ND-500 Monitor Call Mechanism - Deep Analysis¶
Purpose¶
Complete analysis of how ND-500 processes make monitor calls to SINTRAN running on ND-100. When ND-500 needs OS services (I/O, memory, etc.), it triggers an interrupt that causes ND-100 to process the request.
Key Sources: - MP-P2-N500.NPL (lines 656-818, 1251-1439) - Driver and MCHANDEL - CC-P2-N500.NPL - Status routines
1. Overview: The Inter-Processor Call Mechanism¶
sequenceDiagram
participant P as ND-500 Process
participant M as Message Buffer (5MPM)
participant H as Hardware Interface
participant I as ND-100 Interrupt (Lvl 12)
participant D as 5STDRIV Driver
participant K as MCHANDEL
participant S as SINTRAN Kernel
P->>P: Execute MON instruction
P->>M: Write MCNO, parameters to message
P->>M: Set STOPR = MOCALL
P->>M: Set status = ANSWER
P->>H: Signal completion (unlocks interface)
H->>I: Generate Level 12 interrupt
I->>D: 5STDRIV entry
D->>D: Scan execution queue
D->>D: CHN5STATUS (check message)
D->>D: DECOMESS (decode answer)
D->>K: MCHANDEL (dispatch MON call)
alt Fast path (Level 12)
K->>K: Handle directly
K->>M: Write result
K->>P: XACTRDY (reactivate)
else Normal path (Level 1)
K->>S: 5RRTWT (forward to kernel)
S->>S: Process monitor call
S->>M: Write result
S->>P: Restart process
end2. How ND-500 Initiates a Monitor Call¶
2.1 ND-500 Executes MON Instruction¶
When an ND-500 process needs OS services, it executes a MON instruction (monitor call). This causes:
- ND-500 microcode stops execution
- Sets STOPR (stop reason) = MOCALL (or 5FMOCALL for file transfers)
- Writes message to 5MPM shared memory:
- MCNO: Monitor call number
- Parameters in message buffer
- Sets status to ANSWER
- Unlocks the interface (clears 5ILOCK bit in status register)
- Hardware generates interrupt to ND-100
2.2 Message Buffer Layout¶
┌─────────────────────────────────────────┐
│ Message Buffer (in 5MPM) │
├─────────────────────────────────────────┤
│ MICFU │ Microfunction code │
│ STOPR │ Stop reason (MOCALL/TRAP) │
│ MCNO │ Monitor call number │
│ SMCNO │ Saved monitor call number │
│ FUNCV │ Function value (return) │
│ KFLIP │ K flip-flop (error flag) │
│ NUMPA │ Number of parameters │
│ Parameters... │
└─────────────────────────────────────────┘
3. ND-100 Interrupt Handler (5STDRIV)¶
3.1 Driver Entry Point¶
Source: MP-P2-N500.NPL lines 656-698
flowchart TD
A[Level 12 Interrupt] --> B[5STDRIV Entry]
B --> C{CPU alive?}
C -->|No| DONE[CALLID12: Return]
C -->|Yes| D[N500: Main loop]
D --> E{Power fail?}
E -->|Yes| ERR[N500ERR: Restart all]
E -->|No| F[Clear status bits]
F --> G[Read RSTA5]
G --> H{Communication error?}
H -->|Yes| ERR
H -->|No| I[Scan execution queue]
I --> J{Message found?}
J -->|No| K[XACT500: Activate next]
J -->|Yes| L[CHN5STATUS: Check message]
L --> M{Status = ANSWER?}
M -->|Yes| N[DECOMESS: Decode answer]
M -->|No| O[Handle other status]
N --> P[NXTMSG: Next message]
K --> DONE
style A fill:#2196F3,stroke:#1976D2,stroke-width:2px,color:#fff
style L fill:#9C27B0,stroke:#7B1FA2,stroke-width:2px,color:#fff
style N fill:#4CAF50,stroke:#388E3C,stroke-width:2px,color:#fff3.2 Key Code (5STDRIV loop)¶
Line 659: 5STDRIV:
Line 660: IF CPUAVAILABLE NBIT 5ALIVE GO CALLID12
Line 661: N500:
Line 662: DO
Line 663: IF C5STAT/\C5PFMASK >< 0 GO CALLID12 % Power fail check
Line 668: 177377; CALL CLE5STATUS % Read and clear status
Line 678: X:=MAILINK % Start scanning queue
Line 679: DO
Line 680: WHILE X><-1
Line 683: X:=D=:N5MESSAGE % Current message
Line 685: CALL CHN5STATUS; GO N500ERR % Check message status
Line 687: NXTMSG:
Line 692: CC5CPU=:B; CALL XACT500 % Activate next process
Line 693: CALLID12: CALL WT12 % Return from interrupt
Line 694: OD
4. CHN5STATUS - Message Status Check¶
4.1 Status Dispatch¶
Source: MP-P2-N500.NPL lines 730-759
flowchart TD
A[CHN5STATUS Entry<br/>X = Message] --> B[Read N5STATUS]
B --> C{Status value?}
C -->|ANSWER| D[Normal completion]
C -->|5ERANSWER| E[Error answer]
C -->|> 100| F[Restart ND-100 process]
C -->|MSGN500/WAITING| G[Terminate and restart]
D --> H{X = HIMESS?}
H -->|Yes| I[HISTSAMPLE: Histogram]
H -->|No| J[DECOMESS: Decode answer]
E --> K[DECOERRMESS: Error decode]
F --> L[5RRTWT: Restart]
G --> M[XTER500: Terminate]
style A fill:#2196F3,stroke:#1976D2,stroke-width:2px,color:#fff
style J fill:#4CAF50,stroke:#388E3C,stroke-width:2px,color:#fff
style K fill:#F44336,stroke:#D32F2F,stroke-width:2px,color:#fff4.2 Key Code¶
Line 734: CALL RN5STATUS % Read message status
Line 735: IF A=ANSWER THEN % Normal answer?
Line 746: CALL DECOMESS % Decode answer from ND-500
Line 749: ELSE IF A=5ERANSWER THEN
Line 750: CALL DECOERRMESS % Decode error answer
Line 751: ELSE IF A>>100 THEN
Line 752: CALL 5RRTWT % Restart ND-100 process
Line 753: ELSE IF A=MSGN500 OR A=WAITING THEN
Line 755: CALL XTER500; GO LREG % Terminate first
5. DECOMESS - Answer Decoder¶
5.1 Stop Reason Dispatch¶
Source: MP-P2-N500.NPL lines 803-818
flowchart TD
A[DECOMESS Entry] --> B[Read SPFLA special flag]
B --> C{SPFLA set?}
C -->|Yes| D[Jump to special routine]
C -->|No| E[Read MICFU microfunction]
E --> F{MICFU type?}
F -->|3MONCO/3TRACO/3START/3WMONCO| G[Read STOPR]
F -->|Other| H[5RRTWT: Restart]
G --> I{STOPR value?}
I -->|MOCALL| J[MCHANDLE: Monitor call]
I -->|5FMOCALL| J
I -->|TRAPCODE| K[TRAPDECODER: Trap handler]
I -->|Other| H
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style J fill:#4CAF50,stroke:#388E3C,stroke-width:2px,color:#fff
style K fill:#FFA726,stroke:#F57C00,stroke-width:2px,color:#fff5.2 Stop Reason Values (STOPR Field)¶
STOPR field location: Offset 000011 (octal) = 9 (decimal) in message buffer
Source: Verified from ../NPL-SOURCE/SYMBOLS/L07/SYMBOL-1-LIST.SYMB.TXT
| Octal | Decimal | NPL Symbol | Full Name | Meaning |
|---|---|---|---|---|
| 000001 | 1 | MOCAL | MOCALL | Normal MON instruction executed |
| 000002 | 2 | TRAPC | TRAPCODE | Hardware trap occurred (page fault, etc.) |
| 000003 | 3 | 5FMOC | 5FMOCALL | File transfer monitor call |
| 000101 | 65 | - | (TPSTRA return) | Return from N500M RUNN function |
Verification: MOCAL=000001, TRAPC=000002, 5FMOC=000003 confirmed from SYMBOL-1-LIST.SYMB.TXT lines 3921, 249, 747.
Note: Stop reason values are set by ND-500 microcode when the CPU stops. The value 65 (101 octal) is specifically mentioned in the ND-500 Loader/Monitor manual (ND-60.136.04) as the value set by MON 407B (TPSTRA) when returning from the N500M RUNN function.
5.3 Status Register Stop Reason Bits¶
The stop reason is extracted from the status register (RSTA5) bits 10-14:
Status Register RSTA5:
┌────┬────┬────┬────┬────┬────┬────┬────┬────┬────┬────┬────┬────┬────┬────┬────┐
│ 15 │ 14 │ 13 │ 12 │ 11 │ 10 │ 9 │ 8 │ 7 │ 6 │ 5 │ 4 │ 3 │ 2 │ 1 │ 0 │
└────┴────┴────┴────┴────┴────┴────┴────┴────┴────┴────┴────┴────┴────┴────┴────┘
│ │ │ │ │ │ │
└────┴────┴────┴────┘ │ └─ 5ILOCK (interface locked)
│ └─ 5POWOF (power off)
└─ STOPREASON (5 bits, mask 037000 octal = 0x3E00)
Mask: 037000 (octal) = 15872 (decimal) = 0x3E00 (hex)
Extraction: STOPREASON = (RSTA5 >> 10) & 0x1F
6. MCHANDEL - Monitor Call Handler¶
6.1 Overview¶
MCHANDEL (lines 1251-1406) is the central dispatcher for ND-500 monitor calls.
Source: MP-P2-N500.NPL lines 1251-1394
6.2 Monitor Call Dispatch Flow¶
flowchart TD
A[MCHANDEL Entry] --> B[Save stop reason]
B --> C{MON call logging enabled?}
C -->|Yes| D[Log monitor call]
C -->|No| E[Continue]
D --> E
E --> F[Read MCNO from message]
F --> G[Save in SMCNO]
G --> H{MCNO value?}
H -->|2TUSED| I[Return CPU time]
H -->|2CLOCK| J[Return clock data]
H -->|N5SWAP = 377| K[SWPDECODER]
H -->|CERN = 376| L[Execute CERN code]
H -->|500-523| M[Level 12 dispatch table]
H -->|Other| N[NORMMC: Forward to kernel]
M --> O[GOSW dispatch]
O --> P[Execute handler]
P --> Q[Return to NXTMSG]
N --> R[5RRTWT: Restart ND-100 proc]
R --> Q
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style M fill:#9C27B0,stroke:#7B1FA2,stroke-width:2px,color:#fff
style N fill:#4CAF50,stroke:#388E3C,stroke-width:2px,color:#fff6.3 Level 12 Monitor Calls (Fast Path)¶
Monitor calls 500-523 (octal) are handled directly on Level 12 for speed:
Line 1382: IF A >= L12MIN AND A <= L12MAX THEN
Line 1385: 5CMNO-L12MIN GOSW
Line 1386: STAPROC, NSTOPROC, SWITPROC, NINSTR,
Line 1387: NOUTSTR, GERRC, 5SIBMO, SPRIO,
Line 1388: SWMC, DVIO, A5XMSG, B5XMSG,
Line 1389: M5TMOUT, 5MTRANS, M516, M517,
Line 1390: M520, M521, M522, M523;
| MON # (Oct) | Symbol | Purpose |
|---|---|---|
| 500 | STAPROC | Start process |
| 501 | NSTOPROC | Stop process |
| 502 | SWITPROC | Switch process |
| 503 | NINSTR | Input string (DVINST) |
| 504 | NOUTSTR | Output string |
| 505 | GERRC | Get error code |
| 506 | 5SIBMO | SIBAS monitor call |
| 507 | SPRIO | Set priority |
| 510 | SWMC | Switch context |
| 511 | DVIO | Device I/O |
| 512 | A5XMSG | XMSG A function |
| 513 | B5XMSG | XMSG B function |
| 514 | M5TMOUT | Timeout |
| 515 | 5MTRANS | Memory transfer |
| 516-523 | Patch | Reserved for extensions |
6.4 Normal Monitor Calls (Forwarded to Kernel)¶
Monitor calls NOT in the 500-523 range are forwarded to the background monitor:
Line 1393: GO NORMMC % MONITOR CALL SHOULD BE HANDLED BY THE SYSTEM MONITOR.
Line 1277: NORMMC:
Line 1278: IF CSTOPREASON=5FMOCALL THEN
Line 1280: "5FRTBAK"=:PROCAD.MFUNC
Line 1282: FI
Line 1283: CALL 5RRTWT; GO NXTMSG
5RRTWT = "Restart ND-100 RT Wait" - hands off to Level 1 processing.
7. DVIO and DVINST - Device I/O¶
7.1 DVIO (MON 511)¶
DVIO handles device output from ND-500 processes.
Source: MP-P2-N500.NPL lines 1688-1910
flowchart TD
A[DVIO Entry] --> B[Read parameters from message]
B --> C[Get logical unit number]
C --> D{LU valid?}
D -->|No| E[Return error]
D -->|Yes| F[Find device descriptor]
F --> G[Set up I/O request]
G --> H[Queue to device driver]
H --> I[Set process I/O wait]
I --> J[Return to NXTMSG]
style A fill:#2196F3,stroke:#1976D2,stroke-width:2px,color:#fff
style H fill:#4CAF50,stroke:#388E3C,stroke-width:2px,color:#fff7.2 DVINST (MON 503) - NINSTR¶
NINSTR handles device input (called via XNINSTR routine).
Line 1770: IF A=511 THEN % DVIO
Line 1774: X=:N5MESSAGE; CALL XNINSTR % XNINSTR in NINSTR routine
8. Trap Handling (Page Faults)¶
8.1 TRAPDECODER¶
When ND-500 encounters a trap (like page fault), the mechanism is similar:
Source: MP-P2-N500.NPL lines 859-895
flowchart TD
A[TRAPDECODER Entry] --> B[Read TRAPN from message]
B --> C{Trap number?}
C -->|> 53| D[Unknown trap error]
C -->|= 46| E[Page fault]
C -->|Other| F[Report trap error]
E --> G{X = SWMSG?}
G -->|Yes| H[Fatal: Page fault in swapper]
G -->|No| I{Swapper started?}
I -->|No| J[Restart ND-100 process]
I -->|Yes| K[5ACTSWAPPER: Activate swapper]
style A fill:#2196F3,stroke:#1976D2,stroke-width:2px,color:#fff
style E fill:#FFA726,stroke:#F57C00,stroke-width:2px,color:#fff
style K fill:#4CAF50,stroke:#388E3C,stroke-width:2px,color:#fff8.2 Key Code¶
Line 868: ELSE IF D = 46 THEN % PAGE FAULT
Line 869: ITRAPDECODER: IF X>< SWMSG THEN
Line 876: IF A-5SWPROC=0 GO ITRPERR % Page fault in swapper?
Line 877: MSWPFAULT SHZ 10+D
Line 879: CALL 5ACTSWAPPER % Activate the swapper
9. Return Path: Restarting ND-500 Process¶
9.1 MONICO - Return from Monitor Call¶
Source: CC-P2-N500.NPL lines 359-372
After processing a monitor call, the result is returned via MONICO:
Line 363: MONICO: T=:KKFLIP:=5MBBANK; *AAX FUNCV; STDTX % Save function value
Line 364: A:=KKFLIP; *AAX KFLIP-FUNCV; STATX % Set error flag
Line 366: 3MONCO; *AAX -NUMPA; STATX XMICF % Restart after MON call
Line 368: L=:D; MSGN500; CALL WN5STATUS; L:=D % Set status = MSGN500
Line 370: X.PSTAT/\5CLRUNSTATUS+5ACTIVE=:X.PSTAT % Set proc active
9.2 Status Flow Back¶
sequenceDiagram
participant K as Kernel Handler
participant M as Message Buffer
participant X as XACTRDY
participant H as Hardware
participant P as ND-500 Process
K->>M: Write FUNCV (return value)
K->>M: Write KFLIP (error flag)
K->>M: Set MICFU = 3MONCO
K->>M: Set status = MSGN500
K->>K: Set PSTAT = 5ACTIVE
K->>X: Call XACTRDY
X->>X: Select highest priority
X->>H: LMAR5 = bank
X->>H: LMAR5 = message address
X->>H: LCON5 = 5 (ACTIVATE)
H->>P: Resume execution
P->>P: Read return value from message10. Complete Call Sequence Example¶
10.1 Example: ND-500 Process Calls MON DVIO (511)¶
sequenceDiagram
participant P as ND-500 Process
participant CPU as ND-500 CPU
participant MEM as 5MPM Memory
participant HW as PCB 3022
participant INT as ND-100 Level 12
participant DRV as 5STDRIV
participant MCH as MCHANDEL
participant DV as DVIO Handler
participant DEV as Device Driver
P->>CPU: Execute MON 511 (DVIO)
CPU->>CPU: Stop, STOPR = MOCALL
CPU->>MEM: Write MCNO = 511
CPU->>MEM: Write parameters
CPU->>MEM: Set status = ANSWER
CPU->>HW: Clear 5ILOCK (unlock)
HW->>INT: Generate interrupt
INT->>DRV: 5STDRIV entry
DRV->>DRV: Read RSTA5
DRV->>MEM: Scan execution queue
DRV->>DRV: CHN5STATUS
DRV->>DRV: DECOMESS
DRV->>MCH: MCHANDEL (STOPR = MOCALL)
MCH->>MEM: Read MCNO = 511
MCH->>DV: GOSW to DVIO
DV->>MEM: Read I/O parameters
DV->>DEV: Queue I/O request
DV->>MEM: Set process I/O wait
Note over DEV: I/O completes later
DEV->>MEM: Write result
DEV->>MEM: Call MONICO (set MSGN500)
DEV->>HW: XACTRDY → ACTIVATE
HW->>P: Resume ND-500 process
P->>MEM: Read return value11. Key Data Structures¶
11.1 Message Buffer Fields for Monitor Calls¶
Source: N500-SYMBOLS.SYMB.TXT, MP-P2-N500.NPL
| Field | Offset (Oct) | Offset (Dec) | Purpose |
|---|---|---|---|
| STOPR | 000011 | 9 | Stop reason (MOCALL=1, TRAPCODE=2, 5FMOCALL=3) |
| TRAPN | 000016 | 14 | Trap number (46=page fault, >53=unknown) |
| MICFU | @3 index | - | Microfunction code (3MONCO, 3TRACO, etc.) |
| MCNO | - | - | Monitor call number |
| SMCNO | - | - | Saved monitor call number |
| FUNCV | - | - | Function return value (double word) |
| KFLIP | - | - | K flip-flop (0=OK, 1=error) |
| NUMPA | - | - | Number of parameters |
| N5STA | - | - | Process status (ANSWER, MSGN500) |
Note: Offsets marked "-" were not found in symbol files. The @3 notation indicates indexed access via register.
11.2 Microfunction Codes (MICFU Field)¶
Source: N500-SYMBOLS.SYMB.TXT
| Octal | Decimal | Symbol | Full Name | Meaning |
|---|---|---|---|---|
| 000023 | 19 | 3STAR | 3START | Start completion |
| 000024 | 20 | 3MONC | 3MONCO | Monitor call completion |
| 000025 | 21 | 3TRAC | 3TRACO | Trap completion |
| 000026 | 22 | 3WMON | 3WMONCO | Write monitor call completion |
| 000034 | 28 | 3MONO | 3MONO | Monitor output |
| 000046 | 38 | 33MON | 33MON | Extended monitor function |
Additional microfunctions referenced in code:
| Symbol | Meaning |
|---|---|
| 3RMED | Read memory edit |
| 3WMEP | Write memory edit page |
| 3RMEP | Read memory edit page |
| 3WMED | Write memory edit data |
| 3PHSREAD | Physical segment read |
| 3PHSWRITE | Physical segment write |
| 3FITRNSF | File transfer |
| 3SWMESS | Swapper message |
12. Emulator Implementation Notes¶
12.1 What the Emulator Must Implement¶
-
ND-500 MON instruction:
- Stop execution
- Set STOPR, MCNO in message buffer
- Clear 5ILOCK (unlock interface)
-
Interrupt generation:
- Trigger Level 12 interrupt to ND-100 emulator
-
Message buffer handling:
- Read/write all MON call fields
- Track status transitions
-
Driver emulation:
- 5STDRIV queue scanning
- CHN5STATUS dispatch
- MCHANDEL monitor call routing
12.2 Simplified Flow for Emulator¶
ND-500 executes MON:
1. Write MCNO, parameters to message in 5MPM
2. Set STOPR = MOCALL
3. Set status = ANSWER
4. Clear 5ILOCK bit
5. Signal interrupt to ND-100
ND-100 handles interrupt:
1. Read status register (RSTA5)
2. Scan execution queue
3. Find message with status = ANSWER
4. Dispatch based on MCNO:
- 500-523: Handle directly
- Other: Forward to background
Return to ND-500:
1. Write return value (FUNCV)
2. Set status = MSGN500
3. Set MICFU = 3MONCO
4. Call XACTRDY
5. Write LCON5 = 5 to activate
13. Related Documents¶
- ND500-SCHEDULING-ANALYSIS.md - Process scheduling and execution queue
- ND500-SWAPPER-ANALYSIS.md - Swapper and page fault handling
- ND500-IF-USAGE-DEEP-ANALYSIS.md - IOX commands
- MP-P2-N500.md - Main driver module analysis
- ../OS/14-MONITOR-KERNEL-MONCALLS.md - SINTRAN monitor calls
14. Verification Checklist¶
- [x] Inter-processor call mechanism documented
- [x] 5STDRIV driver entry point documented
- [x] CHN5STATUS status dispatch documented
- [x] DECOMESS answer decoder documented
- [x] MCHANDEL monitor call handler documented with Mermaid
- [x] Level 12 fast-path monitor calls listed (500-523)
- [x] Normal monitor call forwarding (NORMMC) documented
- [x] DVIO/DVINST device I/O documented
- [x] Trap handling (page faults) documented
- [x] Return path (MONICO) documented
- [x] Complete call sequence example with sequence diagram
- [x] Key data structures documented
- [x] Emulator implementation notes included
- [x] Stop reason codes verified with actual numeric values from symbols
- [x] Microfunction codes verified with actual numeric values from symbols
- [x] Status register bit layout documented (bits 10-14 = STOPREASON)
- [x] No speculation presented as fact
Document Version: 1.1 Last Updated: 2026-01-29 Sources: - MP-P2-N500.NPL (lines 656-818, 1251-1439) - CC-P2-N500.NPL (lines 359-372) - N500-SYMBOLS.SYMB.TXT (MOCAL, TRAPC, 5FMOC, 3STAR, 3MONC, 3TRAC, 3WMON values) - ND-60.136.04A ND-500 Loader Monitor (TPSTRA stop reason 65)