Intruder Alert!

Copyright © 2025 by Víctor Parada

This is a little game for the 2025 NOMAM's BASIC 10-liners Contest. This program fits in the EXTREM-256 category, and it was written using FastBasic 4.7 for the 8-bits ATARI XL/XE computers line. Development started on 2024-10-31, and it took 3+5+3 days. The final version's date is 2025-03-26.

Description

You are an intruder humanoid that must escape from the maze destroying all the guardian robots.

Instructions

	Press the button to start.
	Use the joystick to move around the maze and reach any of the room exits. There are robots trying to chase and destroy you.
	Use the trigger to shoot to any robot in your way out. You will score 10 points for each destroyed robot. You can perform only one shot at a time. Beware! Robots can dodge shots towards their legs.
	If there are no more robots in a room when you leave it, you will receive a bonus of 100 points. But more robots will chase you in the next room.
	If you stay in a room for too much time, the evil smiling boss will came to chase you. He can't be killed.
	Don't touch the electrified walls, or you will die just like as a robot chased you.
	You have three lives. The game is over when you loose all of them. Press the trigger to play again.

Development of the game

In the search for classic games that had not been ported to a tenliner, I found Berzerk. I thought that it could be really simple to code it and gave it a try. My idea was to use both a modified character set and player/missile sprites for static and moving robots respectively.

My first attempt was to use the sprites from the Atari 2600 version of the game, because the robots could fit ANTIC 7 chars (GR.2) and it could very easy to animate their tracking eye. But I realize there was a problem: it could be hard to draw the maze using that screen resolution, because I should have to manage all 15 posible combinations of wall junctions for random mazes.

Original Berzerk arcade game.

Port of Berzerk for Atari 2600 console.

Trying to solve that, I changed the graphics mode to ANTIC 4 (GR.12), using the whole character as a block for the walls. PLOT and DRAWTO could be used to draw the maze. But, each robot required a tile of 4 chars, and each scan line should be doubled, using more bytes for the redefined charset in the source and wasting some valuable bytes for coding. The good thing is that the tiles for the robots can be placed half the way, giving a less grid-like look of the playfield.

In three days I had a playable prototype of the game in FastBasic, with some animations for the player and the robots. I could replicate the maze generation algorithm, but I did not use a seed for random numbers. Instead, I used the random number generator from the POKEY chip through the RAND() function.

Test using tiles in ANTIC 4 mode.

Robots move smoothly towards the player.

Then, the headache started: I faced a bug while testing, but I couldn't find where in the code it was, because it appeared few times during a lot of hours playing and tracing and debugging. After a couple of days I got frustrated and suspended the project. A couple of weeks later, with a fresh mind, I gave it a new try, added more tracing code but, again, I couldn't find what was going on and dropped the project.

Tracing to find the bug.

Some months later, it was opened the call for the 2025 tenliners contest and I had no fresh ideas. I reviewed some of the many dropped prototype games I've written through the years and this came in front of them. I played it a few times, removed many of the trace code and sat down to think. I was sure there was a border condition that triggered the bug, and it could be related to the hardware registers for collision detection and when the events happens during the screen refresh. If this was true, I could probably be detecting a bit late a collision of the bullet with a robot that it is no longer there. To solve this without adding some extra PAUSE statements to force a screen refresh with the cost of a slower game, I might change the whole game logic.

With less than 3 weeks for the deadline, I decided to rewrite the game logic. It was required to add many new variables to precompute and hold temporary data in order to perform many actions quickly and get the full screen refreshed in just one frame. This was good and bad at the same time: The bug disappeared, but the cost was less coding space because most of the required extra variables were of two letters. I did some statistics of variable usage and reassigned them in order to keep the most used ones as a single letter variable. Anyway, the coding space was almost exhausted and many of the pending features should be discarded and/or some existing features removed to make room for the most important ones.

New code and new color palette.

I added some sound FX, defined the final color palette, restricted and/or released game dynamics and made them all fit the 10 lines of code. After some days, I was satisfied with the result, even when I couldn't include some of the features I wanted to. The game was hard enough in order to make it even harder!

While cleanning and commenting the code for this page, I decided to change the name of the game. Instead of "Berzerk", during a night call with friends from the retro community, I told them that the game I was working on would be called "Krezreb", and it remained in the source code until it was finished. But one of the features I couldn't include was a background sound routine to mimic the "Intruder alert, intruder alert! The humanoid must not escape!" digitized speechs from the original game. Then I thought about a new game name based on those speechs, and some options were "Intruder" and "Intruder humanoid", but "Intruder alert!" was selected.

Download and try

Get the INTRUDER.ATR file and set it as drive 1 in a real Atari (or emulator). Turn the computer on and the game should start after the loading completes. A joystick in port 1 is required.

The code

The abbreviated BASIC code is the following:

The full and expanded BASIC listing is:

o=adr("{binary data}")+14

graphics 28

pmgraphics 2

move o+67,704,9

dpoke dpeek(560)+27,7

move $E000,$8000,$400
move o+172,$8008,48
poke 756,$80

dim r(9),p(800),oh(9) byte,re(19),
  wx(3) byte,wy(3) byte

move o+114,adr(oh),58

s=dpeek(88)+42

o1=o+74

position 3,22
print #6,"INTRUDER ALERT"

data _d()=0,7,0,-7

do

  while strig(0)
  wend

  w=3

  m=1

  li=3

  sc=0

  position 3,22
  print #6,3 rtab(17) sc

  repeat

    poke 77,0

    sound

    mset pmadr(-1),$280,0

    mset dpeek(88),880,0

    mset adr(p),1602,$FF

    t=0
    for q=0 to 3

      color 37+129*(w<>q)
      exec _m peek(o+90+q)
    next q

    color 37
    for n=0 to 19
      sound 0,n,2,n/2

      if n>11 then t=rand(4)

      exec _m peek(o+94+n)
    next n
    sound

    n=0
    while n<m

      c=rand(15)

      if (c-(c>7)) mod 3<2

        c=40*(rand(5)+c/5*7)+rand(5)+c mod 5*7

        if dpeek(c+s)+dpeek(c+s+40)=0

          exec _r c,1

          r(n)=c

          p(c)=n

          inc n
        endif
      endif
    wend

    t=0

    g=0

    x=wx(w)

    y=wy(w)

    u=0

    v=0

    h=0

    i=0

    f=0

    dx=1-2*(w=1)

    dy=0

    gx=0

    gy=0

    d=0

    e=0

    l=0

    oc=500

    k=0

    z=0

    repeat

      t=(t+1) mod 10

      dpoke $800A,re(t)
      dpoke $801A,re(t+10)

      j=stick(0)

      if j<15

        sound 0,j,8,2

        dx=(j&8=0)-(j&4=0)
        dy=(j&2=0)-(j&1=0)
        x=x+dx
        y=y+dy

        sound 0
      endif

      if oc

        dec oc

        if oc=0

          ox=wx(w)
          oy=wy(w)
        endif

      elif t&1

        ox=ox+sgn(x-ox)
        oy=oy+sgn(y-oy)

        sound 2,200,12,9-t
      endif

      if g
        dec g
        if h

          v=v+i
        else

          u=u+i
        endif

        if g=0

          p(r(q))=-1

          r(q)=r(q)+i+39*i*h
          p(r(q))=q

          if rand(3) then exec _w

          if g=0 then l=3
        endif

      elif l=0 and n

        q=rand(n)

        u=r(q) mod 40*4+56
        v=r(q)/40*4+20

        exec _w

        if g then l=1
      endif

      if z
        dec z
        sound 1,200-4*z,6,z
      endif

      if f

        sound 1,40-3*k,6,k
        k=k-(k>0)

        d=d+gx
        e=e+gy

      else

        if strig(0)=0

          gx=2*dx
          gy=2*dy

          d=(x+7-6*(dx<0))&$FE
          e=(y+5)&$FE

          if gx

            fz=3
          else

            fz=$202
          endif

          k=9

          f=1
        endif
      endif

      p1=o+12*(j<15)*(t&1)+24*(dx<0)
      p2=pmadr(0)+y-1

      o2=pmadr(2)+oy-oh(t)

      b1=pmadr(-1)+e
      b2=b1-gy

      r2=pmadr(1)+v-1
      r3=o+48+9*t&1

      pause

      poke $D01E,0

      pmhpos 0,x
      move p1,p2,13

      if f
        pmhpos 4,d
        dpoke b2,0
        dpoke b1,fz
      endif

      if l=1
        exec _b
        exec _r r(q),0
        l=2

      elif l=2
        exec _b

      elif l=3
        exec _r r(q),1
        mset r2,10,0
        l=0
      endif

      if oc=0
        move o1,o2,16
        pmhpos 2,ox
      endif

      pause

      fk=peek($D000)+peek($D008)&2*8
      if fk

        sound 1

        dpoke b1,0

        if fk&$10

          l=0

          g=0

          mset r2,10,0

          c=q

        elif fk&4

          a=(d-56)/4+(e-20)/4*40

          b=a-peek(adr("{binary data}")+peek(s+a))

          c=p(b)

          exec _r r(c),0
          p(b)=-1
        endif

        if fk&$14
          p(r(c))=-1

          if n>1 and c<n-1
            r(c)=r(n-1)
            p(r(c))=c

            if q=n-1 then q=c
          endif

          dec n

          exec _s 1

          z=12

          if n=0 then oc=oc/4
        endif

        f=0
      endif

      ss=peek($D004)+peek($D00C)*16
    until ss

    if ss&$F7

      for a=0 to 14
        sound 0,9,2,a
        move o-13*a&1,pmadr(0)+y-1,
          13
        pause 
        sound
        pause
      next a

      mset pmadr(0)+y,13,0

      dec li

      position 3,22
      print #6,li

    else

      w=(x>wx(1))*3+(y<wy(0))*2+(x<wx(3))

      m=m+(m<10)

      if n=0 then exec _s 10
    endif

  until li=0

loop

proc _s a
  sc=sc+a
  position 5,22
  print #6,rtab(16) sc
endproc

proc _m c

  a=c&31 mod 6*7+2

  b=c&31/6*7

  c=c/32+t
  plot a,b
  drawto a+_d(c),b+_d(3-c)
endproc

proc _w

  a=x-u+1
  b=y-v-2

  if rand(5)=0 or rand(3) and abs(a)>abs(b)

    h=0
    i=sgn(a)
    a=s+r(q)+i+i*(i>0)

    if peek(a)!peek(a+40)=0 then g=4
  else

    h=1
    i=sgn(b)

    if dpeek(s+r(q)+40*(i+i*(i>0)))=0 then g=4
  endif
endproc

proc _r a b
  dpoke s+a,b*$301
  dpoke s+a+40,b*$402
endproc

proc _b
  pmhpos 1,u
  move r3,r2,10
endproc

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