Groups | Search | Server Info | Login | Register

Groups > comp.sys.apple2 > #48855

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

Show all headers | View raw

Here's some information from David A. Lyons dated December 1988:

The slow side of the GS runs at 1MHz, and exists in the first place, for a
very good reason (and complexity is not it!).  The old Apple II video modes
(text, lores graphics, 80-col text, double-lores graphics, hires graphics,
and double-hires graphics) are designed so that the display hardware needs
to read from the display memory (which is in Slow RAM on the GS) once each
microsecond to get data 
to feed to the screen.

When the CPU was running at 1MHz, this worked out perfectly, since the 6502
only needs access to its RAM during half of the clock cycle (which is 1
microsecond with a clock speed of 1MHz).  No cycle stealing of any kind was
needed with a 1MHz CPU. 

As I understand the GS, the FPI (Fast Processor Interface) synchronizes the
fast and slow sides of the system by making the 65816 wait, during access to
slow RAM, for the appropriate part of the slow side's clock cycle.  By
"access to slow RAM" I mean a read or write of a location from $E00000 to
$E1FFFF, or a Write (not a read) to an area of fast RAM that happens to be
shadowed into slow RAM; shadowed RAM normally includes the text screen and
$Cxxx I/O locations, and also the hires/double-hires areas except when
ProDOS 16 or GS/OS is active. 

https://macgui.com/usenet/?group=7&id=8690



Todd P. Whitesel was not a fan of the Mega II and the hardware design of the
IIgs. In February 1990 he wrote:

The //gs was not designed from scratch in any sense of the word. The Mega //
was originally designed to replace the //e chip set but wasn't cheap enough.
The decision to run with the Mega // was the worst made by the //gs design
team, and the next was the way they limited its expansion to the chips that
were available when it first came out. 4 mhz 65816s were quickly available,
and some very easy design changes could have been made which would have
improved the video I/O performance. Since you have implied that you aren't a
hardware guru I won't bore you with details, but I can support my contention
that the //gs hardware was neutered in many places. 

https://macgui.com/usenet/?group=7&id=21427

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

Back to comp.sys.apple2 | Previous | Next — Previous in thread | Next in thread | Find similar

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

csiph-web