Skip to content

MON 270B (octal) — ReadDiskPage (RDPAG)

Reads one or more raw directory pages (2048 bytes each) from a disk into a caller buffer; any page can be read. The directory must be reserved with ReserveDir. It shares one code body with MON 271B WriteDiskPage (WDPAG); the two enter one word apart and the body forks on a read/write skip flag (SSK).

Status: dispatch head byte-proven (GOTAB[270B] = 066276B, a real F1742 entry stub); worker body is real SINTRAN L bytes; the exact MON 270 → worker link crosses an uncarved kernel bridge (see Honest caveats). All addresses/values are octal.

Dispatch path

flowchart LR
    A["User program<br/>MON 270B"] --> B["ENT14 level-14<br/>T = MON number"]
    B --> C["GOTAB[270B] = 066276B<br/>(byte proven)"]
    C --> D["F1742 entry stub<br/>025-S3IRPIT :066276B"]
    D -.uncarved CALLPROC.-> E["RDPAG read worker<br/>006-S3FS :107447B"]
    E --> F["disk page transfer<br/>via JPL I workers"]
    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 × 2.

Role Segment (full disasm) Addr range (octal) Byte offset Symbol Verdict
GOTAB[270] dispatch word commoncode.asm · .hex 071523B (1 word) 59046 GOTAB+270 = 066276B VERIFIED
F1742 entry stub 025-S3IRPIT.asm · .hex 066276B–066301B 29052 F1742 VERIFIED
resident CALLPROC bridge — (uncarved) — — CALLPROC UNVERIFIED
RDPAG/WDPAG read/write worker body 006-S3FS.asm · .hex 107447B–110047B 50766 RDPAG (WDPAG=107451) real bytes; link MISATTRIBUTED

Verify by hand: grep '^107447 ' ../../segments-ref/006-S3FS/006-S3FS.hex → byte offset 50766; then dd if=../../../segments/006-S3FS.bin bs=1 skip=50766 count=4 | od -An -tx1 → f8 10 a8 02 (the stored words 174020 124002 = BSET ZRO SSK / JMP — the RDPAG read entry + skip-over). Confirm the dispatch word: dd if=../../../resident/SINTRAN-DATA_commoncode.bin bs=1 skip=59046 count=2 | od -An -tx1 → 6c be (the stored word 066276 = GOTAB[270]). prove-mon.py 270 reads the same.

Instruction walkthrough

Full listing: 270B-ReadDiskPage.ASM. Region A is the tiny F1742 level-14 stub (4 words: it primes ,B -1 / ,B 17 and clears ,B -2/,B -1 — a parameter-descriptor default, then dispatch continues resident); region B is the shared read/write worker (RDPAG read entry 107447B, WDPAG write entry 107451B).

Read/write entry split (107447–107452) — RDPAG enters at 107447 and clears SSK (read); WDPAG (MON 271B) enters one word later at 107451 and sets SSK (write); one common body follows. 

107447  174020  BSET ZRO SSK   ; RDPAG: skip flag = 0 (READ)
107450  124002  JMP  107452    ; jump over the write-set
107451  174220  BSET ONE SSK   ; WDPAG entry (MON 271B): skip flag = 1 (WRITE)
107452  021047  STD I 47       ; save D (double) -> param slot
Prologue (107453–107456) sets the frame base (107455 SAB 22) and calls resident register-save SPUSH via pointer @107522 = 003752B. Function-code + directory resolve (107457–107466) forks on SSK to load a device function code (107462 SAA 61 write / 107464 SAA 60 read) into ,B 20, then 107466 JPL I 35 (@107523 = 030225B) resolves/validates the directory index. Index + reservation checks (107467–107520) stage the buffer (107467 STX ,B 7), test the directory descriptor and its reserved flag, and on failure load an error code and take an indirect exit (107474 JMP I 31, 107502 JMP I 23, 107520 JMP I 5 → @107525). Page transfer loop (107531–110035) scales the page count to words (107536 SHA ZIN 12 = ×1024, 1 page = 2048 bytes = 1024 words), builds and issues the disk request via JPL I workers (107635/107676), advances the 32-bit disk page address by 1 per page (110026–110034), and loops while pages remain (110035 JMP → 107741). Pointer/data tables (107703–107724, 110036–110047) hold the JPL I targets (SPUSH 003752, directory helper 030225 among them).

Parameter / register contract

Manual-side names/types are from 270B_ReadDiskPage.yaml (MAC: LDT DIRIX / LDX (BUFF / LDA COUNT / COPY SA DD / LDA (PAGNO / MON 270).

Reg / field Dir Meaning Verdict
entry point in 107447B = read; 107451B = write (shared body, SSK split) VERIFIED (bytes)
SSK internal read (0) / write (1) selector, tested at 107460 VERIFIED (bytes)
T = DirIndex in directory index (see GetDirUserIndexes), resolved at 107466 inferred (manual)
X = Buffer in caller buffer (even byte address), staged at 107467 inferred (manual)
A = NoOfPages in page count → words via 107536 SHA ZIN 12 VERIFIED scale; meaning inferred
A = PageAddr in address of the 32-bit disk page address inferred (manual)
function code internal 061 (write) / 060 (read) into ,B 20 VERIFIED (bytes)
error return out standard error code on the fault paths inferred (manual)

This post-CALLPROC body works through ,B frame fields, so the precise T/A/X mapping is inferred from the manual and the caller-side MON 270 wrapper, not byte-proven here.

Pseudo-code (for an emulator)

See 270B-ReadDiskPage.pseudo.c — a pseudo-C model of the handler for emulator authors. Control flow, the read/write (SSK) fork and the page→word scale are byte-verified; the disk-driver worker semantics are inferred from the call structure. Every instruction is translated per the canonical ND-100 Instruction Semantics reference.

Honest caveats

What is byte-proven: GOTAB[270B] = 066276B (level-14 dispatch; prove-mon.py 270 reads the same word); the F1742 stub at 066276B is real code; the RDPAG worker entry at 107447B is real BSET ZRO SSK code with a SSK read/write split and a page→word scale (SHA ZIN 12) matching the manual's 2048-byte page.

What is NOT proven: the link from the F1742 stub (in 025-S3IRPIT) to the RDPAG worker (in 006-S3FS). The F1742 stub is only 4 words and holds no pointer to 107447; the stub→worker transfer is the resident CALLPROC/segment switch, in an uncarved overlay — hence MISATTRIBUTED in the strict sense. This reconciles into one story: the dispatch head and the worker are both genuine SINTRAN L bytes (the body's direct branches all close inside the carve, up to its pointer tables), but the attachment of MON 270 to RDPAG rests on the symbol name + matching read behaviour, not a followed pointer. Confirming it needs a live trace (break at 066276B on a real MON 270, single-step the segment switch, confirm P lands on RDPAG = 107447).

Method: ../../../../../EXTRACTING-RESIDENT-CODE.md §9 · dispatch reality: ../../TASK-05-mismatches.md §G · master map: ../../MON-CALL-INDEX.md.