Rhett Anderson
If you envy the speed with which machine language programmers can manipulate the graphics screen, type in this program to see just how fast BASIC can be.
For years, Atari BASIC programmers have been using strings to manipulate player-missile graphics. Atari BASIC strings are ideally suited for this task. While most BASICs limit the size of strings to 255 characters, Atari BASIC allows huge strings. In addition, Atari BASIC makes sub-string manipulation very easy.
Even if you are familiar with this trick, you may not have realized that it is possible to treat the entire display memory (or playfield, as Atari calls it) as a string. This technique allows you to do some things that are otherwise very slow in Atari BASIC. Among these are
- Erasing or filling the screen or part of the screen with any solid color or pattern.
- Duplicating part of the screen.
- Fast swapping of full or partial screen displays.
Although I have used GRAPHICS 7 for this program, these techniques are easily applied to all the graphics modes—even the text modes.
Typing It In
The program found at the end of this article is written entirely in BASIC. This program assumes that the first variable used will be placed in the first slot in the variable name table. To insure that this is the case, be sure to type NEW before entering the program. Type it in and save it to tape or disk. If you forget to type NEW, or if the program does not operate properly, save the program to tape or disk with the LIST command (see your BASIC manual for an example). Then type NEW and use the ENTER command to retrieve the program. You may now save the program again with the SAVE command.
The program is broken up into several sections. You can use any of these sections in your own programs. Just be sure that lines 10-60 are at the beginning of your program.
You must be sure to dimension the strings you use to the proper size. The amount of memory used by the screen (and therefore by the string) depends upon the screen mode. The size of the screen can be found with this formula:
size = (number of bytes needed per scan line) * (number of pixels available vertically)
| Modes | Bytes per Scan Line |
| 0, 7, 8, 9, 10, 11, 15 | 40 |
| 1, 2, 5, 6 | 10 |
| 3, 4 | 10 |
For example, a GRAPHICS 0 screen would require 40 * 24 bytes of storage, while a GRAPHICS 7 screen requires 40*80.a
As written, the program points
Once the string is reassigned to the screen, any operations you perform on the string will show up on the screen.
Addressing the screen as a string may seem awkward at first, but it is incredibly fast. Once you understand it, you'll be able to write programs that do screen manipulations with machine language speed. In the case of the GRAPHICS 7 screen, a byte represents four pixels. This makes color selection rather odd. Instead of using the COLOR statement to select colors 0–3, the values 0, 85, 170, and 255 represent the four different colors.
Although the memory representation makes working with solid colors rather arcane, it makes working with patterns a breeze (for an example, see the Atari version of Laser Chess ™ in the July 1987 issue of COMPUTE!). In some graphics modes such as mode 15 (available only on XLs and XEs), the two colors blend well enough to appear to be a new color. A patterned screen fill is done in the demo program in lines 320–360. Use this table to replace the numbers in lines 320 and 340 for other color combinations.
| Colors | Values |
| 0 and 1 | 17, 68 |
| 0 and 2 | 34, 136 |
| 0 and 3 | 51, 204 |
| 1 and 2 | 102, 153 |
| 1 and 3 | 119, 221 |
| 2 and 3 | 187, 238 |
For instance, if you want to use colors 2 and 3 in the demo, replace the 102 in line 320 with 187 and replace the 153 in line 340 with 238.
For instructions on entering this program, please refer to "COMPUTE!'s Guide to Typing In Programs" elsewhere in this issue.
Stringing The Atari Screen
NO 1 REM Copyright 1987 COMPUTE! PUBLICATIONS, INC. ALL RIGHTS RESERVED.
LA 2 PRINT "{CLEAR}" : POSITIO N 12, 8 : PRINT "Copyright 1987" : POSITION 7, 10 : PRINT "COMPUTE! Publications, Inc."
GN 3 POSITION 10, 12 : PRINT "A 11 Rights Reserved "
PN 4 FOR 1 = 1 TO 1500 : NEXT I
JO 10 GRAPHICS 7 : CLR : DIM A$ (40 * 80), COL (4), B$ (40 * 80), TEMP$ (40 * 8O) : A$ (40 * 80) = CHR$ (0)
NL 20 VTAB = PEEK (134) + PEEK (135) * 256
MB 30 ATAB = PEEK (140) + PEEK (141) * 256
ND 40 OFFS = PEEK (88) + PEEK (89) * 256 - ATAB
CD 50 HI = INT (OFFS/256) : LO = OF FS - HI * 256
JH 60 POKE VTAB + 2, LO : POKE VT AB + 3, HI
JE 70 REM * * SOLID COLOR FILL * *
NJ 75 PRINT "SOLID COLOR FILL" : GOSUB 1000
PD 80 FOR T = 0 TO 4 : READ COL : COL (T) = COL
BK 90 A$ (1, 1) = CHR$(COL)
BL 100 A$ (2) = A$
NH 105 GOSUB 1000
CF 110 NEXT T
FD 120 DATA 0, 85, 170, 225, 0
PB 200 REM * * FILL PAT OF SCREEN * *
HE 210 PRINT "PARTIAL SCREEN FILL" : FOR T = 0 TO 3 : W HERE = 400 * T + 1 : REM USE 400 BECAUSE 10 LINES OF 40 BYTES EACH
HO 220 A$ (WHERE, WHERE) = CHRS$ (COL (T))
AK 230 A$ (WHERE + 1, WHERE + 399) = A$ (WHERE, WHRE + 398) : REM DO 10 LINES
NL 235 GOSUB 1000
CJ 240 NEXT T
ME 300 REM * * PATTERN FILL *
PD 310 PRINT "PATTERNED SCREEN FILL"
NK 315 GOSUB 1000
PL 320 A * (1, 1) = CHR$ (102)
KO 330 A * (2, 40) = A$ : REM 40 BECAUSE USING GR. 7
CL 340 A$ (41, 41) = CHR$ (153) : REM NEXT LINE DOWN
KA 350 A * (42, B0) = A * (41)
AH 360 A * (81) = A * : REM DUPLICATE FIRST 2 LINES DOWN ENTIRE SCREEN
NL 370 GOSUB 1000
NM 400 REM * * SCREEN DUPLICATION * *
IN 410 PRINT "DUPLICATE PART OF SCREEN "
NL 415 GOSUB 1000
CH 420 FOR X = 0 TO 159 : COLOR X/54
OK 430 PLOT X, 0 : DRAWTO X/2, 79
CP 440 NEXT X
AN 450 FOR 1 = 1 TO 3
MD 560 A$(I * 800 + 1, I * 800 + 800) = A$ : REM 800 FOR 20 LINES
CE 570 NEXT I
ND 580 GOSUB 1000
PJ 600 REM * * FAST SCREEN SAPPING * *
IB 610 PRINT "SCREEN FLIPPING"
KA 620 B * = A *
CE 630 FOR Y = 1 TO 30 : COLOR RND (0) * 3 + 1
JB 640 PLOT RND (0) * 159, RND (0) * 79 : DRAWTO RND (0) * 159, RND (0) * 79
DD 650 NEXT Y
EC 660 FOR K = 1 TO 30
BC 670 TEMP$ = A$ : A$ = B$ = TEMP$
CI 680 NEXT K
OA 690 GOSUB 1000
FJ 700 REM * * PARTIAL SCREEN SWAPPING * *
II 710 PRINT "PARTIAL SCREEN SWAPING" : ? "(TOP WITH BOTTOM"
EG 720 FOR K = 1 TO 30 : B$ = A$ (1, 40 * 40) : A$ (1, 40 * 40) = A$ (40 * 40 + 1) : A$ (40 * 40 + 1) = B$ : NEXT K
IE 800 REM * * ANIMATION * *
AK 810 PRINT "ANIMATION"
BB 820 A$ (1, 1) = CHR$ (0) : A$ (2) = A$
BJ 830 FOR T = 0 TO 3
LE 840 COLOR 1 : FOR Y = 1 TO 10 : COLOR Y : PLOT T + 4, Y + T * 20 : DRAWTO T + 30 + 4, Y + T * 20 : NEXT Y
DA 850 NEXT T
OD 855 GOSUB 1000
KG 860 B$ = A$
GG 870 A$(1, 1) = CHR$(0) : A$(2) = A$
FA 880 X = 10 : Y = 10 : DX = 1 : DY = 1
LE 900 IF X = 100 THEN DX = -1
CM 910 IF X = 5 THEN DX = 1
IN 920 IF X = 5 THEN DY = -1
DB 930 IF Y = 60 THEN DY = 1
AH 940 H = INT (X/4) : L = X - H * 4
LG 950 A$ (H + 40 * Y + 1, H + 40 * Y + 480) = B$ (L * 800 + 1, L * 800 + 480
HJ 960 GOTO 890
KB 1000 FOR DELAY = 1 TO 100 : N EXT DELAY
KC 1010 RETURN