Re: Deadly "Screen of Twos"

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

In article <mtm4.180.000F7090@rsvl.unisys.com>,
Mike McCormick <mtm4@rsvl.unisys.com> wrote:
> I am running a ProDOS8 application (written in Applesoft plus
> assembly patches) that has worked wonderfully for years on my IIGS. 
> 
> Suddenly it has developed a problem when I load a certain large file
> (it's a full screen editor program).
> 
> The symptom is the screen is full of "2"s, the speaker is clicking,
> and the keyboard is hung.

The "screen of twos" is the second text screen, which is enabled by
reading or writing location $C055 (if you're in 40 column mode).
This screen is only used by a very small number of programs.

It sounds like your program has decided to write data into the I/O
space ($C000-$CFFF), which is a very nasty thing to do.  On a IIgs,
there are lots of things you could do to this area which would cause
the machine to hang.


Tracking this down might be difficult.  It could just be a side
effect, e.g. if you accidentally overwrote some zero page locations
used by Applesoft BASIC, your variables might end up in unusual parts
of memory.
-- 
David Empson
dempson@actrix.gen.nz
Snail mail: P.O. Box 27-103, Wellington, New Zealand