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MON 250B (octal) - GetDefaultDir (FDFDI)

Gets the user's default directory: the directory index and the user index are returned for the named user (the name may identify a remote user). All users, all programs, ND-100 and ND-500.

Status: GOTAB dispatch head byte-proven (GOTAB[250B] = 066242B), pointing at the 4-word entry stub F1732 in 025-S3IRPIT; the FDFDI worker body is real SINTRAN L bytes in the file-system segment 006-S3FS (a FILSYS-SYMBOLS symbol). The worker is real executable code with a two-entry SSK idiom (FDFDI/FDINA), a name-parse, a named-vs-current-user branch and the return of the directory/user indexes (it closes at 107057B, bounded by the next symbol WDIEN=107106B). The stub sits inside a shared stub block and does not itself reach FDFDI; the exact MON 250 -> worker link crosses an uncarved kernel bridge (see Honest caveats). All addresses/values are octal.

Dispatch path

flowchart LR
    A["User program<br/>MON 250B"] --> B["ENT14 level-14<br/>T = MON number"]
    B --> C["GOTAB[250B] = 066242B<br/>(byte-proven)"]
    C --> D["F1732 entry stub<br/>025-S3IRPIT :066242B"]
    D -.uncarved CALLPROC.-> E["FDFDI find-default-dir worker<br/>006-S3FS :106732B"]
    E --> F["parse user name; find default<br/>directory; return dir + user index"]
    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. The stub F1732 is a tiny 4-word compiler entry inside a shared stub block (the following F1734=066246B is the MON 254B GetErrorDevice stub); it is not a self-contained named handler and the worker address 106732 does not occur anywhere inside 025-S3IRPIT.

Code location (dispatch path)

Every row is a real region you can open. Byte offset = (addr - loadbase) in octal words x 2; for 025-S3IRPIT (load base 32000B) it is (addr - 32000B) x 2, for 006-S3FS (load base 26000B) it is (addr - 26000B) x 2.

Role Segment (full disasm) Addr range (octal) Byte offset Symbol Verdict
GOTAB[250] dispatch word commoncode.asm - .hex 071503B (1 word) 59014 GOTAB+250 = 066242B VERIFIED
F1732 entry stub 025-S3IRPIT.asm - .hex 066242B-066245B (4 words) 28996 F1732 VERIFIED (GOTAB target); shared stub block
resident CALLPROC bridge - (uncarved) - - CALLPROC UNVERIFIED
FDFDI worker body 006-S3FS.asm - .hex 106732B-107057B (code) + 107060B (pad) + 107061B-107105B (link cells) 50100 FDFDI (SSK=1) / FDINA=106734B (SSK=0) real bytes = CODE; body link MISATTRIBUTED

The worker window is bounded strictly by the next symbol WDIEN=107106B (108 words). Words 106732B-107057B are code, 107060B is a ROP NOOP pad, and 107061B-107105B are the JPL I/JMP I link-cell table (data).

Verify by hand: the GOTAB word: grep '^71503 ' ../../segments-ref/SINTRAN-DATA_commoncode/SINTRAN-DATA_commoncode.hex -> 71503 066242 154 242 59014; then dd if=../../../resident/SINTRAN-DATA_commoncode.bin bs=1 skip=59014 count=2 2>/dev/null | od -An -tx1 -> 6c a2 (word = 066242B). The stub head: dd if=../../../segments/025-S3IRPIT.bin bs=1 skip=28996 count=2 2>/dev/null | od -An -tx1 -> 54 fe (= octal 052376, LDT ,X -2, matching the disassembly). The FDFDI worker entry: grep '^106732 ' ../../segments-ref/006-S3FS/006-S3FS.hex -> byte offset 50100; then dd if=../../../segments/006-S3FS.bin bs=1 skip=50100 count=2 2>/dev/null | od -An -tx1 -> f8 90 (= octal 174220, BSET ONE SSK, the FDFDI entry). prove-mon.py 250 reads the same GOTAB value.

Instruction walkthrough

Full listing: 250B-GetDefaultDir.ASM. Two regions.

Region A - F1732 stub (066242-066245) is the 4-word GOTAB target in 025-S3IRPIT. It sits inside a shared stub block (the following F1734 is the MON 254B stub) and its head (LDT ,X -2 / SKP IF DA GRE ST / JMP 5 / LDA ,B 22) is not a self-contained per-call handler; the real transfer to the worker is the resident CALLPROC, which a static decode cannot follow.

Region B - FDFDI worker (106732-107105) is the functional body. 106732 BSET ONE SSK is the FDFDI entry; the sibling 106734 BSET ZRO SSK is FDINA; both join at 106735 STD I 123. 106741 JPL I 120 -> [107061] is the prologue. 106742-106747 set a decode-name flag from SSK; 106755 JPL I 106 -> [107063] parses the user name. 106764-106765 JAF branches to the named-user path (106766-107005) or the current-user path (107006-107025). The directory index and user index are stored with STT ,B 1 / STA ,B 2 (107034-107035); the tails converge on 107037 SAA -75 / 107040 JMP I 44 -> [107104].

Parameter / register contract

Manual-side names/types are from 250B_GetDefaultDir.yaml.

Reg / field Dir Meaning Verdict
X (UserName) in address of a string holding the user name (16 chars; may identify a remote user) (MAC LDX (USER) inferred (manual)
T (DirectoryIndex) out directory index (MAC STT DIRIX) inferred (manual)
X (UserIndex) out user index in the default directory (MAC STX USRIX) inferred (manual)
B+74 internal decode-name flag set from the SSK entry (STA ,B 74 / STZ ,B 74, LDA ,B 74 / JAF) VERIFIED (bytes); meaning inferred
error return out standard error code in A (107037 SAA -75) VERIFIED (bytes); code value inferred

Pseudo-code (for an emulator)

See 250B-GetDefaultDir.pseudo.c - a pseudo-C model for emulator authors. The control flow (the SSK two-entry idiom, the name-parse, the named-vs-current-user branch and the index returns) is byte-verified; the register/field semantics are inferred from the manual and the code shape.

Every instruction in the pseudo-code is translated against the canonical ND-100 instruction semantics reference (BSET ZRO/ONE SSK skip-flag set/clear, RADD CLD copy idiom, RADD SB DA/DX register add, JAF flag branch, SKP IF DA EQL SX compare, STT/STX ,B frame stores, MIN ,B increment and skip, JPL I/JMP I indirect call/return).

Honest caveats

What is byte-proven: GOTAB[250B] = 066242B (prove-mon.py 250 reads commoncode file byte 0xe686 = 6c a2); that value read as a 025-S3IRPIT address is the F1732 stub, whose bytes decode cleanly (052376B = LDT ,X -2). The FDFDI worker at 106732B in 006-S3FS is real code (first word 174220B = BSET ONE SSK) and it is a find-default-directory routine (name-parse, named-vs-current-user branch, dir + user index return), consistent with GetDefaultDir.

What is NOT proven: the F1732 stub is only 4 words and lands inside a shared stub block, not a dedicated per-call handler; it branches within its own head and the actual transfer to FDFDI is the resident CALLPROC in an uncarved overlay. So the MON 250 -> FDFDI attribution rests on the FDFDI symbol name (Find DeFault DIrectory) + the matching behaviour, not a followed pointer - hence MISATTRIBUTED in the strict sense. The worker's JPL I/JMP I link cells (107061..107105) are a pointer table whose runtime targets are not resolved here. Confirming the dispatch link needs a live trace: issue a real MON 250, single-step through the stub and the resident CALLPROC, and confirm P lands on FDFDI = 106732.

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