ADVENT #12 — minidisc

A lot of the traceability of Alice comes not from the fact that the tape is write-once, but that every read of a cell always returns the same value. There is no thread-visible test for emptiness.

Is this really true, though? Can this be true? My sense is that very few things can be true, though I’m unqualified to offer formal opinion. If I could guarantee this in the system, it would come with a cost. Can I make this cost worth paying? That’s what would mean that the property is preserved in the implementation over time.

There are two obvious ways to leak a full/empty test into an FFI interface. A cooperative FFI can certainly leak this information but only by passing a result back that is memorialized in tape, and which can therefore be replayed. The other would be by forking two calls using the same frame, with one guaranteed to return and the other returning only if a cell was nonempty and seeing which one actually did return. The current implementation manages this by allowing a frame to be used for fork at most one time. How do we manage that without keeping a shadow state or creating yet another write-once tape cell? More on that in an upcoming installment.

Seem needlessly circular, wrapped in a cumbersome protective layer, and limited in utility? Well, Sony MiniDisc has entered the chat. The MiniDisc was everything Sony was looking for in a removable media — its mechanical design was proprietary, its media encoding was proprietary, it incorporated DRM. That wasn’t enough for Sony’s customers outside niche markets. It’s easy to say that it got clobbered by the recordable CD but it really just got walked over without a look back. It was marketed mostly as a digital audio system but there were a couple of data versions (of course incompatible) with the audio discs. The discs were magneto-optical, but not in the same sense as the flopticals we discussed earlier. For reading, they used the Kerr effect — a polarization change as laser light reflects from a magnetic material. For writing, the same laser heated the media to the Curie point to allow low-energy writing and the disk was then written by a conventional magnetic head. This meant that the media could be read indefinitely without contact.

MiniDisc makes a cameo appearance in the movie The Matrix, though it’s never clear just for what. I always thought it was portrayed like a delivery vehicle for a holographic cyber drug. Not just a removeable media in that case, but a consumable one. A dosed media. This may be the purest and most uncut vision of what Sony wanted all along. The movie scene that explains what the disc was for actually was cut, though, and better for it. Early scripts show it was about fixing parking tickets and not getting exotic kicks from digital data.