Two days ago I wrote to Steve Phillips, who worked for GCC and was on the latter-year HyperDrive team in the past. Steve was kind enough to offer me the following information this morning:

HyperDrive intercepted the Mac boot sequence. When the 68000 started up, it was hardcoded to jump to a specific I/O address. By playing some games with address lines, I believe that the HyperDrive swapped the memory map around so that its own ROM was dispatched to instead of the Mac boot ROM. Once our software had control, it could install the hooks which allowed it to intercept disk accesses.

Click to expand...

Click to expand...

This bit of lore is consistent with what I posted in another thread. I met a guy at the Austin Goodwill Computer Works Store who used to program Mac hardware back in the very old days. He said there was a jump in the early portion of the Mac boot code which allowed developers to sieze control of the boot process by adding hardware with code resident at that address to which the Mac jumped.

I got his card, but I've never had any real reason to contact him.