Skip to content

ND-500 Monitor Calls: Activation, Mapping, and Response — Complete Process

Scope: How a monitor call issued by an ND-500 program is activated, routed between the ND-500 and the ND-100, dispatched, handled, and answered — and how to find the code for any MON call. All claims are sourced (NPL line, ND-500 disassembly address, or symbol); items are tagged VERIFIED / UNCERTAIN.

Companion docs (see README.md for the full MON index): - Per-call routing table: ND500-MON-CALL-ROUTING-MAP.md - Message/parameter layout: ../ND500-MONITOR-CALL-PARAMETER-PASSING.md - Call mechanism deep-dive: ../ND500-MONITOR-CALL-MECHANISM.md - Bus/message spec: ../ND500-BUS-INTERFACE-REFERENCE.md - ND-100-side MON dispatch (level 14 / GOTAB): ../../OS/23-MON-CALL-DISPATCH-DEVELOPER-GUIDE.md

0. The three actors (and who does NOT dispatch)

An ND-500 monitor call involves up to three code bodies:

Actor CPU Role Where the code is
ND-500 System Monitor (S3SM5) ND-500 First responder / packager — on a MON trap it builds a message and traps to the ND-100. Its own internal dispatch is UNCERTAIN (not recoverable from the current linear disassembly). The ND-100 does the actual servicing. segment 030-S3SM5.bin (ND-500 code)
ND-100 N500 driver ND-100 The real dispatcher/servicer. MCHANDEL reads the MON number and either services on level 12 (calls 500–515 + special cases) or forwards to the level-1 shadow RT (NORMMC → 5RRTWT) for everything else, then writes results back ../../NPL-SOURCE/NPL/MP-P2-N500.NPL (MCHANDEL/NORMMC lines 1246-1406), 5P-P2-MON60.NPL
ND-500 Swapper ND-500 NOT a MON dispatcher. A paging worker + a SINTRAN client via MON 377B; only sees swap work as messages keyed by a private function code swapper/swapper-k01-pseg.asm

Key correction (VERIFIED from MP-P2-N500.NPL): despite the name, the ND-500 System Monitor does not service these calls on the ND-500 side. S3SM5 packages the call; the ND-100 driver services 500–515 (level 12) and forwards the rest (incl. 410–427) to the ND-100 level-1 shadow RT. See ND500-MON-CALL-ROUTING-MAP.md for the per-call proof.

VERIFIED (swapper is not the dispatcher): the swapper disassembly has no MON entry vector; every MON trap it contains is MON 0B (exit guard, once) or MON 377B (the generic SINTRAN gateway, 15×). It dispatches on a private message function code (DSEG 0x26190, ~29 entries), never on a MON number. It does swap work (RPHS/WPHS page moves) and forwards via MON 377B. See ../swapper/swapper-k01-deep-analysis.md.

So: the ND-500 System Monitor is the universal first-responder; the ND-100 driver is the second-stage handler for forwarded calls; the swapper is neither.

1. The end-to-end flow

flowchart TD
    P["ND-500 program<br/>executes MON nnn"] --> SM["ND-500 System Monitor (S3SM5)<br/>packages the call, traps to ND-100"]
    SM --> MSG["MESSAGE in mailbox memory<br/>MCNO=call#, NUMPA=writeback mask,<br/>MICFU/STOPR; post via 3022 interface"]
    MSG --> INT["ND-100 level-12 interrupt (3022 → ISR)"]
    INT --> MCH["ND-100 MCHANDEL<br/>read MCNO = the MON number<br/>(MP-P2-N500.NPL:1286)"]
    MCH --> D2{"MON number?"}
    D2 -->|"special: 377→swapper,<br/>376→CERN, 347→5SERVER,<br/>333→UDMA, 2TUSED/2CLOCK"| SPEC["dedicated handler<br/>(mostly level 12)"]
    D2 -->|"500..515"| GOSW["GOSW jump table, level 12<br/>STAPROC..5MTRANS (MP-P2-N500.NPL:1385)"]
    D2 -->|"else (incl. 410-427, 231, 516-523)"| NORM["NORMMC → 5RRTWT<br/>level-1 shadow RT ('system monitor')<br/>(MP-P2-N500.NPL:1393)"]
    GOSW --> WB["write results back to ND-500 memory<br/>(only params selected by NUMPA)"]
    SPEC --> WB
    NORM --> WB
    WB --> RESP["post 3MONCO ('restart after monitor call')<br/>message back to the ND-500"]
    RESP --> RES["ND-500 program resumes<br/>after the MON instruction"]

    style P fill:#E3F2FD,stroke:#0D47A1,color:#000
    style RES fill:#E3F2FD,stroke:#0D47A1,color:#000
    style SM fill:#E0F2F1,stroke:#00695C,color:#000
    style MCH fill:#E0F7FA,stroke:#00838F,color:#000
    style D2 fill:#E0F7FA,stroke:#00838F,color:#000
    style MSG fill:#FFF3E0,stroke:#E65100,color:#000
    style INT fill:#FFF3E0,stroke:#E65100,color:#000
    style RESP fill:#FFF3E0,stroke:#E65100,color:#000
    style GOSW fill:#E8F5E9,stroke:#2E7D32,color:#000
    style NORM fill:#E8F5E9,stroke:#2E7D32,color:#000
    style SPEC fill:#E8F5E9,stroke:#2E7D32,color:#000
    style WB fill:#E8F5E9,stroke:#2E7D32,color:#000

The ND-500 System Monitor packages the call (green = ND-500 side); all servicing shown here is on the ND-100. Whether any call is fully serviced ND-500-side without a message is UNCERTAIN (the S3SM5 code is not yet decoded).

2. Activation: how a MON call crosses from the ND-500 to the ND-100

The ND-500 does not interrupt the ND-100 directly per call. It uses the shared message mailbox and the 3022/5015 bus interface.

  1. The ND-500 System Monitor decides a call needs ND-100 service and builds a message in mailbox memory. Field offsets (VERIFIED from symbols / ../ND500-MONITOR-CALL-PARAMETER-PASSING.md):

    Field Offset (oct) Meaning
    MICFU 006 message function / micro-function code
    STOPR 011 stop reason / dispatch selector (MOCALL, 5FMOCALL, …)
    NUMPA 012 write-back mask — which parameters to copy back
    MCNO 013 the monitor-call number
    MSWMC 014 swapper-monitor-call subfield
    SMCNO 037 saved monitor-call number

  2. It posts the message through the 3022 interface (load the MAR, poke the activate bit), which raises a level-12 interrupt on the ND-100. Full electrical/register detail: ../ND500-BUS-INTERFACE-REFERENCE.md.

  3. On the ND-100, the level-12 ISR reads the message STOPR; for MOCALL / 5FMOCALL it enters the monitor-call handler MCHANDLE (a.k.a. 5MONICO). This is how ND-500 monitor calls reach SINTRAN — via the message STOPR field, not a hardware MON trap on the ND-100.

3. Mapping: how the ND-100 decides what to do

Inside MCHANDLE, the ND-100 reads MCNO (the MON number) and splits by range (VERIFIED ../../NPL-SOURCE/NPL/MP-P2-N500.NPL:1269-1393):

SYMBOL L12MIN=500   SYMBOL L12MAX=523
IF A >= L12MIN AND A <= L12MAX THEN         % extended ND-500 calls
   A=:5CMNO; CALL MBSUSPROC
   5CMNO-L12MIN GOSW                        % jump table indexed by (MON - 500)
      STAPROC,  NSTOPROC, SWITPROC, NINSTR,
      NOUTSTR,  GERRC,    5SIBMO,   SPRIO,
      SWMC,     DVIO,     A5XMSG,   B5XMSG,
      M5TMOUT,  5MTRANS,  M516, M517, M520, M521, M522, M523;
FI
GO NORMMC                                    % else: execute as a normal MON call

  • MON 500–523 → the GOSW jump table, handled on level 12. Each slot is a named ND-100 handler:

    MON oct Handler MON oct Handler
    500 764 STAPROC (start process) 510 774 SWMC (swapper MON call)
    501 765 NSTOPROC (stop) 511 777 DVIO (direct virtual I/O)
    502 766 SWITPROC (switch) 512 1000 A5XMSG (XMSG A)
    503 767 NINSTR (DVINST) 513 1001 B5XMSG (XMSG B)
    504 770 NOUTSTR 514 1002 M5TMOUT (timeout)
    505 771 GERRC (get err = gerrcod) 515 1003 5MTRANS (transfer)
    506 772 5SIBMO (SIB) 516–523 1004–1013 M516–M523 (patchable)
    507 773 SPRIO (set priority)
  • Special cases (checked before the range test, VERIFIED MP-P2-N500.NPL): 377→swapper (SWPDECODER), 376→CERN, 347→5SERVER (nucleus), 333→UDMA fast call, plus 2TUSED/2CLOCK — each has a dedicated handler (mostly level 12).
  • MON 516–523 are patch stubs that immediately GO NORMMC (:1397-1402).
  • Everything else → NORMMC (:1393) → 5RRTWT, the ND-100 level-1 shadow RT ("system monitor") that runs the call. This includes the 4xx calls 410, 411, 416, 417, 425, 426, 427 and 231 — VERIFIED forwarded to the ND-100, not handled ND-500-side. (Earlier notes/prompts that called 410–515 "ND-500-side" are corrected by the ND-100 source; see the routing map.)

Per-call detail (first responder, servicer, ND-500-vs-forwarded, evidence lines) is in ND500-MON-CALL-ROUTING-MAP.md.

4. Response: how the ND-100 answers

  1. The handler produces results in the message/parameter area.
  2. Write-back is selective: the NUMPA mask (offset 012₈) is a bit per parameter — bit k set ⇒ parameter k+1 (5AP(k+1)/5DP(k+1)) is copied back into the ND-500's address space. Unset ⇒ left untouched. (VERIFIED, ../ND500-MONITOR-CALL-PARAMETER-PASSING.md §4.)
  3. The ND-100 posts a 3MONCO = "restart after monitor call" message back to the ND-500 (message function codes 3MONCO=24, 3WMONCO=26 "wait monitor call"; VERIFIED ../ND500-BUS-INTERFACE-REFERENCE.md §7.4). The ND-500 program resumes at the instruction after its MON.

5. How to find the code for ANY MON call

ND-500 monitor calls (issued by ND-500 programs): 1. Look up the routing in ND500-MON-CALL-ROUTING-MAP.md. 2. If ND-100-handled (500–523): the handler symbol is in the GOSW table above → read it in MP-P2-N500.NPL (search the symbol, e.g. DVIO, A5XMSG). 3. If ND-100 NORMMC (<500 standard): it runs the normal ND-100 MON handler — see the ND-100 dispatch below. 4. If ND-500-local: disassemble 030-S3SM5.bin with nd500-dis (/home/ronny/repos/ragge/pcc-nd500/bin/nd500-dis) and align N500-SYMBOLS (handler names: FIXSEG, UNFIX, WSEG, STAPROC, …). See ../../tools/sintran-segment-carver/versions/L-VSX-500/segments/030-S3SM5-DISASSEMBLY-PROMPT.md.

ND-100 monitor calls (issued by ND-100 programs) — MON nnn, 0..377: - Dispatch is via level 14 → GOTAB (256-entry jump table). Full method, including reading GOTAB in memory and setting a DAP breakpoint on the dispatch, is in ../../OS/23-MON-CALL-DISPATCH-DEVELOPER-GUIDE.md. - The handler for MON n is GOTAB(n); the resident handler code is reached via the composite level-4 view (see the guide). Undocumented/unclear ND-100 calls are tracked in ../../tools/sintran-segment-carver/ghidra-tasks/TASK-05-undocumented-mon-calls.md.

MON 60B (N500M) — the ND-100 background monitor's own call: the ND-500 Background Monitor (ND-500-MON-J:PROG, ND-100 code) uses MON 60B with a subfunction stub array; decoded in ../ND500-MON-RE-FINDINGS.md.

6. Source references

What Where
ND-100 GOSW dispatch (500–523), NORMMC ../../NPL-SOURCE/NPL/MP-P2-N500.NPL:1269-1393
SEGMONC (execute MON on behalf of ND-500) ../../NPL-SOURCE/NPL/5P-P2-MON60.NPL:2302
MON 60B subfunction interface ../../NPL-SOURCE/NPL/5P-P2-MON60.NPL (FUNCTION-CODE tables)
Message fields, write-back (NUMPA) ../ND500-MONITOR-CALL-PARAMETER-PASSING.md
Message posting, level-12 ISR, 3MONCO/3WMONCO ../ND500-BUS-INTERFACE-REFERENCE.md
ND-500 System Monitor code ../../tools/sintran-segment-carver/versions/L-VSX-500/segments/030-S3SM5.asm
ND-500 Swapper (not a MON dispatcher) ../swapper/swapper-k01-deep-analysis.md
ND-100 MON dispatch (level 14 / GOTAB) ../../OS/23-MON-CALL-DISPATCH-DEVELOPER-GUIDE.md