The early Macs used the IWM, which is basically Wozniak's 1977 Disk II controller reduced to one chip. The same trick with cycle-exact code, that was used in the Apple II, is also used in the Mac.

It's why the cursor stops moving sporadically when writing to a disk. The Mac has a 60 Hz interrupt timer that also tracks the cursor. It needs to be switched off when writing.

There's a story on Folklore.org by Andy Hertzfeld that mentions it in passing:

> Woz's disk technology required that the software feed it new data every 32 microseconds exactly. If we were even a single microsecond early or late, it would cause a glitch in the data and ruin it. In order to write the routines, I needed to know how fast the Macintosh executed each instruction. The manual gave the number of clocks for each instruction, but I wasn't sure how long it took to fetch from memory. So of course, I asked Burrell what the timings were, but I was surprised at his response.

> "I don't know. The Mac is synchronous, just like the Apple II, so each instruction has the same timing, every time you execute it, so you will be able to write disk routines that have exact timing. I don't know what it is, so we'll just measure it. Why don't you write your routine and we'll measure it with the logic analyzer."

-- https://www.folklore.org/Nybbles.html

This reminds me that all of the unusual Apple II disk stuff like spiral tracks and different-sized sectors and different nibbilization schemes were also, at least theoretically, possible on the Mac. I wonder if they were ever used for copy protection?