Computer Combat

Computer Combat is a one-dimensional shooting gallery game where the player manoeuvres a tank along the bottom row of the screen and fires at an enemy unit moving across the display. The player uses keys 5 and 8 to move left and right, and 0 to fire, with a starting ammunition budget of 25 rounds stored in variable B. The enemy scrolls horizontally using AT coordinates, and a random GOSUB at line 155 occasionally fires back at the player, deducting 5 points on a hit. Scoring awards +5 for a hit on the enemy and −1 for a missed shot, with the game ending when ammunition runs out or the enemy completes 20 rows of travel.

Program Analysis

Program Structure

The program is organised into a main game loop, a firing subroutine, and an enemy counter-attack subroutine. Initialisation occurs in lines 5–25, the main loop runs from line 30 to line 95 via GOTO VAL "40", and end-of-game display is at line 100. Two subroutines handle player firing (lines 115–140) and enemy retaliation (lines 155–185).

1. Lines 5–25: Initialise ammunition B=25, score S=0, enemy row M=0, player X position X=16, and enemy column Y=0.
2. Lines 30–95: Main loop — advance enemy row, check row limit, clear screen, randomly trigger counter-attack, draw enemy, advance enemy column, wrap column, draw player tank, read movement keys, check fire key, check ammo, repeat.
3. Lines 115–140: Player fire subroutine — draws a + marker, checks hit/miss, updates score, decrements ammo.
4. Lines 155–185: Enemy counter-attack subroutine — draws a missile character at the enemy column on row 21, checks if it hits the player, penalises score.

Key Variables

Notable Techniques

VAL "number" is used pervasively in place of numeric literals (e.g. GOTO VAL "40", LET B=VAL "25"). This is a well-known memory optimisation on this platform: storing a number as a string literal consumes less RAM than storing it as a floating-point constant in the tokenised line.

Zero-initialisation uses PI-PI rather than the literal 0, exploiting the fact that PI is a single-token keyword, making PI-PI shorter in tokenised form than the numeric constant 0 with its five-byte floating-point representation.

Player movement at line 75 uses a Boolean arithmetic idiom: INKEY$="8" evaluates to 1 (true) or 0 (false), multiplied by 3 to give a step size of 3 columns. This avoids an explicit IF/THEN branch for movement.

The enemy sprite at line 55 uses block graphic characters (\:'\:'\:' — rendered as ▌▘▌▘▌▘) drawn with PRINT AT M,Y. The player tank at line 70 uses \.:\.. (▖▄▖▖ approximating a tank silhouette).

The counter-attack at line 155 uses CHR$ 23 as a missile glyph. On the ZX81/TS1000, character 23 is a block graphic, giving a visual projectile without defining a UDG.

Bugs and Anomalies

• Line 75 reads INKEY$ twice in the same statement for movement, then line 80 reads it again for firing. Because INKEY$ is sampled instantaneously each time, pressing 0 while moving could register, but pressing a movement key simultaneously with 0 may cause the movement read to shadow the fire check depending on key held at that instant — no debouncing is implemented.
• There is no boundary check on X: the player can move off-screen left (below column 0) or right (beyond column 31), potentially causing a PRINT AT error or wrap-around.
• The hit-detection at line 120/125 checks X+1=Y. Given that the player tank sprite is drawn at column X and the enemy at column Y, only a single-column offset constitutes a hit, making the effective target window very narrow.
• Lines 145–150 (PAUSE VAL "50" then GOTO VAL "40") are never reached by any branch in the program; they are dead code.
• Line 35 checks M>VAL "20" but M increments by 2 each iteration starting from 0, so M reaches 22 before triggering, meaning the enemy can briefly appear below row 20 before the game ends.
• After game over at line 100, line 110 calls RUN which resets all variables and restarts — the score flash at line 100 is visible only briefly before the restart, and there is no way to review it.

Content

Appears On

Timex Sinclair Public Domain Library Tape 1003

Assembled by Tim Ward from many sources. Contains programs 10122 – 10175.

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Source Code

   1 REM COMPUTER COMBAT
   2 REM KEYS 5/8 TO MOVE--0 TO FIRE
   5 LET B=VAL "25"
  10 LET S=PI-PI
  15 LET M=PI-PI
  20 LET X=VAL "16"
  25 LET Y=PI-PI
  30 LET M=M+2
  35 IF M>VAL "20" THEN GOTO VAL "100"
  40 CLS 
  45 LET R=INT (RND*5)
  50 IF R=3 THEN GOSUB 155
  55 PRINT AT M,Y;"\:'\:'\:'"
  60 LET Y=Y+VAL "2"
  65 IF Y=VAL "30" THEN GOTO 25
  70 PRINT AT 21,X;" \.:\.. "
  75 LET X=X+(INKEY$="8")*3-(INKEY$="5")*3
  80 IF INKEY$="0" THEN GOSUB 115
  85 IF B<=VAL "0" THEN GOTO VAL "100"
  95 GOTO VAL "40"
 100 PRINT AT 12,10;"SCORE= ";S
 110 RUN 
 115 PRINT AT M,X;"+"
 120 IF X+1<>Y THEN LET S=S-1
 125 IF X+1=Y THEN PRINT AT M,X-1;"***"
 130 IF X+1=Y THEN LET S=S+5
 135 LET B=B-VAL "1"
 140 RETURN 
 145 PAUSE VAL "50"
 150 GOTO VAL "40"
 155 PRINT AT 21,Y;CHR$ 23
 160 IF X=Y THEN GOSUB VAL "170"
 165 RETURN 
 170 PRINT AT 21,Y;"<*>"
 175 PAUSE VAL "40"
 180 LET S=S-VAL "5"
 185 RETURN 
 190 SAVE "1014%6"
 200 LIST 

Note: Type-in program listings on this website use ZMAKEBAS notation for graphics characters.