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CSB FMTowns Guide

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Chaos Strikes Back FM Towns — technical guide for Firestaff agents

This is the FM Towns counterpart to DM1's guide. Read both before changing CSB FM Towns startup, cache, media or runtime code. The games share Phar Lap and TownsOS conventions, but they do not share a title program, CD layout, save body or presentation owner.

Retail media and cache

The admitted CSB F31 disc contains both language trees:

Tree Game data Game program Bootstrap save
English CDATA/GRAPHICS.DAT, CDATA/DUNGEON.DAT CHTWE.EXP CDATA/MINI.DAT
Japanese CJDATA/GRAPHICS.DAT, CJDATA/DUNGEON.DAT CHTWJ.EXP CJDATA/MINI.DAT

src/shared/asset_status_m12.c first verifies the selected hash pair, then materializes the complete ISO inventory and the original FMTOWNS.IMG / FMTOWNS.CUE pair. It must retain that pair for the original TD/TR CD-DA commands; a derived track table or generated PCM is not an equivalent source. The scanner reports each admitted CSB edition, rather than using the selected cache pair as evidence for another platform.

CSB uses its own 31-track Red Book layout. Do not reuse DM1's mixed MODE1/2048-and-audio offset calculation or DM2's Towns music helper. The CSB reader is src/csb/csb_v1_fmtowns_cd.c; M11's live transport boundary is in src/engine/m11_game_view.c.

Startup program graph

TITLE.ANM ──return──> SWITCHTW.EXP ──Game──> CHTWE.EXP / CHTWJ.EXP
                    ├─Story──> ANIMTW.EXP STORY.ANM ──return──┘
                    └─Utility──> UTILE.EXP / UTILJ.EXP

TITLE.ANM, STORY.ANM and ENDING.ANM are F2275/F8288 animation streams. Their Timer-A waits, loop execution and TD/TR requests are source-owned. ENDING.ANM holds its final frame when it returns; it does not route back to the switch page. SWITCHTW.EXP owns the 320×200 language pages, palette, four button streams and the selected handoff. AUTOEXEC.BAT establishes the exit mapping. See ReDMCSB NECIO.C, SWITCH.C, ANIMTOWN.C and STARTUP2.C.

The Game exit is a separate C03 program, not PC 3.4 TITLE.C:

  • F31E admits only CHTWE.EXP (283,936 bytes; FNV-1a 3da136f6).
  • F31J admits only CHTWJ.EXP (284,416 bytes; FNV-1a f937db45).
  • Both enter the C004 Prison wait from STARTUP1.C / ENTRANCE.C F0807.

include/csb_v1_fmtowns_game.h and src/csb/csb_v1_fmtowns_game.c carry that admission receipt. M11 opens only the source-bound C004 entrance and C002/C003 door sequence, then the verified C017/F0128 HUD/viewport session. It must not replay the standalone title timeline after this transition.

F31 MINI.DAT is a native save bootstrap

MINI.DAT is neither an Atari/Amiga GAMEBLOCK nor a substitute DUNGEON.DAT. ReDMCSB CEDTINCD.C F7051 validates its 512-byte C5 header through F7061, then F7057-checks five save parts, reads four external portraits, and passes the remaining dungeon tail to F7063.

Receipt English Japanese
File size 42,776 43,208
FNV-1a 494999c9 284799d1
Header platform F7 F8
Dungeon identity C13 CSB Game C13 CSB Game
Validated party pose map 4, (22,18), direction 2 map 4, (22,18), direction 2
Game time 82 88

The verified save body has native GLOBAL_DATA, active groups, champion and party data, events and timeline parts. The tail begins after the four 464-byte external portraits. Its checksum is validated and can be copied by csb_v1_fmtowns_game_copy_verified_dungeon_tail() into the ordinary real CSB dungeon reader. MAP coordinates are bytes 6/7 (OffsetMapX/Y), not the unreferenced bytes 4/5; this follows ReDMCSB DEFS.H and DUNGEON.C F0154.

This is deliberately not a partial live restore. A valid runtime handoff must bind the F31 CHAMPION_EXCLUDING_PORTRAIT body, external portraits, active groups, events and timeline together. Replacing only the dungeon or only the party pose would create an invented mixed save state. Until those owners are recovered, retain the receipt and source-backed entrance instead.

Utility boundary

UTILE.EXP and UTILJ.EXP are separate C06_CEDT programs. Their P3 envelopes, six-label source pools, C09 icon palette, source-coordinate menu boxes, F31 M653 font material and planar .CMP portrait decoder are verified. That does not authorize an M11 editor: the original EGB screen composition, file-picker, save/portrait transactions and F31J Shift-JIS EGB glyph consumer remain unbound. Keep Utility modal until those specific owners are evidenced.

Files to know

Purpose Path
Game and MINI receipts include/csb_v1_fmtowns_game.h, src/csb/csb_v1_fmtowns_game.c
Switch page and handoffs include/csb_v1_fmtowns_switch.h, src/csb/csb_v1_fmtowns_switch.c
Disc, ISO and CD-DA reader include/csb_v1_fmtowns_cd.h, src/csb/csb_v1_fmtowns_cd.c
Animation stream src/csb/csb_v1_fmtowns_animation.c
Cache materializer src/shared/asset_status_m12.c
M11 startup, entrance and transport src/engine/m11_game_view.c
Japanese C06 text boundary parity-evidence/csb_fmtowns_f31j_text_owner.md
Main remaining work TODO.md (CSB-FMTOWNS-RUNTIME-PARITY)

Non-negotiable boundaries

  • Never use PC 3.4 title, Utility, audio or save code as a FM Towns fallback.
  • Never generate pixels, CD audio, menu labels or save/champion data when the verified F31 owner is missing.
  • Keep the selected CDATA/CJDATA pair ahead of the opposite-language sidecar tree.
  • Keep original data out of the repository. Tests use user-supplied, hash-admitted media and skip safely when it is unavailable.

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