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MON 73B (octal) - SetMaxBytes (SMAX)

Sets the maximum byte pointer of an opened file (the number of bytes minus one). The specified size is stored when the file is closed; a later attempt to read beyond it returns error 3 (end of file). The file must be opened for write, and the call is only relevant for sequential access.

Status: GOTAB dispatch head byte-proven as a real per-call stub (GOTAB[73B] = 121444B -> F1644 in 025-S3IRPIT); the SMAX worker body is real SINTRAN L bytes and calls the SMAXB set-max-bytes primitive; the exact MON 73 -> worker link crosses an uncarved kernel bridge (see Honest caveats). All addresses/values are octal.

Dispatch path

flowchart LR
    A["User program<br/>MON 73B"] --> B["ENT14 level-14<br/>T = MON number"]
    B --> C["GOTAB[73B] = 121444B<br/>(byte-proven)"]
    C --> D["F1644 entry stub<br/>025-S3IRPIT :121444B"]
    D -.uncarved CALLPROC.-> E["SMAX set-max-bytes worker<br/>006-S3FS :103706B"]
    E --> F["SMAXB primitive<br/>006-S3FS :72620B"]
    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.

Code location (dispatch path)

Every row is a real region you can open. Byte offset = (addr - loadbase) in octal words x 2 (equal to the decimal byteoff column of the segment .hex).

Role Segment (full disasm) Addr range (octal) Byte offset Symbol Verdict
GOTAB[73] dispatch word commoncode.asm - .hex 071326B (1 word) 58796 GOTAB+73 = 121444B VERIFIED
F1644 entry stub 025-S3IRPIT.asm - .hex 121444B-121470B (21w) 56904 F1644 VERIFIED
resident CALLPROC bridge - (uncarved) - - CALLPROC UNVERIFIED
SMAX worker body 006-S3FS.asm - .hex 103706B-103717B + tail 103731B-103734B 46988 SMAX real bytes; link MISATTRIBUTED
SMAXB set-max-bytes primitive 006-S3FS.asm - .hex 72620B (call target) 37664 SMAXB called by SMAX (link cell 104026) - VERIFIED

Verify by hand: grep '^103706 ' ../../segments-ref/006-S3FS/006-S3FS.hex -> byte offset 46988; then dd if=../../../segments/006-S3FS.bin bs=1 skip=46988 count=8 | od -An -tx1 -> 22 4d cc 65 cc 59 f0 06 (each pair is one 16-bit word, big-endian on disk = octal 021115 146145 146131 170006 = STD I 115 / RADD CLD SL DA / RADD CLD SB DD / SAB 6, the SMAX entry).

The GOTAB slot itself: dd if=../../../resident/SINTRAN-DATA_commoncode.bin bs=1 skip=58796 count=2 | od -An -tx1 -> a3 24 (= 121444B, the F1644 vector).

The stub bytes: grep '^121444 ' ../../segments-ref/025-S3IRPIT/025-S3IRPIT.hex -> byte offset 56904; then dd if=../../../segments/025-S3IRPIT.bin bs=1 skip=56904 count=2 | od -An -tx1 -> a8 06 (= 124006B = JMP 6, the F1644 entry).

Instruction walkthrough

Full listing: 73B-SetMaxBytes.ASM. Two regions:

Region A - F1644 entry stub (121444-121470, 025-S3IRPIT) is the level-14 entry vectored from GOTAB[73]. It runs an argument/access check (121452 SAT 1, SKP IF DA EQL ST, then 121464 SAT 2), building B/X/D from indirect literals before transferring on through the resident CALLPROC bridge (the JMP I 24 at 121463 and the forward JMPs to 121473B continue into the F1645 dispatch continuation). The stub does not name SMAX; the value 103706 occurs nowhere it dereferences.

Region B - SMAX worker (103706-103717 + shared tail 103731-103734, 006-S3FS) is the functional body. All calls to shared workers are indirect (JPL I / JMP I) through the pointer-word table at 104024-104032; those words are data (link cells), not code, and their resolved worker addresses are annotated in the .ASM.

  • Prologue + parameter copy (103706-103713) - 103706 STD I 115 stashes the caller's double-word parameter (through link cell 104025); 103711 SAB 6 builds the 6-word local frame B; 103712 JPL I 112 -> 003752 is the shared resident prologue worker; 103713 STT I 112 stores the caller's file-number (register T).
  • Set the max byte pointer (103714-103717) - 103714 JPL I 112 -> 072620 (SMAXB, the set-max-bytes primitive) performs the actual update, then 103715 JMP 16 -> 103733 funnels into the store-status tail. The alternate path 103716 MIN ,B 4 / 103717 JMP 12 -> 103731 unwinds and returns.
  • Store status + return (103731-103734) - 103733 STA ,B 2 writes the result word into the caller's status slot B+2; 103734 JMP -3 falls to 103731 SAA -6 (unwind the 6-word frame, matching SAB 6), then 103732 JMP I 76 -> 003776 returns through the resident return cell. This tail is shared with the sibling calls SETBY (MON 74B, 103720B) and SETBC (103735B), which use their own primitives (SBYTE=72622B, SBLOP=72072B, SBLSZ=72351B) in the same link-cell table; those sibling bodies are not part of MON 73B.

The JPL I 112 call to SMAXB (link cell 104026 = 072620) is the byte-level proof that SMAX is the SetMaxBytes worker: it is the entry that drives the file system's set-max-bytes primitive.

Parameter / register contract

Reg / field Dir Meaning Verdict
entry point in 103706B = SMAX worker entry (via F1644 stub) VERIFIED (bytes)
T in file number (from an earlier OpenFile); stored at 103713 STT I 112 VERIFIED (copy); meaning inferred (manual)
D (double) in maximum byte pointer (INTEGER4), saved first (STD I 115) VERIFIED (copy); layout inferred (manual)
local frame B internal SAB 6 = 6-word working frame VERIFIED (bytes)
B+2 out returned status word (STA ,B 2 at 103733) VERIFIED (bytes)
A out error number on the error return inferred (manual MAC example)

The user-visible T/D register convention lives in the caller-side MON 73 wrapper and the uncarved CALLPROC frame, so the precise user-register-to-field assignment is inferred from the manual, not byte-proven here.

Pseudo-code (for an emulator)

See 73B-SetMaxBytes.pseudo.c - a pseudo-C model of the handler for emulator authors. Control flow + the call to the SMAXB primitive are byte-verified; the parameter-field semantics and error-number meanings are inferred from the call structure and the manual. Every instruction is translated per the canonical ../../instruction-semantics/ND100-INSTRUCTION-SEMANTICS.md.

Honest caveats

What is byte-proven: GOTAB[73B] = 121444B (a real level-14 per-call vector, not a fall-through); the F1644 stub at 121444B in 025-S3IRPIT is real code (entry bytes 124006 044353 135035 135035 match the disassembly); the SMAX worker body at 103706B in 006-S3FS is real code (entry bytes 021115 146145 146131 170006 match); and it belongs to the set-size family - it calls SMAXB (072620B, link cell 104026), the set-max-bytes primitive.

What is NOT proven: the link from the F1644 stub (in 025-S3IRPIT) to the SMAX worker (in 006-S3FS). The value 103706 occurs nowhere the stub dereferences; the stub transfers through the resident CALLPROC/segment switch, which lives in an uncarved overlay. So the MON 73 -> SMAX attribution rests on the SMAX symbol name + its call to SMAXB + the matching set-size behaviour, not a followed pointer - hence MISATTRIBUTED in the strict sense. Confirming the link needs a live trace: break at 121444B on a real MON 73, single-step the segment switch, and confirm P lands on SMAX = 103706.

Shared code: SMAX (103706B), SETBY (MON 74B, 103720B) and SETBC (103735B) share one link-cell table (104024-104032) and one error/return tail (103731-103734). Only the SMAX entry body (103706-103717) and the shared tail are MON 73B; the sibling entry bodies are documented in their own folders (e.g. ../074B-SetStartByte/).

The link cells 003752 (resident prologue) and 003776 (resident return) match no FILSYS-SYMBOLS entry; their low addresses suggest resident-monitor / save-restore routines outside the file-system segment and are not resolved here.

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