About this page

Read this before annotating code. What the game is documented to do, with verification status against the binary. Statuses: open (documented, not found yet), traced (in the code, could not be exercised; say what was tried), confirmed (in the code, consistent with the emulator), live (observed directly), differs (the code does something else). "Absent" is not a status.

Sources:

• The cassette inlay (the manual), two scans on the Internet Archive: <https://archive.org/details/Chiller_1984_Mastertronic> and <https://archive.org/details/Chiller_1984_Mastertronic_2648_Instructions>, their OCR text read 28 September 2026. Both say © Mastertronic Limited
• This is the main source.
• The game's own screens, captured from work/play-idle.vsf and from a boot of the contributor's PRG on 28 September 2026 (reference/).
• Lemon64 entry 468, read through the Internet Archive's copy (<https://web.archive.org/web/2022/https://www.lemon64.com/game/chiller>) on 28 September 2026, since the live site answers only a browser check. Credits, control port and magazine reviews.
• Games That Weren't, "Chiller V1", <http://www.gamesthatwerent.com/gtw64/chiller-v1/>, read 28 September 2026. Covers the withdrawn first version and its music.
• Italian Wikipedia, "Chiller (videogioco 1984)", <https://it.wikipedia.org/wiki/Chiller_(videogioco_1984)>, read 28 September 2026, and its screenshot reference/wiki-it-forest.png.
• C64-Wiki has no page for the game: https://www.c64-wiki.com/wiki/Chiller answered 404 on 28 September 2026, and so did Chiller_(Mastertronic), Chiller_(1985) and Chiller_(game). The wiki's search was not reachable. English Wikipedia's "Chiller (video game)" is Exidy's 1986 arcade game, a different game.

What the sources say about who made it: code by Richard Darling and graphics by David Darling (Games That Weren't); "Creator: David Darling, Richard Darling", title screen attributed to James Wilson and music by David Dunn (Lemon64). Ready64 names Jim Wilson for the title screen; CSDb records Jim and James as names for the same artist. The game's own title screen says "PROGRAMMED BY DAVID AND RICHARD DARLING" and does not credit Wilson.

Features

Where names the routine or table and, for live rows, the test (facts.md, Live tests; scripts in work/).

Beyond the documentation

Found in the code, not in the manual (facts.md, Mechanics, Unused code and data, Bugs):

• The way back is the same screens as you left them. Each outward screen is copied into a store as you leave it (screen_store $7FE8), four of them into the RAM under the KERNAL, and the way back reads them from there. A mushroom eaten going out is gone coming back.
• No fall ever hurts. land $53EF compares the fall with a limit and then does nothing: NOPs.
• The ledges crumble. Tiles $4D-$53 step to the next stage every 8th step on them and the last becomes a space.
• The boy's own position is capped at row 22 of 24. move_sprite's own Y clamp holds him there even in free fall over open space. The game still reads rows 23 and 24 for crosses, through the cell below or beside him, so every cross can be taken; see "Can every cross be reached?", below.
• No ending. After the last level the game loads the forest again, score kept.
• The enemies speed up. When every slot's lives are spent, the LEVEL counter goes up and speed_up $5681 halves their delays.
• The border is the player indicator on every screen, not only on the way back, and flickers while poison drains the bar.
• Enemies that could be killed. Hit and kill code with a points table, lives, a scroller and a second-character mode, none reached.
• A silenced sound player whose SID stores all go to $FFFF.
• SILVER CROSSES, in HUD rows that are never shown.
• Bugs: PRESS CTRL FOR MENU never appears (the routine only loads); the jump sound's pitch falls when it should rise; the title flicker writes a level record instead of $D023.

Open questions

• Which release is this? Settled for Gold: the first, withdrawn Thriller release. Its music player and all 4,238 bytes of tune data at $61F2-$727F match Music/v1music.asm in the steward's https://github.com/unorig/Chiller (branch Latest) byte for byte, and about 1,650 differ from v2music.asm; the steward confirmed on 29 September 2026 that V1 is the withdrawn release. The re-release changes the in-play tune and the gate-off compare at $60B6 (3 to 1); its data is in reference/music-v2.json for the page's player.
• Can every cross be reached? Settled for Gold: yes. A Lemon64 user comment (2022) remembered the game as impossible to finish because one cross was out of reach, later fixed by a crack. The three lowest crosses (graveyard-back $079A, ghetto-back $07C5, cinema-back $07CC) were each taken on the game's own code on 29 September 2026 by walking left along the ground, and the steward played this release through all ten levels the same day, taking every cross (facts.md, "Every cross can be taken"). Whether the crack the comment mentions changed anything is not known.
• Which of the manual's enemies is which. Sprites 2-6 are the five enemy slots and 7 the thrown one (facts.md); the manual's ghouls, zombies, ghosts and bats are not named in the code, so matching them to shapes is by eye only.
• The picture nobody placed. Games That Weren't shows a screenshot from Your Commodore that matches no screen in the released game. Noted in case a stray bitmap turns up in the sweep.

How to get from the contributor's own copy to the analysed state. Someone else must be able to follow this exactly.

Tools used

• Emulator: release v3.13.1, v3.13.1-macos-arm64-gui.dmg (tools/vice-mcp), PAL C64 (VIC-II 6569, SID 6581). check-emulator: 56 of 57, failing pause-at-instruction; stop with pause() in kit/c64/vice.py, never a bare vice_execution_pause (kit/skills/c64/tool-vice-mcp/workarounds.md).
• Disassembler: regenerator2000 0.9.20 (the contributor's own instance, answering on :3000).

The image

Chiller (1985)(Mastertronic).prg, supplied by the contributor and copied to work/chiller-1985-mastertronic.prg unchanged: 42,860 bytes, being a two-byte load address of $0801 and 42,858 bytes of program, covering $0801-$AF6A. MD5 71f92e5380d4564acb7970f9caee2502. It is a single program file, so there is no directory listing.

What the file is: the title screen credits "MASTERtronic'S" and the game names itself CHILLER — THE FOREST. No trainer menu, cracktro or protection screen appeared on load. The initial BASIC stub is line 2061, SYS 2061. The stub the loader leaves behind at $0801 is SYS 2072 followed by the text (ANTISOFT), which is a watermark that appeared only after the first stage ran. What ANTISOFT denotes was not established in this step.

From power-on to play

1. vice_autostart the file; no attach is needed first. VICE wraps a bare PRG into a disk image of its own (tools/vice-home/cache/vice/autostart-C64SC.d64), attaches it as unit 8, and runs LOAD"",8,1 and RUN: the name is because the file's is longer than 16 characters. This is VICE's default for any PRG, and the same call as dropping the file on its window.
2. Autostart turns warp mode on. Turn it off before timing anything: tools_call → vice_machine_config_set {"resources": {"WarpMode": 0}} (the typed wrapper rejects the argument).
3. Loader: the BASIC stub runs SYS 2061. The code there rewrites the stub at $0801 to SYS 2072 (ANTISOFT) and hands over at $0818. That address is not the game: it is another copier, which moves $0900-$09FF to $CE00, $0A00-$0AFF to $CF00 and writes $08D0-$08FF into $D000 (VIC setup). The last hand-over, where the game's own code first runs, was not isolated in this step. By policy the loader is not annotated further.
4. No input is needed to start: about 15-18 s after autostart the title screen appears and the play screen follows on its own. No key press, fire button or joystick movement was sent at any point.
5. Snapshots taken:
6. work/play-idle.vsf — the play screen, untouched by input. This is the analysis image.
7. work/entry.vsf — stopped at $0818, the second stub's target, with a stopping checkpoint, before that copier ran.

A hard reset (vice_machine_reset, mode: hard) precedes any boot whose memory is read: an autostart resets the CPU but keeps RAM, and this loader writes over most of its own load area.

Steady state

• Interrupt vectors during play. $0314/5 = $60F5, a RAM address: the music player (music_irq, facts.md), which ends through the KERNAL's $EA31. The hardware IRQ vector $FFFE is still the KERNAL's $FF48, which dispatches through $0314. The game left NMI alone: $FFFA = $FE43, $0318/9 = $FE47, $0316/7 = $FE66, $FFFC = $FCE2, all KERNAL defaults.
• Banking. $01 reads $36 across eight live samples during play, and the snapshot's processor-port byte (file offset 205) is $36 too: BASIC ROM banked out, KERNAL and I/O in. ($0001 in the RAM image reads $00, but that byte is not the port; c64-reference.)
• Where the code runs. Non-stopping checkpoints over 3 s of play (facts.md, Memory layout) put it in $0800-$2FFF, $5000-$7FFF and $C000-$CFFF, with the KERNAL's IRQ path above $E000. Nothing executed in $8000-$BFFF. At play time the load area $0801-$AF6A differs from the file in 40,882 of its 42,858 bytes.
• Consequence for the rest of the run. The image everything is read from is work/play-idle.vsf, not the .prg: the loader moves and overwrites most of what the file holds. The disassembler must hold the snapshot.
• Per-level reloads: not established in this step.

The loader, in a paragraph

The file is a single PRG with a BASIC stub. SYS 2061 runs machine code that rewrites the stub in place to SYS 2072 (ANTISOFT) and passes control to $0818, which is a copier: it installs blocks at $CE00, $CF00 and the VIC registers at $D000 before the game proper starts. The result is a game that runs from $0800-$2FFF, $5000-$7FFF and $C000-$CFFF with the music player on the IRQ at $60F5. The loader's stages before the last hand-over are not annotated, by policy.

Every byte of the C64's 64 KB, one pixel each, 512 to a row. Lit pixels are code, graphics, level data, sound, text, tables and variables; dim areas are screen and working memory.