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MON 1B (octal) - InByte (INBT)

Reads one byte from a character device (a terminal, an opened file, or a word-oriented device where one word is read). The program waits if the device input buffer is empty (unless a no-wait flag is set); for a mass-storage file the next-byte pointer is incremented. Background programs may read logical device 0 (the SINTRAN III command buffer). This is an ND-100 monitor call.

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

CORRECTED 2026-07-13. The previous version of this folder said the call was misattributed (stub-routed), claiming GOTAB[1B] = 120303B routed to an F1607 compiler stub and naming the worker INBT = 032471B in resident commoncode. Both were artefacts of the wrong dispatch model. The 120303B was read out of SINTRAN-DATA_commoncode's fake "GOTAB" - not the real table. Byte-proven: GOTAB[1B] = 071633B = M1, a resident level-14 fast handler (this is one of the 32 GOTAB slots that are NOT MFELL), and MCTAB[1B] = 026576B = YFGET, the filesystem byte-input worker, carved in 006-S3FS. See ../317B-ExecuteCommand/README.md and SINTRAN/CARVING-HANDOFF.md section 3a.

Dispatch path

MON 1B is a fast-path call: its GOTAB slot is a resident level-14 handler (M1), not MFELL. The filesystem byte-input worker YFGET is named by MCTAB[1B].

flowchart LR
    A["User program<br/>MON 1B"] --> B["ENT14 level-14 entry<br/>026-S3IMPIT :072167B"]
    B --> C["GOTAB[1B] = M1<br/>MGOTA=071233B :071234B = 071633B"]
    C --> M["M1 resident level-14<br/>fast byte-input handler<br/>026-S3IMPIT :071633B"]
    B -. "file-byte path (MCTAB)" .-> E["MCTAB[1B] = YFGET<br/>MCTAB=005620B :005621B = 026576B"]
    E --> F["YFGET worker<br/>006-S3FS :026576B"]
    class A blue
    class B,C,M,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

Unlike the 224 MFELL calls, GOTAB[1B] jumps straight to the resident level-14 handler M1 (byte-proven: slot value 071633B, exactly the M1 symbol the skill cites as GOTAB slot 1). MCTAB[1B] = YFGET is the filesystem byte-input worker. The exact runtime hand-off between the M1 fast path and the YFGET file-byte worker is INFERRED (dashed hop) - not byte-proven here.

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
GOTAB[1B] slot 026-S3IMPIT.asm 071234B = 071633B 32056 -> M1 (fast handler) VERIFIED
MCTAB[1B] slot 044-S3IDPIT.asm 005621B = 026576B 1826 -> YFGET VERIFIED
YFGET worker body 006-S3FS.asm 026576B-026640B 764 YFGET VERIFIED

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

dd if=026-S3IMPIT.bin bs=1 skip=32056 count=2 | od -An -tx1   ->  73 9b   (= 071633B = M1, NOT MFELL)
dd if=044-S3IDPIT.bin bs=1 skip=1826  count=2 | od -An -tx1   ->  2d 7e   (= 026576B = YFGET)
dd if=006-S3FS.bin    bs=1 skip=764   count=2 | od -An -tx1   ->  50 26   (= 050046B, YFGET entry: LDT 46)

Instruction walkthrough

Full listing: 1B-InByte.ASM.

Two entries, one body. YFGET (026576B, MON 1B input) and YFPUT (026600B, MON 2B output) are a paired get/put primitive that share a single body from 026601B: - YFGET: LDT 46 then JMP 2 (-> 026601B) - loads a get selector into T. - YFPUT: LDT 45 then falls through to 026601B - loads a put selector into T.

Shared body (026601B onward). STT ,X 30 stores the get/put selector into the open-file control block at ,X 30, then JPL I 44 (-> 026646B) calls the byte-transfer helper. The code sets a status flag (SAA 1 / STA ,B 21), clears a byte counter (STZ ,B 27), reloads a control-block pointer (LDT I 40) and tests it (SKP IF DT UEQ 0), forking between an in-buffer fast return and a refill/error path (JPL I 35 -> 026652B, error codes 132B/133B via SAA 132/SAA 133, branching back to the shared tail at 026555B).

The two-entry structure, the selector store and the transfer-helper call are byte-proven; the detailed buffer-refill logic in the helper is inferred from the get/put pairing.

Parameter / register contract

Reg / field Dir Meaning Verdict
device / file number in selects the input source (terminal, opened file, device) inferred (manual)
T selector internal 46B for get (YFGET); stored into control block ,X 30 VERIFIED (bytes)
byte result out one byte returned (via the transfer helper) VERIFIED (structure); register inferred
,B 21 frame status flag set to 1 VERIFIED (bytes)
,B 27 frame byte counter cleared VERIFIED (bytes)
wait behaviour in/out waits if input buffer empty unless no-wait flag set inferred (manual)

Pseudo-code (for an emulator)

See 1B-InByte.pseudo.c - the two-entry get/put selector, the control- block store and the transfer-helper call are byte-verified; the buffer-refill logic and the M1 fast-path hand-off are inferred.

Honest caveats

What is byte-proven: GOTAB[1B] = 071633B = M1 (a resident level-14 fast handler, NOT MFELL - MON 1B/2B are among the 32 fast GOTAB slots); MCTAB[1B] = 026576B = YFGET; the YFGET entry bytes at 026576B in 006-S3FS match the disassembly (050046B = LDT 46); YFGET/YFPUT are a shared-body get/put pair distinguished by the T selector (46B/45B).

What is NOT proven: the exact runtime relationship between the M1 level-14 fast path and the YFGET file-byte worker (whether M1 tail-calls YFGET for the opened-file case, or the monitor level reaches YFGET via MCTAB independently) - this is the dashed hop and is INFERRED. The buffer-refill logic in the 026646B/026652B helpers is not carved here, and the wait/no-wait and logical-device-0 behaviours come from the manual.

Correction to earlier work. The old folder read GOTAB out of SINTRAN-DATA_commoncode (the fake table) and reported GOTAB[1B] = 120303B -> F1607 with the worker INBT = 032471B. The real GOTAB[1B] = M1 = 071633B and the real worker is MCTAB[1B] = YFGET = 026576B. The 120303B/F1607/INBT reading was the wrong-overlay error.

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