MON 042B (octal) — Undocumented (no symbol)

A low ND-100 monitor call (< 0400B) with no direct level-14 handler: GOTAB[42B] = 000000. The zero slot means MON 042B is a fall-through — it is serviced by the resident second-level dispatch (MFELL → CALLPROC), not by a dedicated entry stub. No code body is carved, so there is no per-call .ASM/.pseudo.c; this document records the proven absence.

Status: dispatch head byte-proven (GOTAB[42B] = 000000, a real SINTRAN L byte pair, matching a live read of the running system); no worker link — the body lives in the uncarved CALLPROC overlay and cannot be pinned to a carved byte range (see Honest caveats). All addresses/values are octal.

• No per-call disassembly: MON 042B is a fall-through (GOTAB[42B] = 000000); there is no entry stub or worker to disassemble in the carved set, so no 042B-Undocumented.ASM is fabricated.
• Bytes live once in the canonical segment layer: ../../segments-ref/.

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Dispatch path

flowchart LR
    A["User program<br/>MON 42B"] --> B["ENT14 level-14<br/>T = MON number"]
    B --> C["GOTAB[42B] = 000000<br/>(byte + live proven)"]
    C -.uncarved CALLPROC.-> D["MFELL -> CALLPROC<br/>second-level dispatch"]
    D --> E["handler body in uncarved<br/>resident overlay (S3MPIT/S3RPIT)"]
    class A blue
    class B,C teal
    class D,E 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 dashed hop (C ⇢ D) is the resident MFELL/CALLPROC second-level dispatch — it is not present in any carved segment, so it is the one link that cannot be followed statically. A 000000 GOTAB slot is fall-through, NOT illegal: MON 14B (OUTBT), a real heavily-used call, also carries GOTAB[14B] = 000000.

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Code location (dispatch path)

Every row that has bytes is a real region you can open. Byte offset = (addr − loadbase) in octal words × 2. GOTAB base = 071233B in commoncode (load 0); entry address = 071233B + MON# = 071275B.

Role	Segment (full disasm)	Addr range (octal)	Byte offset	Symbol	Verdict
GOTAB[42] dispatch word	commoncode.asm · .hex	071275B (1 word)	58746	GOTAB+42B = 000000	VERIFIED (= 000000, fall-through)
resident CALLPROC bridge	— (uncarved)	—	—	MFELL → CALLPROC	UNVERIFIED (outside carved byte set)
fall-through handler body	— (uncarved)	—	—	S3MPIT/S3RPIT overlay	UNVERIFIED (not resolvable)

Verify by hand: grep '^71275 ' ../../segments-ref/SINTRAN-DATA_commoncode/SINTRAN-DATA_commoncode.hex → byte offset 58746; then dd if=../../../resident/SINTRAN-DATA_commoncode.bin bs=1 skip=58746 count=2 2>/dev/null | od -An -tx1 → 00 00 (= octal 000000, the fall-through GOTAB slot).

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Instruction walkthrough

There is no instruction body to walk — that is the point of this entry. Level-14 dispatch (ENT14) loads the MON number into T, indexes GOTAB at 071233B + 42B = 071275B, and reads 000000. A zero handler address is the sentinel that routes the call into the second-level fall-through (MFELL → CALLPROC) instead of jumping to a direct level-14 stub. The CALLPROC second-level table, and the ultimate handler body it selects, live in the memory-resident S3MPIT/S3RPIT overlay whose ENT14/MCALL code pages are not backed in the available carves, so MON 042B's body has no resolvable address from what we have. No .ASM/.pseudo.c is provided rather than fabricate one.

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Parameter / register contract

Reg / field	Dir	Meaning	Verdict
T	in	MON number (42B) used to index GOTAB	VERIFIED (dispatch mechanism)
GOTAB[42B]	internal	000000 — fall-through sentinel, not a handler address	VERIFIED (bytes)
A/X/D	in/out	caller argument/return convention	UNKNOWN — defined by the uncarved CALLPROC handler, not byte-proven
skip return	out	success/error skip convention	UNKNOWN — set by the uncarved handler body

The user-visible register convention lives entirely in the uncarved CALLPROC frame and the handler body it dispatches, so nothing beyond the dispatch mechanism is byte-proven here.

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Honest caveats

What is byte-proven: GOTAB[42B] = 000000 — a real SINTRAN L byte pair in resident/SINTRAN-DATA_commoncode.bin at virtual 071275B (byte offset 58746), matching a live-DAP read of a booted L system. That zero is a genuine fall-through sentinel, not an illegal/undefined call: even-numbered low monitor calls carry 000000 (serviced by the fall-through) while odd-numbered ones (1B..161B) carry direct entry stubs in the S3RPIT 120xxx block.

What is NOT proven: MON 042B's actual handler body. The MFELL → CALLPROC second-level dispatch that services fall-through calls is in the memory-resident S3MPIT/S3RPIT overlay whose ENT14/MCALL code pages are not populated in the available carves, so 042B's body cannot be pinned to an address. It is neither confirmed present nor proven absent — it is outside the carved byte set. The "Undocumented"/TSS-carry-over provenance sometimes attached to this call is not encoded in any source and is not confirmed. Recovering the body needs a live trace (break at the CALLPROC dispatch on a real MON 42B and single-step to the handler) or a carve of the resident overlay.

There is no ANALYSIS-vs-DISPATCH contradiction to reconcile: earlier notes that called GOTAB "unreadable/absent from every carve" were wrong — GOTAB is fully present in commoncode.bin; it is 042B's fall-through body, not the table, that is missing.

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Method: ../../../../../EXTRACTING-RESIDENT-CODE.md §5/7/8 · dispatch reality: ../../TASK-05-mismatches.md · master map: ../../MON-CALL-INDEX.md.