Disk Image Permutation

Discuss the project, or ask a general question
Post Reply
flashfire42
Posts: 4
Joined: Tue Feb 08, 2022 12:42 am

Disk Image Permutation

Post by flashfire42 »

How is Permutation from use of the floppies handled? Is it preferable to image a sealed disk as opposed to one that has been used a lot over the years? was wondering if for example I had a copy of one of the floppies already released and dumped it with the same methods would I get the exact same file?
JonAbbott
Posts: 2938
Joined: Thu Apr 11, 2013 12:13 pm
Location: Essex
Contact:

Re: Disk Image Permutation

Post by JonAbbott »

Provided the disk isn't damaged or suffering from bitrot, it makes no difference.

Unlike other "archive" formats such as IPF stream, which is essentially an analogue image of the flux levels, or ADF which is a dump of 160 tracks. JFD is an application layer digital image of the floppy as seen by the game.

The structure of the JFD file is detailed in !ADFFS.!Structure. They're built up by ADFFS initially recording all calls to Filecore noting the disk address and disk structure used for each call. The JFD recorder then converts that recording into a list of sectors touched, de-dupes that list and then reads them noting any errors.

In the case of sectors with errors, if the error is a partial sector read it will store the partial data and for fatal errors it will simply store the error.

This results in a bare-minimum digital image of the floppy, storing only the sectors the game needs to load. If you subsequently verify a JFD image via ADFS or try to write additional files to the image, ADFS will error on all the sectors not imaged.

To answer your final point. As the JFD file is an application layer image and not a disk layer image, unlike image formats like IPF, it is not possible to write protected floppy JFD images back to a physical floppy.
Post Reply