CURIOSITIES.md

Curiosities — the hidden gems of Subterranean Stryker

Forty years after release, reverse-engineering this 1985 cassette turned up things nobody knew were in there: an easter egg, the developers' signatures, eight sounds that were composed but never played, a level that corrupts memory, and some genuinely clever engineering. This file collects them all in one place; each entry links to the full trace in docs/disasm/ and RE-LOG.md. Where a discovery is implemented in the native port, that's noted inline. (The method that surfaced all of this — running the binary and reading its RAM rather than reading prior write-ups — is in PLAYBOOK.md, which is also why we found things earlier notes missed.)

1. The Star Wars hall of fame — and the developers' signatures

The idle title screen draws a HALL OF FAME ($FCDB) whose default high-score table at $FE0F reads:

Rank	Name	Score
1	somebody	2900
2	Wedge	2820
3	Biggs	2422
4	John D.	1402
5	Luke	488
6	Porkins	487
7	Timothy	442
8	Gof	240

Wedge Antilles, Biggs Darklighter, Luke, Porkins — Star Wars Red Squadron, hiding in the data since 1985. And the last two entries are almost certainly the developers signing their work: Timothy = Tim Follin (music) and Gof = Peter Gough (code). "John D." remains unidentified — if you know who he is, open an issue. (title-menu.md, RE-LOG §59)

The native port ships this exact table as its default (HallOfFame.cs) — beat 240 points and you knock Gof off his own scoreboard.

2. The eight sounds the game never plays

The ROM contains a complete family of sound-effect messages — boss alert, pickup chime, a warning, fuel-low and shield-low alerts, a kill jingle, a 32-byte game-over tune, and five per-level fanfares. The game dutifully QUEUES them at the right moments ($FA0A copies them into a buffer at $FF51)… and then no code anywhere ever plays that buffer. The $FA32 player resets the buffer pointer on entry and only ever plays the title tune; one of the eight ($F93A) isn't even queued by anything — it has no callers at all. The whole system is a development leftover: either the player lost its message mode late in development, or the dispatch was never finished. (sound.md, RE-LOG §63)

We reconstructed them. The message bytes aren't in the title player's (duration, pitch) format — they're groups of pitch bytes separated by $03 (clearest in the game-over data: 1B 58 03 | 58 58 03 | 18 18 03 …), with values in exactly the title player's pitch range. LostSoundReconstructor.cs renders each message through $FA32's pulse-cycle engine (same DJNZ timing, same Follin duty-slide) with documented assumptions for the parts that never shipped (note length, rest length). The results live in assets/extracted/sfx/lost-*.wav.

Hear them in the game: press N in the native port to cycle through three SFX modes:

• OFF (default) — cassette-faithful: those events are silent, exactly as on the original cassette.
• DESIGNED — purpose-built beeper SFX that are distinct and recognisable; regenerable with subterra sfx-render (sfx-*.wav in assets/extracted/sfx/).
• HISTORICAL — the 1985 reconstructed messages, finally audible forty years late (lost-*.wav). Same underlying data, different assumptions about note length; the two sets sound noticeably different.

3. Level 0 is a bug

The per-level entity-record pointer table has six entries, but level 0's pointer sits 3 bytes before level 1's records — its six "records" are level 1's bytes read out of phase, giving sprite addresses in ROM and stray RAM writes ($A0xx). Its scenery pointer is just as broken: it points at the tile bank itself. Level 0 is unreachable in normal play (the level counter increments before the first page); only finishing level 5 — 25 entities, the hardest page — exposes it, which is presumably why it shipped untested. The original would draw garbage and corrupt memory; the port wraps 5 → 1 instead. (entities.md §Level 0, RE-LOG §57)

4. The bitmap IS the collision system

Nothing in this game ever compares coordinates for damage:

• The player takes damage when his XOR-draw lands on non-zero pixels ($DCF5's shadow-carry trick — and the spawn routine $F891 blanks the player's spawn cell precisely so frame one can't false-trigger it).
• Enemy ships and the boss die when their own draw finds the laser beam pattern $EF in the bytes they're about to cover ($E9F0). The laser never checks anything; the targets do.
• We got this wrong once ("the laser hits nothing") before finding the check hiding in the targets' blitter — the correction story and the lesson (absence of one opcode pattern is not absence of a behaviour) are in RE-LOG §58/§62.

(damages.md, laser.md)

5. The boss has no sprite

The boss's sprite-source pointer aims at $EE8E — its own state block. What you see on screen is the boss's current speed byte (written twice per tick) plus neighbouring state values, rendered as horizontal bands that shift as its speed phase cycles. A procedural glitch-creature, zero bytes of artwork. (boss.md §Visual)

6. Entities never move — and four more small gems

• All cave entities are stationary. Every "falling rock" and "flying drone" is a 16-frame animation inside a fixed 16×16 box; the only byte the engine ever writes back to an entity record is its frame counter. (RE-LOG §61)
• The score-parity twinkle. Entity types ≥ $13 swap their sprite pointer to $4800 — the middle of screen memory — whenever bit 0 of the score is clear, sampling whatever pixels happen to be there: a pseudo-random shimmer with zero bytes of state. (entities.md §$F239)
• Title music stops when you touch the keyboard — by design: the player is a synchronous, interrupts-off loop, and its any-key poll is the only way the menu stays responsive. M or N starts it; any key exits it. (sound.md)
• The Follin timbre is two instructions. The famous PWM slide is literally INC E / DEC D once per pulse cycle — duty sweeps while the period holds. (sound.md)
• The 8th ship slot. Each level's init data contains eight ship records; every loop in the game walks seven. Level 1's unused 8th ship (X=$50 Y=$58, alive flag set) has been waiting to spawn since 1985. (RE-LOG §59)