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MON 266B (octal) - ND500MagTape (500MT)

Magnetic-tape access on behalf of an ND-500 program. It is dispatched through the ND-500 System Monitor's numeric MON vector table in 030-S3SM5, in the same contiguous block of real file/device-operation handlers as its siblings 264B (read) and 265B (write).

Status: routing is byte-proven (ND-500 call via the S3SM5 0x60 vector table, slot 0x01CC -> handler file offset 0xA89D, inside the contiguous 260B-277B block of real handlers - not the ND-100 GOTAB). The worker region is real SINTRAN L bytes carved from 030-S3SM5.bin, but it is short (17 bytes to the next handler) and the decode is only partly coherent, so 0xA89D is not a proven instruction boundary - see Honest caveats. ND-100 addresses are octal; ND-500 offsets are hex byte offsets.

Dispatch path

flowchart TD
    A["ND-500 process<br/>MON 266B (magtape)"] --> B["MCHANDEL<br/>MCNO=266B (not 347B, not 500B-523B)"]
    B --> C["NORMMC<br/>forward to System Monitor"]
    C --> D["S3SM5: 0x60 vector table<br/>slot = 0x60 + 2*decimal(266B) = 0x01CC"]
    D --> E["handler stub @ file 0xA89D<br/>ND500MagTape / 500MT (17 bytes)"]
    E -.branch (target outside slice).-> F["shared magtape body<br/>(uncarved in this slice)"]
    X["ND-100 GOTAB[266]<br/>= F1741 = 066266B (SYMBOL-2 stub, data)"] -.not this path.-> B
    class A blue
    class B,C,D teal
    class E,F green
    class X blue
    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 0x60 vector value 0xA89D is the handler file offset directly. The short region and its two branch opcodes give it the shape of a dispatch stub into a shared magtape body (dashed E -> F, target outside the 17-byte slice). The X -> B hop notes that the ND-100 GOTAB[266] slot holds a F1741 descriptor stub (first word 077777, a data/descriptor cell), not this handler.

Code location (dispatch path)

Rows are in execution order. ND-500 rows in 030-S3SM5 are byte-addressed (byte offset is direct, no x2). The ND-100 GOTAB row byte offset = (addr - GOTAB base 071233B) octal words x 2.

Role Segment (full disasm) Addr / file offset Byte offset (dec) Symbol Verdict
GOTAB[266] read (does NOT apply) commoncode.asm · .hex 066266B = F1741 59042 F1741 (SYMBOL-2, descriptor stub; first word 077777) inferred data - ND-100 dispatch descriptor, not the handler
S3SM5 0x60 vector slot 266B 030-S3SM5.asm · .hex file off 0x01CC 460 value 0xA89D (handler offset) VERIFIED - non-zero handler word in the 260B-277B block
Handler stub (ND500MagTape / 500MT) 030-S3SM5.asm · .hex file off 0xA89D..0xA8AE 43165..43181 (17 bytes) (no L07 symbol in the carved window) real bytes; short; decode only partly coherent

Verify by hand: grep '^460 ' ../../segments-ref/030-S3SM5/030-S3SM5.hex -> 460 250 (octal 250 = 0xA8); then the vector slot dd if=../../../segments/030-S3SM5.bin bs=1 skip=460 count=2 | od -An -tx1 -> a8 9d. The handler: grep '^43165 ' ../../segments-ref/030-S3SM5/030-S3SM5.hex -> 43165 071 (octal 071 = 0x39); then dd if=../../../segments/030-S3SM5.bin bs=1 skip=43165 count=8 | od -An -tx1 -> 39 aa 03 6b f1 82 48 6a (matches the .ASM first words).

Instruction walkthrough

Full listing: 266B-ND500MagTape.ASM. The whole region, by file offset into 030-S3SM5.bin:

0xA89D  39 AA        f2 comp r.0xA8          ; entry (UNVERIFIED boundary)
0xA89F  03           noop
0xA8A0  6B F1 82     d4 -   DESC(r2) r.0x8    ; subtract (selector check?)
0xA8A3  48 6A        by stz b.0xA8
0xA8A5  D9 2C 40     if << go $0x2C40         ; conditional branch (target outside slice)
0xA8A8  C0 05        go $0x5                  ; unconditional branch (target outside slice)
0xA8AA  AA 05 BA     w3 mulad $0x5,r.0xE8     ; last full op inside the 17-byte window

Readable structure: a compare and a subtract followed by a conditional and an unconditional branch, both targeting past this short slice - the classic shape of a dispatch stub that checks a small selector then jumps into a larger shared magtape body. That is consistent with ND500MagTape behaviour, but the branch targets are not resolvable inside the 17-byte window and 0xA89D is not a proven instruction boundary, so this is INFERRED, not proven. The raw bytes are ground truth; the mnemonics are unreliable where marked.

Parameter / register contract

This is an ND-500 call; argument transport is the ND-500 MON message block, not ND-100 A/X/T registers. The manual (ND-860228-2 EN, section 2.14) lists the call name-only.

Field Dir Meaning Verdict
MON number in 266B routes via MCHANDEL -> NORMMC -> S3SM5 0x60 vector slot 0x01CC VERIFIED (bytes)
tape function selector in a compare/subtract near entry looks like a selector check; value not attributed UNVERIFIED
status out branch leaves this slice into an uncarved shared body; no status field attributed UNVERIFIED

Full YAML contract: 266B_ND500MagTape.yaml (marked STUB - name/short only).

Pseudo-code (for an emulator)

See 266B-ND500MagTape.pseudo.c - a pseudo-C model for emulator authors. Every modelled line gives the real ND-500 operation from the instruction-semantics reference: ../../instruction-semantics/ND500-INSTRUCTION-SEMANTICS.md (register model, addressing modes, branch conditions; note C=1 means no-borrow, i.e. inverted). Only the routing and the dispatch-stub shape (two branch opcodes leaving the slice) are relied upon; the selector values, argument block, and status/error contract are UNVERIFIED and are not modelled as behaviour.

Honest caveats

What is byte-proven: MON 266B is an ND-500 call, forwarded to the ND-500 System Monitor. In 030-S3SM5 the 0x60 vector table is indexed by octal MON number (file_byte = 0x60 + 2*decimal(N)), verified independently by the routine map (../../030-S3SM5-routine-map.md section 2). Slot 0x01CC (byte 460) reads 0xA89D, a non-zero handler offset in the contiguous 260B-277B block of real file/device-operation handlers. That block, and the carved bytes at 0xA89D, are real bytes on disk.

What is NOT proven: the exact entry alignment, the branch targets, and the body semantics. The region is only 17 bytes (to the next non-zero slot 270B=0xA8AE), the two branch opcodes point outside the slice, and 0xA89D is not a proven instruction boundary. So the disassembly is only partly coherent; the RAW BYTES are ground truth but the decoded op sequence and the "dispatch stub" reading are INFERRED. The selector values, argument block, and error/skip contract are entirely UNVERIFIED (the manual gives only the name ND500MagTape / 500MT; the call is absent from the available NPL source). Confirming the entry and the shared body needs an S3SM5 symbol map or a live trace; treat the routing as reliable and the exact entry/body as provisional.

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