What's poke 559,0: problem with published game program

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2 total messages Started by Kim Wallen {Psychology} Tue, 11 Dec 1984 03:11

#454

Author: Kim Wallen {Psychology}
Date: Tue, 11 Dec 1984 03:11

28 lines
1512 bytes

I am trying to get a game program from the latest issue of COMPUTE!
running, but it seems to bomb on the part when the program tries to
modify the internal character set.  The relevant code is:

	2000 POKE 559,0:DIM S4(1024)
	2010 A=ADR(S$):B=INT(A/512+1)*2:CBASE=B*256-A+1
	2020 FOR I=0 TO 511
	2040 S$(CBASE+I,CBASE+I)=CHR$(PEEK(I+57344)):NEXT I:H=16:V=23

The poke 559,0 blanks the screen and it never comes back.  If I leave the
poke out then I get an error message at line 2040 that there is no 
FOR associated with the next.  Can anyone see anything wrong with
the code here?  It checks out using computes checksum program, but
it doesn't run.  Aftyer typing in 11 blocks of code I sort of want the
thing to run now.

This is on an 800XL.  Any help would be greatly appreciated.

Kim Wallen
Emory University
Atlanta, GA 30322

{akgua ga-tech sb1}!emory!emoryu1!psykw

or

{akgua ga-tech sb1)!emory!kim

Re: Orphaned Response

#534

Author: bsdaniel
Date: Thu, 24 Jan 1985 20:54

16 lines
1600 bytes

The poke address that you mention controls the Direct Memory Access
of the graphics processor, ANTIC.  Poking a 0 to this address turns
off the ANTIC processor and thus the screen goes black. There is
another value to poke to this address to turn the screen back on
but since I don't have my reference manual handy, you'll either
have to look in a manual (if you have one) or examine the program
listing for another poke to 559 (which may have been accidently
left out of the listing and is why your program won't work).
The purpose of turning off the screen is that when you redefine
the character set, the screen becomes rather messy during this
redefinition process, and a black screen is better than a messy
screen. You can leave the poke 559,0 out if you like.  It is
there only for cosmetic purposes and is not neccesary to the
execution of the program. I hope this has helped you. I have
owned an ATARI since 1980 and am pretty familiar with this
machine.

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