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CHDSI / WXDIR - directory master-block processing (COMPLETE carve)

Byte-verified reverse-engineering of the SINTRAN III L07 / L-VSX-500 directory master-block ("extended-info") processing in segment 006-S3FS (load base 26000B). Covers EVERY path through:

Addr Symbol Role
37702B WXDIR recompute additive checksum + write block 0 back
37763B CHDSI read block 0, checksum, capacity, owner interlock, rebuild, stamp, write-back
40162B REENB release directory (clear entered bit + write back) - mirror of the stamp

All addresses octal. On-disk multi-byte values are big-endian words. Every claim is graded VERIFIED (proven from carved bytes), INFERRED (strong reasoning), or OPEN (crosses an uncarved boundary).

Offset of octal address A in the .bin: (A - 26000B) * 2 bytes.

1. The master block (extended-info, 8 words at front of directory page 0)

Disk words 1750B..1757B:

Word Meaning Touched by
0 additive checksum of words 1..7 CHDSI recompute, WXDIR store
1..3 directory identity / dates (not read here) -
4 flag word; bit15 (field 170 DA) = "entered" CHDSI set, REENB clear
5 owner system number CHDSI stamp
6..7 capacity = pages available (32-bit) CHDSI compare / rebuild

The checksum is a plain 16-bit additive sum, NOT XOR - proven twice: the compare loop in CHDSI (040011 ADD ,X 0) and the recompute loop in WXDIR (037716 ADD ,X 0) are identical ADD loops over words 1..7. VERIFIED.

2. CHDSI - complete path map (each branch -> outcome)

Entry: T = entering system number (ENDIR local 26, referenced as local 34/50 here).

CHDSI 37763B
  037767  prologue (003752, OPEN)
  037770  JPL 050323  get param/datafield + resource word (OPEN)
            |-- fail --> 040135 error exit (propagate callee A)
  037772  local16 := param word (option/resource bits)
  037773  BSKP ZRO 100 DA  (bit6 of param = resource-reserve bit)
            |-- bit6 SET --> 040131 EARLY SUCCESS (no page-0 I/O at all)
  040000  JPL RXDIR 37643B   read directory BLOCK 0 (D=0) into 8-word block
            |-- fail --> 040135 error exit (propagate RXDIR A)
  --- checksum recompute (sum of words 1..7) + compare word0 ---
  040017  computed == stored ?
            |-- NO  (040020) --> 040063 REBUILD
            |-- sum==0 (040021) --> 040063 REBUILD
            |-- YES --> capacity compare
  --- GOOD checksum: capacity compare vs device geometry (040022-040061) ---
  040023  JPL 037101  get geometry capacity (in-seg)   fail-> 040135
  040030-040033  stored cap == geometry cap ?
            |-- MATCH --> 040062 --> 040100 owner interlock
            |-- DIFFER --> resident capacity-adjust sequence:
                 040036 JPL 050124  (OPEN)  fail-> 040135
                 040044 JPL 050223  (OPEN)  fail-> 040135 ; bit4 set-> 040134 generic
                 040050 JPL 037565  (in-seg) fail-> 040135
                 040060 JPL 050226  (OPEN)  fail-> 040135
                 040062 --> 040100 owner interlock
  --- BAD/zero checksum: REBUILD / self-heal (040063-040077) ---
  040063-040071  zero all 8 words of the block
  040075  JPL 037101  get geometry capacity            fail-> 040135
  040077  words 6-7 := geometry capacity
          fall through --> 040100 owner interlock
  --- OWNER INTERLOCK (join 040100) ---
  040102-040104  param bit3 SET  -> STAMP (force/override, bypass interlock)
  040105-040107  entering system == 0 -> STAMP (no interlock for system 0)
  040110-040112  flag bit15 clear (not entered) -> STAMP
  040113-040114  owner word == 0 (unowned) -> STAMP
  040115-040116  owner == entering system (re-enter) -> STAMP
  040117         else REJECT: LDA (=003203 sentinel) --> 040135 error exit
  --- STAMP (040121-040127) ---
  040122  word5 := entering system (owner)
  040124  word4 bit15 := 1 (entered)
  040127  JPL WXDIR 37702B  recompute checksum + write block 0 back
            |-- fail --> 040135 (WXDIR returns 35B)
  040131  MIN ,B 4 -> SUCCESS return

Owner-interlock accept/reject logic (VERIFIED control flow)

The mount is REJECTED (returns the 003203 sentinel at 040117) only when ALL of the following hold:

  1. param bit3 (force/override) is clear, AND
  2. entering system number is non-zero, AND
  3. flag word bit15 "entered" is set, AND
  4. stored owner word is non-zero, AND
  5. stored owner differs from the entering system number.

If any one fails, control reaches the STAMP (accept). So: a force/override request, system 0, a not-yet-entered block, an unowned block, or a re-enter by the same owner all accept; only a genuine cross-system collision rejects. (The two leading overrides - param bit3 and system==0 - are additional guards above the flag/owner check documented in enter-directory.md section 5.4.)

3. WXDIR - complete path map

WXDIR 37702B
  037706  prologue (003752, OPEN)
  037714-037721  checksum recompute loop (ADD words 1..7)
  037723  word0 := computed checksum
  037727  JPL CL1DB 35240B   release/flush (OPEN edge)
  037730  JPL RCBLO 35766B   reserve cache buffer for block 0
            |-- fail (037731) --> 037750 error
  037733-037736  copy the 8-word block into the page buffer (JPL 001224, OPEN)
  037741  JPL WCBLO 36357B   WRITE cache block 0 back to device
            |-- fail (037742) --> 037746 error
  037743  MIN ,B 4 -> SUCCESS
  037746  JPL 047365 (resident error handler, OPEN)
  037747  SAA 35  -> return ERROR 35B "master block transfer error"

Note vs prior anchor: the actual WCBLO call is at 037741 (pool word 037760 = 036357B); instruction 037727 is the preceding CL1DB call. The SAA 35 (error 35B) at 037747 is confirmed.

4. REENB - release (mirror of the stamp)

REENB 40162B
  040167  JPL 050323 param helper        fail-> 040210
  040171  resource bit6 set -> 040205 early success
  040176  JPL RXDIR 37643B  read block 0 fail-> 040210
  040201  BSET ZRO 170 DA   CLEAR flag bit15 "entered"
  040203  JPL WXDIR 37702B  write back   fail-> 040210
  040205  MIN ,B 4 -> SUCCESS

VERIFIED: release re-reads the block, clears only bit15, and writes back via WXDIR (so the checksum stays valid). It does NOT clear the owner word.

5. Error / return codes (each code + trigger)

Code Where Trigger Verdict
35B WXDIR 037747 (SAA 35) RCBLO reserve OR WCBLO write of block 0 fails VERIFIED (byte 170435 dd-checked)
CHDSI 040127->040130 propagates WXDIR 35B when the stamp write-back fails VERIFIED
003203 CHDSI 040117 (LDA 37 -> word 040156) cross-system owner reject (interlock, all 5 conditions) value VERIFIED (003203 dd-checked); its operator-visible errno mapping is OPEN - it is a resident-pointer-shaped word, not a clean file-system errno
003204 CHDSI 040134 (LDA 25 -> word 040161) generic sub-step failure in the capacity-adjust sequence (040047, 040052) value VERIFIED (003204); errno mapping OPEN
propagated A CHDSI 040135 (STA ,B 2) reached from 037771/040001/040024/040037/040045/040051/040061/040076/040130 the failing sub-call's own A is stored and returned VERIFIED (control flow)
REENB -16 / status REENB 040210 RXDIR or WXDIR failure during release VERIFIED (control flow)

Important consequence (already noted in enter-directory.md 5.3): a bad or zero checksum does NOT raise a mount error - CHDSI self-heals (zeroes the block, writes geometry capacity) and proceeds to stamp + WXDIR. So a garbage page-0 read surfaces as a write failure (35B) or a re-fail on next enter, not as a "checksum error".

The 003203/003204 sentinels are the single OPEN item in this carve: the bytes decisively show what value is loaded and returned, but that value does not map to a small file-system errno in the routine's own literal pool. The reference-manual candidates for the owner reject remain 032B "Directory entered" / 034B "Unit occupied" (INFERRED, pending a live DAP trace of the value the command interpreter prints for a cross-system enter).

6. VERIFIED / INFERRED / OPEN summary

Item Verdict
Checksum = 16-bit additive sum of words 1..7 (CHDSI compare + WXDIR recompute) VERIFIED
CHDSI reads block 0 via RXDIR before any validation VERIFIED
Bad/zero checksum -> zero 8 words + write geometry capacity (self-heal, not reject) VERIFIED
Good checksum -> capacity compare; on mismatch run resident adjust sequence VERIFIED (flow); adjust-helper semantics OPEN (resident 050124/050223/050226)
Owner interlock: reject iff (not-override) AND (sys!=0) AND entered AND owned AND owner!=me VERIFIED (control flow)
param bit3 = force/override bypass; param bit6 = resource-reserve early-success INFERRED (meaning); branch VERIFIED
Stamp: word5 := owner, word4 bit15 := 1 VERIFIED
WXDIR: recompute -> store word0 -> RCBLO -> copy -> WCBLO write block 0 VERIFIED
WXDIR error 35B on transfer failure VERIFIED (byte)
REENB clears bit15 only, keeps owner, writes back VERIFIED
Owner-reject return code 003203 / generic 003204 value VERIFIED; errno mapping OPEN
Resident prologue 003752 / epilogue 003776 / helper 050323 / copy 001224 / handler 047365 OPEN (uncarved resident segment)
RCBLO / WCBLO / CL1DB device transfer of block 0 (via datafield ,X 14) VERIFIED request; driver primitive OPEN (see RCBLO carve)

7. dd spot-check (verified against 006-S3FS.bin)

Command form: off = (A-26000B)*2; read 2 big-endian bytes.

Addr Off Bytes Octal Instruction OK
037763 10214 22 6c 021154 CHDSI entry STD I 154 OK
037702 10116 22 28 021050 WXDIR entry STD I 50 OK
037747 10190 f1 1d 170435 WXDIR SAA 35 (err 35B) OK
037773 10230 fa 45 175105 CHDSI BSKP ZRO 100 DA (resource bit6) OK
040011 10258 64 00 062000 CHDSI ADD ,X 0 (checksum add) OK
040117 10398 48 1f 044037 CHDSI LDA 37 (reject code load) OK
040124 10408 f8 fd 174375 CHDSI BSET ONE 170 DA (set entered bit15) OK
040143 10438 3f a3 037643 pool: RXDIR ptr OK
040157 10462 3f c2 037702 pool: WXDIR ptr OK
037760 10208 3c ef 036357 pool: WCBLO ptr OK
040156 10460 06 83 003203 reject sentinel word OK
040161 10466 06 84 003204 generic-error word OK
040142 10436 00 06 000006 const 6 (datafield offset) OK

All 13 words reproduce byte-for-byte.