Discovering Atari's "Hidden" Graphics
Gregory L. Kopp
Indianapolis, IN
If you were a stumbling, beginning BASIC programmer like I was, you probably tried to enter a few "improper" graphics commands which resulted in curious and unexpected displays on your television screen. Before I understood the function and proper use of POKE 756 (which allows one to display Atari lower case letters and special graphics characters in text modes 1 and 2, I stubbornly tried to put CONTROL characters on-screen without the requisite POKE, which produced only seemingly random keyboard characters and frustration instead.
Much later, the thought nevertheless occurred to me that I might have actually accidently discovered some "hidden" (or at least undocumented) graphics capability of my Atari. In the experimental binge to which owners of microcomputers are sometimes given, I used the PRINT #6; command to enter each keyboard character while pressing CTRL at the same time. Discovery! Although the Atari special graphics characters appeared in the PRINT #6; statement, the actual screen display consisted of keyboard characters, but not the characters for the keys I entered.
Dutifully noting the results (first chart below), I pondered the apparent micro-fluke, these "hidden" characters, then asked myself the inevitable scientific question: "So what?" Two uses came fairly quickly to mind — the first purely cosmetic, the second functional.
10 GR. 2 + 16 20 X = 0 30 FOR L = 1 TO 50 40 RC = INT(15 * RND(0)) : RS=(255 * RND(0)) 50 SETCOLOR 0, RC, 6 60 SOUND 0, RS, 10, 4 70 POSITION 5, 4 80 PRINT #6; "1" (use CTRL = Q) 90 FOR W = 1 TO 25 : NEXT W 100 X = X + 1 : IF X = 4 THEN X = 0 110 NEXT L 120 SOUND 0, 0, 0, 0 130 GR. 2 + 16 140 POSITION 5, 4 150 PRINT #6; "1
If one could change these hidden characters from "default green" to other colors, one could eliminate the irksome problem encountered in modes 1 and 2 of having punctuation and numbers displayed in different colors than the text lettering. The INVERSE key! Sure enough, PRINTing the graphics characters in inverse changed my hidden green characters to red. Now I could choose from normal character (yellow), inverse normal (blue), CTRL character (green) and inverse CTRL (red). Experimenting further, I discovered one could achieve any Atari color by use of a SETCOLOR 0 to 3 or POKE 709 to 711 command to change each respective character. No more would I have to sheepishly explain to those not-of-the-computer-persuasion why my apostrophe or my "I" was blue while my text was red!
So much for cosmetics. If you are not bothered by the inconsistent color text problem, then use the last two paragraphs as speed-reading exercises. However, if you have purchased software such as Iridis 2 or Datasoft's Character Generator, you may already have thought of the second application. Instead of redefining one's lower case (and thereby "losing" it) to achieve new characters, one may use "hidden graphics" to redefine the number set, selected punctuation marks, or arithmetic signs. While this could be done normally, using "hidden graphics" allows one to display numbers, punctuation, or signs in four colors instead of only two! (If you have not run the above program yet, try it. Then try to produce four different color I's the conventional way.)
A Second Approach
Now enter and run the following program:
10 X = 0: Y = 0: Z = 65
20 GR. 2 + 16
30 FOR AZ = 0 TO 25
40 SOUND 0, 255-AZ * 10, AZ + 8,8
50 COLOR Z
60 PLOT X,Y:IF X = 18 THEN X = 0: Y = Y + 170 X = X + 1 : Z = Z + 1
80 FOR W = 1 TO 50 : NEXT W
90 NEXT AZ
100 IF Z = 91 THEN Z = 193 : X = 0 : Y = 2 : GOTO 30
110 IF Z = 219 THEN Z = 225 : X = 0 : Y = 4 : GOTO 30
120 IF Z = 219 THEN Z = 225 : X = 0 : Y = 6 : GOTO 30
130 FOR W = 1 TO 500 : NEXT W
140 POSITION 2,9 : PRINT #6; "COLOR
STATEMENT"
150 PRINT #6; "GRAPHICS!"
160 FOR W = 1 TO 1000 : NEXT W
If you are trying to figure out how we got all those alphabet characters using PLOT and COLOR statements, read on.
As any intermediate programmer can tell you, one cannot plot points in modes 1 and 2. you get absolutely nothing displayed if you try it. Of course the stumbling beginner might think the reason one gets nothing is that one did not enter a COLOR statement. Sandwiching COLOR 1 between the lines and trying again, he discovers that he has plotted an "!" instead of a point. "Pixel-head!" he chides himself. "You can't use PLOT in modes 1 and 2!" He notes this in his reference manual and ranks himself a step closer to intermediate programmer, missing the opportunity to discover more hidden graphics.
The Atari will plot a character in modes 1 and 2 at whatever position the programmer commands. The nature and color of that character are determined by a single COLOR statement. Using the COLOR Statement Graphics Chart, you can display any Atari keyboard character (POKE 756 for lower case) by using the associated COLOR statement, then plotting X,Y coordinates to place it at the desired position on the screen.
Once again, SETCOLOR 0 to 3 or POKE 709 to 711 can be used to color each individual character, including lower case characters which are normally limited to only two colors. (Note: these SETCOLOR's and POKE's work only when using GR. 1 or 2 + 16.) Again, redefined characters may be used and this time manipulated arithmetically. Game-writers, rejoice!
While the PRINT #6; approach displays numbers, punctuation and arithmetic signs, the COLOR/PLOT technique allows access to upper and lower case letters as well. Preference for one method over the other will vary from user to user and application to application, as you will see once you have tried them a few times.
| Table A. "HIDDEN GRAPHICS" CHART | |||||
| To Get Character | Press CTRL + Key | Character | (Default) | SE. | POKE |
| 0 | P | normal | yellow | 0 | 708 |
| 1 | Q | "hidden" | green | 1 | 709 |
| 2 | R | inverse, | |||
| 3 | S | normal | blue | 2 | 710 |
| 4 | T | inverse, | |||
| 5 | U | "hidden" | red | 3 | 711 |
| 56 | V | ||||
| 7 | W | ||||
| 8 | X | ||||
| 9 | Y | ||||
| : | Z | ||||
| ! | A | ||||
| " | B | ||||
| # | C | ||||
| $ | D | ||||
| % | E | ||||
| & | F | ||||
| ' | G | ||||
| ( | H | ||||
| ) | I | ||||
| * | J | ||||
| + | K | ||||
| ' | L | ||||
| - | M | ||||
| . | N | ||||
| / | O | ||||
| [ | ; | ||||
| @ | . |
| Other | Color* | Press Keys |
| ^ | green | ESC then DELETE · BACKS |
| red | ESC then CTRL + DELETE · BACKS | |
| < | green | ESC then CTRL + | - |
| red | ESC then SHIFT + DELETE · BACKS | |
| > | green | ESC then CTRL + — |
| red | ESC then CLR · SET · TAB | |
| = | green | ESC then CTRL + | = |
| red | ESC then SHIFT + INSERT · > | |
| ? | green | ESC then CTRL + —* |
| red | ESC then SHIFT + CLR · SET · TAB | |
| - | green | ESC then CLR · SET · TAB |
| red | ESC then CTRL + INSERT · > | |
| ; | green | ESC then ESC |
| ] | red | ESC then CTRL + ".2 |
| * greens manipulated by SE.1 and POKE 709 reds manipulated by SE.3 and POKE 711 |
| Table B. COLOR STATEMENT GRAPHICS CHART | |||||
| Use Color Number | |||||
| To Get | SE. | 0 | 1 | 2 | 3 |
| Character | POKE | 708 | 709 | 710 | 711 |
| green | yellow | red | blue | ||
| ! | 1 | 33 | 129 | 161 | |
| " | 2 | 34 | 130 | 162 | |
| # | 3 | 35 | 131 | 163 | |
| ! | 4 | 36 | 132 | 164 | |
| % | 5 | 37 | 133 | 165 | |
| & | 6 | 38 | 134 | 166 | |
| ' | 7 | 39 | 135 | 167 | |
| ( | 8 | 40 | 136 | 168 | |
| ) | 9 | 41 | 137 | 169 | |
| * | 10 | 42 | 138 | 170 | |
| + | 11 | 43 | 139 | 171 | |
| ' | 12 | 44 | 140 | 172 | |
| - | 13 | 45 | 141 | 173 | |
| . | 14 | 46 | 142 | 174 | |
| / | 15 | 47 | 143 | 175 | |
| 0 | 16 | 48 | 144 | 176 | |
| 1 | 17 | 49 | 145 | 177 | |
| 2 | 18 | 50 | 146 | 178 | |
| 3 | 19 | 51 | 147 | 179 | |
| 4 | 20 | 52 | 148 | 180 | |
| 5 | 21 | 53 | 149 | 181 | |
| 6 | 22 | 54 | 150 | 182 | |
| 7 | 23 | 55 | 151 | 183 | |
| 8 | 24 | 56 | 152 | 184 | |
| 9 | 25 | 57 | 153 | 185 | |
| : | 26 | 58 | 154 | 186 | |
| ; | 27 | 59 | * | 187 | |
| < | 28 | 60 | * | 188 | |
| = | 29 | 61 | 157 | 189 | |
| > | 30 | 62 | 158 | 190 | |
| ? | 31 | 63 | 159 | 191 | |
| @ | 96 | 224 | * | 192 | |
| [ | 91 | 123 | 219 | 251 | |
| ] | 93 | * | 221 | 253 | |
| \ | 92 | 124 | 220 | 252 | |
| ^ | 94 | 126 | 222 | 254 | |
| - | 95 | 127 | 223 | 255 | |
| A | 97 | 65 | 225 | 193 | |
| B | 98 | 66 | 226 | 194 | |
| C | 99 | 67 | 227 | 195 | |
| D | 100 | 68 | 228 | 196 | |
| E | 101 | 69 | 229 | 197 |
| Table C. COLOR STATEMENT GRAPHICS CHART (Cont.) | ||||
| Character | Color Number | |||
| green | yellow | red | blue | |
| F | 102 | 70 | 230 | 198 |
| G | 103 | 71 | 231 | 199 |
| H | 104 | 72 | 232 | 200 |
| I | 105 | 73 | 233 | 201 |
| J | 106 | 74 | 234 | 202 |
| K | 107 | 75 | 235 | 203 |
| L | 108 | 76 | 236 | 204 |
| M | 109 | 77 | 237 | 205 |
| N | 110 | 78 | 238 | 206 |
| O | 111 | 79 | 239 | 207 |
| P | 112 | 80 | 240 | 208 |
| Q | 113 | 81 | 241 | 209 |
| R | 114 | 82 | 242 | 210 |
| S | 115 | 83 | 243 | 211 |
| T | 116 | 84 | 244 | 212 |
| U | 117 | 85 | 245 | 213 |
| V | 118 | 86 | 246 | 214 |
| W | 119 | 87 | 247 | 215 |
| X | 120 | 88 | 248 | 216 |
| Y | 121 | 89 | 249 | 217 |
| Z | 122 | 90 | 250 | 218 |
| *Writing color statements that would logically appear in these positions displays nothing on the screen. |
Atari Program Listings
COMPUTE! is starting a new, standardized Atari program listing format. All the editing and cursor-control characters are spelled out (e.g., CLEAR for clear screen) and surrounded by brackets.
Other characters, such as CTRL-T, the "ball" character, will be listed as the "normal" character within brackets: {T}. A series of identical control characters will be indicated by a number within the brackets, e.g. 5 DOWN for five cursor downs and 12 R for twelve CTRL-R's. Two control characters, {=} and {-} should be shifted. Any reverse-field text will be enclosed in vertical lines, | like this |. (Press the Atari logo key for each vertical line.) We expect that this convention will permit easy, unambiguous program typing.