PSX Mode2/2352 disc geometry Confirmed

At a glance

Magic

none - a 12-byte sync pattern opens every sector; files are found through ISO9660

Where on disc

the whole .bin image; the Primary Volume Descriptor sits at LBA 16

Stride

2352 bytes per sector, 2048 of them file content at byte 24

Parser

crates/iso (raw.rs sector reader, iso9660.rs walker, write.rs EDC/ECC re-encode, relayout.rs)

Confidence

Confirmed - textbook CD-ROM XA / ECMA-130 geometry; every disc-gated test in the workspace reads through it

Used by

extraction pipeline, disc patcher, in-browser ROM patcher

Overview

A CD image (.bin) is a flat run of sectors addressed by LBA - the logical block address, a sector number counting from the start of the disc. "Mode2/2352" names the sector flavour PlayStation discs use: mode 2 of the CD-ROM XA standard, stored as full 2352-byte raw sectors rather than pre-stripped 2048-byte ones. Reading a file means knowing its start LBA and peeling the 2048 user-data bytes out of each sector.

Two things live on top of that geometry:

• ISO9660 - the standard CD filesystem that turns sector numbers into the 45 named files on the disc.
• Relayout - growing a file by whole sectors, which the official PAL discs did to fit longer translated dialogue and which a community language pack needs when its text does not fit in place.

Sector layout

Each sector is 2352 bytes. Only the 2048-byte slice in the middle is file content; the rest is the sync mark and address the drive uses to find the sector, a subheader (meaningful for streamed XA audio and video), and the error-detection and error-correction codes that let a scratched disc still read.

RawDisc::read_sector(lba) returns just the user-data slice; read_user_data(lba, count, buf) reads a contiguous run.

const SECTOR_SIZE: usize = 2352;
const USER_DATA_OFFSET: usize = 24;
const USER_DATA_SIZE: usize = 2048;

ISO9660 walk

On top of the sector layer sits ISO9660, the standard CD filesystem - it turns "sector 16" into named files and directories. The layout is textbook:

• Primary Volume Descriptor at LBA 16.
• Root directory record at PVD offset 156.
• Each directory record begins with a length byte; records pad to even lengths.

The walker (crates/iso/src/iso9660.rs) is iterative and yields stable-sorted file paths. The Legend of Legaia (USA) disc produces 45 files:

CDNAME.TXT  DMY.DAT  PROT.DAT  SCUS_942.54  SYSTEM.CNF
MOV/MV1.STR ... MV6.STR
XA/XA1.XA  ... XA34.XA

Full-ISO relayout

Growing a file by whole sectors - PROT.DAT, which the official PAL discs grew at mastering to fit longer localized dialogue - moves every file after it, so every on-disc reference to those files has to be rewritten. Legaia's disc makes this a small, well-bounded edit. Implementation: legaia_iso::relayout (generic ISO9660 + ECMA-130; embeds no game bytes).

The LBA reference graph

Two facts make the relayout safe (how we know):

1. PROT.DAT's internal TOC is PROT.DAT-relative, not absolute disc LBAs. Entry 0's TOC start LBA is identical on every region's disc even though PROT.DAT sits at a different disc LBA per region. Growing an interior entry needs only an internal-TOC shift (PROT.DAT TOC), not a disc-wide cascade.
2. No file is located by a hardcoded absolute LBA in the executable. Every file is found by ISO9660 name lookup; no post-PROT.DAT file's disc LBA appears as a literal in any USA or PAL executable.

When PROT.DAT grows by G sectors the cascade reduces to one rule: every ISO9660 LBA value > prot_lba gains +G; PROT.DAT's directory-record size gains +G*2048; the PVD volume-space size gains +G.

Subdirectory extents that live after PROT.DAT (MOV and XA on the retail disc) relocate too, so their self . record and file records are patched at the extent's new position.

How we know

Full page: docs/formats/disc.md.

See also