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Emulating the Early Macintosh Floppy Drive
I have been working on an emulator for early (Motorola 68000-powered) Macintosh computers. While implementing the disk drive, I noticed documentation was scattered and hard to find. Now that I have a working implementation, this post is my attempt to document everything in one place.
The 68000 CPU interfaces with the disk drive through a controller, called ‘IWM’ (Integrated Woz Machine, a reference to Apple co-founder Steve Wozniak, who designed it). In the Macintosh 128K and 512K with the 400K disk drives, the speed of the spindle motor is controlled by software using a PWM signal that is written to the LSB of every 16-bit slot in the sound buffer. At boot, the Macintosh ROM performs an initial calibration by setting the drive to two different speeds and reading back the tachometer signal (TACH register).
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