3 ms·
Let me see if I understand this. The disk authoring machine would lay down a true spiral, right? And the software controlling the read head would know this, and
by function_seven 8y ago
Let me see if I understand this. The disk authoring machine would lay down a true spiral, right? And the software controlling the read head would know this, and follow/emulate that spiral by stepping the head in a 1/4 track every 90°? So while the continuous spiral moved underneath the read head, the read head would mostly follow it but with some "quantization" error? (Not large enough to cause issue with reading, but impossible for an OS-level utility to replicate when copying the disk)
- kazinator 8y agoThe thing is, if the OS-level utility replicated that with quarter-track increments so that the same data is read back, it would be fooled. The consumer Apple II Drive (Disk II) has only quarter track increments; it cannot follow a smooth spiral. If a copy protection is based on a smooth spiral, it has to somehow depend on detecting the quantization errors due to the track skew, the absence of which reveals that there is a quantized, piece-wise spiral. If the skew didn't cause any issues with reading, then the software couldn't tell the difference between the smooth spiral and the fake staircased one.
- function_seven 8y agoI'm thinking that the OS-level diskcopy tool doesn't expect to see such a wacky layout, and would just fail to track along what it expects to be concentric circles. Only the software that's aware of the custom layout would be able to read the disk, by taking direct low level control of it. And if the data is laid out in a true spiral, mimicking that with 1/4 shifts every 90 deg. might be close enough to never lose track of the, erm, track.