Star Blast

This program implements a space shooter game where the player controls a cannon (UDG “a”) at the bottom of the screen, firing projectiles (UDG “b”) upward to hit stars (UDG “c”) arranged in four scrolling rows. Three custom UDG characters are defined via POKE USR: a triangular cannon shape, a vertical bullet, and an eight-pointed star. The four rows of stars scroll horizontally at different rates — row k shifts by k characters every 50 timer ticks — creating increasing difficulty. The STICK command (encoded as |) reads joystick input for movement and firing, and the score increments by 100 for each successful hit, with the time limit extending by 500 ticks each wave.

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Program Analysis

Program Structure

The program is organized into clearly separated segments by line number range:

1. 200–300: Initialization — screen clear, variable setup, score label.
2. 500–770: UDG character definition via POKE USR.
3. 900–980: Wave initialization — star array reset, initial star placement, screen draw.
4. 1010–1310: Main game loop — joystick polling, player movement, projectile update, timing logic.
5. 3000–3070: Subroutine: populate star field array.
6. 4000–4030: Subroutine: render all four star rows to screen.
7. 5000–5080: Subroutine: scroll star rows.
8. 6000–6130: Subroutine: collision detection between projectile and stars.
9. 7000–7030: Subroutine: score update and sound effect.
10. 9000–9020: End-of-level handler, resets and restarts game.

UDG Character Definitions

Three User-Defined Graphics are created using POKE USR and BIN literals:

UDG	Purpose	Description
\a	Cannon / player	Wide triangle shape, narrow top, broad base — resembles a spaceship or cannon barrel.
\b	Projectile / bullet	Thin vertical line with a slight fork at the bottom — a stylized missile.
\c	Star / target	Radiating pattern with a solid cross and diagonal hints — an 8-point star. Note: line 710 contains BIN 011111110 (9 digits), which the interpreter silently truncates to 8 bits, effectively the same as BIN 11111110.

Joystick Input via STICK

The program uses the TS2068 STICK keyword (encoded as | in the source). |(1,1) polls joystick port 1: a return value of 4 moves the cannon left (line 1010), 8 moves it right (line 1020), and |(2,1)=1 on the second axis triggers a shot (line 1030). This makes the game entirely joystick-driven with no keyboard fallback.

Star Field and Scrolling

The star field is stored in a 4×32 string array a$, where each row corresponds to one of four display rows on screen, each separated by a blank line. The scrolling subroutine at line 5000 rotates row k left by k characters using string slicing — so row 1 scrolls slowest and row 4 scrolls fastest. This differential scrolling is achieved with:

• h$=a$(k,1 TO k) — save the leading k characters.
• a$(k,1 TO 32-k)=a$(k,k+1 TO 32) — shift left.
• a$(k,32-k+1 TO )=h$ — wrap saved characters to the end.

Scrolling is triggered every 50 timer increments (checked at line 1265 with IF tm/50=INT(tm/50)), giving a periodic rather than continuous scroll.

Projectile and Collision Detection

When fired, the projectile starts at row 19, column sta (the cannon’s position) and moves up by 2 rows per loop iteration (LET s=s-2, line 1250). Collision is checked only when the projectile is in the upper half of the screen (s<=7) and on an odd row (s/2<>INT(s/2)), since star rows occupy rows 1, 3, 5, and 7. The subroutine at 6000 checks a$(k,c+1)="\c" for each of the four star rows: the column offset of c+1 is because the array is 1-indexed while screen columns are 0-indexed. A hit clears the array cell, erases the character from the screen, decrements left, and adds 100 to the score.

Wave Progression and Difficulty

The variable n controls the number of stars placed per wave, initialized to 15 and incremented by 5 each new wave up to a cap of 100 (line 3000). The time limit limit is extended by 500 ticks per wave. The condition at line 1270 (IF tm=limit) ends the current wave and loops back to line 200 for a full reset rather than continuing from where the player left off — meaning there is no persistent score accumulation across waves.

Notable Bugs and Anomalies

• Line 1280 checks IF left=0 THEN GO TO 1300, but line 1300 is simply GO TO 1010 — the branch to 1300 is identical in behavior to falling through to line 1290, which also goes to 1010. The intent may have been to trigger a new wave when all stars are cleared, but lines 1310–1320 (which call GO SUB 4000 and loop back) are never reached by any execution path.
• Line 710 uses BIN 011111110, a 9-bit binary literal. The interpreter reads only the rightmost 8 bits, yielding 11111110 (254) instead of the likely intended 01111110 (126). This causes the top of the star UDG to have its leftmost pixel lit unexpectedly.
• The projectile column variable is named c, which shadows no keyword but is also used without initialization if pr is ever non-zero from a prior cycle without a fresh shot — in practice the flow prevents this.