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From: mil@mendel.acc.Virginia.EDU (Maria I. Lasaga)
Newsgroups: comp.sys.mac.programmer
Subject: More Help with ScrnBase, Please
Message-ID: <1990Aug27.170838.199@murdoch.acc.Virginia.EDU>
Date: 27 Aug 90 17:08:38 GMT
Sender: news@murdoch.acc.Virginia.EDU
Distribution: usa
Organization: University of Virginia
Lines: 72




Many thanks to those of you who helped me figure out what I was doing wrong
in the code I posted previously.  Basically, I was using @ to set a pointer
to the global variable ScrnBase rather than Pointer().  I was also using $727  
as the base address for the alternate memory buffer, and so I was writing over
other important system info; I should have been using $72700.

Now, I have set a pointer to the memory location ($824) correctly.  To do
so, I have used a handle, because the memory location $824 contains an
address itself, which is what I want to change.  

So, I set my handle as a pointer to the location ($824).  I know this works,
because when I check with Lightsbug (Think Pascal debugger) it tells me that
the value of MyHandle^ is the same as screenbits.baseaddr.  So I know that I 
have set up a handle that points to the location $824 which did, indeed, 
contain the memory location of the main screen buffer.  

Here's where I now run into a STRANGE problem.  When I then try to set
MyHandle^ as a pointer to memory location $727000 (the base for the alternate
screen buffer) instead of what $824 had originally pointed to, it doesn't 
work, but in an odd way:  When I check with Lightsbug, it tells me that 
MyHandle^ is still the _same_ as screenbits.baseaddr.  But when I use the 
Instant window and ask it to write the value of MyHandle^, it correctly 
tells me that it is $727000.  What's going on?  Can anyone give me a clue?  
(I list the short code I am using below.)

Any help would be greatly appreciated.  I am very frustrated with this 
problem.  I don't get why Apple doesn't put out a tech note on how to
use the alternate video buffer on the "lower" macs.  Granted, the Mac II's 
don't have an alternate buffer, but not everyone is into software development  
for use on all machines.  Besides, there is a lot of software available
that doesn't run on the earlier macs.  Why not allow us to develop some
software that doesn't run on the Mac II?  I myself purchased an SE because it 
had an alternate slot to which I could attach an external monitor, and 
because it had an alternate video buffer.  I need to use these two options 
for research purposes.  Yet I can't find documentation anywhere that tells 
me how to take advantage of either of these two options!!  I may as well 
have purchased a Mac Plus.  (I wish I could have afforded a Mac II.)  
Allright, enough venting.  ANY help on figuring out how to access 
the alternate screen buffer of an SE, or on what is wrong with the following
code, would be welcomed:



 
TYPE
  ahandle = ^ptr;
  
VAR
  MyHandle: ahandle;
  

BEGIN
 MyHandle := pointer($824);  { the location of the global variable, }
                             { ScrnBase, that holds the address of } 
                             { the main screen buffer. }

 MyHandle^ := pointer($72700); { Attempt to set ScrnBase to the address }  
                               { of the alternate screen buffer.}

END.


--------------------------------------------------------------------------
Maria Lasaga
Dept. of Psychology
Gilmer Hall
University of Virginia
Charlottesville, Virginia 22903                  mil@virginia.bitnet
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