Newsgroups: comp.sys.mac.programmer
Path: utzoo!utgpu!news-server.csri.toronto.edu!mailrus!wuarchive!psuvax1!swatsun!jackiw
From: jackiw@cs.swarthmore.edu (Nick Jackiw)
Subject: Re: Help with ScrnBase (again)
Message-ID: <14CPAYB@cs.swarthmore.edu>
Reply-To: jackiw@cs.swarthmore.edu (Nick Jackiw)
Organization: Visual Geometry Project, Swarthmore College, PA
References: <1990Aug23.172022.20545@murdoch.acc.Virginia.EDU>
Distribution: usa
Date: Thu, 23 Aug 90 18:59:14 GMT

mil@mendel.acc.Virginia.EDU (Maria I. Lasaga) writes:
> Can anyone tell me why the following code bombs?  Any clues
> would be appreciated.

DISCLAIMER: I've never played with ScrnBase and don't, off the
top of my head, know how to do it.  The following observations
are "clues"--not answers.

	> 
	> var
	>   oldport: grafptr;
	>    offscreen: grafport;
	>   mypointer: ptr;

"Ptr" is defined in the Memory Manager as ^SignedByte.  You are
trying to stuff either a word or a longword into myPointer.
Depending on your compiler, you may just be sticking the low-order
byte of this word or longword into myPointer^.  Try instead
declaring myPointer as either ^longint or ^integer, depending
on whether you want to write four bytes or two.

	>   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 := @myaddress;

I highly doubt that location $727 is the base address of an alternate
memory buffer.  Why?  Because a B&W 9" screen is about 20K worth of
data, and the area between $727 and $727+20K is filled with important
low-memory globals.  Not exactly information you want to be drawing
pictures over.  Could location $727 *contain* the address of the
alternate screen buffer?  If so, you'll want to set myBitmap.baseAddr
to longintPtr($727)^, instead of to $727 itself.

	>  mybitmap.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;

This stuff is wrong for the above reasons.  You're setting location $824
to value $727, and I suspect you want to set location $824 to value of
location $727.  If not, as soon as you write data, the 20K following location
$727 is going to be scrambled--including, of course, location $824.

Hope this puts you on the right track.



-- 
------------------------
Nick Jackiw		jackiw@cs.swarthmore.edu  "Just break out the
Visual Geometry Project	jackiw@swarthmr.bitnet     rum so we seem natural!"
Swarthmore College, PA 19081-1397				-F. Franklin