From: mitch2gs@hotmail.com   
      
   kegs@provalid.com (Kent Dickey) wrote:   
      
   >In article ,   
   >Mitchell Spector   wrote:   
   >   
   >>    Then I saw James Lewis' project to build a single chip Apple II, and   
   >>read his claim the Mega II's primary (sole?) function is to provides 8-bit   
   >>video modes. So, that got me curious. What, exactly, is allowing the   
   >>IIGS to emulate the Apple IIe?   
   >   
   >Here's how the IIgs works:   
   [snip]   
   >So, the FPI is the magic that talks to the slow Apple II side.  The Mega II   
   >is just one of the things it talks to, which is just most of Apple //e   
   >logic rolled into one chip to take less board space.   
   >   
   >The Mega II is MMU, IOU, and non-SHR video generation on a IIgs, but   
   >only for banks $E0 and $E1.  So the FPI has to ALSO have the MMU logic   
   >in it since it affects fast side memory as well.  One way to think of   
   >the Mega II is it controls the memory in banks $E0 and $E1, and that is   
   >the Apple //e part, so it's doing the MMU features for that memory.  But   
   >the FPI is handling all other banks, so it has replicated the MMU logic,   
   >too (aux/main and LC switches are all in the FPI, too, as well as the   
   >Mega II).  The FPI just replicates the needed logic to get the Apple //e   
   >compatibility right.  The FPI handles shadowing, passing writes to the   
   >video memory in banks $00 and $01 to also go to the Mega II to update   
   >the bank $E0 and $E1 memory.  So this is likely where some confusion   
   >arises--the FPI also has to do lots of things for Apple //e   
   >compatibility, and does it in parallel, duplicating some Mega II   
   >functionality.   
      
       Thanks for providing all that, it provides more of a picture of what's   
   going on behind the scenes when emulating an Apple IIe on a IIGS.   
      
       I think the short answer falls somewhere between what James Lewis   
   claimed in 2022, and what Apple claimed back in 1986. Neither are   
   entirely correct. It's somewhere in the middle.   
      
       Would it be a fair assessment to say, then, the Mega II is not solely   
   responsible for emulating the Apple IIe, but nor is it almost completely   
   sitting unused (jn 8-bit emulation mode) and just merely "spitting out"   
   some classic Apple II video modes as needed?   
      
       This is unlike the Gemini chip on the Apple IIe PDS card. In that   
   situation this Apple IIe-on-a-chip takes over the host machine and   
   the Macintosh literally becomes an Apple IIe (for the most part, video   
   of course is emulated by QuickDraw, and there is still a custom GUI   
   control panel accessible when it's suspended).   
      
       I think the misconception has been: when the IIGS goes into 8-bit   
   emulation mode, the Mega II kicks in and takes over full control like   
   the Gemini--it does NOT. Likely why, perhaps, some IIGS-specific   
   features can still be accessible under Applesoft BASIC (through peeks   
   and pokes, I've seen the Ensoniq and SHR graphics used). Or how   
   there's been hybrid games possible, essentially 8-bit software with   
   some glorified audio/visual effects mixed in (e.g. Paperboy, Gauntlet,   
   John Madden's Football).   
      
       Interestingly, on a side note, it makes it sound like the Mega II is   
   more tied into native IIGS operations than one might think, and kind   
   of dispells the notion the IIGS isn't really an Apple II at heart. I've   
   seen the IIGS looked at as if it were an entirely new platform, that   
   just happens to have backwards compatibility with the Apple IIe.   
   Maybe that would be another discussion, is the IIGS really an Apple II? :)   
      
   Mitchell Spector   
      
   --- SoupGate-Win32 v1.05   
    * Origin: you cannot sedate... all the things you hate (1:229/2)