Scredit

32K Disk
by Ron Hodge

Smooth scrolling is the most dramatic graphics capability of the Atari system. If you've been following Kyle Peacock's recent series, Fine Scrolling (issues 13-16), you've no doubt sat entranced while the screen moved about smoothly under joystick control.

After deciding to set up your own scrolling game or utility, you whip out Create-A-Font (issue 16) and create the character set that will make your screen sparkle. Then, it suddenly dawns on you how do you design a scrolling screen when you can't even see all of it?

I was faced with the same problem, and the result was Scredit (short for Scrolling Screen Editor). Scredit is a screen editor that allows you to design a screen simply by typing in the characters to the screen, then SAVEing the screen as a binary file.

My particular need was for a screen in Graphics Mode 2, 24 TV screens wide. While your screen may not be the same size or mode, I think you'll find it easy to modify the program for your own needs. Let's take a took at how Scredit works.

Scredit starts with a menu of options you'll need to get started. After taking a look at the disk directory with option 4, press 1 or 3 to LOAD in a screen or character set. The menu will remind you whether you're LOADing or SAVEing a file, and ask for the filename. Prefix the filename with D:. Scredit will confirm that the LOAD is complete, or report any disk I/O errors, using standard error numbers.

If you change your mind about LOADing or SAVEing a file, simply press RETURN when asked for the filename. You'll get an error message and a request for your next option. (You may LOAD or SAVE a file from any drive, but only look at the directory on the drive 1.)

After LOADing your files, press 5 to exit to the editor. Your cursor will be in the upper left corner of the screen. To move it around, use the CONTROL-ARROW keys or press the trigger while moving the joystick. If you want to place a character on screen, press the appropriate key, or the space bar to erase a character. All CONTROL-KEY combinations other than the cursor arrows are ignored by Scredit.

Press OPTION to change the image priority of the cursor. You have two methods to see which character is under the cursor. Pressing START will toggle the screen between the character set chosen and the internal character set. Pressing SELECT will display the character in the text window, along with its hex value.

The hex value will show you how the character derived its color. If the high bit is set, it shows an inverse character. If the next-highest bit is set, the character is in lower caps. For example:

C=$23upper case
C=$63small caps (+$40)
C=$A3inverse (+$80)
C=$E3inv + small caps (+$40+$80)

Move the joystick to scroll the screen. The cursor will move if the trigger is pressed. You can get back to the menu by pressing ESCAPE. Flipping to the menu and back is also convenient for positioning the cursor at the beginning of the screen, since you always enter the editor at the beginning of the screen.

That's all there is to using Scredit. It shouldn't be hard to modify Scredit for other screen sizes or modes. Screen and player/missile parameters, in addition to the display list, are all the changes needed. The logic flow remains the same. Since horizontal and vertical scrolling are so similar, the routines to maintain cursor position for vertical or horizontal scrolling are similar to the current routines.

One last thought. Since the logic for Scredit is so minimal -- about 2300 bytes -- it could be coresident with a program under development. Then you could flip between the program you're fine-tuning and Scredit to make screen changes as needed.

BASIC Listing

SCREDIT.LST is available in ATASCII format.

10 REM *** SCREDIT ***
20 DATA 0,1,2,3,4,5,6,7,8,9,0,0,0,0,0,
0,0,10,11,12,13,14,15
30 DIM DAT$(91),HEX(22):FOR X=0 TO 22:
READ N:HEX(X)=N:NEXT X:LINE=990:RESTOR
E 1000:TRAP 110:? "CHECKING DATA"
40 LINE=LINE+10:? "LINE:";LINE:READ DA
T$:IF LEN(DAT$)<>90 THEN 160
50 DATLIN=PEEK(183)+PEEK(184)*256:IF D
ATLIN<>LINE THEN ? "LINE ";LINE;" MISS
ING!":END 
60 FOR X=1 TO 89 STEP 2:D1=ASC(DAT$(X,
X))-48:D2=ASC(DAT$(X+1,X+1))-48:BYTE=H
EX(D1)*16+HEX(D2)
70 IF PASS=2 THEN PUT #1,BYTE:NEXT X:R
EAD CHKSUM:GOTO 40
80 TOTAL=TOTAL+BYTE:IF TOTAL>999 THEN 
TOTAL=TOTAL-1000
90 NEXT X:READ CHKSUM:IF TOTAL=CHKSUM 
THEN 40
100 GOTO 160
110 IF PEEK(195)<>6 THEN 160
120 IF PASS=2 THEN PUT #1,224:PUT #1,2
:PUT #1,225:PUT #1,2:PUT #1,0:PUT #1,6
4:CLOSE #1:END 
130 ? "INSERT DISK WITH DOS, PRESS RET
URN";:DIM IN$(1):INPUT IN$:OPEN #1,8,0
,"D:AUTORUN.SYS"
140 PUT #1,255:PUT #1,255:PUT #1,0:PUT
 #1,64:PUT #1,80:PUT #1,73
150 ? :? "WRITING FILE":PASS=2:LINE=99
0:RESTORE 1000:TRAP 110:GOTO 40
160 ? "BAD DATA: LINE ";LINE:END 
1000 DATA 202843202F40207940201C4320A3
4020DC40207D41A580C93590EED0062028434C
F144202843203C432040434C,627
1010 DATA 0040A220A9039D4203A94D9D4403
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1030 DATA 9D4503A9049D4A03A9009D4B0320
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1120 DATA 429810034C5442A580C931D004A9
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1130 DATA 43A92FA04420C1404C0243A202A9
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1140 DATA 40A9039D4203A9C99D4403A9449D
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E84CBE4220E242A200C90A90,711
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1170 DATA A90C855460201443A951A04420C1
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1190 DATA 2A007D1D1D7F1E1E1E1E53435245
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1200 DATA 41442053435245454E9B7F322D20
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1240 DATA 4D504C455445449B1D7F53415645
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1250 DATA 494E55459B7FC5D2D2CFD23A2020
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1260 DATA 2020202020202020202020202020
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1270 DATA 437C435943202020202000000000
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000000000000000000000000,866
1280 DATA 0000000000A00FB9AE4899F00088
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1290 DATA 04D48583A95A8DC002A9308D00D0
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1300 DATA 07D4A9038D1DD0A0A1A90099FF73
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1310 DATA 3E8D2F02A9478D0002A9488D0102
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1320 DATA 2901D00E20B6474C62E4AD8402F0
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1330 DATA 464C4746C90BD00620BD464C4746
C90ED00620ED464C4746C90DD006201E474C47
46AD1FD0C903D0062018484C,399
1340 DATA 1046C905D009206C482029484C10
46C906D00B20BE48A9048D1FD04C4746ADFC02
C9FFF01DC91CD0034C0C4920,702
1350 DATA CF48B01120E848B00F205A46206D
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1360 DATA CAD0F74CB545A230A9009D48039D
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1370 DATA A000A5F191FAA4F8C013F015B013
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1380 DATA F238E91385F2B006A5F3E90085F3
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1390 DATA 65F285F2A5F3690085F3A9C885FD
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1400 DATA A5F318691285F3A900207F47A0BF
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1410 DATA 1085FCA9FF207F4760A90085F9A5
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4760AD7802C907D00E208A47,478
1420 DATA B01AA90885F0A98085FE60C90BD0
0D209E47B008A90085F0A91485FE60A4FCA210
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1430 DATA 04A5F5D008E6F4D002E6F5186038
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1490 DATA 42287442507442787441000628CA
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1500 DATA E08DF4026048293FA005D9E248F0
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1510 DATA 463860C986D00520BD463860C987
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1520 DATA D44C004040002060000000000000
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Checksum Data
10 DATA 886,957,808,431,727,198,599,55
3,272,701,611,124,947,676,36,8526
160 DATA 165,400,643,736,739,594,825,6
10,609,552,355,530,502,378,633,8271
1140 DATA 619,561,666,493,760,573,382,
415,551,888,586,695,839,484,647,9159
1290 DATA 812,789,983,768,887,783,939,
678,46,956,72,145,914,964,983,10719
1440 DATA 982,990,696,217,152,410,681,
680,498,5306

Listing 2 appears on a separate page.

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Original text copyright 1984 by ANALOG Computing. Reprinted with permission by the Digital ANALOG Archive.