Re: DSK to NIB Instruction

[pausch@saaf.se (Paul Schlyter)]

In article <didmu2$evg$1@unlnews.unl.edu>,
Ed Eastman  <noone@nowhere.net> wrote:
>Paul Schlyter wrote:
>> Andy McFadden  <fadden@fadden.com> wrote:
>>>Knut Roll-Lund <kr-lund@nogarbage.online.no> wrote:
>>>Sadly, information about sync bytes is not retained.
>> WOuldn't it be safe to assume that any FF bytes are sync bytes?  Or at
>> least any sequence of at neast <N> FF bytes where <N> is a suitable
>> number.
>
>Well yes, but was it a 1 or 2 bit inserted sync byte?
>
>So tell me, is the nibble image a string of bytes or stream of bits to 
>be reprocessed? Does the emulator do bit processing or does it simply 
>deal with the bytes returned?

AFAIK it deals with bytes only, and as long as you've chosen an appropriate
start-of-track it will work.

This of course means that the nib image files aren't an accurate
representation of the bits on the floppy.  To become more accurate,
nib files should really be a bit stream rather than a byte stream,
where sync nibbles are stored as 10 instead of 8 bits.  Of course
all normal nibbles might then start at any bit within a byte, and
the lowest-level processing of the nib file would be much slower
(to read one nibble one would usually have to read two bytes from
the nib file and then shift and mask these two bytes into the "nibble"
byte one wanted to read).

>This will make a difference when you are 
>trying to emulate a nibble count or other copy protection influences. 
>If the emulators are not prepared to drop extra bits while processing, 
>then there is no point to worrying about sync bytes.  De protection/ 
>normalization is your friend.
>
>Hmm thought, thoguhts...
>
>Has anyone considered making a new Prodos format from the nib image to 
>only have one header and make a track of A000 bytes?
>
>It would not be a 16 sector 'disk' but it would hold a bit more...
>
>Thankx,
>Ed


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