Re: A2GS Accelerator Thoughts

[doug.cotton@cmdweb.com (Doug Cotton)]

In article <1998071100354300.UAA16304@ladder01.news.aol.com>,
terryolsen@aol.com (TerryOlsen) wrote:

> Well, if everything is to be loaded onto an accellerator card...why not just
> build a replacement IIgs motherboard that goes 40mhz???  Sounds like it would
> be much more elegant than spiderwebbing an accelerator board in there.

That's a good question, Terry.

Replacing the motherboard completely is actually a nice idea (one which we
considered on the Commodore as well). But there are a number of problems
that make it less attactive in some ways. First, the size of the board is
large, and it contains a great many connectors and components that would
raise the cost considerably over an 'add-on'. In addition, there are
several custom chips that may not be easily available or may be very
expensive to procure. In addition, there are copyright issues with regard
to the system ROMs.

Some of these problems could be overcome by having the user swap chips from
their existing board to the new one, but this makes for a trouble-prone
installation process for users that aren't hardware-friendly. And if some
of the chips are soldered, it becomes nearly impossible for many users to
perform the installation.

On the Commodore, we got lucky. Our C-64 version just plugs into the
cartridge port and takes control of the machine from there. The 128 was a
little more difficult, since some necessary signals were not available at
the port. For this we had to make an adapter card that replaced the MMU
(which is always socketed) and also had to include 5 jumper clip
connections from the adapter to pick up certain signals on the host CPU.
The adapter and accelerator card communicate through a couple of port lines
that are unused at certain times, so no new connections had to be made
between the internal adapter and the external accelerator.

The reason I say we 'got lucky' with this design is because the Commodore
64/128 machines were pretty much designed to allow external devices to take
control of the machine (it's easy to supplant the internal ROMs with
external ones via existing memory configurations in these computers).

I don't know if the GS has any similar abilities, and that certainly could
make creating a fairly seamless accelerator much more difficult. However, I
suspect that the architecture of the GS is otherwise a bit simpler than
that of the Commodore 128, so if the former can be overcome I think the
rest would be not all that difficult.

-- 
Doug Cotton <doug.cotton@cmdweb.com>

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