Re: Hopefully last dumb newbie question.

[dempson@actrix.gen.nz (David Empson)]

Jeff Blakeney <CUTjefbla@bconnex.net> wrote:

> On Thu, 04 Jun 1998 13:43:07 -0500, Slick <Slick_RCBD@hotmail.com>
> wrote:
> 
> >Now the question. What exactly does the Alternate Display Mode DA do? I
> >think it has something to do with the secondary text screen (Address
> >$800-C00 on earlier models), but am not sure. None of the books at my
> >library have anything about it. I do know about the shadowing to banks
> >$e0 and $e1, and have looked at assembly language programming on the
> >IIGS. I just never had the money to buy ORCA or APW. I've used the Hyper
> >C (ProDOS 8, and free) assembler to write programs.
> 
> If I remember correctly, text page 2 doesn't get shadowed or the
> softswitch for changing to text page 2 doesn't work or something like
> that.

In the original IIgs motherboard (ROM 00 or ROM 01), there is no support
for hardware shadowing of text page 2 ($0800-$0BFF).

> The Alternate Display CDA allows you to enable or disable another
> shadowing routine, which is slower than the hardware shadowing, but allows
> certain older 8 bit programs to use text page 2 so that they will work
> correctly.

Yep.  Alternate Display Mode works by copying the contents of bank 0
$0800-$0BFF into the actual display memory in bank $E0.  This is done
with a vertical blanking interrupt handler, so it noticeably slows down
the computer, and won't work if the program locks out interrupts.

Note, however, that the 1MB IIgs motherboard (ROM 3) _does_ have
hardware shadowing for text page 2, and on this machine Alternate
Display Mode simply turns on the hardware option (no speed or
compatibility penalties).

> If an 8 bit program crashes or doesn't run correctly, try enabling the
> Alternate Display mode and try it again.  It just might work.

If the program tries to use text page 2 you will see a text screen full
of '2' characters.  This is the only situation in which turning on
Alternate Display Mode will help.

-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P.O. Box 27-103, Wellington, New Zealand