by Paul Lay

Issue 18

Nov/Dec 85

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If you want to write a good space game you need a good background and what better than a scrolling starfield such as used in STAR RAIDERS or GALAXIAN?

The programs presented here give you various starfields to which you can add your own game play. Listing 1 is a simple scrolling star field which uses one missile and a Display List Interrupt service routine to provide an excellent set of scrolling stars which are independent of the playfield. The routine is shorter than it looks as lines 160 - 290 are merely a demo to illustrate how printing to the screen does not affect the stars. Listing 2 is a better version that uses a much more complex Display List Interrupt to give a 3-D effect. Lines 10 - 440 provide the basic routine with the remainder of the program giving a demonstration to show the independence of the stars from the playfield. A further demonstration of the use of this routine is given in Listing 3. This includes some machine code routines to allow you to steer through the stars by using a joystick in port 1. To run this one delete lines 450 - 570 from Listing 2 and add the new lines.

Finally for those of you interested in exploring the routines further I have included the assembler source code for the display list interrupt.

Listing 1

AtariLister - requires Java

Listing 2

AtariLister - requires Java

Listing 3

AtariLister - requires Java

 

Listing 4 - Assembler source code

 

1000 *=$0600 1010 SETUP 1020 PLA 1030 LDA #$00 1040 STA LINECOUNT 1050 REPEAT 1060 LDA $D40B 1070 BNE REPEAT 1080 LDA #192 1090 STA $D40E 1100 RTS 1110 DLI1 1120 PHA 1130 TXA 1140 PHA 1150 TYA 1160 PHA 1170 LDX LINECOUNT 1180 INC LINECOUNT 1190 LDA POSITIONS,X 1200 TAY 1210 STA $D40A 1220 STA $D004 1222 ORA #$08 1224 STA $D012 1230 CPY #120 1240 BCC SKIP1 1241 CPY #136 1242 BCC SKIP1+1 1250 INY 1260 INY 1270 SKIP1 1280 DEY 1290 TYA 1300 CPX #$00 1310 BEQ EXIT1 1320 STA POSITIONS-1,X 1330 EXIT1 1340 CPX #$2F 1350 BNE CONT1 1360 LDA $D20A 1370 STA POSITIONS+$2F 1380 LDA #DLI2&$00FF 1390 STA 512 1400 LDA #DLI2/$0100 1410 STA 513 1420 CONT1 1430 PLA 1440 TAY 1450 PLA 1460 TAX

1470 PLA 1480 RTI 1490 DLI2 1500 PHA 1510 TXA 1520 PHA 1530 TYA 1540 PHA 1550 LDX LINECOUNT 1560 INC LINECOUNT 1570 LDA POSITIONS,X 1580 TAY 1590 STA $D40A 1600 STA $D004 1602 ORA #$08 1604 STA $D012 1610 CPY #120 1620 BCC SKIP2 1621 CPY #136 1622 BCC SKIP2+1 1630 INY 1640 INY 1650 SKIP2 1660 DEY 1670 TYA 1680 CPX #$5F 1690 BEQ EXIT2 1700 LDA LAST 1710 STY LAST 1720 STA POSITIONS,X 1730 JMP EXIT3 1740 EXIT2 1750 LDA #$00 1760 STA LINECOUNT 1770 LDA $D20A 1780 STA LAST 1790 LDA #DLI1&$00FF 1800 STA 512 1810 LDA #DLI1/$0100 1820 STA 513 1830 EXIT3 1840 PLA 1850 TAY 1860 PLA 1870 TAX 1880 PLA 1890 RTI 1900 LINECOUNT=$CC 1910 LAST=$CD 1920 POSITIONS=$400

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