MON 246B (octal) - ReserveDir (RESDI)¶

Reserves a directory for special use; other users cannot open files on it. The directory must be entered and all its files must be closed. Release it with ReleaseDir (MON 247B). User RT and user SYSTEM.

Status: GOTAB dispatch head byte-proven as fall-through (GOTAB[246B] = 000000, no per-call stub); the RESDI worker body is real SINTRAN L bytes in the file-system segment 006-S3FS (a FILSYS-SYMBOLS symbol). It is a shared reserve/release body: the SSK skip flag selects reserve (RESDI, SSK=0, this call) vs release (RELDI, SSK=1, MON 247B). It closes at 107426B, bounded by the next symbol 500RD=107434B. The exact MON 246 -> worker link crosses an uncarved kernel bridge (see Honest caveats). All addresses/values are octal.

Full disassembly: 246B-ReserveDir.ASM - the actual code (the shared RESDI/RELDI worker; there is no entry stub because the GOTAB slot is zero).
Bytes live once in the canonical segment layer: ../../segments-ref/.

Dispatch path¶

flowchart LR
    A["User program<br/>MON 246B"] --> B["ENT14 level-14<br/>T = MON number"]
    B --> C["GOTAB[246B] = 000000<br/>(byte-proven: fall-through)"]
    C -.uncarved MFELL / CALLPROC.-> E["RESDI reserve worker<br/>006-S3FS :107401B (SSK=0)"]
    E --> F["find directory; reserve<br/>(RELDI=107403B is the release twin)"]
    class A blue
    class B,C blue
    class E,F green
    classDef blue fill:#E3F2FD,stroke:#0D47A1,color:#0D47A1
    classDef teal fill:#E0F7FA,stroke:#00838F,color:#00838F
    classDef green fill:#E8F5E9,stroke:#2E7D32,color:#2E7D32

The GOTAB slot is zero, so there is no per-call entry stub. The dashed hop (C -> E) is the resident MFELL/CALLPROC fall-through second-level dispatch - it is not present in any carved segment, so it is the one link that cannot be followed statically.

Code location (dispatch path)¶

Every row is a real region you can open. Byte offset = (addr - loadbase) in octal words x 2; for 006-S3FS (load base 26000B) it is (addr - 26000B) x 2.

Role	Segment (full disasm)	Addr range (octal)	Byte offset	Symbol	Verdict
GOTAB[246] dispatch word	commoncode.asm - .hex	`071501B` (1 word)	59010	`GOTAB+246` = `000000`	VERIFIED (fall-through)
resident MFELL/CALLPROC bridge	- (uncarved)	-	-	`MFELL`/`CALLPROC`	UNVERIFIED
RESDI/RELDI shared worker body	006-S3FS.asm - .hex	`107401B-107426B` (code) + `107427B` (pad) + `107430B-107433B` (link cells)	50690	`RESDI` (SSK=0) / `RELDI`=107403B (SSK=1)	real bytes = CODE; body link MISATTRIBUTED

The window is bounded strictly by the next symbol 500RD=107434B (27 words). Words 107401B-107426B are code, 107427B is a ROP NOOP pad, and 107430B-107433B are the JPL I/JMP I link-cell table (data).

Verify by hand: grep '^107401 ' ../../segments-ref/006-S3FS/006-S3FS.hex -> byte offset 50690; then dd if=../../../segments/006-S3FS.bin bs=1 skip=50690 count=2 2>/dev/null | od -An -tx1 -> f8 10 (the stored word = octal 174020, BSET ZRO SSK, the RESDI reserve entry). The GOTAB slot itself: grep '^71501 ' ../../segments-ref/SINTRAN-DATA_commoncode/SINTRAN-DATA_commoncode.hex -> 71501 000000 000 000 59010; then dd if=../../../resident/SINTRAN-DATA_commoncode.bin bs=1 skip=59010 count=2 2>/dev/null | od -An -tx1 -> 00 00 (= 000000, fall-through). prove-mon.py 246 reads the same GOTAB zero.

Instruction walkthrough¶

Full listing: 246B-ReserveDir.ASM. The body is the shared RESDI/RELDI worker (there is no F16xx stub because GOTAB[246] = 0).

Two-entry prologue (107401-107410) - 107401 BSET ZRO SSK clears the skip flag (RESDI reserve entry, this call); the sibling 107403 BSET ONE SSK sets it (RELDI release entry, MON 247B). Both join at 107404 STD I 23 (save the pair); 107405-107406 copy link/frame; 107407 SAB 6; 107410 JPL I 20 -> [107430] calls the resident prologue worker (find directory).

Reserve/release branch (107411-107421) - 107411 LDX ,B 5 / 107412 LDX ,X 31 walk into the directory datafield; 107413 BSKP ONE SSK selects the release resident worker (107415 JPL I 14 -> [107431]) or the reserve resident worker (107420 JPL I 12 -> [107432]).

Return (107422-107426) - 107422 MIN ,B 4 advances the return; 107423 SAA -6 loads a standard error code; 107424 JMP I 7 -> [107433] returns indirectly; the error tail 107425-107426 stores the error code first.

Parameter / register contract¶

Manual-side names/types are from 246B_ReserveDir.yaml.

Reg / field	Dir	Meaning	Verdict
`T` (DirectoryIndex)	in	directory index, from `@LIST-DIRECTORIES` (`MAC` `LDT DIRIX`)	inferred (manual)
`SSK` flag	internal	reserve (SSK=0, RESDI) vs release (SSK=1, RELDI) selector (`BSET ZRO/ONE SSK`, `BSKP ONE SSK`)	VERIFIED (bytes)
`B+5` / `X+31`	internal	directory datafield + reservation-word offset (`LDX ,B 5` / `LDX ,X 31`)	VERIFIED (bytes); meaning inferred
error return	out	standard error code in `A` (`107423 SAA -6`)	VERIFIED (bytes); code value inferred

Pseudo-code (for an emulator)¶

See 246B-ReserveDir.pseudo.c - a pseudo-C model for emulator authors. The control flow (the SSK reserve/release discriminator, the find-directory prologue, the datafield walk and the branch) is byte-verified; the directory-index register meaning is inferred from the manual.

Every instruction in the pseudo-code is translated against the canonical ND-100 instruction semantics reference (BSET ZRO/ONE SSK skip-flag set/clear, RADD CLD copy idiom, BSKP ONE SSK skip, LDX ,B/LDX ,X indexed loads, MIN ,B increment and skip, JPL I/JMP I indirect call/return).

Honest caveats¶

What is byte-proven: GOTAB[246B] = 000000 (level-14 fall-through; prove-mon.py 246 reads commoncode file byte 0xe682 = 00 00); the RESDI worker body at 107401B in 006-S3FS is real code (first word 174020B = BSET ZRO SSK) and it is a shared reserve/release directory routine (find-directory prologue, SSK-selected reserve vs release resident call), consistent with ReserveDir.

Which segment and why: RESDI=107401B is a FILSYS-SYMBOLS symbol in the file-system segment 006-S3FS; its release twin RELDI=107403B shares the same body two words later. The window 107401B-107433B is bounded strictly by 500RD=107434B (27 words): 107401-107426 are code, 107427 is a ROP NOOP pad, 107430-107433 are the link-cell table.

What is NOT proven: the link from the zero GOTAB slot to the RESDI worker. Because the vector is zero there is no stub to disassemble; dispatch drops into the resident MFELL/CALLPROC second-level path, which lives in an uncarved overlay - hence MISATTRIBUTED in the strict sense: the attribution rests on the RESDI symbol name (REServe DIrectory) + the matching behaviour + the RELDI twin, not a followed pointer. The JPL I/JMP I link cells (107430..107433) are a pointer table whose runtime targets are not resolved here. Confirming the dispatch link needs a live trace of a real MON 246.

Method: ../../../../../EXTRACTING-RESIDENT-CODE.md - dispatch reality: ../../TASK-05-mismatches.md - master map: ../../MON-CALL-INDEX.md.