Jalapeno <jalapeno1@mac.com> wrote:

> In article <1213181.YYNYKHMJ@news.telusplanet.net>,
>  Simon Williams <DON'Temail@luddite.ca> wrote:
> 
> > And furthermore... (not sure if this is even a valid question) does the
> > GS run P8 applications directly or is there some kind of emulator in
> > ROM ???
> 
> The MEGA II on the motherboard is a nearly complete IIe in a chip. I'm
> sure David E. could give a good explanation (and likely has, so just do
> a google search for all his articles, print them out, sort them, take
> them to a copy shop and have them bound into a reference manual ;o)

:-)

To summarise: the Mega II is an ASIC (Application Specific Integrated
Circuit) which provides most of the hardware from a IIe or IIc
motherboard.  In combination with a power supply, a CPU, 16KB of ROM,
128KB of dynamic RAM and possibly some support circuitry, it gives you a
complete 8-bit Apple II.

If you compare it to the IIe or IIc, the Mega II is the equivalent of
the IOU and MMU chips, plus the video and keyboard ROMs (supporting all
display languages) and probably the keyboard encoder, together with some
other bits and pieces.  It also includes a mouse controller.  Its mouse
and keyboard support are not used in the IIgs - they are implemented
through a separate ADB microcontroller.

The Mega II doesn't include a disk controller - a separate IWM chip is
used.

Apart from the IIgs, the Mega II was also used in the IIe emulation card
for the Macintosh LC.  It might have been used on the Video Overlay Card
as well (I have a broken one around here somewhere, but it would take
too long to find it).  If Apple had continued to develop the 8-bit Apple
II models, they could have incorporated the Mega II into future models,
saving a fair amount of space on the motherboard (potentially allowing
an even smaller IIc, for example).

In the IIgs, the Mega II is responsible for the "slow" side of the
computer, including most of the I/O space, RAM banks $E0 and $E1, and
all video modes except Super Hi-res.

The Mega II is supplemented with other ASICs to implement the "fast"
side of the computer and other features which are unique to the IIgs.
The two major components are the FPI (Fast Processor Interface) and VGC
(Video Graphics Controller).  The FPI was renamed as the CYA (probably
"Control Your Apple") in the ROM 3 IIgs.

The FPI/CYA is responsible for the ROM, "fast" RAM on the motherboard
and memory expansion slot, and coordinating the CPU with the rest of the
machine, controlling its clock speed and handling sychronization.  It
also provides the shadowed video and I/O mechanisms which allows banks 0
and 1 to behave like an 8-bit Apple II.

The VGC implements Super Hi-res graphics mode and provides support for
the RGB monitor.  (It is also used on the Video Overlay Card.)

There are various other components in the IIgs that provide support for
specific areas, including the real time clock and parameter RAM chip,
the Ensoniq digital oscillator chip, the ADB/keyboard microcontroller,
the IWM disk controller, the Z8530 Serial Communication Controller, etc.

-- 
David Empson
dempson@actrix.gen.nz