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RXDIR-CACHE-COMPLETE - the complete block-0 read path through the page cache

Full every-path carve of the SINTRAN III VSX/500 L07 routines that turn a filesystem "read directory block 0" request into a device transfer, in segment 006-S3FS (load base 26000B, sha256(006-S3FS.bin)=b4a563d0...):

Symbol Addr Role
RXDIR 037643B read page-0 extended-info via the cache; sets block := 0
RCBLO 035766B reserve/read a cache block (HIT / MISS / alloc / dispatch)
GSIZE 037101B in-core configured disk size (NO device I/O)
COMPP 036616B RCBLO's cache compare/search helper ([036064]=036616)
G3NWT 034371B get-new cache entry / wait ([036066]=034371)
R3BUF 035112B RELEASE (invalidate) data-cache buffers for a device
R3IBU 035102B RELEASE (invalidate) index-cache buffers for a device
CL1DB 035240B release one cache buffer (used by RXDIR and RCBLO error exit)

This EXTENDS the sibling folder ../RCBLO/ (which already carved RCBLO + GSIZE and answered "why no page-0 read is issued") to add the full RXDIR body, the R3BUF/R3IBU release logic, and to name the cache helpers RCBLO depends on (COMPP, G3NWT). It also closes the boundary that enter-directory.md Section 4 left at the JPL I ,B 10 dispatch.

Grades. VERIFIED = re-read from carved 006-S3FS.bin bytes (offset = (addr - 26000B)*2 decimal, big-endian). INFERRED = reasoned from bytes + architecture. OPEN = crosses an uncarved resident overlay or a runtime binding. All addresses/values are octal. Disassembly: RXDIR-CACHE-COMPLETE.ASM; pseudo-code: RXDIR-CACHE-COMPLETE.pseudo.c.

1. The call graph (every edge byte-verified except the runtime ,X 14 hop)

CHDSI 037763B --- 040000: JPL I 143 -> [040143]=037643 ---> RXDIR 037643B
RXDIR 037643B
   037647: JPL I 25  -> [037674]=003752   reserve/prologue
   037651: RADD CLD 0 DD                   BLOCK NUMBER := 0        (VERIFIED)
   037652: JPL I 23  -> [037675]=035766 ---> RCBLO 035766B (read cache block 0)
   037661: JPL I 16  -> [037677]=001224    copy 8-word ext-info out of buffer
   037665: JPL I 13  -> [037700]=035240 ---> CL1DB  (release the borrowed buffer)
RCBLO 035766B
   035773: JPL I 63  -> [036056]=037101 ---> GSIZE   (in-core size, NO I/O)
   036011: JPL I 53  -> [036064]=036616 ---> COMPP   (cache compare/search)
   036020: JPL I 46  -> [036066]=034371 ---> G3NWT   (get-new entry / wait)
   036135/036142/036167/036174: JPL I ,B 10 --->  DEVICE TRANSFER (RUNTIME ,X 14)
   036203: JPL I 15  -> [036220]=035240 ---> CL1DB   (release on error)

2. RXDIR (037643B) - every path

RXDIR is small and linear (body 037643-037672; 037673 ROP NOOP separates it from its pointer pool; next routine WXDIR 037702B).

  1. Prologue (037643-037647): save link, 7-word frame, call resident reserve/prologue 003752.
  2. Block := 0 (037650-037651): RADD CLD 0 DA / RADD CLD 0 DD set both A and the D (block-number) register to 0. VERIFIED byte 146101.
  3. Read via cache (037652): JPL I 23 -> [037675]=035766 = RCBLO with block 0. VERIFIED [037675]=035766.
    • RCBLO error (037653 JMP 16 -> 037671): store status in ,B 2, unwind, return error. This is RXDIR's ONLY error path - it forwards whatever RCBLO returned (GSIZE-fail, descriptor-fail, COMPP-fail, geometry-error, or a driver-transfer error) straight up to CHDSI.
  4. Extract ext-info (037654-037661): X = returned buffer, A = A+21B (ext-info lives at buffer offset 21B), word count T := 10B (8. words), JPL I 16 -> [037677]=001224 = resident block-copy into the caller's area (,B 0).
  5. Release the buffer (037662-037665): JPL I 13 -> [037700]=035240 = CL1DB. RXDIR does NOT keep the page-0 buffer - it copies the 8 ext-info words out and immediately frees it. VERIFIED [037700]=035240.
  6. OK exit (037666-037670): ok-marker MIN ,B 4, unwind -7, return via [037701]=003776 (resident epilogue).

So RXDIR itself has exactly two exits: OK (037670) and error (037671->037667), and issues no device I/O of its own - all I/O is inside RCBLO.

3. RCBLO cache HIT / MISS / alloc decision map

RCBLO is the page cache. Its structure (bytes in the .ASM):

prologue (035766-035774)
  |  035773 GSIZE ; 035774 GSIZE-fail --------------------> ERROR STUB 036205
descriptor + classify (035775-036004)
  |  036000 descriptor-fail ------------------------------> ERROR STUB 036205
COMPP cache search (036011)
  |  036012 COMPP-fail -----------------------------------> ERROR STUB 036205
G3NWT get-new + stamp candidate (036020-036030)
block compare (036031-036042):  hi ,X5 / lo ,X6 / device
  |  any mismatch ----------------------------------------> MISS  036072
HIT gate (036043-036046):  clean/valid ?
  |  dirty/invalid ---------------------------------------> MISS  036072
HIT (036047-036053): reserve/wait, NO device read ------->  SUCCESS 036144
MISS (036072-...):
   stamp block/device (036072-036075), reserve/wait (036077)
   load transfer_fn = unit ,X14 -> local 10 (036100-036102)
   geometry math (036103-036117):  out-of-range -> A:=100B -> ERROR EPI 036177
   036122 descriptor2-fail -------------------------------> ERROR EPI 036177
   4 device-transfer dispatches (below)
SUCCESS (036144-036160): link buffer into cache, return X=buffer
ERROR EPILOGUE (036177-036204): CL1DB release + return error
ERROR STUB (036205-036206): store status + return error

Decision summary

Situation Path Device read?
Block 0 already cached, clean HIT 036047-036053 -> 036144 NO
Block 0 cached but dirty/invalid falls to MISS 036072 YES (rewrite/read)
Block 0 not cached MISS 036072 -> dispatch YES
GSIZE / descriptor / COMPP fails ERROR STUB 036205 NO
Geometry out of range 036116 A:=100B -> ERROR EPI 036177 NO
Driver transfer returns error ERROR EPI 036177 (CL1DB) attempted

The HIT path returning a stale/clean buffer WITHOUT a device read is exactly the locus-(2) candidate for "why no page-0 read is issued" identified in ../RCBLO/README.md Section 4d.

4. The four dispatch sites and what distinguishes them

All four are the SAME instruction 135410 JPL I ,B 10 (VERIFIED) - an indirect call through frame word B+10, which was loaded at 036100-036102 from the unit descriptor's datafield word 14 (LDA ,X 14). What differs is the function selector "ABFUN" each site builds from the operation code in ,B 12:

# Addr ABFUN arithmetic (VERIFIED) On driver error On driver OK
1 036135 (op & 065B) + 066B 036136 -> 036177 error epi fall to #2 (036137)
2 036142 (op & 054B) + 063B -> 036161 (#3 group) -> 036144 SUCCESS
3 036167 (op & 032B) + 066B 036170 -> 036177 error epi fall to #4
4 036174 (op & 022B) + 063B 036175 -> 036177 error epi 036176 -> 036144 SUCCESS
  • The AND masks (065 / 054 / 032 / 022) and the two base constants (066B and 063B) are VERIFIED from the bytes.
  • The two bases pair up: 066B on sites #1/#3, 063B on sites #2/#4. INFERRED: 066B/063B select two adjacent datafield sub-entries of the driver's transfer block (an even/odd read-vs-companion selector), i.e. ABFUN is a datafield-relative selector, NOT a raw SCSI opcode. The mapping ABFUN -> SCSDISK function number (0=READ, 1=WRITE, 4=SEEK, 42=READ FORMAT) is OPEN (that translation lives in the uncarved driver).
  • Which site runs is driven by the skip-return status of the previous dispatch (site #1 ok falls into #2; #2 ok goes to success; #2 error chains to #3/#4). This chaining is VERIFIED from the branch bytes; the semantic meaning of each site (READ vs a companion/verify op) is INFERRED.

The exact ABFUN value for a block-0 read

op (frame word ,B 12) is loaded at 036001 from the device-descriptor getter 050124 return, NOT from RXDIR (RXDIR passes A=0, but RCBLO overwrites A with the descriptor status before it reaches ,B 12). So the concrete numeric op at mount time is a RUNTIME input and cannot be fixed from static 006-S3FS bytes.

  • VERIFIED: the primary/READ dispatch (#1, 036135) emits ABFUN = (op & 065B) + 066B.
  • INFERRED: for a plain read the op-class bits masked by 065B are clear, in which case ABFUN(#1) = 066B (the base). 066B is the datafield READ sub-entry selector, not a SCSI opcode.
  • OPEN: the numeric SCSI function that 066B resolves to inside the driver.

This is the honest answer the no-assumptions rule demands: the arithmetic is byte-fixed; the input operand and the driver-side numeric meaning are not in this segment.

5. GSIZE (037101B) - in-core, no I/O

Save link, 7-word frame, resident prologue 003752, then three in-core parameter getters (050124, 050223, 050220), a geometry-bit test (037124 BSKP ONE 100 DT) that may call low helper 000215, a shift (037132), and 037134 RMPY to compute the 32-bit size, stored in ,B 2. Proof of no device I/O: there is no JPL I ,B (no datafield dispatch) anywhere in the body. Error path 037141-037142. VERIFIED entry 021042, VERIFIED 141216 RMPY.

6. R3BUF (035112B) / R3IBU (035102B) - bulk buffer RELEASE

R3BUF/R3IBU are the release counterpart to the cache getters. Two entries share one body:

  • R3IBU 035102B sets flavour 2 (index buffers), R3BUF 035112B sets flavour 1 (data buffers); both converge at 035121.
  • The body walks the cache-buffer list (035121-035123 list head via resident iterator 010500), and for each buffer that matches the requested device (,X 5 == ,B 1, 035132-035135) and passes the flavour/busy gate (035136-035147) it:
    1. invalidates the buffer: ,X 4 := -1, ,X 5 := -1 (035153-035154), and
    2. unlinks it from the LRU doubly-linked list via the forward ,X 22 / back ,X 23 pointers (head-of-list case 035160-035167, mid-list case 035170-035177), then pushes it onto the free list (035200-035210).
  • Loop continues (035211-035212) until the T=-1 end sentinel (035124-035126), then releases the reserve and returns (035213-035217).

INFERRED role: dismount / cache-flush - drop every cached block for a device so a later mount re-reads from the disk. VERIFIED entries 021116 (R3IBU) / 021106 (R3BUF); VERIFIED shared-body head 045101. The exact list-head data addresses are I-displacement literals into the segment data pool (not carved as symbols here).

Relation to the mount bug: R3BUF is the routine a correct dismount/re-mount would call to invalidate a stale page-0 buffer. If block 0 is left in the cache clean from a prior probe and R3BUF is NOT called between operations, RCBLO's HIT path (Section 3) returns it with no device read - consistent with the "page-0 read never enqueued" symptom. (INFERRED; a live DAP trace settles it, per ../RCBLO/README.md Section 4.)

7. dd verification (15 load-bearing words)

Re-read from versions/L-VSX-500/segments/006-S3FS.bin, byte offset = (addr - 26000B)*2 decimal, big-endian. All matched expected:

Addr Off (dec) hex Octal val Meaning
037643 10054 2218 021030 RXDIR entry STD I 30
037651 10066 cc41 146101 RXDIR block := 0 (RADD CLD 0 DD)
037652 10068 ba13 135023 RXDIR JPL I 23 -> RCBLO
037675 10106 3bf6 035766 RXDIR ptr -> RCBLO
037700 10112 3aa0 035240 RXDIR ptr -> CL1DB
035766 8172 2236 021066 RCBLO entry STD I 66
036135 8378 bb08 135410 RCBLO JPL I ,B 10 (dispatch)
036056 8284 3e41 037101 RCBLO ptr -> GSIZE
036064 8296 3d8e 036616 RCBLO ptr -> COMPP
036220 8480 3aa0 035240 RCBLO ptr -> CL1DB
037101 9346 2222 021042 GSIZE entry STD I 42
037134 9400 c28e 141216 GSIZE RMPY SD DT (compute size)
035112 7316 2246 021106 R3BUF entry STD I 106
035102 7300 224e 021116 R3IBU entry STD I 116
035121 7330 4a41 045101 R3BUF shared body LDA I 101

Command form (Git Bash): off=$(( (8#ADDR - 8#26000) * 2 )); dd if=006-S3FS.bin bs=1 skip=$off count=2 | od -An -tx1.

8. VERIFIED / INFERRED / OPEN summary

# Claim Verdict
1 RXDIR 037643B sets block := 0 (146101) and calls RCBLO via [037675]=035766 VERIFIED (dd)
2 RXDIR copies 8-word ext-info out (001224) then releases the buffer via CL1DB [037700]=035240 VERIFIED (dd)
3 RXDIR has exactly two exits (OK 037670 / error 037671) and issues no I/O itself VERIFIED
4 RCBLO HIT path (036047-036053) returns a cached buffer with NO device read VERIFIED
5 RCBLO MISS path loads transfer_fn from unit ,X 14 (036100-036102) and dispatches VERIFIED
6 4 dispatch sites = same 135410 JPL I ,B 10; ABFUN = (op&065)+066 / (op&054)+063 / (op&032)+066 / (op&022)+063 VERIFIED (arith)
7 op (,B 12) comes from descriptor getter 050124 at runtime, not from RXDIR VERIFIED
8 ABFUN(#1) for a read = 066B (op read-class bits clear) INFERRED
9 ABFUN is a datafield sub-entry selector, not a raw SCSI opcode; ABFUN->SCSI function map OPEN (uncarved driver)
10 GSIZE 037101B is in-core (no datafield dispatch); RMPY 037134 computes size VERIFIED
11 COMPP 036616B = RCBLO's cache compare/search; G3NWT 034371B = get-new/wait VERIFIED (symbol + call ptr)
12 R3BUF 035112 / R3IBU 035102 = bulk RELEASE (invalidate + LRU unlink) of a device's cache buffers VERIFIED (structure) / INFERRED (dismount role)
13 Dispatch target ,X 14 -> SCSDISK -> SCLLD (INITO/SCWAQ) INFERRED (NPL) / OPEN (runtime pointer, foreign segment)

Provenance. Carved 006-S3FS SINTRAN L07 bytes, load base 26000B, sha256(006-S3FS.bin)=b4a563d0...; symbols SINTRAN/NPL-SOURCE/SYMBOLS/L07/FILSYS-SYMBOLS.SYMB.TXT (RXDIR 037643B, RCBLO 035766B, GSIZE 037101B, COMPP 036616B, G3NWT 034371B, R3BUF 035112B, R3IBU 035102B, G3BUF 034643B, G3IBU 034633B, CL1DB 035240B); driver logic (different revision, INFERRED) SINTRAN/NPL-SOURCE/NPL/IP-P2-SCSI-DISK.NPL / IP-P2-SCSI-DRIV.NPL.

See also