Clive Invaders

This program implements a Space Invaders-style arcade game in BASIC, featuring a single alien invader that descends the screen while the player controls a base along the bottom row. The alien sprite is drawn using five rows of block graphics characters, and the player fires a laser beam upward using a FOR/NEXT loop that draws and erases a column of block characters. Collision detection is handled by comparing the horizontal column positions of the laser beam against the alien’s bounding box across several lines (1040–1070 and 4015–4030). The program tracks both a current score (S1) and a high score (S2), updating the high score only when the current score exceeds it. Alien movement includes a random vertical speed-up mechanic controlled by a random variable Z1, and the alien also fires back at the player via a bomb drawn with a second sprite routine (lines 4000–4050).

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Program Analysis

Program Structure

The code is organised into a series of functional sections accessed via GOTO and GOSUB:

1. Lines 1–8: Entry point; asks for instructions and initialises scores.
2. Lines 8–165: Game field setup — draws the border, scoreline, and initialises all game variables.
3. Lines 165–320: Main game loop — reads player input, moves alien and base, fires alien bomb, checks for collision.
4. Lines 1000–1200: Player laser fire subroutine.
5. Lines 2000–2047: Game Over handler — updates high score and prompts to replay.
6. Lines 4000–4050: Alien bomb subroutine.
7. Lines 5000–5510: Alien animation subroutines (alternate leg positions for sprite).
8. Lines 6000–6050: Player base destroyed handler.
9. Lines 7000–7028: End-game alien victory animation loop.
10. Lines 9000–9080: Instructions screen and title display; GOSUB 9038 from line 2 jumps into the middle of the subroutine, landing at line 9050.

Sprite Design

The alien is a six-row sprite (including blank guard rows) constructed from block graphic escape sequences. The five meaningful rows use variables G$, A$, B$, C$, and D$. The player base is two rows stored in E$ and F$. Animation of the alien’s “legs” is achieved by toggling D$ between two values through subroutines at lines 5000 and 5500, controlled by the frame counter F.

Key BASIC Idioms and Techniques

• Mid-subroutine entry: GOSUB 9038 at line 2 deliberately jumps into the middle of the subroutine block (between lines 9030 and 9050), bypassing the GOTO 6 at 9030 and landing at the PRINT at line 9050. This is an intentional trick to share code.
• Boundary clamping: Lines 200–212 clamp D (base column), B (alien column), and are placed after input reading but before rendering — a typical guard pattern.
• Random alien movement: H is set to INT(RND*3) each frame; line 205 maps the value 2 to −1, giving the alien a left/right random walk of {0, 1, −1}. Variable Z1 (1–10) occasionally causes the alien to skip a row (if Z1=10 it subtracts 2 from A before adding 1, but then does an extra iteration via a direct GOTO 230 bypassing the full loop).
• Laser beam: The subroutine at line 1000 fires from the base upward using a FOR I=17 TO A+4 STEP -1 loop, drawing \ : (a half-block character) at each row, creating an animated upward beam. The beam is then erased in a second loop.
• Score persistence within session: S2 stores the high score and is only updated at line 2030 when S1>S2, persisting across replays without file I/O.
• Inverse video for UI: All score and UI text uses %X inverse-video characters throughout, consistent with the ZX81/TS1000 display style of the era.

Collision Detection

Two collision systems are present. Player-laser-hits-alien is checked in lines 1040–1070 by comparing the laser column J (set to D+4) against four consecutive alien columns (B+1 through B+4); a hit triggers the kill sequence at line 1100. Alien-bomb-hits-base is checked in lines 4015–4030 by comparing B+3 against four base columns (D+3 through D+6); a hit triggers the base destruction at line 6000.

Anomalies and Notable Points

• Line 34 draws the bottom border using \..\.. block sequences (▄ characters) at row 21, but the main play area is also bounded by the alien reaching A+5=C (i.e. row 20), so the border is decorative rather than a collision barrier.
• The alien bomb counter E is incremented by 0.2 per frame (line 318) and by 0.5 on a kill (line 1190). When E>=5 (line 281), the bomb fires. After firing, E is reduced by a random amount (line 4040: E=E-INT(RND*5)+1). Note that operator precedence means this computes E - (INT(RND*5)) + 1, so the minimum reduction is 1 and maximum is 4, not 0–4 as likely intended.
• The SLOW command at line 165 (ZX81/TS1000 specific) is called on every pass through the main loop rather than once at startup, which is redundant but harmless.
• Line 9070 contains a SAVE "1015%7" statement that is never reached by normal program flow; it exists purely to save the program to tape.
• The victory animation loop at lines 7000–7028 is infinite with no exit path, requiring a manual break.

Variable Summary

Variable	Purpose
A	Alien top row position
B	Alien column position
C	Base row position (fixed at 20)
D	Base column position
E	Alien bomb charge counter
F	Alien animation frame counter / bomb state flag
H	Alien horizontal direction (−1, 0, 1)
S1	Current score
S2	High score
G$–F$	Sprite row strings for alien and base