Re: Can the IIgs output colour on its composite monitor output?

["Roger Johnstone" <rojaws@mac.com>]

In article <yE%T6.114845$ff.856055@news-server.bigpond.net.au>, "ralmin" 
<xbiber@bigpond.net.au> wrote:

> We're still talking on different wavelengths.
>
> I'm not talking about GS/OS at all.
>
> When you press Ctrl-OA-Esc, you get a list of desk accessories.
>
> One is "Control Panel", another is "Alternate Display Mode".
>
> Within "Control Panel", there are a number of control panels, one of which
> is "Display", which has the settings you're talking about, that is
> Color/Monochrome, 40/80 column, etc.
>
> But I'm not talking about the Control Panel at all.
>
> I'm talking about the Desk Accessory called "Alternate Display Mode". It is
> simply On or Off and has no other settings.
>
> What does it do?
>
> Ralmin.

It fixes an errata/bug/missing feature. The IIgs has its video display
memory in banks $E0 and $E1. The older Apple IIs have display memory in the
first 128KB, equivalent to banks 0 and 1 on a IIgs. All non-IIgs programs
write to this old memory location. On a IIgs these banks are shadowed, that
is every write to the video areas in banks 0/1 is automatically copied to
$E0/E1. This ensures the real video buffer is always written to.

Shadowing is a hardware function, so it doesn't slow the computer down
(well, just a tiny bit). Not all of the bank 0/1 area is shadowed, just the
parts that are used for video. The areas that are shadowed are text page 1,
hi-res page 1, hi-res page 2 plus the auxilliary pages (for 80-column text
and double hi-res) plus the IIgs's super hi-res. The one missing is text
page 2!

This does not often cause any problem since very, very few programs ever
used it. Indeed Applesoft BASIC starts its program memory in text page 2,
which is another reason it's rarely used. However, if a program does use
text page 2 you'll just get a screen full of '2's. Apple fixed it by adding
the Alternate Display Mode desk accessory to ROM. Turn it on and it will
continually copy text page 2 to the correct bank, but it will slow your IIgs
down a lot too since it has to do it with a software routine.

In the 1MB IIgs (ROM 3) they fixed the shadowing problem so Alternate
Display Mode was no longer needed.

--
Roger Johnstone, Invercargill, New Zealand

Apple II - Future Cop:LAPD - Warcraft II
http://homepage.mac.com/rojaws
______________________________________________________________________

There are two ways to write error-free programs. Only the third one works.