Apple IIc Plus MIG Chip

The MIG is a custom chip in the Apple IIc Plus that provides signals for accessing 3.5" disk drives and access to a 2K static RAM chip mainly used to buffer 3.5" sector data for reading and writing. The RAM is also used for the accelerator.

MIG RAM

Access to the MIG RAM is via a 512-byte window at $CE00 (mirrored at $DE00) that becomes visible only when the aux firmware bank is selected. The first 32 bytes of the window address the RAM, the rest are control signals or unknown purpose. The visible 32-byte page of the window is selected as follows:

  • Access $CEA0 - Set page 0
  • Access $CE20 - Increment page

Other locations in the MIG likely change the state of the head select and other drive signals.

The following page usages have been noted:

  • Page 0 and 1 - Used by the 3.5" firmware.
  • Page 2 - Used by the accelerator firmware.
  • Page 3+ - Used by the 3.5" firmware.

The contents of the MIG RAM can be viewed using my MIG Inspector.