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From: mil@mendel.acc.Virginia.EDU (Maria I. Lasaga)
Newsgroups: comp.sys.mac.programmer
Subject: Help with ScrnBase
Message-ID: <1990Aug23.171125.20436@murdoch.acc.Virginia.EDU>
Date: 23 Aug 90 17:11:25 GMT
Sender: news@murdoch.acc.Virginia.EDU
Distribution: usa
Organization: University of Virginia
Lines: 60


Can anyone tell me why the following code bombs?  Any clues
would be appreciated.

Essentially, I am trying to switch between the main and alternate video 
buffers of an SE. (I know that newer Mac models don't have this option, but 
I need to use it for a very specific application.)  I am trying to get
around using assembly language because I haven't used it in a while and am 
a bit rusty, and because I don't have the time to relearn it.
(IM III-20 suggests how one might alternate between the main and
alternate video buffers in assembly language, but I can't make sense of
it.)  

So here's the code that bombs.  The machine just freezes.  I assume I 
am overwriting parts of memory I shouldn't, but I don't get how I am
doing this:




var
   offscreen: grafport;
  mypointer: ptr;
  mybitmap: bitmap;
  myaddress: longint;

begin

{--offscreen bitmap created here--}


 eraserect(theport^.portrect);     {  clearing the area of memory...  }

 myaddress := $727;     {starting location for alternate memory buffer on SE}
 mybitmap.baseaddr := @myaddresvkcPmybitmap.rowbytes := offscreen.portbits.rowbytes;
 mybitmap.bounds := offscreen.portbits.bounds;
 copybits(offscreen.portbits, mybitmap, 
         offscreen.portrect, offscreen.portrect, srccopy, nil);
		 
                  {copies an image previously drawn in an offscreen portrect}
				  
 myaddress := $824;   {supposedly the location of the global variable,}
                      {ScrnBase, that holds the address of the main screen }
                      {buffer. }
					  
 mypointer := @myaddress;
 mypointer^ := $727;   {Attempt to set ScrnBase to the address of the }
                       {alternate screen buffer.}
end;




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