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Mapping the Atari cover

APPENDIX EIGHT

Display Lists

A display list is a short program for the ANTIC chip, telling it how to display data on the screen. This program includes such instructions as how many blank lines to place on the screen for top boundaries, where the screen display data is stored, what mode the line(s) to be displayed are in, whether or not there is an interrupt to execute and where to find the display list itself. There are nine pre-programmed display lists (ten with the GTIA) you use in BASIC, one for each GRAPHICS mode. You can examine the display lists for each mode by running the program at location 560. You can change these lists to suit your own needs without much effort. It is quite easy to design and implement your own display list once you know where it's located and what the proper instructions are. Certain techniques, such as horizontal and vertical fine scrolling, require that you modify the display list in order to properly display your screen data. Sometimes you want to be able to display data in more than one mode or mix graphics and text in the same screen. These are all done by modifying the display list. The smallest display list is for GRAPHICS 2, so I'll use it as an example. It consists of a mere twenty odd bytes, but the format is the same for every list; it's just the instructions that change. Use the program listed in the Memory Map to examine the list or use a simple two-liner such as: 10 GRAPHICS 2: P = PEEK(560) + PEEK (561) * 256 20 FOR N = 0 TO 23: PRINT PEEK(P + N);" ";: NEXT N When you RUN this example, you should get this: 112 112 112 71 112 158 7 7 7 7 7 7 7 7 7 66 96 159 2 2 2 65 88 158 Or something similar depending on your available memory. If you change the GR.2 to GR.2 + 16, you will get: 112 112 112 71 112 158 7 7 7 7 7 7 7 7 7 7 7 65 92 158 The display list instruction set is discussed at location 560, but here's a chart to summarize it: Instruction BASIC Scan Pixels Bytes Comments Decimal Hex mode lines line line Blank instructions 0 0 -- 1 -- -- 1 blank line 16 10 -- 2 -- -- 2 blank lines 32 20 -- 3 -- -- 3 blank lines 48 30 -- 4 -- -- 4 blank lines 64 40 -- 5 -- -- 5 blank lines 80 50 -- 6 -- -- 6 blank lines 96 60 -- 7 -- -- 7 blank lines 112 70 -- 8 -- -- 8 blank lines Display instructions 2 2 0 8 40 40 text mode 0 3 3 -- 10 40 40 text mode * 4 4 -- 8 40 40 text mode * 5 5 -- 16 40 40 text mode * 6 6 1 8 20 20 text mode l 7 7 2 16 20 20 text mode 2 8 8 3 8 40 10 graphics mode 3 9 9 4 4 80 10 graphics mode 4 10 A 5 4 80 20 graphics mode 5 11 B 6 2 160 20 graphics mode 6 12 C -- 1 160 20 graphics mode * 13 D 7 2 160 40 graphics mode 7 14 E -- 1 160 40 graphics mode * 15 F 8 1 320 40 graphics mode 8 Jump instructions (three bytes long) 1 1 -- -- -- -- jump to location 65 41 -- -- -- -- jump and wait for VBLANK Modes marked with an asterisk (*) have no equivalent in BASIC. These are the instructions in the display list. You can alter the display instructions by setting the bits for horizontal or vertical scroll, load memory scan (tells ANTIC where the next line(s) to be displayed are in memory and what mode to use for them) and enable a display list interrupt. These are: Function add decimal hex bit Vertical scroll 16 10 4 Horizontal scroll 32 20 5 Load memory scan 64 40 6 Display list interrupt 128 80 7 The LMS instruction is a three-byte instruction; the second and third bytes are the LSB and MSB of the address where the line or screen data is to be displayed. You can add any or all of these modifications to the text or graphics mode instructions. You can only add the interrupt modification to blank line or jump instructions. The two bytes that follow the jump instructions are the LSB and MSB of the address to which the ANTIC jumps to continue or repeat the list. So let's analyze the DL for GRAPHICS 2 that we printed above: 112 These three instructions print 112 24 blank scan lines at the top 112 of the screen 71 GR.2 with LMS instruction added 112 Address of the first line of screen data 158 158 * 256 + 112 = 40560 7 Display the rest of the data in 7 GR.2, so we have a total of 7 ten GR.2 lines, or 10 * 16 = 7 160 scan lines used. 7 7 7 7 7 66 GR.0 with LMS instruction added 96 Address of the text window at bottom 159 159 * 256 + 96 = 40800 2 GR.0 for text window, so we have 2 a total of four lines 2 65 Jump and wait for vertical blank 88 Address of display list itself 158 158 * 256 + 88 = 40536 (return to the top of this list) Now examine the list for GR.2 + 16. You can see that it adds two 7's to replace the GR.0 lines at the bottom of the screen. A little math shows us that the screen in both cases has a total of 192 scan lines. That's an important number; if you want your screen to come out properly, you must insure that you get as close to this figure as possible; otherwise you'll end up with blank lines at the bottom of your screen, or worse -- in the display itself. You will find the value 112 in every Atari display list. The three of them are used to bring the display to a readable location on your set. Try replacing one or more of them with a zero to see what happens without them. The jump instructions are also used to skip across a 1K boundary, since the DL itself cannot cross a 1K boundary without such a jump. Also, DL data cannot cross a 4K boundary, so you must use an LMS instruction before crossing one. The critical factor in designing your own display list is to make sure that the data and the scan lines match. This may require you to manipulate your data so that you have the proper number of bytes per line so that the display appears correctly on the screen. Here are the number of bits per pixel for each of the ANTIC modes: Mode Bits per decimal hex BASIC pixel 2 2 0 8 text modes 3 3 -- 8 4 4 -- 8 5 5 -- 8 6 6 1 8 7 7 2 8 8 8 3 2 graphics modes 9 9 4 1 10 A 5 2 11 B 6 1 12 C -- 1 13 D 7 2 14 E -- 2 15 F 8 1 You can have as many DL's as you wish, using the jump/vertical blank instruction at the end of the DL to tell ANTIC where your new DL is located. When placing your new DL (page six, unless used for other routines, is a good protected place to put it), do a POKE 559,0 to disable the DL fetch instructions, then POKE it with the proper value to turn it back on afterwards. Be inventive and create your own screens with varied lines of text and graphics. I suggest that you read De Re Atari and Your Atari 400/800 for more information. The latter has a few good examples of altered display lists and tells how to create them. Two DL utilities are The Next Step from Online and Tricky Tutorial #1 from Santa Cruz.

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