Anti-Aircraft Gunner

This is a shooting-gallery game in which the player fires at a moving target sliding across the screen. The program uses FAST mode to fill the screen with inverse-space characters as a border effect, then switches to SLOW mode for gameplay. A target moves left to right across row 3 in steps of two columns, with a random delay (controlled by a FOR/NEXT busy-wait loop seeded by RND) governing its speed. The player presses “V” to shoot, and the program checks whether the target is at column 14 for a hit; scores for shots fired and hits are maintained in variables Q and D respectively. After 20 shots the game ends with a flashing score display achieved by alternating PRINT statements in a tight loop.

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Program Analysis

Program Structure

The program divides into five logical phases:

1. Initialisation (lines 3–8): Variables D (hits), Q (shots) and A (loop counter) are zeroed; FAST mode is entered.
2. Screen fill (lines 10–20): A FOR-less loop prints rows of inverse-space characters 21 times to build a border/background, then falls through to line 25.
3. Target animation (lines 25–80): The target slides from column 0 to column 29 in steps of 2, with a random speed delay regenerated each pass.
4. Shot handling (lines 90–140): A keypress at “V” triggers shot logic, checks for a hit at column 14, updates scores, and re-enters the animation loop.
5. Game over (lines 500–560): After 20 shots, a flashing score is displayed, and the program saves itself and re-runs.

Screen Fill Technique

Rather than using a conventional FOR/NEXT loop, the background is drawn with an unstructured loop: lines 10–20 print a full row of inverse spaces, increment A, and branch back or forward depending on whether A has reached 21. Running in FAST mode makes this fill effectively instantaneous from the user’s perspective. The result is a screen packed with inverse-space characters serving as a dark backdrop.

Target Movement and Speed

The target is a pair of inverse-space characters printed at row 3 using PRINT AT 3,C;"% % \.' (an inverse space, space, inverse space, and a block graphic ▞). Column C steps from 0 to 29 in increments of 2 inside the outer FOR C loop. Between each column step a busy-wait FOR E=1 TO F … NEXT E provides the delay, where F=INT(RND*10)/2 produces a random integer 0–4, giving variable target speed that is re-randomised each full pass.

Shooting Mechanic and Hit Detection

Within the delay loop at line 52 the program polls INKEY$ for “V”. On a keypress, execution jumps to line 90, which draws a “bullet” column at column 16 (rows 18 and 19) and simultaneously redraws the player sprite at row 4/18 column 16. The hit test at line 96 is simply IF C=14 THEN GOTO 106; because the target steps by 2 and the bullet is fixed at column 16, a hit is registered when the target occupies column 14 — meaning the graphic overlap occurs one step before the bullet column. This is a deliberate approximation rather than a true collision check.

Hit Animation

Lines 106–108 draw an explosion using block graphics centred on column 15:

• Line 106 uses \' (▘) and \ ' (▝) combined with \. (▖) and \ . (▗) to paint a four-quadrant burst pattern.
• Line 108 overwrites with \:' (▛) and \:. (▙) variants to animate a second frame.
• Both lines also reprint surrounding inverse-space characters to restore the background after the animation.

Score Display and Game Over

Shots (Q) and hits (D) are displayed with inline concatenation: PRINT AT 18,2;Q;"% SHOTS" and AT 18,23;D;"% HITS" (the % being an inverse space used as a separator). When Q>20, the game-over routine at line 500 enters a tight loop alternating between printing blank spaces and the hit count at position (18,23), creating a simple flashing effect without any explicit timing mechanism other than the loop itself.

Save and Auto-Run

Line 550 saves the program under a filename that includes inverse characters ("1026%6" where %6 is an inverse digit), which on the ZX81/TS1000 encodes an auto-run flag so the program starts automatically when loaded. Line 560 immediately re-runs the program after saving, so a fresh game begins without user intervention.

Variables Summary

Variable	Purpose
A	Screen-fill loop counter (0–21, then discarded)
C	Current target column (FOR loop, 0–29 step 2)
D	Hit counter
E	Speed delay loop index
F	Random delay value (0–4)
Q	Shot counter
Y	Short busy-wait loop at game over (lines 522–524)

Notable Bugs and Anomalies

• After a hit (line 106–140), the outer FOR C loop is re-entered at line 40 with C reset to 0, but the FOR statement re-initialises C cleanly, so this is not a bug — it simply restarts the target from the left.
• On a miss (no hit at C=14), line 102 jumps back into the middle of the FOR E loop at line 55, bypassing the FOR statement. On the ZX81 this reuses the existing E loop descriptor rather than reinitialising it, which could cause the delay to run from wherever E was rather than from 1. This is a subtle timing irregularity.
• Line 36 prints a gun/player sprite at rows 19–20 column 16 every time a new random speed is chosen, not just once — this is harmless but redundant.
• The CLEAR at line 540 is never reached because the game-over loop at lines 510–530 never exits.