Philips/Data Recovery Project/converting backup tapes
This page explains how I got from 1600 bpi magtapes to PTS6875/6876 Diskimages.
The tapes were created by the DOS program SUM, taking a backup of either 1 User, or of the complete disc ( to be more precise : the part of the disc used by the various Users). The output tape was unlabelled. If a full backup was specified, it would also back-up the system part of the disc, as a User called SAG, containing the DOS operating system.
The first job is to restore the tape to a PC disc. I used the 'InterMedia for Windows' program to do this, but you can use any program able to read tapes.
For PTS the recordsize is 410, but I've seen P800 backups with a recordsize of 512. For the remainder of this page, I will only discuss the PTS backups.
The first segment of the first granule (a granule being defined as 8 consecutive segments) of the back-up files contains the volume label, creation date, and other staic info.
What you need to find now, is the Volume ID, as that will be the name of the directory where the tape is restored to.
In segment 8, you can find the CATALOG record (full backups only), showing the User-id's.
If there is no CATALOG record, the backup contains just 1 user.
Now, before you continue, you should familiarize yourself with how the P800 series, including the PTS series, organized its discs.
In the beginning, it looks quite confusing, but it is in fact quite easy to understand.
To keep it simple, I'll summarize the structure for a User-id.
In CATALOG, you will find a User-id, followed by a granule pointer. The first segment of that granule contains a number of filenames, each followed by a new granule pointer. The referred segment contains a number of pointers, pointing at the data belonging to that file. Each of thise pointers is a start point for 8 segments, containing the data.
Back to the back-up tape.
When it was created, the data on the disc could be located anywhere on the disc, very much like the Defragment problem with Windows. The Back-up procedure would access the data sequentially, so the files were written one at a time. However, the segment references were copied too. I truly hope that the Restore program (RUM), would take this into account when a tape was to be restored.
The program I developped for restoring tapes, DOSTOSS, uses the original segment references, so the disc image created by the program, corresponds to the original disc.
... and there is more to come