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Old 18 August 2010, 01:00   #1
prowler
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Location: Sidcup, England
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DiskSqueeze questions

@dirkies,

I have generated valid DiskSqueeze archives from ADF images of the Amiga Format Subscriber disks 81-87 and compared them with the invalid archives previously on the EAB File Server.

The new archives were prepared in WinUAE emulation by mounting each image in turn in DF0:, and running the DiskSqueeze v1.23 application.

Select Action: Pack
Select Device to Pack: DF0:
Is the disk using a mixed filesystem? No
Do you want to add a filelist to the archive? No
Do you want to add a File_ID.DIZ to the archive? No

Seven new archives were generated from the disk images, and the integrity of each archive tested OK.

The Image.dsq file extracted from each archive using the LZX command line tool was found to be identical with that extracted from the corresponding invalid archive.

Comparing the new, valid archives with the old ones showed them to be identical, except for twelve bytes at offset $1C (differences shown in bold type):

Code:
             Offset
81.dsq         $1C  8B C5 0D 43 D0 29 A7 8A 0C F1 FB 10   Valid
530534 bytes        18 10 5B 94 D0 29 A7 8A 86 00 74 10  Invalid
Image.dsq
929024 bytes
 
82.dsq         $1C  8B C5 2C 01 A0 30 E4 73 78 14 D6 24   Valid
836836 bytes        18 10 5B FA A0 30 E4 73 CC E8 8F 8D  Invalid
Image.dsq
929552 bytes
 
83.dsq         $1C  8B C5 30 CF 5F 18 2E 05 59 1E F9 BF   Valid
793600 bytes        18 10 5C 58 5F 18 2E 05 EB 58 51 5F  Invalid
Image.dsq
907376 bytes
 
84.dsq         $1C  8B C5 34 87 17 EF 38 89 4F B6 3D 53   Valid
788336 bytes        18 10 5C E2 17 EF 38 89 33 64 13 6B  Invalid
Image.dsq
899984 bytes
 
85.dsq         $1C  8B C5 38 7B 62 49 33 80 07 DD 85 E8   Valid
820668 bytes        18 10 6A 5F 62 49 33 80 1B 38 6E E7  Invalid
Image.dsq
926912 bytes
 
86.dsq         $1C  8B C5 3B 14 7E 49 42 B1 E6 01 0E 02   Valid
769920 bytes        18 10 6A CD 7E 49 42 B1 69 40 B8 77  Invalid
Image.dsq
916352 bytes
 
87.dsq         $1C  8B C5 3D 45 88 B8 38 5C A7 BE 9A 27   Valid
773004 bytes        18 10 6B 4F 88 B8 38 5C 60 D0 65 D4  Invalid
Image.dsq
921632 bytes
To enable me to fix similar archives in the future, could you please advise me of the purpose of these bytes in the file header?

I assume that the first four bytes are the file's date and time stamp, as they increase progressively for both the valid and invalid archives.

The four middle bytes I assume are the files' checksums, as they don't change, but I'm mystified about the last four bytes and I'm assuming that it is those which are breaking the invalid archives.

Thanks in advance for any help you can offer.
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