Working with Old Diskettes


I've done a lot of work recently with my old 360KB diskettes and I am reminded how much I hate these things and how frustrating they can be. Hard drives have really made computing so much more productive and enjoyable. Here is some background information on diskettes and some tricks for using them that you might find useful. None of this is specific to the PCjr.

Background

The PCjr uses a 5.25 inch double density diskette drive that is capable of holding 360 KB of data. This was the standard type of diskette drive for IBM compatible personal computers at the time. Other personal computers such as the Apple ][ and the Commodore 64 also used double density 5.25 inch diskettes with disk drives of varying capacities. High density 5.25 inch diskette drives holding 1.2 MB of data came later in August 1984 when the IBM PC AT was announced. Double density 3.5 inch diskette drives were available on some PC clones, and then first used by IBM on the IBM PC Convertible in 1984 or 1985. Other personal computers (the Macintosh in particular) used 3.5 inch diskette drives, but with a different formatting scheme. High density 3.5 inch diskette drives holding 1.44 MB and quad-density 3.5 inch diskette drives holding 2.88 MB came later on.

Anatomy of a Diskette

Diskettes are round magnetic media in square protective jackets. The diskette drive clamps the media in the center, and rotates it to gain access to the different areas of the diskette. Nothing fancy here.

Diskettes are either single sided or double sided. Most diskettes are double sided, meaning that both surfaces are suitable for recording. Early diskettes may be single sided, meaning that only one surface was certified for data storage.

Data is laid out on the surface of a diskette in tracks and sectors. A track is a concentric ring at a fixed location from the center of the diskette. A sector is a small area of a track. Unlike a vinyl record (which was current technology when diskettes were invented) or a CD-ROM, there are multiple tracks on a diskette, not just one spiral. A stepper motor in the diskette drive positions the drive heads to the desired track.

Data is written to the diskette a sector at a time. There is no way to change just one byte of data - to change one byte, the sector containing the byte must be read, the new byte needs to be put in the buffer holding the sector, and the entire sector must be written out. Sector size is set by the diskette drive and controller. IBM PC clones generally use a 512 byte sector.

A diskette drive "formats" a diskette before it stores data on it. The formatting process can be thought of as laying out "street signs", so that data stored on the diskette can be found later. The "street signs" are numbers, identifying areas on the diskette. Diskettes can be formatted in many different ways, but in general only a few well-known formats are used. The format used depends on the diskette drive and the diskette.

The Double Density 5.25 Inch Diskette Drive:

IBM introduced the PC with a single sided double density 5.25 inch diskette drive. This drive formatted diskettes to a 160 KB capacity. This drive was quickly replaced with double sided double density drives capable of formatting a diskette to 360KB in size. The PC XT and PCjr used the newer double sided drives. Below are the formatting parameters for this drive:

Capacity Tracks/Sides/Sectors DOS Format command flag: Comments:
160 KB 40/1/8 /f160 single sided: lowest common denominator
180 KB 40/1/9 /f180 single sided
320 KB 40/2/8 /f320 oddball format - not used too often
360 KB 40/2/9 /f360 "normal" format for 360KB disk drive

Sector size for IBM compatible computers is 512 bytes. Knowing the number of sides being used, the number of tracks, and the number of sectors in a track, you can compute the amount of data a diskette holds by using simple multiplication. For example, the 360 KB format shown above uses 40 tracks * 2 sides * 9 sectors per track * 512 byes per sector, for a total of 368640 bytes on a diskette. (368640 = 360 KB)

This drive used double density diskettes.

A 5.25 inch diskette is fragile compared to a newer 3.5 inch diskette. The outer jacket is very flexible, and there is a large oval shaped hole where the media is completely exposed. The 5.25 inch diskette is write protected by placing an opaque "tab" over the write enable notch on the side of the diskette. (Scotch tape might not work because both optical and mechanical sensors were used on the write protect mechanism!).

Be careful when inserting a 5.25" diskette! When you close the drive door or lever, the drive clamps onto the diskette at the center. Close the door or lever slowly to allow the diskette to center on the spindle; if you close it too fast you might not get the diskette clamped on-center, which will make it impossible for the diskette drive heads to follow the circular tracks!

The design flaws of the 5.25 inch diskette were corrected on the 3.5" diskette. The 3.5" diskette has a harder casing, the media is protected by a shutter, and the hub ring was replaced by a much sturdier mechanism.

An improvement: The High Density 5.25 Inch Diskette Drive

In 1984 IBM introduced its first high density diskette drive. This diskette drive was designed to use "high density" media, which allowed for data to be packed more tightly on the diskette. This in turn increased storage capacity. Below are the parameters for the new format that this diskette drive used for high density media:

1200 KB 80/2/15 /f1200 normal format for high density media

The new diskette drive could also read and write double density media - it had the circuitry to handle both types of media. When using double density media, the existing formatting schemes were used.

A Different Improvement: The Double Density 3.5 Inch Diskette Drive

The standard 5.25 inch diskette was fragile and inconvenient to work with. The 3.5 inch diskette drive was introduced as an alternative to the 5.25 inch diskette drive. It was especially popular for portable systems, where space was at a premium. Below are the parameters for the double density 3.5 inch diskette drive:

720 KB 80/2/9 /f720 normal format for double density 3.5 inch drive

This format looks amazingly similar to the format used by the double density 5.25 inch diskette drive, except that it has twice the number of tracks. This made it easy for computer manufacturers to transition to this format - it was a very minor change. Electrically, the new diskette drives looked like the standard diskette drives.

Both Improvements Together: The High Density 3.5 Inch Diskette Drive

The double density 3.5 inch diskette was successful; the diskettes held more than the original double density 5.25 inch diskettes, the media was protected better, and they were convenient to use. Moving to high density media made for the most successful type of diskette in personal computer history - the ubiquitous 1.44 MB diskette. Below are the parameters for the high density 3.5 inch diskette drive:

1440 KB 80/2/15 /f1440 normal format for high density 3.5 inch drives

More on Diskette Drive Density:

As mentioned above, there are two major types of floppy disk media:

"Quad Density" media is also available, but it is not in widespread use, and probably never will be. The CD-ROM pretty much killed it.

The media type that you use depends on the diskette drive that you have. In general, high density drives can use high density or double density disks. Double density drives can only use double density diskettes; high density media in a double density drive is not supported. Here is a table:

Drive Type Read Write Comments
5.25 inch double density Double density only Double density only  
5.25 inch high density High density and double density High density and double density Do not write to double density disks! see below
3.5 inch double density Double density only Double density only  
3.5 inch high density High density and double density High density and double density  

If you read the chart carefully, you will notice an inconsistency. A 5.25" High density drive can read and write double density disks, but the comment says not to try writing. Although the hardware can do it, it is not recommended. Here is why:

Double density 5.25" drives use 40 tracks per side. When high density 5.25" drives were introduced, they doubled the number of tracks to 80 per side. To do this, a narrower disk drive head had to be used.

A high density 5.25" drive can read a low density diskette just fine, even though the drive head is narrow compared to the track. However, if you try to write to the diskette, you will write a new track of data that is much narrower than the existing data. This will generally make the diskette unreadable in a double density drive, because the double density drive has the old "fat" drive head that will see a narrow data track laid on top of a normal sized data track. Other high density drives may not deal with this diskette well either, depending on their calibration.

If using a double density 5.25" disk in a high density drive, only do reads from it - make sure that you do not write to it!

This does not apply to 3.5" diskettes. 3.5" drives all use 80 tracks per side, whether they are low density or high density. Therefore, they do not have the head width problem that the 5.25" drives do.

The format type that you select determines what type of media you need. The formats used on high density diskettes can not be used on double density diskettes - double density diskettes were not designed to pack data in that closely. If it works, it will be very unreliable. And likewise you should not use high density diskettes in a double density drive; a double density drive does not have the circuitry to write to high density media. If it works, it will be unreliable too. Below is the unified table with the legal formatting options for the different drives and media types:

Drive Type Media Type Formats allowed
Double Density 5.25 inch drive Double Density media Read and write 160, 180, 320, and 360 KB formats
Double Density 5.25 inch drive High Density media DO NOT USE - NOT SUPPORTED
High Density 5.25 inch drive Double Density media Read 160, 180, 320 or 360 KB formats - do not write
High Density 5.25 inch drive High Density media Read and write 1200 KB format



Double Density 3.5 inch drive Double Density media Read and write 720 KB format
Double Density 3.5 inch drive High Density media DO NOT USE - NOT SUPPORTED
High Density 3.5 inch drive Double Density media Read and write 720 KB format
High Density 3.5 inch drive High Density media Reamd and write 1440 KB format

How can you tell double density media from high density media? The diskette should be labeled with either a 'DD' or an 'HD.' Sometimes the words "Double Density" or "High Density" are used. The formatted capacity may also be specified. In the absense of any markings, a double density 5.25 inch diskette usually has a "hub ring" which reinforces the diskette where the diskette drive clamps the media while a high density diskette does not usually have a hub ring. You can't see a hub ring on a 3.5 inch diskette because the mechanism is different, so you just have to hope the diskette is labelled correctly. On a 3.5 inch diskette you can also tell a double density disk by the lack of a square hole on the opposite side of the write-protect mechanism. (High density diskettes have another hole, where double density diskettes do not.) (Thanks to Octavio Alvarez for the tip.)

Old double density diskette drives always assume that they have double density media. They will try to write to a high density diskette, but it will either fail or not work reliably. High density diskette drives usually have a "media sensor" that will tell it what type of diskette is inserted. This allows the diskette drive to fail if it is told to format a low density disk with a high density format. Some diskette drives and controller combinations did not implement this correctly, so do not rely on the hardware to prevent you from making a mistake.

Archiving Diskettes:

Every person who grew up on floppy diskettes knows the value of a good backup. Floppies are very unreliable compared to hard disk technology. Almost half of my diskette collection is devoted to backups, and that is true even though I use file compression on almost everything.

There are many ways to make backup copies of your floppy diskettes. Here are the methods I use:

File copying

You can back up selected files by copying them to another floppy diskette, to a hard drive, to a Zip drive, or to anything that DOS or Windows thinks is a filesystem. No tricks here. If you don't copy all of the files that you might need, then you run the risk of losing them. You might also miss hidden files on the diskette. Individual file copying really isn't recommended as a backup strategy because it is too easy to make an incomplete backup.

Archivers

Archivers are like file copying, except that you can copy multiple files at a time and they are usually compressed. The result is a single file, which is an archive of the files that you specified. The utility that archives the files also has a way to extract files from the archive. The archive file takes up less storage, is easier to manage than multiple files, and takes less time to transmit.

PkZip is the most common file archiver in use today. Other file archivers include InfoZip, Tar, LhArc, Arc, Zoo, etc.

As with file copying, you need to be careful about preserving directory hierarchies and storing hidden files. Just like with file copying, a forgotten file is potentially a lost file.

Diskcopy

This is a DOS command that makes an exact copy of your diskette on another diskette.. No tricks here either. Diskcopy does not work with copy protected diskettes. Diskcopy also requires that the target and source diskettes are the same type and capacity. Diskcopy can not handle errors on the diskettes.

CopyIIPC/CopyWrite/TeleDisk/Etc.

These programs are like diskcopy, except that they can accurately reproduce copy-protected diskettes. These programs were usually purchased to make backups of copy protected games. They can be used on non-copy protected disks that have read errors.

Alas, many of these programs do not work on modern hardware. The most likely culprit is their timing routines, which were geared towards PCs and ATs, not fast 80486 class machines and the latest Pentium IV behemoths. All of these also work in straight DOS. I doubt that any of them work under Windows.

Diskette images

Several programs exist that allow you to create an "image" of a diskette. An image is a copy of the sectors of the diskette that can be used to recreate the diskette. The image is usually stored in a file. The file can then be moved around and copied like any other file. A companion program is used to create a diskette from the image file. While the image of the diskette can be stored anywhere, usually the image must be written back to the same type of diskette that it came from.

Diskette imaging programs make it very easy to backup your diskettes to another medium, such as Zip or CD-R. You get a complete backup, and you probably get the benefit of putting it on something more reliable than a diskette.

Back when I was using a PCjr, I used all of these methods except for diskette images. Now that I want to preserve my diskette collection onto CD-R, I am using zip files and diskette images. For each diskette I create a zip file with all of the files of the diskette and a diskette image. The zip file will allow me to retrive specific files on any computer with a CD-ROM and the zip program. The diskette image serves as a backup of the diskette, without the bulk of 150 extra (and flakey) diskettes laying around. The diskette images can be compressed to save space.

For non-copy protected diskettes I'm creating a raw binary image. A raw binary image is just all of the sectors of the diskette, copied into a single file. No control data is added and no sectors are skipped. Raw diskette images are readily written by the Linux 'dd' command, the "rawrite" program under DOS, IBM's "loaddskf" program, and several other programs.

I want to point out two programs in particular. DITU (available on the downloads page) is small, easy to use, and runs under DOS, Windows, Etc. It is great for making images of standard DOS diskettes without read errors or copy protection. DskImage is my program for creating disk images, inspired by DITU. You will find that it is very similar to DITU, except it places more emphasis on trying to recover from read errors on diskettes. (I recommend using DskImage over DITU because of the enhanced error recovery.)

For copy protected diskettes I'm using a program called TeleDisk. TeleDisk was shareware years ago, and it can handle most forms of copy protection. Many copy protected diskettes do not have a DOS readable filesystem on them, so the zip technique is not possible on these diskettes.

The ultimate disk imager is the Central Point Option Board, which can make a disk image of a copy protected disk. It can also create a copy protected disk from the disk image. The Central Point Option Board runs on older machines (80386 or less) and there is no modern equivalent that I know of.

Whatever method you choose, remember to test it thoroughly! You need to be able to get your data into it and out of it reliably.

Handling Diskette Errors

Diskettes can go bad. Usually they start by developing "soft" errors, which are recoverable by retrying the operation several times. Eventually the soft errors turn to "hard" errors that can not be recovered.

How do you know when a diskette is starting to go bad? Your first clue will be an error message like "Disk error reading Drive x, Abort, Retry, or Ignore?" When you start to see that happen to a diskette, it's time to do something.

If you get your files off the diskette safely, there are some steps you can take with the diskette. It's not necessarily bad; it just may need some help:

It doesn't pay to try to reuse a floppy that format reports has bad sectors. If format can't handle the diskette, it has reached it's end of life. The diskette may be physically damaged, and you may lose more data in the near future.

Diskette Drive Malfunctions

Not every floppy diskette error is caused by the floppy diskette. The drives are all old now and they wear out too. Here are the problems I've run into:

Your ability to handle these problems is probably very limited. I listed them here so that you would aware, and not start tossing diskettes as soon as they look bad. It helps to have a second or third diskette drive to validate bad diskettes against.


Created in July 8th 2001, Last updated June 14th, 2014
(C)opyright Michael B. Brutman, mbbrutman at gmail.com

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