Entry 242

Overlays and DATA.BIN, their last unknowns

2026-09-27format

Filesdocs/formats/prg.mddocs/formats/data-bin.md

Asked for: settle the docs' open questions, starting with the format pages. docs/formats/prg.md and data-bin.md are now solid.

The overlays

• overlay_id at run time. Nothing reads it. mwLoadOverlay(path, dest) reads only three header fields at dest: bss_size (+0x14) and the constructor table (+0x18, +0x1c). dest comes from overlay_tbl[slot - 1] in ccThLoadOverlay, and every call passes slot 1 (0x00400800). No instruction in the executable or an overlay loads from 0x00400804.
• Which file. The event instruction overlay num is the only caller of ccLoadOverlay: 0 DEMO, 1 DESKTOP, 2 TOPPAGE, 3 GCMN.
• Destructors. The static constructors register destructors on __global_destructor_chain through __register_global_object, and nothing else reads the chain. There is no __destroy_global_chain, and _exit goes straight to the kernel. So no destructor ever runs, when an overlay is replaced or at exit. The stale records of a replaced overlay are never walked.

Mutation, Outbreak and Quarantine load the same way (each through its own overlay_tbl).

DATA.BIN

• usize 0. ccFileListLoad (0x00164ea8) wires its two threads by it:

  • a usize other than 0: FileReadTh fills the compressed ring and UngzipTh inflates into the ring ccStream reads;
  • 0: FileReadTh fills ccStream's ring itself, and UngzipTh gets no ccUngzip and never decodes.

  So 0 means stored without compression.

• Category 19. ccAddFileList gives a category-19 entry offset 0, csize -1 and usize 0, in directCCSTbl. ccFileListLoad opens cdrom0:\DATA\ + name, sizes it with sceLseek, and reads it raw. The two unknowns were one: a usize of 0 is category 19's.

  No volume adds a category-19 entry:

  • no {19, name} pair in any executable or overlay (a scan of every word pair);
  • the lists built at run time take constant categories (7, 9, 11, 15 to 18);
  • every store of the constant 19 was looked at, and none is a file list.

  No disc has a loose .CCS in DATA.