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 MDBUS '816 Address Bus (low 16 bits) SBUS | PDS Address PDS Data 6502 Address 6502 Data | http://www.classiccmp.org/cini/pdf/Apple/Apple%20IIgs%20schematic.pdf 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 | http://www.1000bit.it/support/manuali/apple/technotes/aiie/tn.aiie.10.html |