SINTRAN III on-disk filesystem format (Phases 1-4)

Byte-exact layouts of the four on-disk structures that make up a SINTRAN III directory device, each grounded in real disk bytes (SMD0.IMG, volume PACK-ONE) and cross-checked against the NDFS C reader, the ndtool inspector, and the carved 006-S3FS filesystem code. See the Filesystem foundation for the full source map and phased plan.

Evidence tags used throughout: VERIFIED (real-disk bytes and/or the producing code), INFERRED (NDFS/doc only), OPEN (unresolved, with the source that would settle it). On-disk multi-byte values are big-endian words.

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The four structures

#	Document	Structure	Size	Anchor
1	directory-label.md	Master block / directory label	32 B	page 0, byte 2016
1b	extended-info-block.md	Extended-info block (checksum, flag word, system number, capacity)	16 B	page 0, byte 2000
2	object-entry.md	Object (file) entry	64 B	object file (indexed)
3	user-entry.md	User (account) entry	64 B	user file (indexed)
4	page-bitmap.md	Page allocation bitmap (bit file)	1 bit/page	bit file (contiguous)

Page 0 also carries the raw boot code ahead of the master block: boot-sector.md - the page-0 map and the three ND-100 boot-code formats (BPUN, FLOMON, raw binary bootstrap), with the real SMD0.IMG boot sector decoded as evidence and the PIOF/IOF + IOX/IOXT detection logic. VERIFIED: SMD0.IMG / PACK-ONE is a raw SMD/ECC bootstrap (corrects the earlier "boot format None" reading).

flowchart TB
    MB["Master block - page 0 byte 2016<br/>name + 3 block pointers"]
    MB -->|object_file_ptr - indexed| OF["Object file<br/>64-byte object entries<br/>32 per page"]
    MB -->|user_file_ptr - indexed| UF["User file<br/>64-byte user entries<br/>32 per page"]
    MB -->|bit_file_ptr - contiguous| BF["Bit file<br/>1 bit / page, 0=free 1=used"]
    OF -->|user index byte 34| UF
    OF -->|file pointer bytes 60-63| DATA["File data pages"]
    BF -.tracks allocation of.-> DATA

    classDef orange fill:#FFF3E0,stroke:#E65100,color:#E65100;
    classDef green fill:#E8F5E9,stroke:#2E7D32,color:#2E7D32;
    classDef purple fill:#F3E5F5,stroke:#7B1FA2,color:#7B1FA2;
    classDef teal fill:#E0F7FA,stroke:#00838F,color:#00838F;
    classDef blue fill:#E3F2FD,stroke:#0D47A1,color:#0D47A1;
    class MB orange
    class OF green
    class UF purple
    class BF teal
    class DATA blue

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Block-pointer encoding (shared by all four)

Every 4-byte block pointer is a big-endian value: top 2 bits = type, bottom 30 bits = block/page id. Types: 0 contiguous, 1 indexed, 2 sub-indexed, 3 reserved. Full derivation in directory-label.md 3.2. VERIFIED.

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Verification status (real disk PACK-ONE)

Structure	Byte decode	Cross-reader (ndtool)	Producing code (006-S3FS)
Master block	VERIFIED (2 disks)	-i volume/name match	GMAIN/WDIRE/CRDIR
Object entry	VERIFIED (3 entries)	--stat field-for-field	ROBJE/COBJE/WOBJE
User entry	VERIFIED (3 entries, all 10 users)	-u / --friends match	RUSER/WUSER/GUSEN
Page bitmap	VERIFIED (popcount = ndtool used/free)	-i 14277/24123 match	GPAGE/ALPAG/RLPAG

Real disk vs NDFS - one discrepancy: on the SMD system disk the label points the bit file at block 18468, but the NDFS image_creator SMD template hard-codes 18472. The on-disk pointer is authoritative (the reader follows it); only the NDFS creator template differs, because PACK-ONE was written by genuine SINTRAN CRDIR. Details in directory-label.md 7.

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Open questions carried forward

• Master block: unreserved_pages semantics vs the bitmap free count; relationship of the 32-byte label to the 24-word directory entry MON 244 returns.
• Object entry: extra header-byte flag bits (SEGFIL0 = 0x90); byte 33; next_version/prev_version chaining.
• User entry: bytes 38-39 and 42-47 (byte 47 mxobl/acobl); password hashing.
• Bitmap: allocation search direction (upward vs highest-range) and the 0-6 reservation in the carved GPAGE/ALPAG (OPEN-Q3).

See the foundation README section 6 for the full list. Allocation-code and CRDIR analysis (Phases 5-7) live under ../code-logic/.

The extended-info block's checksum, flag word, system number and pages-available are now kernel-proven (WXDIR/CHDSI) - see extended-info-block.md and the report validation in ../NDFS-VALIDATION.md.