Bad Bytes in .DCM -- can anything be salvaged?

[+Larry]

While trying to un-DISKCOMM some very old 720K disks this morning, I've found that a couple of the DCM files have bad bytes in them. Does anyone know if there is any way to salvage any of the files in the DCM? If I look at the archive using a disk editor, I can see that the disk directory is intact. These DCM's were written using an XF551 probably close to 30 years ago -- I should have done this a long time ago!

 

Anyone know anything about this?

 

-Larry

[+Larry]

Well, to answer this myself... Since these are game DCM's and I can see the intact directory sectors, probably the most expedient thing to do is to re-create the DCM archive from other sources. Since most of these are commonly available, I should only lose one or two .EXE game files. (These files were still called .EXE when these were created in the '80's.)

 

Maybe as an academic exercise it would be interesting to see if the archive can be salvaged, but not worth spending a lot of time on.

 

-Larry

[1050]

Could be a long shot, but you might try Jindroush's ACVT

on it while turning it into an ATR for example. I seem

to remember him saying he put some repair code in there

for DCM files. But that was a looong time ago.

 

http://www.atarimax.com/jindroush.atari.org/asoft.html

[Rybags]

I don't get it - DCM is supposed to be compressed so how would you see directory entries?

 

Maybe it's just simple RLE, in which case such things would be easy to see.

The problem though with most compression is that even a single bit of corruption can render part or all of the archive unusable though I suspect in this case it's likely the leadup data would be OK.

Edited July 2, 2017 by Rybags

[+Nezgar]

Maybe it's like SpartaDOS SCOPY - I think it only 'compresses' empty sectors...

[lemiel]

Altirra should open them if they are not totally damaged.

[+Larry]

Yes, things like Disk Communicator, Scrunch, Shrink, etc. had a main purpose of allowing a disk to be sent via modem as a file. The compression was very mild and mainly removing empty sectors to cut down on the transmission time/cost. I did double-check two of the defective DCM files and they both have their directories visible at the normal location. I remember that some DCM's were then Arc'd which cut down the size tremendously. I took one of the bad DCM's and Zipped it and it cut the size nearly in half. The DCM was of a DD 180K disk that was nearly full.

 

I may have to dig out Ernest Schreurs' format description of DISKCOMM. Some nice utilities there, if you don't have them already. Right now, I'm getting about 25% defective DCM's. Not bad sectors, just "compression" errors, I'd say. They have probably been there since the day they were created.

 

http://home.planet.nl/~ernest/atarixle.html

 

@lemiel and @1050-

Thanks, I'll try those suggestions.

 

-Larry

[Mclaneinc]

The only issue with lots of the old utils is that they won't work on a 64bit OS, I suppose I could make a VM to do it but its a load of trouble for so very little...

 

Yes, I'm lazy

[Atari_Ace]

If you post one or two examples of the corrupt files I could take a look. I wrote a dcm2atr in Perl many years ago to better understand the format which could be adapted to salvage some of the data.

[+Larry]

PM sent...

[phaeron]

The DCM format uses very lightweight compression -- it is a mix of run-length encoding and repeating head/tail bytes from the previous sector. Most of the disk should be recoverable with a bit of elbow grease and a hex editor.

[+Larry]

I ended up with two bad DCM's out of 57. Originally there were three bad, but I tried one again and it expanded properly. The other two were still bad -- "Bad bytes in this file."

 

Altirra won't open it, and neither will APE nor Imagic. Haven't tried Jindroush's utility yet, but will.

 

Just for grins... does anyone have any earlier versions of Disk Communicator -- 1.x or 2.x ?

 

-Larry

[+Larry]

Well, Atari_Ace got these straightened out! He found that somehow the files were mislabeled, and there was a DCM inside of an ATR. (?) And once he got that separated, Altirra accepted the DCM. I then did much the same thing, and watched as Altirra instantly opened the archive. So all is well, and I owe a big thanks to Atari_Ace and of course, to Phaeron whose Altirra ultimately made this pretty easy!

 

-Larry