then you have found _nothing_, and should scan further for
another valid address mark.  It is certain that there will not be a valid data
mark before another address mark occurs, since you have either just passed
it (and misread the $AD) or have somehow managed to misread something else
as a $D5 and $AA, since the $D5 cannot validly occur except in prologs.

Scanning for a data header is only justified when you have _just now_ found
an address mark.  Once you have reason to be  uncertain of your position on
the track, the only appropriate thing to look for is an address header.

-michael

Check out amazing quality 8-bit Apple sound on my
Home page:  http://members.aol.com/MJMahon/