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MON 122B (octal) - RESRV (ReserveResource)

Reserve a device or file for the calling program only. Some devices have a separate input part and output part; each MON 122 reserves one part. Released by MON 123 (ReleaseResource) or automatically on normal RT-program termination. ND-100 monitor call, manual section for RESRV.

Status: byte-verified (dispatch + worker body). The worker is real carved code in 003-S3CP, next to its sibling RELES (MON 123B). All addresses/values are octal.

CORRECTED 2026-07-13. The previous version of this folder said the worker was misattributed / not-carved, "zero-filled" at 037103B. That was an artefact of the wrong dispatch model. MON calls are not dispatched through the level-14 GOTAB (it is MFELL for 224 of 256 calls); they go through MCTAB @ 005620B (segment 044-S3IDPIT). MCTAB[122B] = 037103B = RESRV, and that worker IS carved - in 003-S3CP. The old analysis read 037103B in SINTRAN-DATA_commoncode (zeros there - wrong overlay) and wrongly concluded "absent". See ../317B-ExecuteCommand/README.md and SINTRAN/CARVING-HANDOFF.md section 3a.

REFINEMENT 2026-07-14 (see ../../kernel-carving/PRSRV-124B/README.md). The overlay/bytes here are CORRECT: 003-S3CP @ 037103B is byte-identical to the resident System Monitor 071-S3SM. Two interpretation notes: (1) the 037110B-037155B command-builder block is really the shared executor EXECC 037110B (the entry stub RESRV is the 5 words 037103-037107 in front of it); (2) the two MON 70B dispatches resolve to MCTAB[070]=050673B=COMSB, the @-command interpreter. Its force-sibling PRSRV (124B) is a DIFFERENT shape - a 2-word trampoline to LEAV2 027417B, not this command builder - so do not assume RESRV and PRSRV share a body.

Dispatch path

flowchart LR
    A["User program<br/>MON 122B"] --> B["ENT14 level-14<br/>026-S3IMPIT :072167B"]
    B --> C["GOTAB[122B] = MFELL<br/>:071355B"]
    C --> D["MFELL level switch<br/>:072114B -> CALLP 032201B"]
    D --> E["MCTAB[122B] = RESRV<br/>044-S3IDPIT :005742B = 037103B"]
    E --> F["RESRV worker<br/>003-S3CP :037103B"]
    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

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[122B] slot 044-S3IDPIT.asm 005742B = 037103B 1988 -> RESRV VERIFIED
RESRV worker body 003-S3CP.asm 037103B-037155B 7302 RESRV VERIFIED

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

dd if=044-S3IDPIT.bin bs=1 skip=1988 count=2 | od -An -tx1   ->  3e 43   (= 037103B = RESRV)
dd if=003-S3CP.bin    bs=1 skip=7302 count=2 | od -An -tx1   ->  3d fe   (= 036776B, RESRV entry)

Instruction walkthrough

Full listing: 122B-ReserveResource.ASM.

Prologue (037103B-037117B). A short register/context preamble, then EXECC (037110B) loads a flag word and takes an early EXIT (037112B) if it is clear. Otherwise it saves the link (T := L), and stages a command-string pointer into D.

Command dispatch (037120B-037124B). Calls a helper (JPL I 45) that builds a SINTRAN command line, then issues MON 70B (CallCommand) to run a RESERVE command through the command processor.

Byte-scan loop (037125B-037155B). Walks the string byte-by-byte (LBYT), comparing each byte to delimiter 47B; for each delimited field it re-issues MON 70B. The loop ends at 037156B, which is the RELES boundary. The exact per-field semantics are inferred; the two MON 70B dispatches and the loop control are byte-proven.

Parameter / register contract

Reg / field Dir Meaning Verdict
A in command/parameter-string pointer used to build the RESERVE command VERIFIED (bytes: LDA 46 / MON 70)
mem[053] flag in gate checked at EXECC; clear -> early EXIT VERIFIED (bytes)
L in/out return link saved through T across the helper call VERIFIED (bytes)
DeviceNo / IOFlag / WaitFlag / Status in/out manual parameter list for the reservation itself inferred (manual)
error return out standard error code inferred (manual)

Pseudo-code (for an emulator)

See 122B-ReserveResource.pseudo.c - the EXECC gate, the two MON 70B (CallCommand) dispatches, and the byte-scan loop are byte-verified; the resource-table semantics are inferred from the manual.

Honest caveats

What is byte-proven: MCTAB[122B] = 037103B = RESRV; the RESRV entry bytes at 037103B in 003-S3CP match the disassembly; the routine is a thin wrapper that builds and runs a RESERVE command line via MON 70B inside a byte-scan loop, sharing infrastructure with its neighbour RELES (MON 123B).

What is NOT proven: the resource-table layout and the manual's DeviceNo/IOFlag/WaitFlag/Status parameter list - the actual locking happens inside the command processor MON 70B calls, not in these bytes. The old "zero-filled / not carved" verdict was simply the wrong overlay (commoncode instead of 003-S3CP) and is withdrawn.

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