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MON 077B (octal) - SetStartBlock (SETBC)

Sets the next block to be read or written in an already-opened file (block-pointer positioning). The first block of a file is number 0; the standard block size is 512 bytes. The call is equivalent to SetStartByte with the block number multiplied by the block size. This is an ND-100 (and ND-500) monitor call.

Status: byte-verified (dispatch + worker body). The worker is real carved code in 006-S3FS. All addresses/values are octal.

CORRECTED 2026-07-13. The previous version of this folder said the call was misattributed, claiming GOTAB[077B] = 121516B routed to an F1646 "compiler stub" in 025-S3IRPIT, and it named the worker SETBL = 030164B. Both were artefacts of the wrong dispatch model. MON calls take their worker from MCTAB @ 005620B (segment 044-S3IDPIT), not from that GOTAB (which is MFELL for 224 of 256 calls). MCTAB[077B] = 103735B = SETBC (byte-proven), and that worker is carved in 006-S3FS, part of the SETBC / SBSIZ / RMAX file-parameter family. The old SETBL = 030164B name and the F1646 stub came from the disproven "uncarved bridge" story. See ../317B-ExecuteCommand/README.md and SINTRAN/CARVING-HANDOFF.md section 3a.

Dispatch path

flowchart LR
    A["User program<br/>MON 077B"] --> B["ENT14 level-14 entry<br/>026-S3IMPIT :072167B"]
    B --> C["GOTAB[077B] = MFELL<br/>MGOTA=071233B :071332B"]
    C --> D["MFELL level switch<br/>026-S3IMPIT :072114B -> CALLP 032201B"]
    D --> E["MCTAB[077B] = SETBC<br/>MCTAB=005620B :005717B = 103735B"]
    E --> F["SETBC worker<br/>006-S3FS :103735B"]
    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[077B] = MFELL (byte-proven, 74 4c); MFELL is a program-level switch, not a subroutine call. The monitor level then indexes MCTAB and reaches SETBC.

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[077B] slot 026-S3IMPIT.asm 071332B = 072114B 32180 -> MFELL VERIFIED
MCTAB[077B] slot 044-S3IDPIT.asm 005717B = 103735B 1950 -> SETBC VERIFIED
SETBC worker body 006-S3FS.asm 103735B-103751B 47034 SETBC VERIFIED

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

dd if=026-S3IMPIT.bin bs=1 skip=32180 count=2 | od -An -tx1   ->  74 4c   (= 072114B = MFELL)
dd if=044-S3IDPIT.bin bs=1 skip=1950  count=2 | od -An -tx1   ->  87 dd   (= 103735B = SETBC)
dd if=006-S3FS.bin    bs=1 skip=47034 count=2 | od -An -tx1   ->  22 36   (= 021066B, SETBC entry: STD I 66)

Instruction walkthrough

Full listing: 77B-SetStartBlock.ASM.

Prologue (103735B-103741B). STD I 66 / RADD CLD SL DA / RADD CLD SB DD / SAB 6 save the caller's registers and build the frame; JPL I 63 (-> 104024B) calls the shared FILSYS open-file lookup helper, and STT I 63 stores the resolved control-block pointer.

Set the block pointer (103743B-103751B). JPL I 66 (-> 104031B) is the helper that writes the new position into the open-file control block; on the success path JMP 4 (-> 103750B) stores the result (STA ,B 2) and returns, while the error path (103745B-103747B, MIN ,B 4 / SAA -6 / JMP I 61 -> 104030B) reports a file error.

Sibling family. SETBC (103735B) is the first of three near-identical entries - SBSIZ (103752B) and RMAX (103767B) - that share the same STD I / SAB 6 / JPL I skeleton and set/read different open-file parameters. That family structure is what identifies SETBC as the block-pointer setter.

The prologue, the two helper calls and the error/success fork are byte-proven; the precise semantics of the 104024B / 104031B helpers are inferred from the family structure.

Parameter / register contract

Reg / field Dir Meaning Verdict
file number in selects the open-file control block (resolved via JPL I 63) VERIFIED (bytes); meaning inferred (manual)
block number in new next-block position written to the control block VERIFIED (structure); byte layout inferred
,B 2 out result / status stored on the success path VERIFIED (bytes)
,B 4 out error counter (MIN ,B 4) on the failure path VERIFIED (bytes)
return out via saved link; error path 104030B VERIFIED (bytes)

Pseudo-code (for an emulator)

See 77B-SetStartBlock.pseudo.c - the prologue, the open-file lookup, the set-position helper call and the error/success fork are byte-verified; the helper internals are inferred from the SETBC/SBSIZ/RMAX family.

Honest caveats

What is byte-proven: GOTAB[077B] = MFELL; MCTAB[077B] = 103735B = SETBC; the SETBC entry bytes at 103735B in 006-S3FS match the disassembly (021066B = STD I 66); the worker resolves an open file, calls a helper to write a new position, and forks on error/success. It is byte-adjacent to its siblings SBSIZ (103752B) and RMAX (103767B).

What is NOT proven: the exact control-block field written (block vs byte units), and the internals of the 104024B / 104031B helpers, which are reached by JPL I and not carved in this folder.

Correction to earlier work. The old folder read the disproven GOTAB, reported GOTAB[077B] = 121516B -> F1646 and named the worker SETBL = 030164B. The byte-proven worker is MCTAB[077B] = SETBC = 103735B. The F1646/SETBL reading was the wrong-overlay error.

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