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MON 13B (octal) - ClearInBuffer (CIBUF)

Clears a device's input (ring) buffer: character input from terminals and other character devices is queued in a per-device input ring, and MON 13B discards whatever is currently queued for the addressed logical device. It shares one code body with MON 14B ClearOutBuffer (COBUF); the two enter one word apart (CIBUF=044120B, COBUF=044125B) and the body selects the input vs output side by which entry it is jumped to.

Status: dispatch head byte-proven (GOTAB[13B]=120454B); worker body is real SINTRAN L bytes; the MON 13B -> worker link is NOT byte-followable (the dispatch target is data, not a stub - see Honest caveats). All addresses/values are octal.

Dispatch path

flowchart LR
    A["User program<br/>MON 13B"] --> B["ENT14 level-14<br/>T = MON number"]
    B --> C["GOTAB[13B] = 120454B<br/>(byte + live proven)"]
    C --> D["120454B target<br/>025-S3IRPIT : DATA, not a stub"]
    D -.uncarved CALLPROC.-> E["CIBUF clear worker<br/>commoncode :044120B"]
    E --> F["per-buffer reset<br/>via JPL I -> 044222B"]
    class A blue
    class B,C,D teal
    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 dashed hop (D ⇢ E) is the resident CALLPROC/segment-switch - it is not present in any carved segment, so it is the one link that cannot be followed statically. For MON 13B this is worse than a normal fall-through: the dispatch target 120454B decodes as data (FDV/STA/STZ), not a jump stub, so there is not even a pointer to dereference.

Code location (dispatch path)

Every row is a real region you can open. Byte offset = (addr - loadbase) in octal words x 2.

Role Segment (full disasm) Addr range (octal) Byte offset Symbol Verdict
GOTAB[13] dispatch word commoncode.asm · .hex 071246B (1 word) 58700 GOTAB+13 = 120454B VERIFIED
Dispatch target (data) 025-S3IRPIT.asm · .hex 120454B-120461B 55896 F1614 (SYMBOL-2, cross-revision) MISATTRIBUTED (data, not a stub)
resident CALLPROC bridge — (uncarved) — — CALLPROC UNVERIFIED
CIBUF clear worker body commoncode.asm · .hex 044113B-044156B (entry CIBUF=044120B) 37014 CIBUF (SYMBOL-1) real bytes; link MISATTRIBUTED

Verify by hand (worker entry): grep '^44113 ' ../../segments-ref/SINTRAN-DATA_commoncode/SINTRAN-DATA_commoncode.hex -> byte offset 37014; then dd if=../../../resident/SINTRAN-DATA_commoncode.bin bs=1 skip=37014 count=8 | od -An -tx1 -> 59 80 d1 04 2c 00 d1 08 (= octal 054600 150404 026000 150410 …, the LDX ,B -200 / POF / LDD ,X 0 / PON prologue).

Verify by hand (GOTAB target is data): grep '^120454' ../../segments-ref/025-S3IRPIT/025-S3IRPIT.hex -> byte offset 55896; then dd if=../../../segments/025-S3IRPIT.bin bs=1 skip=55896 count=4 | od -An -tx1 -> 9c 1e 08 07 (= octal 116036 004007 = FDV ,X 36 / STA 7: data, no jump).

Ground truth for the GOTAB word: python3 scripts/prove-mon.py 013.

Instruction walkthrough

Full listing: 013B-ClearInBuffer.ASM. The functional body is the CIBUF/COBUF shared clear worker (Region B); Region A is the GOTAB[13] dispatch target, shown to be data rather than a followable stub.

Prologue (044113-044117) - stage one device/buffer table entry for an atomic test. LDX ,B -200 sets X to the table base at (B-200); the POF / LDD ,X 0 / PON bracket is the standard SINTRAN idiom for reading a device datafield atomically against interrupt-level updates; SAT -1 seeds T with the scan sentinel / mask.

Input-side entry + test (044120-044124) - CIBUF enters here. SKP IF DA UEQ ST breaks out to the tail (JMP 31 -> 044152) when the scanned entry equals the sentinel; otherwise BSKP ONE 160 DA tests a status bit and routes to the input clear arm or the second (output-style) arm at 044137.

Output-side entry + first clear arm (044125-044136) - COBUF (MON 14B) enters at 044125 AND 73. The arm masks the datafield, compares magnitude (SKP IF DA MLST ST), updates a counter at (B-54), tests the shift-carry condition, and on match calls the shared per-buffer reset routine via indirect JPL I 65 -> 044222B, then rejoins the loop.

Second arm (044137-044147) - structurally identical, updating the counter at (B-16) and calling the same reset routine via indirect JPL I 53 -> 044222B.

Loop advance + tail (044150-044156) - AAX 2 advances the table cursor by one 2-word entry and JMP -35 -> 044114 re-reads atomically; the sentinel match drops into the tail at 044152, which loads result/status words for the exit path.

Both JPL I calls resolve to 044222B (outside this window - an indirect target, so the carve is still control-flow closed). The exact skip-return to the MON caller is part of the CALLPROC epilogue and is not contained in this window.

Parameter / register contract

Reg / field Dir Meaning Verdict
T in logical device number (LDT DEVNO; MON 13) VERIFIED (manual MAC example)
A out error / standard error code on error return VERIFIED (manual: "Error number in register A")
return convention out skip return: MON 13 then JMP ERROR (error); fall-through = normal VERIFIED (manual MAC example)
entry point in 044120B = input ring (CIBUF); 044125B = output ring (COBUF, MON 14B) VERIFIED (bytes)
datafield table base internal (B-200) -> X, entry X+0 double word, atomic read VERIFIED (bytes 044113-044116)
counters updated internal (B-54), (B-16) during clear VERIFIED (bytes 044131/044143)
shared reset routine internal 044222B via indirect JPL I (target not carved) VERIFIED (bytes; target off-window)
CALLPROC call-in (how T=device -> (B-200) table base) in register handoff not traced inferred
that MON 13B enters at CIBUF=044120B — dispatch not byte-followable (GOTAB[13]=120454B != 044120B) UNVERIFIED

Input vs output side is selected by which entry the dispatcher jumps to, not by a parameter - VERIFIED from the two labelled entries; the dispatch selection itself is UNVERIFIED.

Pseudo-code (for an emulator)

See 013B-ClearInBuffer.pseudo.c - a pseudo-C model of the handler for emulator authors. Control flow, the atomic datafield read, and the two clear arms are byte-verified; the status-bit meanings and the reset routine at 044222B are inferred from the call structure.

Every instruction in the .pseudo.c is translated against the canonical ND100-INSTRUCTION-SEMANTICS.md (bare LDx/LDT/AND disp = P-relative mem[P+disp], not literals; SKP/BSKP skip polarity; RADD CLD SD DA = A = D; T/X transfers = physical EL).

Honest caveats

What is byte-proven: GOTAB[13B] = 120454B (level-14 dispatch word in commoncode, matches a live read of the running system); the carved worker at 044113B..044156B is an exact slice of commoncode.bin (0 mismatches) and its entry bytes match the disassembly; 044120B is CIBUF and 044125B is COBUF in SYMBOL-1-LIST, and the code there is a genuine ND-100 ring-buffer clear routine (atomic POF/LDD/PON read, BSKP ONE bit tests, indirect JPL I reset calls); the manual-side contract (T=device, A=error, skip return) is solid.

What is NOT proven: the link from the dispatch head to the CIBUF body. GOTAB[13]=120454B does not equal CIBUF=044120B, and 120454B is not a followable stub - in commoncode.bin it reads 000000 (inside a runtime-populated zero run 103031B..170177B), and in the 025-S3IRPIT overlay it reads FDV/STA/STZ data (the SYMBOL-2-LIST label F1614 there is from a different NPL revision than the overlay bytes, so it is misattributed). There is no direct jump and no indirect pointer word at 120454B to dereference; the stub->worker transfer is the resident CALLPROC/segment switch, which lives in an uncarved overlay. So the MON 13B -> CIBUF attribution rests on the symbol name + the ring-buffer behaviour, not a followed pointer. (This reconciles the two prior notes: the earlier "GOTAB[13B]=MFELL fall-through" story is withdrawn - there is no MFELL symbol in the L07 lists, and GOTAB[13] is the concrete address 120454B.) Confirming the chain needs a live trace: break at the level-14 handler on a real MON 13, single-step the resident CALLPROC second-level dispatch, and confirm P lands on CIBUF=044120B; also capture the T(device) -> (B-200) register handoff.

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