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February 6, 2015

The Original IBM PC’s Storage Secret

Filed under: Main — admin @ 12:01 am

“Eight hundred dollars! Are you serious?” Yes, Radio Shack was serious. That was the cost of upgrading my TRS-80 Model III with a floppy disk drive. The upgrade also boosted system RAM from 16KB up to the maximum 48KB. But I wanted that disk drive!

More important than the disk drive was the issue of how information was stored and retrieved on a computer — technically, a microcomputer back then — without any mass storage devices. It’s something you don’t even consider today.

Forget the hard drive. Tandy offered its first Radio Shack TRS-80 hard drive a few months after I purchased my Model III. It cost about $10,000 for 5MB of storage. (Curiously, that was flat storage; the TRS-80’s operating system lacked a hierarchical directory structure.) I had no idea what I’d do with 5MB, let alone where I’d find the $10K. So a floppy drive that ate 180KB diskettes would be good enough for me.

Being a restaurant slave, I had to work and work to save up for the $800 floppy drive upgrade. In the meantime, I used what most other early adopters used: A cassette tape.

I purchased the special Radio Shack tape recorder, model CTR-80A, where the CTR probably stood for Computer Tape Recorder. Figure 1 illustrates the gizmo, which I’ve retained for over 30 years now — the only relic from my first computer purchase.

TRS-80 cassette recorder

Figure 1. My first computer storage device. The kids broke off the cassette cover a few years back.

While it may seem odd to use a cassette recorder for computer data, it worked. The information was recorded by translating digital signals to audio, which is exactly what a dialup modem does. The process was slow and it went like this:

  1. Rewind the tape.
  2. Type the CLOAD command, followed by the filename.
  3. Press the PLAY button on the recorder.
  4. Wait.
  5. When the program had loaded, press the STOP button on the recorder.

It took a while to load a program, especially when you had several programs stored on a single cassette. Mostly I used the cassettes to load games, but I did save some of my early programs.

The secret of original IBM PC is that it also used cassette tape to store and retrieve files.

I purchased my PC from my employer at the time. It was the original, model 5150. On the back was a Cassette Port. In fact, for years, PC DOS included the CAS device, similar to CON, PRN, and other standard I/O devices. You could write programs that accessed that Cassette Port. I never did, but I could.

IBM discontinued the Cassette Port on the next generation PC. I asked a computer dealer if he knew anyone who ever used the Cassette Port. He said that all the IBM PCs were sold with one or two floppy drives. He knew no one who used tape to store files.

As I recall, I didn’t even think to hook up my PC to the CTR-80 to see whether or not it worked. I mean, the thing had floppy drives! Two of them! And both stored a beefy 320KB of data. I was in heaven! But, yes, once upon a time, the IBM PC could use cassette tape for file storage.

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