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The Mega II, was Re: How does the Apple IIGS emulate a IIe?

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Mitchell Spector wrote:
> 
>     On a side note, if the Mega II was NOT responsible for Apple IIe
> emulation, and mainly just used as an I/O controller that bottlenecked
> the IIGS bus and video draws to 1 MHz, one questions why on earth
> Apple didn't scrap the Mega II and design a replacement chip for the
> IIGS in all those years? It seems like it was added to the IIGS simply 
> because it happened to be sitting unused in Apple's development 
> tool box (the Mega II was originally developed for other purposes).
> 

In March 1990, Todd P. Whitesel said: "The Mega II has to run at 1 Mhz and
in the GS that's a big problem. If they had implemented its functions in a
more distributed manner then the only part of the machine that would run
slow would be the actual expansion slots. When the GS was originally
designed they barely had the technological capability to do this [chip
design], but for a couple of years now it is has been well within Apple's
custom chip capability. 

https://macgui.com/usenet/?group=1&id=24939


He further said: "The Mega II "Apple II on a chip" is the Ball and Chain of
the GS -- it was originally designed for a low cost //e but wasn't cheap
enough to make the //e any cheaper. (to Apple, apparently. Certainly not to
us.)

When they get rid of it and implement the logic where it belongs (i.e. all
over the machine and integrated into the custom chips that handle each part
of the system already) it will blow away the performance limitations of the
current design and cost a hell of a lot less. 

Do not assume that the IIGS is the best that the Apple II can do. You would
be doing the machine a serious injustice, and there are a number of specific
examples which are so trivial to fix that they would have done so long ago
had 
they been given more than miserable funding for the project. "

https://macgui.com/usenet/?group=1&id=24989 (at the bottom of his message)



In May 1990, Brian Willoughby explained: "The Mega II is not bad.  The Mega
// exists to support the old Apple video (along with some other I/O specific
hardware).  The Mega // runs at 1 MHz purely because that is the rate at
which the video information is needed for every screen mode except Super
HiRes.  It would do no good to speed up the Mega //, because the
functionality it provides would not be improved at all. 

The only thing that needs improvement is the way in which the Mega //
accesses video RAM.  Here is where the bottleneck appears.  If Apple Co.
were to integrate the Video RAM from the Mac SE/030 into the GS, then the
CPU could access video RAM with no contention from the Mega //.  In other
words,dual port RAM would no longer require the the CPU to slow down to 1
MHz for video I/O. 

What we need is not a replacement for the Mega //, but a way for the Mega //
to share RAM with a very fast processor without adding wait states. 

https://macgui.com/usenet/?group=1&id=43028

-- 
]DF$
The New Apple II User's Guide:
https://macgui.com/newa2guide/

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Thread

How does the Apple IIGS emulate a IIe? Mitchell Spector <mitch2gs@hotmail.com> - 2024-02-11 02:38 -0500
  Re: How does the Apple IIGS emulate a IIe? Antoine Vignau <ntn.vignau@gmail.com> - 2024-02-11 11:12 -0800
  The Mega II, was Re: How does the Apple IIGS emulate a IIe? D Finnigan <dog_cow@macgui.com> - 2024-02-13 17:56 +0000
    Re: The Mega II, was Re: How does the Apple IIGS emulate aIIe? D Finnigan <dog_cow@macgui.com> - 2024-02-13 18:04 +0000
      Re: The Mega II, was Re: How does the Apple IIGS emulate aIIe? Antoine Vignau <ntn.vignau@gmail.com> - 2024-02-13 11:42 -0800
    Re: The Mega II, was Re: How does the Apple IIGS emulate a IIe? Yeechang Lee <ylee@columbia.edu> - 2024-02-13 12:11 -0800
  Re: How does the Apple IIGS emulate a IIe? kelvin@pro-kegs.uucp (Kelvin Sherlock) - 2024-02-16 00:31 -0500
    Re: How does the Apple IIGS emulate a IIe? D Finnigan <dog_cow@macgui.com> - 2024-02-16 13:59 +0000
      Re: How does the Apple IIGS emulate a IIe? D Finnigan <dog_cow@macgui.com> - 2024-02-16 14:05 +0000
        Re: How does the Apple IIGS emulate a IIe? Mitchell Spector <mitch2gs@hotmail.com> - 2024-02-17 23:58 -0500
          Re: How does the Apple IIGS emulate a IIe? D Finnigan <dog_cow@macgui.com> - 2024-02-19 14:50 +0000
      Re: How does the Apple IIGS emulate a IIe? kegs@provalid.com (Kent Dickey) - 2024-02-19 21:56 +0000
        Re: How does the Apple IIGS emulate a IIe? D Finnigan <dog_cow@macgui.com> - 2024-02-20 14:09 +0000
          Re: How does the Apple IIGS emulate a IIe? kegs@provalid.com (Kent Dickey) - 2024-02-20 16:55 +0000
            Re: How does the Apple IIGS emulate a IIe? D Finnigan <dog_cow@macgui.com> - 2024-02-20 18:32 +0000
              Re: How does the Apple IIGS emulate a IIe? "James Hall [VE3MYZ] FN25dj. 73" <james.hall.ottawa@gmail.com> - 2024-02-21 23:34 -0800
  Re: How does the Apple IIGS emulate a IIe? David Schmidt <schmidtd@my-deja.com> - 2024-02-16 09:28 -0500
  Re: How does the Apple IIGS emulate a IIe? kegs@provalid.com (Kent Dickey) - 2024-02-19 21:45 +0000
    Re: How does the Apple IIGS emulate a IIe? Mitchell Spector <mitch2gs@hotmail.com> - 2024-02-19 19:37 -0500
      Re: How does the Apple IIGS emulate a IIe? Steve Nickolas <usotsuki@buric.co> - 2024-02-19 22:38 -0500

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