Technical  Notes  on
Preserving, Running and Studying
IBM® Personal Computer™ DOS
Versions 1.00 (1981) and 1.10 (1982)

Copyright © 2005 by Daniel B. Sedory

NOT to be reproduced in any form without Permission of the Author !


Versions 1.00 and 1.10 of IBM® Personal Computer™ DOS are troublesome for both collectors and digital historians1 to work with! Unlike later versions, the media they were shipped on not only used 8 sectors per track (instead of the later 9 SPT used by the more common 180/360 KiB diskettes), but they neither contain a true BPB area (BIOS Parameter Block) nor what has become the standard Signature Word (AA55h) at the end of all other Boot Records.

So, apart from the fact that it can be difficult to locate a working 5-1/4 inch floppy drive these days, many utility programs and all Windows® Operating Systems consider these 160 KiB diskettes to be incorrectly formatted! If your DOS OS also has trouble with these diskettes, the problem is usually with the computer's BIOS2. This means you may not be able to run nor even extract any files from an IBM PC DOS 1.xx diskette using any version of DOS. If your computer suffers from this problem, it will still be able to perform a directory listing of all their files though! That's because the Directory contents will always be contained within sectors 4 through 7, thus avoiding the errors that occur when your OS (using info from the BIOS) simply assumes the diskette must have 9 sectors per track (instead of the correct 8 SPT), and passes that along to the drive controller; forcing the floppy drive to look in the wrong physical location for any sector number greater than 8. If you try to Explore one of these diskettes under Windows™ 2000/XP, a warning or error dialog-box will pop up! And if you try to access the diskette at a Command-line prompt, it will result in this error message:

The disk media is not recognized. It may not be formatted.

As we mentioned on our DOS 1.00 index page, it's possible to use the BOCHS Emulator to access every sector of a 160 KiB disk's image file, but you still need a way to create those image files!

You should, of course, never try to image (make an exact copy a diskette) nor read any data from an original OS diskette unless you first make sure the diskette has been properly write-protected, that the drive's write protect mechanism is functioning correctly, and that the drive will not damage the diskette! Therefore, you should first test your floppy drive with some spare diskette of no concern to you and examine it for any physical damage, then make sure the PC can not write to it when its write-protect notch has been appropriately covered. (Note: It would be highly unlikely for you to find any of the earliest DOS distribution diskettes that did not have one of these write-protect notches at the edge of the diskette; only much later on did software vendors release disks that were intended to never be written to!) Today, it would be almost impossible to find one of the thick opaque sticky tabs that were always included with boxes of blank 5-1/4 diskettes. These tabs were supposed to remain firmly in place so a mechanical pin could never press through the notched portion of the diskette. Later on, drive manufacturers began using a small LED (with a beam of light) and receptor/sensor (that recognized the light) instead of a pin, so these tabs had to be opaque to block that light as well! If your diskette already has a tab on it, we'd advise you to check its condition before relying upon it; the adhesive may have become useless, depending on its age and reliability! If so, you may wish to reinforce the original tab by placing a special NON-permanent tape over it (that will not leave any sticky residue on the diskette, should you decide to remove it and/or sell it as a 'collectible' item).

Although a PC's write-protection mechanism is generally reliable, it could easily be called into question during a legal trial. Why? Because it may not only depend upon the drive, but also the motherboard interface and even the operating system! That's why forensic experts avoid any mistakes and legal hassles by using a method that's guaranteed to always work; this method is also employed by collectors or historians who care a great deal about their original media and/or have forensics training: You must either locate or physically alter a disk drive so it's impossible for the drive to ever write to a diskette! To do this, the write signal line coming from the motherboard must be cut open on the drive and kept at the specified DC signal level by attaching either a pull-up or pull-down resistor to the appropriate voltage level; this would normally require the skills of an electronics technician and enough knowledge of a PC's drive electronics to accomplish correctly. A more simplistic approach which defeats only the write-protect mechanism of the drive, still leaves it dependent upon other factors; though perhaps more reliable than using only a write-protect tab, assuming the job is done correctly.

In either case, the work must be thoroughly tested before making use of it. Thus, the forensic method employed by experts allows you to be confident that nothing could be written to the disk and avoids the necessity of placing anything sticky on its surface, keeping it in its pristine state.

Creating a Working Copy

We don't recall now exactly how it was done, perhaps we borrowed someone's old IBM PC with a BIOS that could still understand 160 KiB drives, but after making an image of our original diskette, we were able to write it back to an old 360 KiB diskette as a 160 KiB diskette (under DOS) so we had a working copy that could boot-up on another computer with a 5.25-inch floppy drive.

Note, however, that both the BASIC.COM and the BASICA.COM programs from these diskettes will most likely cause any computer other than a true IBM® PC to lock-up during execution! The reason is that those programs were written to make use of IBM® ROM BASIC, which is only found in IBM machines! If you wish to attempt to run the *.BAS programs from these diskettes on Non-IBM PC, you will need to use an OEM copy of BASIC made by Microsoft® for the "clone" machines. These programs were most often distributed with the filename GWBASIC.EXE.



1 Digital Historian. Someone who is meticulous enough to deal with all the technical aspects of computer data, such as a lab technician trained in computer forensics, and one who also feels it's important for history to try preserving the actual binary code on the original media (if possible; or image files otherwise) of an early operating system. Ray Duncan, in a review of a few trade books on the history of the PC, made the following insightful (or should we say hindsightful) comments about the loss of both early computing hardware and software:

It's pathetic how many of the artifacts of early digital computing have already been consigned to the junkpiles and paper shredders, in spite of people such as Gwen Bell at the Boston Computer Museum who have been making an earnest effort in recent years to recover and safeguard some of the most precious machines, program listings, and documentation. I'm as guilty of this heedless destruction of our heritage as anyone. When I owned my first Imsai 8080 computer, I had a copy of the source code for Gates and Allen's original 4K ROM Basic, and I threw it away when I sold the Imsai because I couldn't imagine ever needing it!
[From: "It Takes a Great Deal of History to Produce a Little Literature," Dr. Dobb's Journal, March, 1992.]

2 The BIOS code in your computer has a great deal more to do with its operation than most people think! At one point in time, we were unable to access 160 KiB diskettes even under a real 16-bit DOS OSs with a particular computer. After we "flashed" that computer's BIOS chip with some newer BIOS code in order to access hard drives up to 137 GB (it had been limited to only about 32 GB with the original BIOS code), we were later surprised to discover the computer could then access beyond the first 8 sectors of any 160 KiB diskette! Thus, we concluded that changing the BIOS code had affected the operation of our 5-1/4 inch floppy drive as far as being able to read from and write to diskette media having only 8 sectors per track.


Updated: 25 September 2005 (25-09-2005).
Last Update: February 13, 2015 (13-02-2015).

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