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​mg_notes:iie_card:gemini_vs_megaii [2019/08/26 03:56] (current)
M.G. created
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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 \\ 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 |