Hardware / Delta Products / XOR / Floppy Disks


This is really retro stuff!

I bought the following two 8" disks on ebay several weeks ago because I was interested in them. Of course, with floppy disks the question always comes up: "Are they still readable?" In the course of the last years I made the experience that almost every old 8" floppy disk is "basically" still readable, if it looked very good in the ebay offer. This was also the case with the following two disks. Only with the DISK B there was a small error in the directory.

delpa products cp/m
Delta Products CP/M 2.2 on 8" floppy disks

And now the question: "Who or what is Delta Products?" I've never heard that name before. My active experience with computers starts in 1986 and actually only IBM compatible computers and software.

But that's what the Internet is for. To be brief, the company Delta Products built the so-called XOR computers around 1983.

More information on Delta Products aka XOR on Herb Johnsons website (S-100) and the links below. In the following an advertisement from BYTE Magazine, issue 09-1983, where the name Delta Products no longer appears, but U.S. Micro Sales.

Reading an 38 year old floppy disk

This is for me the most exciting thing about retro computing. Are the disks still readable after almost 40 years? Information on reading from 8" floppy disks ...

For old CP/M diskettes, UniForm-PC and/or 22DISK is always the first choice. Once you have found the right floppy disk format, the rest is very quick. Under UniForm-PC it is called "EXO" and with 22DISK it is called "EXO1".

The reading is unproblematic. Actually, that was it. Well, not quite, a little curiosity is still there. To analyze old floppy disks the program ImageDisk (IMD) by Dave Dunfield is perfectly suited.

So, start IMD, set the drive and number of tracks and select the alignment test. IMD always starts with track 0 during this analysis and in this case shows me the SINGLE DENSITY (SD) format with 26 sectors and 128 bytes per sector. Why that? 22DISK and the disk label say DOUBLE DENSITY (DD) and 512 bytes per sector!

With IMD I have now randomly selected track xy and ... no format readable anymore. Strange. By chance I called the re-analyze and see, the format was now DD with 16 sectors per track and 512 bytes each.

After some research on the internet for the XOR my results were confirmed: Double density disks are formatted with 512 byte sectors and 16 sectors per track but track zero is always the standard IBM 3740 format (SD).

This issue is also covered in the manual below. It is mentioned in dozens of places that track zero is always in SD (FM) format.


The EXO CP/M disk format has causally nothing to do with the XOR computers. The DD parameters 16 sectors/track and 512 byte/sector coincide here by chance. The original EXO format does not support track zero in SD format, for example.

But track zero does not seem to bother either UniForm-PC or 22Disk while reading the 8" XOR floppy. Since the table of contents (FAT?) starts at track 2, sector 1, the content is readable by 22DISK and UniForm-PC.

According to Andy Johnson-Laird, tracks 0 and 1 of a bootable CP/M floppy disk contain the bootstrap loader, CCP, BDOS and BIOS. But I don't know that much about this subject. I don't care, it worked!

EXO Systems

XOR computers are so-called S-100 systems and have a different floppy controller. Also the DEC format RX02 is such a freak with SD/FM and DD/MFM on one floppy disk!

EXO Systems

Conclusion of today's Discovery Day

It was fun to rediscover old things. Maybe I will run into an old XOR one day? ;-)



Format: 8" / T0: SS, SD, 26x128 (IBM 3740) / T1-77: SS, DD, 16x512

ImageDisk is so well programmed that it can also store an image of this combined format. It analyzes track by track and writes each one correctly into the image file.

The following images are saved with a Mitsubishi M2896-63-02M 8" disk drive. This drive was also used in some XOR computers.

External links