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Gemini vs Mega II

The Gemini chip of the Apple //e Card is often compared to the Mega II chip of the Apple IIgs. The descriptions often vary from “newer version of” or “slightly revised” when comparing the the two. I believe these statements to be incorrect, and will attempt to demonstrate such via a direct comparison:

Criteria Mega II Gemini References and Notes
Pin Count 84 128
Busses ADBUS
'816 Address Bus (low 16 bits)
PDS Address
PDS Data
6502 Address
6502 Data
SBUS → Slotmaker on IIgs
CPU Control No Yes On IIgs, these are generated by the FPI.
DRAM bus width 8 bits 9 bits
Video Circuit Yes No See IIgs Schematic, above.
Floating Bus Yes No
Slot bus signals Yes No Gemini only supports real I/O for an IWM in “slot” 6.
3.5DRIVE and HDSEL No Yes VGC handles these on IIgs.
Mouse Interface Yes No
Traps/CPU Pause No Yes