Aliens

This is a Space Invaders-style shoot-em-up game in which the player defends against randomly appearing aliens using a joystick-controlled rocket launcher. The program defines four User Defined Graphics (UDGs) for the alien sprite (\a), the launcher base (\l), the missile (\d), and an explosion (\e) via POKEd bitmap data in the subroutine starting at line 1000. Joystick input is handled using the STICK keyword, reading both axes of the left joystick for movement and fire detection. Alien appearance speed is governed by the system timer peeked at address 23672, with spawn rate increasing according to a skill level variable `a` entered by the player (1–5). The SOUND keyword drives multi-channel audio for missile launch and hit effects, while hit detection relies on ATTR to check the color attributes of the cell above the missile’s current position.

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Program Analysis

Program Structure

The program is organized into a main game loop and several subroutines:

1. Lines 5–9: Header remarks and startup beep.
2. Lines 10–20: Two initialization subroutines — UDG definition (line 1000) and instructions/skill selection (line 1200).
3. Lines 30–90: Screen setup; draws the ground strip using PAPER 4, scatters stars with OVER 1, and initializes key variables.
4. Lines 100–370: Main game loop — alien spawning, launcher movement, missile firing and travel, hit detection, and scoring.
5. Lines 400–440: Game-over screen with replay prompt.
6. Lines 1000–1160: UDG definition subroutine, loading four characters.
7. Lines 1200–1320: Instructions display and skill level input.
8. Line 9999: SAVE with autostart.

UDG Definitions

Four UDGs are defined at lines 1000–1160 by POKEing bitmap data into USR "x" addresses:

UDG	Variable	Role	Bitmap data
\a	a	Alien sprite	129,126,219,126,60,60,90,129
\l	l	Launcher base	24,24,24,24,60,126,255,90
\d	d	Missile	16,16,16,16,16,16,58,40
\e	e	Explosion	137,74,52,204,51,44,82,145

Note that the loop variable names (a, l, d, e) collide with other game variables used later. In particular, l is reused as the launcher column position starting at line 80, and a is reused as the skill level at line 1310. This is safe because the UDG subroutine completes before these variables take on their game roles, but it is a potential maintenance hazard.

Joystick Input (STICK Keyword)

The TS2068-specific STICK keyword (represented as | in the source) reads joystick state. The program uses two forms:

• stick(1,1) at lines 205 and 231 reads the left joystick’s directional axis and fire button to control horizontal launcher movement and firing.
• Direction 8 = right, direction 4 = left (standard Kempston-style encoding). The launcher position l is updated by adding and subtracting boolean results of these comparisons.
• Fire detection at line 231 checks stick(2,1)=1; if not firing, line 241 checks |(2,1)=0 to loop back without firing.

Alien Spawning and Timing

The system frame counter at address 23672 is used as a software timer. Line 110 resets it with POKE 23672,0, and line 120 reads it back with PEEK 23672. When the counter reaches the threshold 80-(a*6), a new alien is printed at a random screen position using PAPER 8; INK 2 (transparent paper). Skill level a ranges from approximately 0.67 to 4.67 (after the VAL a$-(.33) calculation at line 1310), making the threshold vary between roughly 52 and 76 frames. Higher skill levels cause more frequent alien appearances. The alien counter f1 increments each spawn; when it exceeds 10, the game ends.

Missile Mechanics and Hit Detection

When fire is detected (line 231), a FOR loop from line 250 walks the missile UDG \d upward from row 18 to row 0. At each step the missile is printed in INK 0 (black), then erased. Hit detection at line 276 uses ATTR (c-1,l) to check the color attribute of the cell directly ahead of the missile. The value 106 corresponds to BRIGHT 1 + PAPER 8 + INK 2 (i.e., the alien’s attribute), which triggers the hit routine at line 300. On a hit, f1 is decremented and a FLASH 1 explosion UDG \e is displayed briefly.

SOUND Usage

The TS2068 SOUND keyword is used for multi-register AY-3-8910 sound effects:

• Line 265: Missile travel sound — registers 6, 7, 8, 9, 10, 13 configured for noise-based propulsion sound.
• Line 285: Missile miss — resets registers 8 and 7.
• Lines 331–333: Hit explosion — registers 6, 7, 8, 9, 10, 12, 13, then silenced.
• Line 351: Post-hit silence — resets registers 8 and 7.

Starfield Background

Lines 50–75 draw 20 random stars using PLOT with OVER 1 (XOR plotting mode). The Y coordinate is constrained to RND*120+40, keeping stars above the ground strip (which occupies the lower portion of the screen). Using OVER 1 means re-plotting a star position would erase it, a side effect not guarded against but statistically unlikely with only 20 points.

Skill Level Calculation

At line 1310, the skill level is computed as LET a=VAL a$-(.33). Subtracting 0.33 means skill 1 yields a≈0.67 and skill 5 yields a≈4.67. This fractional offset slightly softens the lowest difficulty while keeping the spawn threshold formula 80-(a*6) continuous and non-integer, producing slightly irregular timer thresholds — likely an intentional tuning choice.

Notable Bugs and Anomalies

• Line 1280 vs 1290: Line 1280 collects INKEY$ into a$, but line 1290 immediately overwrites it with an INPUT statement, making the INKEY$ at 1280 redundant. The validation GO TO 1280 at line 1300 will loop back to re-execute the INKEY$ before re-prompting, which is slightly awkward but functional.
• Launcher position guard: Line 220 prevents l exceeding 31, but uses l=l-1 rather than LET l=31, which could theoretically allow values of exactly 32 briefly if the joystick increments by more than 1 per iteration.
• Line 350: PRINT AT 21,29; INK 4;"\::" prints a block graphic over the alien count display, apparently intended as a visual flash effect, but the count is restored two lines later at line 360.
• Commented-out line 400: The original game-over message "You're Dead" is preserved as a REM comment and replaced by "They got you!" at line 401.