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Article #70 (419 is last):
Newsgroups: freenet.sci.comp.atari.questions
From: ag971
Subject: Basic help/where is zork?
Date: Sun Jul 30 09:10:11 1989


   Help!  I have an Atari 800 XL, I'm writing in Revision C Basic
(because B has a BUG, and A is less powerful).  My question is:
how could I have High Resolution Graphics on the first 1/2 of the
screen and text on the bottom half of the screen simultaneously?
   Also--where could I find the Zork Trilogy for my computer?
Thanks a lot.
                              John Rowland

** Answered by DOUG WOKOUN (aa384) on Fri Mar 31 20:32:11 1989 **

Basic REV.A also had a bug, just like
REV.B  The Rev. B bug scrambled
programs when a multiple of 256 bytes
were inserted into a program...this
was CREATED by Atari while trying
to fix a bug in Rev.A where deleting
256 bytes from a program cause a
crash.  


To get a Display List with mixed
graphics modes...


First you need to find out where
the Display list is....

DL=PEEK(560)+PEEK(561)*256

The first three bytes at location DL
are to move the screen down...(112)

The next three bytes are a LMS
(load memory scan with the graphics
mode and screen memory location)

79 is the LMS for gr.8
and the next two bytes are the
LSB,MSB for the display memory.

for each scan line of Gr.8, use
a 15 for that byte in the display
list....then to switch to gr.0,
use another LMS (66) and the
new memory location....you cannot
have more than 4K of screen memory
without a LMS

then, put several lines of gr.0 (2)
and top it off with a jump and
wait for vertical blank

(65) followed by the address of the
start of the display list.

To map this out:


Start > 112
        112
        112
         79
        LSB (screen memory)
        MSB (screen memory)
         15
         15
         15
          ... until you get halfway
              down the screen...
              95 of these should do
              it.
         66
        LSB (screen memory for Gr.0)
        MSB (screen memory for Gr.0)
          2
          2
          2   (12 of these to get
          2... to the bottom)
         65
        LSB (START)
        MSB (START)

You can get more information from
Compute!'s Mapping the Atari which
has an excellent section on Display
List Interrupts.   There are several
DL codes for blank scan lines and
other graphics modes unavailable
with BASIC.

Try B&G for Zork and other software
.  It is about the only place left
locally.

            Thanks for asking.

  Doug Wokoun
     aa384
  Atari SysOp


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