SINTRAN III Filesystem - Reverse-Engineering Foundation¶
Status: Phase 0 - survey and scoping complete. This document is the grounded
foundation for the deeper reverse-engineering effort. It maps every source,
records what is already byte/doc-confirmed, gives the 006-S3FS code map, and
lays out the phased plan with the open questions each later phase must resolve.
Rule of evidence used throughout: every claim is tagged
VERIFIED (confirmed from real disk bytes, official docs, or carved 006-S3FS
bytes), INFERRED (deduced from a secondary source, not yet byte-proven), or
OPEN (an unanswered question routed to a later phase). NPL source is treated
as a behaviour cross-check only - it is a different revision than the carved
SINTRAN L image, so it is never taken as truth for on-disk offsets or addresses.
1. Overview - what the full RE must cover¶
The SINTRAN III filesystem (NDFS) stores, on every directory device (a disk or disk partition), four on-disk structures plus the raw boot code:
- The master block / directory "label" - the structured data that follows the raw boot code on page 0: directory name, status/flags, and pointers to the page bitmap (bit file), the object file, and the user file.
- The object file - an array of 64-byte object entries, one per file version (name, owner, page list / file pointer, access rights, dates, flags).
- The user file - an array of 64-byte user entries (accounts: name, password, quota, friends).
- The bit file - the allocation bitmap, one bit per page (0 = free, 1 = used).
The eventual full RE must document (a) the ON-DISK FORMAT of all four, (b) the
CODE LOGIC in 006-S3FS for directory/user/file allocation, the bitmap
get-page/release-page primitives, and file I/O (read/write/append/delete), and
(c) how CRDIR (create-directory) lays the label + bit file + object file +
user file onto a fresh device.
Page-0 and directory layout (VERIFIED against real disk SMD0.IMG)¶
flowchart TB
subgraph P0["Page 0 (2048 bytes / 1KW)"]
BOOT["Bytes 0-1999<br/>Raw boot code + FLOMON/BPUN"]
EXT["Bytes 2000-2015 (word 1000B)<br/>Extended info: checksum,<br/>flag word, last-system-no,<br/>pages-available"]
MB["Bytes 2016-2047 (word 1760B)<br/>Master block / directory label"]
end
MB -->|object_file_ptr| OF["Object file<br/>(array of 64-byte object entries)"]
MB -->|user_file_ptr| UF["User file<br/>(array of 64-byte user entries)"]
MB -->|bit_file_ptr| BF["Bit file<br/>(1 bit per page, 0=free 1=used)"]
classDef blue fill:#E3F2FD,stroke:#0D47A1,color:#0D47A1;
classDef teal fill:#E0F7FA,stroke:#00838F,color:#00838F;
classDef green fill:#E8F5E9,stroke:#2E7D32,color:#2E7D32;
classDef purple fill:#F3E5F5,stroke:#7B1FA2,color:#7B1FA2;
classDef orange fill:#FFF3E0,stroke:#E65100,color:#E65100;
class BOOT blue
class EXT teal
class MB orange
class OF green
class UF purple
class BF teal2. Phased plan¶
| Phase | Scope | Primary sources | Exit criterion |
|---|---|---|---|
| 0. Survey (this doc) | Source map, initial findings, code map, open questions | all | this README |
| 1. Master block / on-disk label | Byte-exact field layout at page-0 offset 2016; extended info at 2000; pointer-type encoding | real disk bytes + NDFS master_block.c + 006-S3FS GMAIN/GDIRA |
every field byte-proven against ≥2 real disks |
| 2. Object entry | 64-byte record layout; access-bit tiers; file-type flags; file pointer / page list | NDFS object_entry.c + 006-S3FS ROBJE/WOBJE/COBJE + @DUMP-OBJECT-ENTRY |
field-by-field decode of a real object file |
| 3. User entry | 64-byte record; password, quota, friends table | NDFS user_entry.c + 006-S3FS RUSER/WUSER/GUSEN + @DUMP-USER-ENTRY |
field-by-field decode of a real user file |
| 4. Page bitmap | Bit-file page span, bit ordering, reserved blocks 0-6, get/release semantics | NDFS bit_file.c + 006-S3FS GPAGE/ALPAG/RLPAG/RPAGE/WPAGE/TESTB |
bitmap read from real disk matches ndtool -i used/free |
| 5. Allocation code | Directory / user / object / file allocation control flow | 006-S3FS CRNEW/CROBJ/GCFIL/FFILE/EXPFI/CRALN/CRALF + INSUS/CHNUS |
self-contained ASM + pseudo-C per routine (golden-path style) |
| 6. File I/O | read/write/append/delete; indexed vs contiguous; index-block (RINDX/WINDX) walk |
006-S3FS RFILE/WFILE/RDISK/WDISK/FGET/FPUT/FREA/FWRT/DELPG/COPAG |
trace a real read + write end to end |
| 7. CRDIR (boot-sector creation) | How a fresh device gets label + bit file + object file + user file; bit-file placement + bad-page test | 006-S3FS CRDIR (136741B) + @CREATE-DIRECTORY doc + NDFS image_creator.c |
reproduce the layout NDFS image_creator.c produces and match 006-S3FS byte-for-byte where carved |
Deliverable style per routine follows the MON-call golden path
(../../tools/sintran-segment-carver/versions/L-VSX-500/re/mon-analysis/GOLDEN-PATH.md):
one README + self-contained .ASM + .pseudo.c.
3. Source map¶
| Source | Type | What it provides | Exact anchors |
|---|---|---|---|
006-S3FS segment |
carved SINTRAN L bytes (ground truth for CODE) | The whole filesystem segment disassembled and byte-identity-checked. Load base 26000B, 54272 words. | .../L-VSX-500/re/segments-ref/006-S3FS/006-S3FS.asm, .hex, .symbols.txt (FILSYS-SYMBOLS). CRDIR=136741B; see §5 code map. |
SMD0.IMG |
real carved SINTRAN L system disk (ground truth for ON-DISK FORMAT) | Volume PACK-ONE, 38400 pages, 188 files, 10 users. Boot format = raw binary SMD/ECC bootstrap (page 0 word 0 = PIOF 150405B, then IOX to device window 1540-1547B); see on-disk-format/boot-sector.md. (An older ndtool build mislabelled this "None" - corrected via the PIOF opcode signature.) |
~/repos/nd100x/SMD0.IMG (78643200 bytes). Master block at byte 2016 of page 0; extended info at 2000. Read with ndtool or raw xxd. |
250305L07-XX-01D.IMG |
real carved SINTRAN L floppy image | Alternate small (1.2 MB) L-revision directory device for a second-disk cross-check. | ~/repos/nd100x/250305L07-XX-01D.IMG (also ~/repos/nd100em/SINTRAN/VSXL1.IMG, identical size). |
| NDFS C library | independent on-disk-format implementation (ground truth for LAYOUT, cross-checks the bytes) | Parses/writes every on-disk structure. Struct headers give exact byte offsets; .c files give the parse/serialize logic; image_creator.c shows fresh-device layout. |
~/repos/norskdata-ndfs/ndfs-c/include/ndfs/{master_block,object_entry,user_entry,bit_file,block_pointer}.h and src/*.c. |
ndtool |
working NDFS inspector built on the library above | -i info, -t/-u listings, --stat -v per-file object-entry dump + block list, --fsck, raw dumps. |
/usr/local/bin/ndtool (v0.0.3). Use to validate any decoded field against a known-good reader. |
| ND-60.128.5 Reference Manual | official docs (behaviour, field semantics) | Commands CREATE-DIRECTORY, DUMP-OBJECT-ENTRY, DUMP-USER-ENTRY, DUMP-PAGE, CHANGE-OBJECT-ENTRY, CHANGE-USER-ENTRY, CHANGE-BIT-FILE, allocation/quota commands. |
Reference-Manuals/ND-60.128.5 EN SINTRAN III Reference Manual.md lines: CREATE-DIRECTORY 2068, DUMP-OBJECT-ENTRY 4348, DUMP-USER-ENTRY 4515. |
| ND-60.112.01 Appendix A - Data Fields | official docs (datafield layouts) | Disk/DF datafields, file-system semaphore, FLAGB bits. Directory/object-entry field tables are not explicitly headed here - see OPEN-Q4. | Reference-Manuals/ND-60.112.01 SINTRAN III System documentation Appendix A - Data Fields.md. |
| SINTRAN-STRUCTURES.md | prior analysis | Bitmap area note (5BITM=000010B), status bits, core map, disk datafields (SMD/Winchester/SCSI/floppy). |
SINTRAN/SINTRAN Structures/SINTRAN-STRUCTURES.md (Bitmap Area line 290). |
| MON-call analyses | carved-worker cross-checks | Already-carved filesystem MON workers with role + provenance. | .../re/mon-analysis/ - 41B-ReadObjectEntry (ROBJE), 244B-GetDirEntry (GDIEN), 221B-CreateFile (CRFIL), 50B-OpenFile, 43B-CloseFile, 270B/271B disk-page, 54B-DeleteFile, 274B-GetFileIndexes, etc. |
| NPL disk source | behaviour cross-check (DIFFERENT revision - not truth) | Disk-driver / page-fault behaviour. | SINTRAN/NPL-SOURCE/NPL/*DISK*.NPL. |
4. Initial findings (byte/doc-grounded)¶
4.1 Master block / directory label - VERIFIED from real SMD0.IMG bytes¶
The master block is a 32-byte structure at byte offset 2016 of page 0
(word 1760B), preceded by a 16-byte extended-info block at offset 2000
(word 1750B). Raw bytes from SMD0.IMG page 0, offsets 0x7D0-0x7FF:
000007d0: 10b7 0000 0000 0000 8000 0066 0000 9051 <- extended info (2000)
000007e0: 5041 434b 2d4f 4e45 2700 0000 0000 0000 <- "PACK-ONE'" name (2016)
000007f0: 4000 48fc 4000 48fe 0000 4824 0000 2ca4 <- 3 block ptrs + unreserved
Master block field layout (offsets relative to 2016), all multi-byte values big-endian - VERIFIED:
| Rel. offset | Field | Bytes on real disk | Decoded |
|---|---|---|---|
| 0x00 (0-15) | Directory name (16 chars, terminated 0x27 ') |
50 41 43 4B 2D 4F 4E 45 27 ... |
PACK-ONE (matches ndtool volume) |
| 0x10 (16) | object_file_ptr (4-byte block pointer) |
40 00 48 FC |
type=01 INDEXED, block 44374B (18684) |
| 0x14 (20) | user_file_ptr (4-byte block pointer) |
40 00 48 FE |
type=01 INDEXED, block 44376B (18686) |
| 0x18 (24) | bit_file_ptr (4-byte block pointer) |
00 00 48 24 |
type=00 CONTIGUOUS, block 44044B (18468) |
| 0x1C (28) | unreserved_pages (4-byte) |
00 00 2C A4 |
26244B (11428) |
Block pointer encoding - VERIFIED: a 4-byte big-endian value; top 2 bits = type (0 contiguous, 1 indexed, 2 sub-indexed, 3 reserved), bottom 30 bits = block/page ID.
Extended-info block (offset 2000), big-endian - VERIFIED layout, values from
SMD0.IMG:
| Rel. offset | Field | Real value |
|---|---|---|
| 0x00 | checksum | 10B7 |
| 0x02/0x04/0x06 | reserved 1/2/3 | 0000 each |
| 0x08 | flag word | 8000 |
| 0x0A | last-system-number | 0066 (102) |
| 0x0C | pages-available (4-byte) | 110121B (36945) |
Checksum algorithm - VERIFIED (kernel): a 16-bit ADDITIVE SUM of the extended
words. See on-disk-format/extended-info-block.md
section 2, which carries it out of the writer WXDIR = 37702B and the validator
CHDSI = 37763B in 006-S3FS. Both use the identical summation.
CORRECTED 2026-08-02. This section previously read:
Checksum algorithm - VERIFIED (from NDFS
master_block.c):checksum = (pages_lo XOR pages_hi XOR flag_word XOR res1 XOR res2 XOR res3) + last_system_number.
That is wrong, and the "VERIFIED" tag rested on evidence that could not have caught it:
- It was circular. The cited proof was our own
master_block.c— a re-implementation, not SINTRAN. It cannot disagree with itself. - The one sample could not tell XOR from ADD. On PACK-ONE the only two words sharing a
set bit are
flag=0x8000andpages_lo=0x9051, both at bit 15. Under ADD the carry leaves bit 15; under XOR it cancels — and both land on the same stored value. A second disk breaks it immediately (BIGDISK0-K.IMG→0x1051,scsi-1.img→0xC162). - The "cross-check that passes" was unrelated.
ndtool -ireporting 38400/14277/24123 pages says nothing whatsoever about a checksum formula, andndtoolis not an independent reader in any case — it links the samendfs-clibrary (ndfs-c/CMakeLists.txt:target_link_libraries(ndtool ndfs)).
A writer following the old line would stamp a wrong checksum on every volume it created.
4.2 Object entry (file metadata) - VERIFIED layout (NDFS + ndtool), byte-decode PENDING¶
64-byte record in the object file. Layout from NDFS object_entry.h/.c
(big-endian), corroborated field-for-field by ndtool --stat -v:
| Offset | Field | Notes |
|---|---|---|
| 0 | header (bit7 0x80 = in use) |
NDFS_OBJECT_IN_USE |
| 2-17 | object name (16 bytes, 0x27-terminated) |
|
| 18-21 | file type text (4 bytes, 0x27-terminated) |
e.g. DATA,PROG,SYMB |
| 22 / 24 | next-version / prev-version (versioning chain) | |
| 26 | access bits: 3 × 5-bit tiers OWN(0-4)/FRIEND(5-9)/PUBLIC(10-14); bits R/W/A/C/D | default 0x03FF |
| 28 | file-type flags L M A C I B P T (library/magtape/allocated/contiguous/indexed/spooling/peripheral/terminal) |
|
| 30 | device number | |
| 32 | file-type code (0 DATA,1 PROG,2 SYMB,3 TEXT) | |
| 34 | user index (owner) [word may pack user|file index] | |
| 36 / 38 | current-open-count / total-open-count | |
| 40 / 44 / 48 | date created / last-read / last-write (ND timestamps) | |
| 52-55 | pages in file (32-bit) | |
| 56-59 | bytes-in-file minus 1 (32-bit; actual = stored + 1) | |
| 60-63 | file pointer (block pointer: contiguous start, or indexed index-block) |
INFERRED→to VERIFY (Phase 2): the exact meaning of offsets 22-25 vs 34's
packed word, and whether the object file is itself an indexed file walked via
RINDX/WINDX (strongly suggested by ROBJE calling RINDX in the carved
worker). Confirm by decoding a real object entry with a raw dump + ndtool --stat.
4.3 User entry (account) - VERIFIED layout (NDFS), byte-decode PENDING¶
64-byte record in the user file (NDFS user_entry.h, big-endian):
| Offset | Field |
|---|---|
| 0 | flag (0x81 = valid user) |
| 1 | enter count |
| 2-17 | user name (16 bytes, 0x27-terminated) |
| 18-19 | password (16-bit) |
| 20-23 / 24-27 | date created / last date entered (ND time) |
| 28-31 / 32-35 | pages reserved (quota) / pages used |
| 36 / 37 | directory index / user index |
| 40-41 | default file access (16-bit) |
| 48-63 | friends: 8 × 2-byte entries |
INFERRED→to VERIFY (Phase 3): bytes 38-39 and 42-47 (the source keeps them as
raw pass-through; user_entry.h hints byte 47 = mxobl/acobl). Resolve against
006-S3FS RUSER/WUSER and @DUMP-USER-ENTRY of a real user.
4.4 Page bitmap (bit file) - VERIFIED semantics, byte-decode PENDING¶
- One bit per page, 0 = free, 1 = used (NDFS
bit_file.h). VERIFIED. - Blocks 0-6 are reserved (system); first allocatable block = 7
(
NDFS_FIRST_ALLOC_BLOCK). VERIFIED in NDFS; must confirm the same reservation in006-S3FSALPAG/GPAGE(Phase 4). - Contiguous-file allocation searches for a free range; single-page allocation
takes the first free bit from block 7 up. INFERRED (NDFS behaviour) - verify
the search direction against
006-S3FS, since the doc says contiguous files are placed in the highest page range (@CREATE-FILErule 3), which is the opposite direction and is an OPEN question (§6, Q3). - Prior analysis
SINTRAN-STRUCTURES.mdnotes a bitmap area constant5BITM=000010B- relationship to the on-disk bit file is OPEN.
4.5 Directory entry returned by MON 244 - VERIFIED size¶
The in-memory / returned directory entry is 24 words (42 bytes)
(from the carved 244B-GetDirEntry/GDIEN analysis). This is distinct from the
32-byte on-disk master block; the relationship between the two (which fields
overlap) is an OPEN question for Phase 1.
5. 006-S3FS filesystem code map¶
Segment load base 26000B. Byte offset of octal address A in the .hex:
(A - 26000B)_octal_as_decimal * 2. All addresses octal. One-line roles are from
the FILSYS symbol names + carved MON-worker cross-checks; bodies not yet
individually verified are marked so in their phase.
Master block / directory¶
| Addr | Sym | Role (one line) |
|---|---|---|
| 30225B | GDIRA |
Get directory address (base of a directory's in-core datafield) |
| 30235B | GNAMA |
Get name address |
| 47402B | GDIRI |
Get directory index |
| 47653B | GMAIN |
Get main directory |
| 47716B | WDIRE |
Write directory (entry/label) |
| 107106B / 107111B | WDIEN / GDIEN |
Write / get directory entry (GDIEN = MON 244) |
| 107401B / 107403B | RESDI / RELDI |
Reserve / release directory |
| 136741B | CRDIR |
Create directory - lays down label + bit/object/user files (Phase 7) |
Directory datafield accessors (offset getters into the directory DF, 43263B-43775B):
DDEVN DLOGU DFRFL DUSEN DSPAC DNUSE DPASS DACCE DFNAM DPAGE DNFIL DDVNU DOBJI DBLSZ DBYTP DOUTF ...
Object entry¶
| Addr | Sym | Role |
|---|---|---|
| 55563B / 55566B / 55750B | FOBJB / ROBJE / WOBJE |
Find / read (MON 41) / write object block |
| 56307B / 56326B | ROBJB / GOBJI |
Read object block / get object index |
| 61502B | COBJE |
Change object entry |
| 63726B / 64146B | CROBJ / DLOBJ |
Create / delete object |
| 104035B / 104037B / 104410B | MROBJ / DROBJ / DWOBJ |
MON read / direct read / direct write object entry |
User entry¶
| Addr | Sym | Role |
|---|---|---|
| 53174B / 53243B | TUSEN / FUSEB |
Test / find user block |
| 53246B / 53410B | RUSER / WUSER |
Read / write user entry |
| 53721B | RUSEB |
Read user block |
| 55111B | GUSEN |
Get user entry |
| 55206B | CUSED |
Change used (pages) |
| 62206B / 62314B | CHNUS / INSUS |
Change / insert user |
| 105010B | MRUSE |
MON read user entry |
Page bitmap / allocation primitives¶
| Addr | Sym | Role |
|---|---|---|
| 50627B | ALPAG |
Allocate page (mark used in bit file) |
| 50632B / 50635B | XRLPA / RLPAG |
Release page |
| 51025B | TPAGF |
Test page free |
| 51120B | RSPAG |
Reserve/set page |
| 51353B / 51355B | TESTB / TESTP |
Test bit / test page |
| 76205B | GPAGE |
Get (allocate) a page - core bitmap primitive |
| 101707B / 101711B | RPAGE / WPAGE |
Read / write a bit-file page |
| 60147B / 60151B | DLSPA / DLPAG |
Release / delete page(s) |
| 74510B | DFPAG |
Deallocate/free page |
File index blocks (indexed files)¶
| Addr | Sym | Role |
|---|---|---|
| 51451B / 51453B | GP5IX / RINDX |
Get index / read index block |
| 52066B | FINDX |
Find index (walk index block for a file page) |
| 52163B | PRKEY |
Process key |
| 52501B | WINDX |
Write index block |
File create / open / close / delete / rename / expand¶
| Addr | Sym | Role |
|---|---|---|
| 64410B / 64670B | CRNEW / GCFIL |
Create new / get-create file |
| 65144B | FFILE |
Find file |
| 67432B / 67612B | FOPEN / FCLOS |
File open / close |
| 103026B / 103034B / 103037B | DOPEN / OPFIL / OLDOP |
Direct-open / open-file / open-old |
| 103350B / 103355B | BCLOS / CLOFI |
Backup-close / close file |
| 105555B | EXPFI |
Expand file |
| 105560B / 105562B | CRALN / CRALF |
Create allocated (indexed / contiguous) |
| 106060B / 106063B | MRNFI / MDLFI |
Rename / delete file |
File I/O¶
| Addr | Sym | Role |
|---|---|---|
| 102021B / 102023B | RDISK / WDISK |
Read / write disk (page-level) |
| 102130B / 102132B | RFILE / WFILE |
Read / write file |
| 102517B / 102625B | FGET / FPUT |
File get / put (byte/record) |
| 77000B / 77230B | REBUF / WRBUF |
Read / write buffer |
| 77542B / 100130B | FREA / FWRT |
File read / write core |
| 100566B / 100570B | FDREA / FDWRT |
File direct read / write |
| 107447B / 107451B | RDPAG / WDPAG |
Read / write disk page |
| 110050B / 110472B | COPAG / DELPG |
Copy page / delete page |
Queue / lock plumbing (context, not FS structure)¶
FCSTA INIQ RELQ WLOCQ LOCQ REAQ WRIQ APPQ TAKQ UNLQ (26000B-26331B) - the
file-system request-queue and lock primitives that the I/O paths run on.
6. Open questions - what each later phase needs¶
-
OPEN-Q1 (Phase 1): Confirm the master-block layout against a second real directory device (
250305L07-XX-01D.IMG) and against the006-S3FSwriterGMAIN/WDIRE/CRDIRbytes, so the offsets are proven from the producing code, not only from the NDFS reader + one disk. Need: byte-trace of whereCRDIRstores the three block pointers. -
OPEN-Q2 (Phase 1): How does the on-disk 32-byte master block relate to the 24-word (42-byte) directory entry that MON 244 (
GDIEN) returns? Which fields are the same, which are in-core only? Need:GDIEN/GDIRAfield trace. -
OPEN-Q3 (Phase 4): Bitmap allocation direction. NDFS allocates upward from block 7, but
@CREATE-FILEdocuments contiguous files placed in the highest address range. Resolve the realGPAGE/ALPAGsearch direction and the role of the reserved blocks 0-6 in006-S3FS. Need:GPAGEcontrol-flow trace. -
OPEN-Q4 (Phase 2/3): Appendix A (Data Fields) does not contain an explicit object-entry / user-entry field table under a findable heading. Locate the authoritative on-disk field table in the ND System Documentation set (or accept NDFS +
ndtool+ the carved code as the authority). Need: doc search of the full ND-60.112 / System Supervisor manuals. -
OPEN-Q5 (Phase 7): CRDIR (136741B) is a JPL-heavy routine dispatching through an in-body pointer table (words at 137014B+). Full decode requires carving the referenced sub-routines and matching the bit-file bad-page test ("write/compare three test patterns", per the
@CREATE-DIRECTORYdoc) against the carved bytes. Need: closure-checked carve ofCRDIR+ callees, then compare the produced layout to NDFSimage_creator.c(bit_file_block = pages/2, object/user index blocks adjacent).
7. Subfolders¶
on-disk-format/- master block, object entry, user entry, bitmap byte layouts (Phases 1-4), plusboot-sector.md- the page-0 boot code and the three ND-100 boot formats (BPUN / FLOMON / raw binary bootstrap), with the realSMD0.IMGboot sector decoded, andextended-info-block.md- the 16-byte page-0 extended-info block (checksum, flag word, system number, capacity) decoded from the producing kernel routinesWXDIR/CHDSI.code-logic/- allocation, bitmap primitives, file I/O, CRDIR (Phases 5-7).create-directory.md- fullCRDIRcreate-directory walk-through (label, bit/object/user files, bad-page test, checksum), andcreate-directory-placement.md- the VERIFIED bit-file placement formulabit = 9*floor(floor(pages/2)/9)(roundpages/2down to a multiple of 9), the supplied-address-vs-default branch, the all-image data table, and the concrete NDFS fix forbit_file_block = pages/2.boot-creation.md- the boot half of making a bootable device (separate from the filesystem structure): how the page-0 bootstrap is produced per device class (SMD [real bytes], floppy FLOMON/BPUN [real bytes], Winchester [derived], SCSI [derived]), the mass-storage-load / binary-load firmware contract and ALD presets, and how SINTRAN writes it (DUMP-BOOTSTRAPfor floppy; pack-to-pack copy for hard disk).NDFS-VALIDATION.md- point-by-point validation of the independent NDFS on-disk-format report against the SINTRAN kernel bytes + real disk: confirmed / corrected / still-open per claim, and the 7 ranked open questions answered (the checksum is a 16-bit additive sum, not XOR; flag-word bit 15 = "directory entered"; bad checksum triggers rebuild, not rejection).code-logic/enter-directory.md- the full end-to-end@ENTER-DIRECTORY(mount) trace from the carved bytes: command dispatch ->ENDIR140176B (unit reserve +GDIRAdatafield) ->CHDSI37763B -> the page-0 read (RXDIR/RCBLO, block 0, dispatched through the datafield transfer pointer) -> checksum/capacity/owner-interlock/flag-stamp ->WXDIRwrite-back, with the disk-controller read contract and the mount error codes.
Last updated: Phase 0 survey. Evidence base: real SMD0.IMG bytes, NDFS C
library, carved 006-S3FS SINTRAN L bytes, ND-60.128.5 official docs.