A reader recently asked about the “native” format of Central Point Backup (CP Backup) floppies. Diskettes formatted in this manner are somewhat tricky to work with as they do not use a standard DOS format. A session with Sydex’s AnaDisk helped shed some light on this format. An old (1992) backup on 5¼” floppies was used for analysis.
When the DIR command is ran on a CP Backup floppy, a listing of largely zero-length files is shown. If the listing is not sorted (standard DOS behavior), a message spells out that the floppy does not use normal DOS format and needs to be reformatted for DOS use. If the directory listing is sorted (4DOS or similar), the message may prove difficult to decipher.
The above should imply that CP Backup formats floppies in a way that DOS can recognize, but not work with. AnaDisk immediately shows what the “problem” is: The 5¼” diskette is formatted with a standard BPB, FATs, and a root directory, but shows that all clusters are bad. What’s worse, the BPB indicates a standard 15 sectors per track (SPT) format, but in reality the diskette is formatted with 16 SPT. This will cause severe confusion to any tool (including DOS itself) which expects the BPB to reflect the true format.
AnaDisk is obviously not thrown off by this. It can dump CP Backup floppies and creates images of size 1,310,720 bytes. That is of course larger than a standard 1,228,800 byte image and explains how CP Backup stores more data per diskette in its native format.
Other tools, such as IBM’s XDFCOPY, fare rather worse than AnaDisk. A standard-size 1.2MB image is produced, which means one sector on each track was not imaged. Norton’s DiskEdit has the same problem and only accesses 15 sectors per track instead of 16. Since 5¼” diskettes are somewhat difficult to use with with recent Windows versions, but the assumption is that Windows would have trouble with this format as well. A quick check with Windows XP and rawwritewin produced an unexpected result—a 1.44MB size image, of unknown integrity.
It is worth noting that CP Backup is not nearly as extreme as XDF or 2M in providing DOS-compatible disk structures. It creates standard-size FATs and root directory, which wastes several sectors. On the other hand, unlike XDF or 2M, CP Backup uses a uniform geometry which has at least some chance of working on non-DOS systems, and can be reasonably represented by a raw image.
No interleaving or sector slipping appears to be used. CP Backup presumably gains its speed primarily from using optimized disk access which efficiently transfers data and does not waste time between disk revolutions.
It should be mentioned that CP Backup can utilize three different modes of floppy access: DOS compatible (slow), medium speed, and high speed. Backups created in high speed mode (which is the default mode of operation) cannot be restored using the ‘compatible’ mode.
To complicate matters, most if not all CP Backup versions (such as those delivered with PC DOS 6.3 or 7.0) have trouble running on fast CPUs in the “native” modes. Which means that users with disks created in high-speed CP Backup mode find themselves in an unpleasant situation—medium/high speed mode doesn’t work, and compatible mode won’t read the disks. More about this problem later.