The Commodore 64’s Forgotten CD-ROM

With a 12 year production run, the Commodore 64 is not just one of the most popular computers ever — it still holds the title of the best selling single computer model of all time. That successful run did spill over into the early 1990s, but the height of the machine’s popularity was firmly planted in the 1980s. Being a budget-friendly, consumer-oriented computer, you could find the typical mass storage options of the time available for it, like floppy and tape drives. But certainly nothing like a CD-ROM drive was available, as they did not really enter the market for mainstream users until the mid-1990s.

Or so you probably thought. But mass storage on CD-ROM was a real thing for the Commodore 64, although it was not very popular at the time and few are aware it ever existed. So that we can all have a look at this forgotten technology, YouTuber Wolfgang Kierdorf recently took a look at a CD-ROM loaded with classic games like Impossible Mission, M.U.L.E., and Lode Runner. In total, the disc contains 10 games and 10 audio tracks. Try that with a 1541 disk drive!

Sounds amazing, right? Yes, but this system is not everything that it seems to be at first blush. When you think of CD-ROMs, you think of digital data and relatively fast load times, but that is not how this storage device works. Rather, it is much more like a glorified cassette tape drive. In fact, the CD-ROM comes with an adapter that plugs into the Commodore’s Cassette Port. The other side of the adapter has an RCA plug that connects to the audio output of any standard CD player.

By now, you may have guessed the trick. Data is encoded into an analog audio signal, which the computer demodulates in the same way as it does with a cassette tape. In fact, the standard “LOAD” command is even used to transfer data from the CD to the Commodore’s memory. So while the data is stored in digital format on the CD-ROM, it is played as analog audio, then demodulated back into digital data. And the loads are just as slow as you remember cassette loads being.

Even still, there was a pretty good use case for this system. Lots of software could be stored on a single, reliable disc. And there was a pretty nifty system for choosing the software to load. The first track of the disc contained a menu that allowed the user to cursor around and select a game. When the game was selected, it would specify which track should be played. After the user manually started that track on the CD player and pressed a button on the computer, the load would start.

Combining CD-ROM storage with the Commodore 64 is an odd technology mashup that feels something like the existence of a liger or a tigon — a thing that is possible, but that should not naturally be. In any case, now you know, so you can win the trivia competition at the next Vintage Computer Festival or bore — uhh, I mean amaze — your friends with obscure facts.