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MON 331B (octal) - DiskMirroring (MSYSU)

Internal-use disk-mirroring control (manual section 2.14). A sub-function index selects one of 16 DIMIR operations that manage disk-mirror tags, headers, layout and locks. Not intended for user programs. This is an ND-100 monitor call.

Status: partial. GOTAB[331B] = 112532B (byte-proven) routes to the DIA10 disk-device entry stub in overlay 025-S3IRPIT - real code that saves the caller register block and switches page context (TRR PCR). The worker MSYSU = 132567B (same overlay) is real code: a 16-way sub-function jump table whose shape matches the DiskMirroring contract. The DIA10 -> MSYSU link crosses the TRR PCR context switch / resident CALLPROC bridge (uncarved), so it is attributed by symbol + the 16-way dispatch, not a followed pointer (see Honest caveats). All addresses/values are octal.

Dispatch path

flowchart LR
    A["User/system program<br/>MON 331B"] --> B["ENT14 level-14<br/>T = MON number"]
    B --> C["GOTAB[331B] = 112532B<br/>(byte-proven)"]
    C --> D["DIA10 entry stub<br/>025-S3IRPIT :112532B"]
    D -.uncarved TRR PCR / CALLPROC.-> E["MSYSU worker<br/>025-S3IRPIT :132567B"]
    E --> F["16-way DIMIR dispatch<br/>tags / headers / layout / locks"]
    class A blue
    class B,C,D teal
    class E teal
    class 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 dashed hop (D ⇢ E) is the page-context switch (TRR PCR) plus the resident CALLPROC second-level dispatch - not present in any carved segment, so the exact DIA10 -> MSYSU transfer cannot be followed statically. Both endpoints (the DIA10 stub and the MSYSU body) are real bytes in 025-S3IRPIT.

Code location (dispatch path)

Every row is a real region you can open. Byte offset = (addr − loadbase) in octal words × 2 (decimal); commoncode load base is 0, 025-S3IRPIT load base is 32000B.

Role Segment (full disasm) Addr range (octal) Byte offset Symbol Verdict
GOTAB[331] dispatch word commoncode.asm · .hex 071564B (1 word) 59112 GOTAB+331 = 112532B VERIFIED
DIA10 entry stub 025-S3IRPIT.asm · .hex 112532B-112560B (25 words) 49844 DIA10 VERIFIED (real stub)
context switch / CALLPROC bridge — (uncarved) — — TRR PCR / CALLPROC UNVERIFIED
MSYSU worker body 025-S3IRPIT.asm · .hex 132567B-132733B (code) 66286 MSYSU real bytes; link inferred

Verify by hand (GOTAB word): grep '^71564 ' ../../segments-ref/SINTRAN-DATA_commoncode/SINTRAN-DATA_commoncode.hex → 71564 112532 225 132 59112; then dd if=../../../resident/SINTRAN-DATA_commoncode.bin bs=1 skip=59112 count=2 2>/dev/null | od -An -tx1 → 95 5a (= octal 112532, the DIA10 dispatch address).

Verify by hand (DIA10 stub): grep '^112532 ' ../../segments-ref/025-S3IRPIT/025-S3IRPIT.hex → byte offset 49844, value 006017; then dd if=../../../segments/025-S3IRPIT.bin bs=1 skip=49844 count=2 2>/dev/null | od -An -tx1 → 0c 0f (= octal 006017, STA ,X 17, the stub's first word).

Verify by hand (MSYSU worker): grep '^132567 ' ../../segments-ref/025-S3IRPIT/025-S3IRPIT.hex → byte offset 66286, value 135145; then dd if=../../../segments/025-S3IRPIT.bin bs=1 skip=66286 count=2 2>/dev/null | od -An -tx1 → ba 65 (= octal 135145, JPL I 145, the worker's first word). prove-mon.py 331 reports the same GOTAB[331]=112532 -> DIA10.

Instruction walkthrough

Full listing: 331B-DiskMirroring.ASM. All addresses octal; X = saved register block, B = per-call disk datafield (roles inferred from the access pattern).

DIA10 entry stub (112532-112560) — the level-14 disk entry. It saves the caller's A/T/L into a register block (STA ,X 17 / STT ,X 0 / RADD CLD SL DX), toggles the interrupt system (IOF / ION) around a TRR PCR page-context switch, re-fetches the worker block pointer (LDX I 120 -> [112661B]), and stores an initial status word (STA ,B 12). It then JMP 62 -> 112630 into the shared disk-stub tail. VERIFIED (bytes).

MSYSU 16-way dispatcher (132567-132644) — after mirror-state setup (JPL I 145/142/137 through pointer words) and a mirror-flag check, 132620 LDA ,B 20 fetches the sub-function index, SAT 17 / SKP IF DT MGRE SA bounds it to 0..17, and 132624 RADD SA DP performs P = P + index - a computed jump into the 16-entry JMP table at 132625-132644. Each entry vectors to a JPL I through a pointer word ([132747]..[132765]) - the 16 DIMIR sub-function workers. VERIFIED (bytes); the operation each index performs is inferred.

Status store + return (132723-132733) — the success path sets SAA 0, increments a counter (MIN ,B 7), stores the status word (STA ,B 12), and returns via the caller link word (JMP I 7 -> [132742]). Words 132734-132765 are the JPL-I pointer/data table (they disassemble as bogus instructions); their final callees live outside this carve.

Parameter / register contract

Manual-side names/types are from 331B_DiskMirroring.yaml.

Reg / field Dir Meaning Verdict
SubFunction in disk-mirror sub-function index, one of 16 DIMIR functions (0..17 octal) VERIFIED range (132621-132624); meaning inferred (manual)
,B 20 in sub-function index fetched by the worker VERIFIED (132620)
,B 25 work mirror-state word cleared/tested during setup VERIFIED (132601/132617/132727)
,B 7 work per-call counter incremented on success VERIFIED (132724)
,B 12 out status word stored to caller (error code or 0) VERIFIED (132610/132726)
,X 0/17 work saved register block (MON number, caller A) VERIFIED (DIA10 stub)
error return out standard error code (appendix A) inferred (manual)

The 16 concrete DIMIR operations are reached through the JPL I pointer words and need live X/B context; their final callees are outside the carved window and are not resolvable from these bytes.

Pseudo-code (for an emulator)

See 331B-DiskMirroring.pseudo.c — a pseudo-C model for emulator authors. The DIA10 register-block save + TRR PCR context switch and the MSYSU 16-way jump table + status store are byte-verified; the meaning of each DIMIR sub-function and the caller-side parameter convention are inferred from the manual.

Every instruction in the .pseudo.c is translated against the canonical ND100-INSTRUCTION-SEMANTICS.md (RADD CLD SL DX = X = L COPY; RADD SA DP = P = P + A computed jump; SKP IF DT MGRE SA = unsigned skip when 17 >= index; bare LDA 132/LDA 101 = P-relative mem[P+disp], not literals; MIN ,B 7 = increment-and-skip-on-zero; MST PIE = masked-set of the interrupt-enable register).

Honest caveats

What is byte-proven: GOTAB[331B] = 112532B routes to DIA10 (real code, first word 006017 = STA ,X 17); the DIA10 stub's register-block save + IOF/TRR PCR/ION critical section; the MSYSU worker at 132567B (first word 135145 = JPL I 145); the 0..17 sub-function range check and the RADD SA DP computed jump into a 16-entry table; the status store at ,B 12; return via the caller link word.

What is NOT proven: the DIA10 -> MSYSU link and the 16 concrete DIMIR operations. DIA10 switches page context with TRR PCR and re-enters through the resident CALLPROC bridge, which lives in an uncarved overlay; the 16 sub-function workers sit past the JPL I pointer cells, also outside this carve. Attributing the body to MSYSU rests on the symbol name (MSYSU = Mirror SYStem Utility) plus the 16-way dispatch shape that matches the manual's "16 DIMIR functions", not a followed pointer.

This reconciles into one story: the dispatch head (GOTAB[331] -> DIA10) is solid; the DIA10 stub and the MSYSU 16-way dispatcher are both real bytes in 025-S3IRPIT; the transfer between them and the leaf DIMIR workers cross the uncarved context-switch/pointer layer. Confirming the exact link needs a live trace (break at 112532B on a real MON 331, single-step the TRR PCR switch, and record where P lands).

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