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MON 405B (octal) — SwitchUserBreak (USTRK)

Switches the user-defined ESCAPE handling of an ND-500 program on or off. When on, pressing the ESCAPE key transfers control to a user routine and lets the register set be saved (read back later with GetUserRegisters, MON 420B). This is an ND-500 monitor call (octal >= 0400), not an ND-100 native call.

Status: routing byte-proven (ND-500 call via the S3SM5 0x60 vector table, not the ND-100 GOTAB); the handler body is real SINTRAN L bytes carved from 030-S3SM5.bin at the exact vector value 0xBA31 (no +5 correction needed), but the byte stream does not decode as a clean subroutine — the window is at least partly misaligned, so the body is UNVERIFIED past the routing. See Honest caveats. ND-100 addresses are octal; ND-500 offsets are hex byte offsets (the code is byte-addressed).

Dispatch path

flowchart TD
    A["ND-500 process<br/>MON 405B (USTRK)"] --> B["MCHANDEL<br/>MCNO=405B (not 500B-523B level-12)"]
    B --> C["NORMMC<br/>forward to System Monitor"]
    C --> D["S3SM5: 0x60 vector table<br/>slot = 0x60 + 2*405B = 0x026A"]
    D --> E["handler @ file 0xBA31<br/>USTRK body (vector value, exact)"]
    X["ND-100 GOTAB[405]<br/>= 056155B = DT73R"] -.MISATTRIBUTED<br/>device-table data, not code.-> B
    class A blue
    class B,C,D teal
    class E 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

Unlike 410B (whose raw vector pointed into an error string and needed a +5 correction), the 405B vector value 0xBA31 points directly at the handler entry. The X ⇢ B hop marks the ND-100 GOTAB[405] read as a red herring — a coincidental read into device-table data (DT73R), not a handler.

Code location (dispatch path)

Rows are in execution order. Byte offset for the ND-100 GOTAB row = (addr − loadbase) octal words × 2; ND-500 rows in 030-S3SM5 are byte-addressed (byte offset is direct, no ×2).

Role Segment (full disasm) Addr / file offset Byte offset (dec) Symbol Verdict
GOTAB[405] read (does NOT apply) commoncode.asm · .hex 071640B (=071233B+405B) 59200 reads 056155B = DT73R (device table) MISATTRIBUTED — coincidental read into device-table data, not a handler
S3SM5 0x60 vector slot 405B 030-S3SM5.asm · .hex file off 0x026A 618 value 0xBA31 (points AT entry) VERIFIED (routing)
Handler body (USTRK) 030-S3SM5.asm · .hex file off 0xBA31..0xBA6B 47665..47723 (59 bytes) USTRK (L07 name only; not in carved window) real bytes; alignment UNVERIFIED

Verify by hand: grep '^47665 ' ../../segments-ref/030-S3SM5/030-S3SM5.hex → 47665 064 (octal 064 = 0x34); then dd if=../../../segments/030-S3SM5.bin bs=1 skip=47665 count=2 | od -An -tx1 → 34 ba (matches the .ASM first line). The vector slot: dd if=../../../segments/030-S3SM5.bin bs=1 skip=618 count=2 | od -An -tx1 → ba 31 (the vector value, which equals the entry offset).

Instruction walkthrough

Full listing: 405B-SwitchUserBreak.ASM. The 59-byte region carries a short routine: frame-field compares/loads, an entsn at 0xBA52, and forward go branches — a skeleton consistent with a small "set the user-break flag and stash the handler address" routine (the documented USTRK behaviour). But the stream opens with w1 comp (not an ENT* prologue) and byte 0xBA4C is an undecodable opcode (0x00F2), so the aligned decode is not trustworthy past the first few bytes. This is ND-500 code; the RAW BYTES are ground truth but the instruction boundaries inside the window are not proven — the individual mnemonics are provisional.

Parameter / register contract

This is an ND-500 call; argument transport is the ND-500 MON message block (CALLG Func, Address), not ND-100 A/X/T registers.

Field Dir Meaning Verdict
MON number in 405B routes via MCHANDEL → NORMMC → S3SM5 0x60 vector slot 0x026A VERIFIED (bytes)
Func (INTEGER2) in 1 = enable user ESCAPE handling, 0 = disable inferred (manual)
Address (INTEGER2) in program address entered when ESCAPE is pressed inferred (manual)
status / skip out not attributed to a byte-proven field (window misaligned) UNVERIFIED

The user-visible argument layout lives in the ND-500 MON caller wrapper and the S3SM5 message frame; the two documented IN parameters are from the manual, not byte-proven against this carve.

Pseudo-code (for an emulator)

See 405B-SwitchUserBreak.pseudo.c — a pseudo-C model for emulator authors. Because the byte stream does not decode as a clean subroutine, only the documented on/off + handler-address contract is expressed; no instruction-level behaviour is modelled as verified. Instruction semantics reference: ../../instruction-semantics/ND500-INSTRUCTION-SEMANTICS.md (register model, addressing modes, branch conditions; C=1 means no-borrow, inverted). The routing is byte-verified; the body is provisional.

Honest caveats

What is byte-proven: 405B is an ND-500 call, NOT an ND-100 native MON call. The prove-mon.py 405 GOTAB[405] result (056155B = DT73R) is a device-table datafield (the DT70..DT75 device series in SYMBOL-2-LIST.SYMB.TXT) — a red herring, not a handler. The S3SM5 0x60 vector slot for 405B (0x026A) holds 0xBA31, and that value points directly at the carved handler bytes at file offset 0xBA31 — routing and entry offset are real bytes on disk.

What is NOT proven: the exact instruction alignment / behaviour of the body. The window opens with w1 comp rather than an ENT* prologue and contains an undecodable opcode at 0xBA4C, so the aligned mnemonics are untrustworthy even though the raw bytes are ground truth (reference Sec.9). The L07 USTRK symbol confirms the name/behaviour only; its address does not fall inside the carved 030-S3SM5.bin window, so it cannot anchor the entry byte-exactly beyond the vector. The argument block and status/skip contract are UNVERIFIED. Confirming the body needs an S3SM5 symbol map or a clean-boundary re-carve; treat the routing as reliable and the body as provisional.

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