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From: ww0n#@andrew.cmu.edu.UUCP
Newsgroups: comp.sys.m6809
Subject: Disabling BASIC's screen refresh
Message-ID: <4UK9X1y00UoBz7o0Lw@andrew.cmu.edu>
Date: Fri, 13-Mar-87 18:39:44 EST
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Posted: Fri Mar 13 18:39:44 1987
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Something I have always wanted to straighten out is the proper way to prevent
BASIC from resetting special screen settings.  For example, I use:

	POKE &HE045, PEEK(&HE045) OR 16
	POKE &HE047, 0
	ATTR 3,2

to set my COCO 3 up for a monochrome monitor in 80-column mode.  For the
regular old 32-column mode, I use:

	POKE &H95C9, 255
	POKE &HFF22, &H3D
	POKE &HE033, 16
	POKE &HE035, 0

which sets up the COCO 3 for monochrome and lowercase.  The important point
here is my use of POKE &H95C9, 255 to prevent BASIC from resetting the value
I POKEd into $FF22.  The method you see most of the time is:

	POKE 359, x		or
	POKE &H167, x

where x=0 or 57 or some other value claimed to do the job.  Don't use this
method!  Address $167 is a jump vector which points to the disk i/o routines
and funny things happen when it is changed.  I disassembled the code which
actually resets the screen and found:

	$95C5	LDA	>$FF22
	$95C8	ANDA	#$07
	$95CA	STA	>$FF22

Therefore, when you change the value at $95C9, you change what is ANDed with
the contents of the PIA register at $FF22.  By using 255, no change is made
to the current contents.  Of course, you could NOP out all the code or get
fancy and do something else -- the choice is yours -- this is just a quick,
easy method.


Walter L. Wimer, III
Carnegie Mellon University

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