wdao@castor.usc.edu (Walter Dao) (07/04/89)
The amiga disk drive as any other drive can only write a given number of WORDS (2 bytes) on any single track. On my drive I can write and can only write about $31d8 (+/- 2) bytes. (they are composing the raw MFM bytes of the sectors, headers and gap .) Now I have encountered disks written by other amiga computers which had more bytes on them (the abacus disk had 320a bytes on track) or less bytes on disk (like Thexder which had 30ca or something ... (forgot)). now my question : how can I slow/speed up the motor (?) write spead (?) of my disk drive so that I can have more/less bytes storable on a track I have tried to play with the PRECOMP stuff but that didnt help, I was still stuck with being able to only write 31d8 bytes on a track. I even tried to alter the data written by the DMA (like writting only 00 on a full track , then writing less than 31d8 bytes on that same track. (lets say 3000 bytes) but I still got a track of 31d8 bytes storable on it.. any Hardware modification is definitely out of the question. I have read and reread the HArdware manual but could find any lead. anyway ... I am waiting for any replies ... 8-#
dillon@POSTGRES.BERKELEY.EDU (Matt Dillon) (07/06/89)
E-Gad! Talk about fine tuning. These deviations are due to
minute deviations in motor speed. You *must* allow for the worst
case which is why there is padded gap at the end of the track when
it gets written out (i.e. no gaps in between the sectors but a gap
at the end of the track).
I'm not sure exactly what C-A does but this is the right way
to do it (numbers picked at random) for writing a track:
buffer: <padding ~200 bytes> <data ~12K> <padding ~200 bytes>
Proper operation is where the padding at the end partially overwrites
padding at the beginning. The amount of padding must allow for the
worst case and best case motor speed deviation. I think if you
look at the spec'd allowed deviation for the drives and convert it
to bytes you will find the numbers fit within the spec.
You can easily *read* data written on a slightly faster or slower drive
because the drive uses a phase-locked loop to clock the data
and the PLL 'locks' onto the frequency of the data on the track.
This is the whole reason for MFM encoding in the first place... you
must *somehow* encode a clock onto the track so the drive's PLL
stays locked.
To encode more actual data onto the drive you can try a different
encoding method. The old commodore PET/CBM drives used a form of
GCR encoding.
-Matt
:The amiga disk drive as any other drive can only write a given number
:of WORDS (2 bytes) on any single track. On my drive I can write and can only
:write about $31d8 (+/- 2) bytes. (they are composing the raw MFM bytes of the
:sectors, headers and gap .)
:Now I have encountered disks written by other amiga computers which had
:more bytes on them (the abacus disk had 320a bytes on track) or less bytes
:on disk (like Thexder which had 30ca or something ... (forgot)).
:
:now my question : how can I slow/speed up the motor (?) write spead (?)
:of my disk drive so that I can have more/less bytes storable on a track
:
:I have tried to play with the PRECOMP stuff but that didnt help, I was still
:stuck with being able to only write 31d8 bytes on a track.
:I even tried to alter the data written by the DMA (like writting only
:00 on a full track , then writing less than 31d8 bytes on that same track.
:(lets say 3000 bytes) but I still got a track of 31d8 bytes storable on it..
:
:any Hardware modification is definitely out of the question.
:I have read and reread the HArdware manual but could find any lead.
:
:anyway ... I am waiting for any replies ...
:
:
: 8-#jesup@cbmvax.UUCP (Randell Jesup) (07/07/89)
In article <8907051954.AA04521@postgres.Berkeley.EDU> dillon@POSTGRES.BERKELEY.EDU (Matt Dillon) writes: > I'm not sure exactly what C-A does but this is the right way > to do it (numbers picked at random) for writing a track: > > buffer: <padding ~200 bytes> <data ~12K> <padding ~200 bytes> Actually, it's 830 data (1660 mfm encoded) bytes of gap written at the beginning of the track. The gap must be large enough so the track will partially overwrite it. The maximum number of data bytes that can be guaranteed to be stored on a track (including headers, sync words, etc) is about 5999 bytes, because of timing and disk speed variations. Drives are speced at +-1.5%, the amiga clock (for various reasons) is speced at +-5%, and the amiga clock is 2.5% fast nominally. The nominal number of data bytes on a 1-meg floppy in 6250. > To encode more actual data onto the drive you can try a different > encoding method. The old commodore PET/CBM drives used a form of > GCR encoding. > > -Matt True, but to use GCR on the amiga you must drop back to the slow bitrate, which cuts your storage in half to start with. There are other MFM-style encodings that would still follow the rules about clock bits correctly, but they are non-standard, and would be much harder to encode/ decode. -- Randell Jesup, Keeper of AmigaDos, Commodore Engineering. {uunet|rutgers}!cbmvax!jesup, jesup@cbmvax.cbm.commodore.com BIX: rjesup Common phrase heard at Amiga Devcon '89: "It's in there!"
cmcmanis%pepper@Sun.COM (Chuck McManis) (07/07/89)
In article <4285@merlin.usc.edu> wdao@castor.usc.edu (Walter Dao) writes: >The amiga disk drive as any other drive can only write a given number >of WORDS (2 bytes) on any single track. > I can write ... $31d8 (+/- 2) bytes. > I have encountered disks ... had 320a bytes ... or 30ca . One of the more common copy protection methods. The Amiga can synchronize to a slightly faster data clock rate and read more bytes of the disk than it can write to them. The copy protection comes from the fact that an unmodified Amiga drive cannot write that same data, hence can't make a copy. Seems like Walter has clearly taken up the challenge of writing a protected disk copy program ala NIB or Marauder. Usually this is a good project since it will teach you a lot about disk formats, copy protection, and how data can and cannot be stored on magnetic media. [ If you use this knowledge to pirate programs then you will be considered scum until you repent. ] >now my question : how can I slow/speed up the motor (?) write spead (?) >of my disk drive so that I can have more/less bytes storable on a track Answer : Not easily. Try taking the drive out of your machine and twiddling with the speed potentiometer(sp?). Actually, when embarking on an endeavor such as yours, the proper equipment is essential. Pick up a logic analyzer and an oscilloscope, and connect them to an "exposed" drive (out of the case). That will let you monitor most anything. On a slightly different but similar note : CADVision has Designer X-Cad out for $120 at my local dealer. This seems to be 100% of X-Cad but with a new label (including the Commodore chicken head ?!?!) It is also dongle protected. I want to buy this program, but won't until I have a patch to eliminate the dongle requirement. If anyone knows how to do this send me some mail, thanks. Music-X is _not_ copy protected and I appreciate that immensely. --Chuck McManis uucp: {anywhere}!sun!cmcmanis BIX: cmcmanis ARPAnet: cmcmanis@sun.com These opinions are my own and no one elses, but you knew that didn't you. "A most excellent barbarian ... Genghis Kahn!"