A.N.A.L.O.G. ISSUE 36 / NOVEMBER 1985 / PAGE 33

You knew if had been too quiet. Nothing had shown on the scanner for the whole watch. That in and of itself wasn’t unusual, but intelligence had reported increased enemy activity. It seemed that a major move to capture and destroy the gunbases that protected the Interior was being planned.

Further, the enemy had developed a new type of intelligent robot, which could stand the shock of being parachuted to Earth and, once there, could team up with other robots to destroy the gunbases. Intelligence reports indicated that each robot could carry one-quarter of the explosives necessary to pierce the armor of the gunbase you manned.

The anticipated plan is that the enemy choppers will drop robots, which, if they land successfully, will wait until three more robots have also landed, then team up to destroy your base. Since radio silence must be maintained, the robots only “know” about other robots in their direct line of sight, so four robots must land successfully on one side of your base.

These robots are not invulnerable, however. If one parachuting robot lands on another, the one underneath will be crushed and immobilized.

Your gunbase is a pretty awesome weapon. The gun is mounted on a concrete pedestal and is aimed by your joystick. The missiles unleashed by your fire button are steerable — they will travel left and right if you press the joystick control in the appropriate direction, and rise toward the top of your scanner screen if you center the stick.

The missiles are powerful, capable of obliterating the enemy’s helicopters, as well as the robots. One strategic trick, learned in advanced gunnery class, is to use a missile to destroy a parachute by careful aiming, thus causing the robot to plummet to Earth, destroying any robots that happen to be beneath it.

This is really the only method of destroying robots that are already on the ground. The enemy has split the attack into levels, and each level is faster and fiercer than the previous one. Duty calls, so plug your joystick into port 1 and prepare to defend your home as the attack commences! Good luck.

Each procedure is commented with a brief description of what it does. Some of the procedures illustrate interesting programming tricks, however, and I want to expand on them.

The first is the procedure Title(). As stated, it prints the title screen. Notice that it checks the location of the vertical scan VCOUNT and puts color information directly into the hardware registers COLPF0-COLPF3. This causes each scan line on the TV screen to be drawn in a different color. Action! is so fast that you can do this without resorting to machine language display list interrupts.

By using the built-in jiffy timer (RTCLOCK), which advances by one each time a new screen is drawn, in the equation to compute what color is actually used, the colors are made to “scroll” up the screen, providing a rather neat effect. The speed of the scroll is determined by the RSH portion of the color term. RSH essentially does a divide, so the more times you RSH the RTCLOCK, the slower the scroll will be.

The other interesting procedure is MoveTroopers(), which moves the robots down the screen. As you can see by looking at the program listing, Sneak Attack is written in graphics 0, with a redefined character set. Yet the robots scroll smoothly down.

The way it works is this: each robot is two characters high (chute and robot) and is initially put on the screen by simply printing three characters one above the other — the two characters which make up the shape and a third character which is initially blank. These three characters appear one after the other in the character set.

To move the robot in what looks like a smooth scroll down the screen, the 16 bytes which make up the shape (two characters at 8 bytes per character) are shifted 1 byte further into the 24 bytes of the three characters which were printed on the screen.

This “dynamic character redefinition” goes on until the figure has been shifted 8 bytes down, at which time the top character of the three is blank, and the 16-byte figure resides in the lower two characters. Then, you move the 16 bytes back into the top two characters, and then print the three characters one position lower on the screen.

The shifting of 16 byte blocks is done using MOVEBLOCK. The location of the character set and the location of the 16 bytes which make up the shape are passed to MOVEBLOCK by using the names of the arrays which contain the data. Used in this manner, array names are treated as the memory addresses of the data in the array.

I’ve been programming Atari home computers for four years. The very first video game I ever saw running on a home computer was a little something from Sierra (then On-Line), called Sabotage. It was only available on the Apple and was never translated.

I’ve always enjoyed Sabotage and several times tried to program something similar myself. I was never very successful, mostly because BASIC just isn’t up to the job. But Action! is, and I think you’ll enjoy this version of a venerable game.

One more thing. The end is worth waiting for.

*David Plotkin is a Project Engineer for Chevron
U.S.A., with a Master’s in Chemical Engineering. He
bought his Atari in 1980 and is interested in programming
and design of games, as well as word processing.
His work has been seen in ANALOG Computing,
Compute! and other computer magazines.*

; Sneak Attack by David Plotkin MODULE BYTE ChrBase=756,Max,Bkgrnd=710, Fate=53770,Level=[1],CursIn=752, Stick=632,Ps,Loud=[0],Indx=[0], DownL=[0],DownR=[0],Loud1=[0], Snd1=$D208,Snd2=$D20F,Freq=[169], Wsync=$D40A,Colbk=$D018, Nmien=$D40E,Hard=[15], Consol=53279 CARD Scrn=88,RamSet,HiMem=$2E5, Score=[0],Comp=[300],Sdlst=560, Vdslst=512 CARD ARRAY Linept(24) BYTE ARRAY Charset,Chopperstatus(30), Chopperx(30),Choppery(30), Expx(60),Expy(60),ExpStatus(60), TrStatus(30),Trx(30),Try(30), MisStatus(30),Misx(30),Misy(30), Ll(20),Rr(20),Dlist, ShapeTable(0)= [254 16 124 71 127 12 62 0 127 8 62 226 254 24 126 0 96 96 48 48 24 60 231 255 24 24 24 24 24 60 231 255 6 6 12 12 24 60 231 255 128 85 17 66 24 170 91 131 60 126 255 255 195 66 36 24 60 36 24 255 60 24 36 102 0 0 0 0 0 0 0 0 60 36 24 255 60 24 36 102 60 36 219 255 60 24 36 102 60 60 24 60 60 24 24 28 60 60 24 60 60 60 102 195] PROC Download() ;Step back HiMem and move the ;character set into RAM CARD Index BYTE Val RamSet=(HiMem-$400)&$FC00 ChrBase=RamSet RSH 8 HiMem=RamSet FOR Index=0 TO 1023 DO Val=Peek(57344+Index) Poke(RamSet+Index,Val) OD Charset=RamSet RETURN PROC Dlint() ;the display list interrupt routine [$48 $8A $48 $98 $48] Wsync=1 Colbk=50 [$68 $A8 $68 $AA $68 $40] PROC ScoreLine() ;set up the dli Dlist=Sdlst Vdslst=Dlint Dlist(27)=130 Nmien=$C0 RETURN PROC Update() ;print score and level Position(1,23) Print("Score: ") Position(8,23) PrintC(Score) Position(18,23) Print("Level: ") Position(25,23) PrintB(Level) RETURN PROC Title() BYTE colpf0=53270,colpf1=53271, colpf2=53273,colpf3=53273, rtclock=20,vcount=54283 Graphics(18) Position(3,4) PrintD(6,"SNEAK ATTACK") Position(8,5) PrintD(6,"BY") Position(3,7) PrintD(6,"david plotkin") Position(3,9) PrintD(6,"PRESS start") WHILE Consol<>6 DO colpf3=Fate Wsync=0 colpf0=128-vcount+rtclock RSH 2 colpf1=vcount+rtclock RSH 2 OD RETURN PROC Gr0Init() ;Set up the address of each screen ;line and initialize CARD xx Graphics(0) CursIn=1 Print(" ") FOR xx=0 TO 23 DO Linept(xx)=Scrn+(40*xx) OD FOR xx=0 TO 29 DO Chopperstatus(xx)=0 Chopperx(xx)=0 Choppery(xx)=0 Misx(xx)=0 Misy(xx)=0 MisStatus(xx)=0 TrStatus(xx)=0 OD FOR xx=0 TO 59 DO ExpStatus(xx)=0 OD FOR xx=0 TO 19 DO Ll(xx)=0 Rr(xx)=0 OD Bkgrnd=0 Update() RETURN PROC Plot0(BYTE x,y,ch) ;Plot a char at location x,y BYTE ARRAY line line=Linept(y) line(x)=ch RETURN BYTE FUNC Locate0(BYTE x,y) ;Returns the value of the char at x,y BYTE ARRAY line line=Linept(y) RETURN(line(x)) PROC Noise() ;the explosion noises IF Loud=0 AND Loud1=0 AND Freq=169 THEN RETURN FI IF Loud THEN Loud==-2 Sound(0,90,8,Loud) FI IF Loud1 THEN Loud1==-2 Sound(1,150,8,Loud1) FI IF Freq<168 THEN Freq==+8 Sound(2,Freq,10,4) ELSE Freq=169 Sound(2,0,0,0) FI RETURN PROC HitChute(BYTE wh) ;see which chute was hit by missile wh BYTE lp FOR lp=0 TO 29 DO IF Misx(wh)=Trx(lp) AND (Misy(wh)=Try(lp) OR Misy(wh)=Try(lp)+1) THEN TrStatus(lp)=2 Plot0(Trx(lp),Try(lp),0) Plot0(Trx(lp),Try(lp)+1,10) Plot0(Trx(lp),Try(lp)+2,0) EXIT FI OD IF Try(lp) LSH 3 < Freq THEN Freq=Try(lp) LSH 3 FI RETURN PROC HitMan(BYTE wh) ;see which man was hit by missile wh BYTE lp FOR lp=0 TO 29 DO IF Misx(wh)=Trx(lp) AND (Misy(wh)=Try(lp)+1 OR Misy(wh)=Try(lp)+2) THEN TrStatus(lp)=3 Plot0(Trx(lp),Try(lp)+1,6) Plot0(Trx(lp),Try(lp),0) Plot0(Trx(lp),Try(lp)+2,0) FI OD Loud1=12 RETURN PROC ExplodeChopper(BYTE lp) ;explosions in place of Chopper lp BYTE lq FOR lq=0 TO 59 STEP 2 DO ;find empty IF ExpStatus(lq)=0 THEN ExpStatus(lq)=1 ExpStatus(lq+1)=1 Expx(lq)=Chopperx(lp) Expx(lq+1)=Chopperx(lp)+1 Expy(lq)=Choppery(lp) Expy(lq+1)=Choppery(lp) Chopperstatus(lp)=0 Plot0(Expx(lq),Expy(lq),6) Plot0(Expx(lq+1),Expy(lq+1),6) EXIT FI OD RETURN PROC HitChopper(BYTE wh) ;which chopper was hit by missile wh BYTE lp FOR lp=0 TO 29 DO IF Misy(wh)=Choppery(lp) AND (Misx(wh)=Chopperx(lp) OR Misx(wh)=Chopperx(lp)+1) THEN ExplodeChopper(lp) EXIT FI OD Loud=12 RETURN PROC MissileHit(BYTE wh) ;see if missile wh hit anything BYTE dum dum=Locate0(Misx(wh),Misy(wh)) IF dum=0 THEN Plot0(Misx(wh),Misy(wh),84) RETURN FI MisStatus(wh)=0 IF dum=1 OR dum=2 THEN HitChopper(wh) Score==+1 ELSEIF (dum=7 AND Indx<6 OR dum=8 AND Indx>3) THEN HitChute(wh) Score==+2 ELSEIF (dum=8 AND Indx<4 OR dum=9 AND Indx>1) THEN HitMan(wh) Score==+1 FI RETURN PROC Modify() ;Modify the RAM character set CARD xx FOR xx=0 TO 103 DO Charset(xx+8)=ShapeTable(xx) OD RETURN PROC LaunchTrooper(BYTE wh) ;drop a paratrooper from chopper wh BYTE lp IF Fate>240-(Level LSH 1) THEN FOR lp=0 TO 29 DO ;find MT trooper IF TrStatus(lp)=0 THEN ;got one TrStatus(lp)=1 Trx(lp)=Chopperx(wh) IF Trx(lp)=0 THEN Trx(lp)=1 FI Try(lp)=Choppery(wh)+1 Plot0(Trx(lp),Try(lp),7) Plot0(Trx(lp),Try(lp)+1,8) Plot0(Trx(lp),Try(lp)+2,9) EXIT FI OD FI RETURN PROC EraseChopper(BYTE wh) ;erase chopper number wh Plot0(Chopperx(wh),Choppery(wh),0) Plot0(Chopperx(wh)+1,Choppery(wh),0) Chopperstatus(wh)=0 Chopperx(wh)=0 Choppery(wh)=0 RETURN PROC DrawChopper(BYTE wh) ;draw chopper number wh Plot0(Chopperx(wh),Choppery(wh),1) Plot0(Chopperx(wh)+1,Choppery(wh),2) RETURN PROC ClearScreen() ;clear the screen BYTE lp FOR lp=0 TO 29 DO IF Chopperstatus(lp) THEN EraseChopper(lp) FI IF TrStatus(lp) THEN TrStatus(lp)=0 Plot0(Trx(lp),Try(lp),0) Plot0(Trx(lp),Try(lp)+1,0) Plot0(Trx(lp),Try(lp)+2,0) FI IF MisStatus(lp)=1 THEN MisStatus(lp)=0 Plot0(Misx(lp),Misy(lp),0) FI OD FOR lp=0 TO 59 STEP 2 DO IF ExpStatus(lp)=1 THEN ExpStatus(lp)=0 ExpStatus(lp+1)=0 Plot0(Expx(lp),Expy(lp),0) Plot0(Expx(lp+1),Expy(lp+1),0) FI OD RETURN PROC MoveChopper() ;move the choppers BYTE lp,ps=[0] FOR lp=0 TO 29 DO IF Chopperstatus(lp)=1 THEN ;right IF Chopperx(lp)=38 THEN EraseChopper(lp) ELSE Plot0(Chopperx(lp), Choppery(lp),0) Chopperx(lp)==+1 DrawChopper(lp) LaunchTrooper(lp) FI FI IF Chopperstatus(lp)=2 THEN ;left IF Chopperx(lp)=0 THEN EraseChopper(lp) ELSE Plot0(Chopperx(lp)+1, Choppery(lp),0) Chopperx(lp)==-1 DrawChopper(lp) LaunchTrooper(lp) FI FI OD IF ps=0 THEN Charset(8)=56 Charset(16)=28 ps=1 ELSE ps=0 Charset(8)=254 Charset(16)=127 FI RETURN PROC LaunchChopper() ;Decide whether to send off a new ;chopper, which side, how high up BYTE lp IF Fate>230-(Level LSH 1) THEN FOR lp=0 TO 29 DO ;find MT chopper IF Chopperstatus(lp)=0 THEN Choppery(lp)=Rand(Hard) IF Fate>128 THEN Chopperx(lp)=38 ;right side Chopperstatus(lp)=2 ELSE Chopperx(lp)=0 ;left side Chopperstatus(lp)=1 FI DrawChopper(lp) EXIT FI OD FI RETURN PROC DrawBase() ;draw the base BYTE lp FOR lp=19 TO 21 DO Plot0(lp,22,128) OD Plot0(20,21,4) RETURN PROC AimGun() ;read the joystick and move the base IF Stick=11 THEN Ps=3 ELSEIF Stick=7 THEN Ps=5 ELSE Ps=4 FI Plot0(20,21,Ps) RETURN PROC Shoot() ;send off a bullet BYTE trig=644,lp,flg=[0] IF trig=1 OR flg=0 THEN flg=1 RETURN FI FOR lp=0 TO 29 DO ;find empty shot IF MisStatus(lp)=0 THEN ;got one MisStatus(lp)=1 Misy(lp)=20 IF Ps=3 THEN Misx(lp)=19 ELSEIF Ps=5 THEN Misx(lp)=21 ELSE Misx(lp)=20 FI MissileHit(lp) EXIT FI OD flg=0 RETURN PROC MoveShots() ;move the fired bullets BYTE lp FOR lp=0 TO 29 DO ;for each shot IF MisStatus(lp)=1 THEN Plot0(Misx(lp),Misy(lp),0) IF Stick=11 THEN Misx(lp)==-1 ELSEIF Stick=7 THEN Misx(lp)==+1 ELSE Misy(lp)==-1 FI IF (Misx(lp)<>39 AND Misy(lp)<>255 AND Misx(lp)<>0) THEN MissileHit(lp) ELSE MisStatus(lp)=0 FI FI OD RETURN PROC MoveExplosions() ;move the explosions BYTE lp FOR lp=0 TO 59 STEP 2 DO IF ExpStatus(lp)=1 THEN Plot0(Expx(lp),Expy(lp),0) Plot0(Expx(lp+1),Expy(lp+1),0) Expy(lp)==+1 Expy(lp+1)==+1 Expx(lp)==-1 Expx(lp+1)==+1 IF Expy(lp)<>22 AND Expx(lp)<>0 AND Expx(lp+1)<>39 THEN Plot0(Expx(lp),Expy(lp),6) Plot0(Expx(lp+1),Expy(lp+1),6) ELSE ExpStatus(lp)=0 ExpStatus(lp+1)=0 FI FI OD RETURN PROC BaseExplode() ;explode the base BYTE ARRAY endx(0)=[16 24 17 23 20], endy(0)=[22 22 19 19 17] BYTE lp,time=20 color=38 FOR lp=0 TO 4 DO Plot(20,22) DrawTo(endx(lp),endy(lp)) OD FOR lp=0 TO 16 DO Sound(0,Fate,8,16-lp) Sound(1,Fate,8,16-lp) time=0 DO UNTIL time=15 OD OD SndRst() color=32 FOR lp=0 TO 4 DO Plot(20,22) DrawTo(endx(lp),endy(lp)) OD RETURN PROC EndRight() ;move the troopers from the right ;to the base BYTE lp,lq,nn,time=20 FOR lp=0 TO 19 DO IF Rr(lp)=1 THEN lq=21+lp WHILE lq>20 DO IF nn=12 THEN nn=13 ELSE nn=12 FI Plot0(lq,22,nn) time=0 DO UNTIL time=10 OD Plot0(lq,22,0) lq==-1 OD Plot0(21,22,11) FI OD FOR lp=0 TO 3 DO Plot0(21,22-lp,11) time=0 DO UNTIL time=10 OD OD BaseExplode() RETURN PROC EndLeft() ;Move the troopers from the left to ;the base BYTE lp,lq,lc,nn,time=20 FOR lp=0 TO 19 DO lq=19-lp IF Ll(lq)=1 THEN FOR lc=lq TO 19 DO IF nn=12 THEN nn=13 ELSE nn=12 FI Plot0(lc,22,nn) time=0 DO UNTIL time=10 OD Plot0(lc,22,0) OD Plot0(19,22,11) FI OD FOR lp=0 TO 3 DO Plot0(19,22-lp,11) time=0 DO UNTIL time=10 OD OD BaseExplode() RETURN PROC EndPrint() ;print the end of game message and ;test for new game BYTE trig=644,lp Position(10,7) Print("Game Over...Final Score:") Position(15,8) PrintC(Score) Position(15,9) Print("FINAL LEVEL :") PrintB(Level) Position(10,20) Print("Press FIRE to play again") DO UNTIL trig=0 OD DownL=0 DownR=0 Put(125) FOR lp=0 TO 19 DO Ll(lp)=0 Rr(lp)=0 OD Score=0 Level=1 DrawBase() Update() Hard=15 RETURN PROC GameOverTwo() ;game over when four troopers down BYTE lp SndRst() ClearScreen() Loud=0 Loud1=0 Freq=169 FOR lp=0 TO 19 DO IF Ll(lp)=1 THEN Plot0(lp,22,11) FI IF Rr(lp)=1 THEN Plot0(lp+21,22,11) FI OD IF DownL=4 THEN EndLeft() ELSE EndRight() FI EndPrint() RETURN PROC GameOverOne() ;game over when trooper lands on base BYTE lp SndRst() ClearScreen() Loud=0 Loud1=0 Freq=169 FOR lp=0 TO 19 DO IF Ll(lp)=1 THEN Plot0(lp,22,11) FI IF Rr(lp)=1 THEN Plot0(lp+21,22,11) FI OD BaseExplode() EndPrint() RETURN PROC TrooperDown(BYTE wh) ;redraw trooper wh at bottom of screen BYTE cc TrStatus(wh)=0 cc=Trx(wh) Plot0(Trx(wh),Try(wh),0) ;erase chute Plot0(Trx(wh),Try(wh)+1,11) ;replace IF Trx(wh)<20 AND Ll(cc)=0 THEN Ll(cc)=1 DownL==+1 ELSEIF Trx(wh)>20 AND Rr(cc-21)=0 THEN Rr(cc-21)=1 DownR==+1 ELSEIF Trx(wh)=20 THEN GameOverOne() FI IF DownL=4 OR DownR=4 THEN GameOverTwo() FI RETURN PROC TrooperFall() ;make trooper fall when chute hit BYTE lp,qq,cc FOR lp=0 TO 29 DO IF TrStatus(lp)=2 THEN Plot0(Trx(lp),Try(lp)+1,0) Try(lp)==+1 IF Try(lp)=21 THEN cc=Trx(lp) IF Trx(lp)<20 AND Ll(cc)=1 THEN DownL==-1 Ll(cc)=0 ELSEIF Trx(lp)>20 AND Rr(cc-21)=1 THEN Rr(cc-21)=0 DownR==-1 FI FI IF (Try(lp)<22 AND Trx(lp)<>20) OR (Try(lp)<20 AND Trx(lp)=20) THEN Plot0(Trx(lp),Try(lp)+1,10) ELSE TrStatus(lp)=0 FI FI OD RETURN PROC MoveTroopers() ;move paratroopers down screen BYTE lp,qq BYTE ARRAY Trooper(0)= [60 126 255 255 195 66 36 24 60 36 24 255 60 24 36 102 0 0 0 0 0 0 0 0] FOR lp=0 TO Indx DO Charset(56+lp)=0 OD MoveBlock(Charset+56+Indx+1, Trooper,16) Indx==+1 IF Indx<8 THEN RETURN FI Indx=0 FOR lp=0 TO 29 DO IF TrStatus(lp)=1 THEN Plot0(Trx(lp),Try(lp),0) Try(lp)==+1 IF Try(lp)=21 THEN TrooperDown(lp) FI FI IF TrStatus(lp)=3 THEN TrStatus(lp)=0 Plot0(Trx(lp),Try(lp)+1,0) FI OD MoveBlock(Charset+56,Trooper,24) FOR lp=0 TO 29 DO IF TrStatus(lp)=1 THEN Plot0(Trx(lp),Try(lp),7) Plot0(Trx(lp),Try(lp)+1,8) Plot0(Trx(lp),Try(lp)+2,9) FI OD RETURN PROC NewLevel() ;go to higher level BYTE lp,time=20 Level==+1 IF Level>100 THEN Level=100 FI SndRst() Loud=0 Loud1=0 Freq=169 Comp==+300 FOR lp=10 TO 150 STEP 10 DO Sound(0,lp,10,4) Sound(1,lp+10,10,4) time=0 DO UNTIL time=2 OD OD Position(25,23) PrintB(Level) IF Level>8 THEN Hard=19 FI SndRst() RETURN PROC Main() BYTE time=20,lp,ch=764 Title() Gr0Init() Snd1=0 Snd2=3 Download() Modify() DrawBase() ScoreLine() DO LaunchChopper() MoveChopper() MoveExplosions() Noise() TrooperFall() MoveTroopers() Position(8,23) PrintC(Score) IF Score>Comp THEN NewLevel() FI time=0 FOR lp=2 TO 6 STEP 2 DO AimGun() Shoot() MoveShots() DO UNTIL time=lp OD OD OD RETURN