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MON 254B (octal) - GetErrorDevice (GERDV)

Gets the logical device number of the error device - the terminal that outputs system errors and RT program messages (normally the console). If the error device is reserved by an RT program, the call also returns the address of that program's RT description (0 = unreserved). Available on the ND-100 and ND-500. The worker is GERDV = 102525B in segment 025-S3IRPIT.

Status: byte-verified (dispatch + worker body). All addresses/values are octal.

CORRECTED 2026-07-13. The previous version of this folder said the worker was misattributed: it read a "GOTAB" out of SINTRAN-DATA_commoncode, got GOTAB[254B]=066246B vectoring to an F1734 entry stub, and could only attach GERDV by name across an "uncarved CALLPROC bridge". That GOTAB-in-commoncode table is fiction - its slot 0 is 000000, not the real GOTAB slot-0 MFELL=072114B. MON calls dispatch through MCTAB @ 005620B (segment 044-S3IDPIT), indexed directly by N. MCTAB[254B] = 102525B = GERDV - the SAME worker the old folder found by name, now proven directly from the table slot, no stub and no uncarved bridge. See ../317B-ExecuteCommand/README.md and SINTRAN/CARVING-HANDOFF.md section 3a.

Dispatch path

flowchart LR
    A["User program<br/>MON 254B"] --> B["ENT14 level-14<br/>026-S3IMPIT :072167B"]
    B --> C["GOTAB[254B] = MFELL<br/>MGOTA=071233B"]
    C --> D["MFELL level switch<br/>:072114B -> CALLP 032201B"]
    D --> E["MCTAB[254B] = GERDV<br/>044-S3IDPIT :006074B = 102525B"]
    E --> F["GERDV worker<br/>025-S3IRPIT :102525B"]
    class A blue
    class B,C,D,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

There is no dashed hop. GOTAB[254B] is MFELL (as it is for 224 of the 256 calls); MFELL switches program level (writes CALLP=032201B into the monitor level's P) and the monitor level dispatches through MCTAB.

Code location (dispatch path)

Byte offset = (addr - loadbase) in octal words x 2 (decimal). Offsets reproduced with dd.

Role Segment Addr (octal) Byte offset Symbol Verdict
MCTAB[254B] slot 044-S3IDPIT.asm - .hex 006074B = 102525B 2168 -> GERDV VERIFIED
GERDV worker body 025-S3IRPIT.asm - .hex 102525B-102554B 41642 GERDV (SYMBOL-2-LIST) VERIFIED

Verify by hand (from tools/sintran-segment-carver/versions/L-VSX-500/segments/): 

dd if=044-S3IDPIT.bin bs=1 skip=2168  count=2 | od -An -tx1   ->  85 55   (= 102525B = GERDV)
dd if=025-S3IRPIT.bin bs=1 skip=41642 count=2 | od -An -tx1   ->  ba 1a   (= 135032B, GERDV entry JPL I 32)
The worker block runs 102525B-102554B (24 words), bounded by the next symbol HIL14=102555B.

Instruction walkthrough

Full listing: 254B-GetErrorDevice.ASM.

GERDV worker body (102525B-102554B). It calls a resident prologue (JPL I 32), then enables the interrupt-enable bits (SAA 4 / MST PIE), fetches the error-device descriptor through an indirect pointer (LDX I 30), stages fields (LDA ,X 15/21, a RADD CLD copy, a SWAP SD DA full exchange, STZ ,X clears), calls shared resident workers (JPL I 21, JPL I 15, JPL I 13), and stores the two outputs into the caller's B-frame: the logical device number (STA ,B 12) and the RT description address (STA ,B 13). The JPL I link cells (102557/102561/102562) lie past the carved window in the following HIL14 region.

Parameter / register contract

Manual-side names/types are from 254B_GetErrorDevice.yaml.

Reg / field Dir Meaning Verdict
A (ErrorDevice) out logical device number of the error device (MAC example STA ERDEV) inferred (manual)
D (RTProgram) out RT description address of the reserving RT program; 0 = unreserved (MAC COPY SD DA) inferred (manual)
B+12 internal/out worker stores the device number here (102546 STA ,B 12) VERIFIED (bytes); field meaning inferred
B+13 internal/out worker stores the RT description address here (102552 STA ,B 13) VERIFIED (bytes); field meaning inferred
error return out standard error code (appendix A) inferred (manual)

The descriptor read and the two STA ,B output stores are VERIFIED from bytes; the mapping onto the user-visible A=ErrorDevice / D=RTProgram contract lives in the caller-side wrapper and is inferred from the manual, not byte-proven here.

Pseudo-code (for an emulator)

See 254B-GetErrorDevice.pseudo.c - the GERDV worker body is byte-verified; the field semantics (which frame word is the device number vs the RT description address) are inferred from the manual. Every instruction is translated per the canonical ND100-INSTRUCTION-SEMANTICS.md (RADD CLD copy idiom, MST PIE masked-set, SWAP SD DA full exchange, indirect LDX I/LDA I loads, STZ ,X clears, JPL I indirect call).

Honest caveats

What is byte-proven: MCTAB[254B] @006074B = 102525B = GERDV; the GERDV entry bytes at 102525B in 025-S3IRPIT (135032B = JPL I 32) match the disassembly; the 24-word block (102525B-102554B, bounded by HIL14=102555B) has coherent descriptor staging, resident-worker calls and two STA ,B output stores, exactly matching GetErrorDevice (a device number and an RT description address).

Which overlay and why: GERDV=102525B is a SYMBOL-2-LIST symbol, and 025-S3IRPIT is the SYMBOL-2-LIST code overlay; it sits directly after MSG=102453B (the MON 32B worker) in the same overlay. The parallel 026-S3IMPIT overlay holds unrelated data at 102525B.

What is NOT proven: the semantic label of each B-frame word (the ErrorDevice / RTProgram assignment is inferred from the manual MAC example, not byte-isolated from this carve), and the JPL I link cells (102557/102561/102562) which lie past the carved window in the following HIL14 region and are not resolved here.

Correction to earlier work. The old folder's GOTAB[254B]=066246B -> F1734 chain was the wrong-overlay artefact described above; MCTAB[254B] is the real link, byte-proven, so the verdict is upgraded from misattributed to VERIFIED. The F1734 bytes are real code but are not on the MON 254B path.

Method: ../../../../../EXTRACTING-RESIDENT-CODE.md - master map: ../../MON-CALL-INDEX.md.