ND-500 Monitor Call Parameter Passing and Response Mechanism¶

Purpose¶

Complete documentation of how monitor call parameters are passed between ND-500 and ND-100, how responses are returned, and what happens during monitor call processing. All information verified from NPL source code with line numbers.

1. Overview: The Complete Monitor Call Lifecycle¶

When an ND-500 process executes a MON instruction, the following sequence occurs:

sequenceDiagram
    participant ND500 as ND-500 CPU
    participant Micro as ND-500 Microcode
    participant 5MPM as 5MPM (Shared Memory)
    participant IF as PCB 3022 Interface
    participant ND100 as ND-100 CPU
    participant Driver as ND-500 Driver
    participant Shadow as Shadow RT-Program

    ND500->>Micro: Execute MON instruction
    Micro->>5MPM: Write parameters to message buffer
    Micro->>Micro: Set STOPREASON = MOCALL (1)
    Micro->>IF: Assert interrupt, stop CPU
    IF->>ND100: Level 12 interrupt
    ND100->>Driver: N500 driver entry (NXTMSG)
    Driver->>5MPM: Read STOPREASON from STOPR offset
    Driver->>Driver: MCHANDEL: Dispatch by mon call number

    alt Simple Mon Call (e.g., TIME-USED)
        Driver->>5MPM: Process and write result to FUNCV
        Driver->>Driver: MONICO: Set restart parameters
    else Complex Mon Call (requires RT-program)
        Driver->>5MPM: Set process status to 5INMCALL
        Driver->>Shadow: 5RRTWT: Restart shadow RT-program
        Shadow->>Shadow: Process monitor call
        Shadow->>5MPM: Write results to message buffer
        Shadow->>Driver: Signal completion (MONICO/5MONICO)
    end

    Driver->>5MPM: Write MICFU = 3MONCO (restart)
    Driver->>Driver: XACT500: Activate next process
    IF->>Micro: Resume execution
    Micro->>ND500: Continue after MON instruction

2. Message Buffer Parameter Layout¶

Source: N500-SYMBOLS.SYMB.TXT

2.1 Complete Message Buffer Offsets¶

Offset (Oct)	Offset (Dec)	Symbol	Size	Purpose
000002	2	N5STA	1	Process status (MSGN500, N5IOWAIT, etc.)
000006	6	MICFU	1	Microfunction code for restart
000007	7	H500A	1	ND-500 auxiliary status
000011	9	STOPR	1	Stop reason (copied from status bits 10-14)
000011	9	KFLIP	1	Error flag (K flip-flop)
000012	10	NUMPA	1	Parameter write-back mask
000013	11	FUNCV	2	Function return value (double word)
000013	11	MCNO	1	Monitor call number (same offset as FUNCV)
000037	31	SMCNO	1	Saved monitor call number
000060	48	XMICF	1	Extended microfunction
000100	64	5AP1	2	Parameter 1 (input, double word)
000101	65	5DP1	2	Parameter 1 (output, double word)
000102	66	5AP2	2	Parameter 2 (input, double word)
000103	67	5DP2	2	Parameter 2 (output, double word)
000104	68	5AP3	2	Parameter 3 (input, double word)
000105	69	5DP3	2	Parameter 3 (output, double word)
000106	70	5AP4	2	Parameter 4 (input, double word)
000107	71	5DP4	2	Parameter 4 (output, double word)

2.2 Parameter Reading Example¶

Source: MP-P2-N500.NPL lines 1455-1465 (N5FUD routine for MON 333 UDMA)

%-------------------------- GET PARAMETERS
       T:=5MBBANK; *LDATX X5SND              % GET 500 PROC. NO
       A=:N5SE
       *AAX 5PPA2; LDDTX                     % GET BUFFER ADDRESS
       AD=:DBUA;  *AAX 5AP1-5PPA2; LDDTX     % GET FUNCTION (parameter 1)
       IF A >< 0 GO UERR; A:=D=:UFU
       *AAX 5AP2-5AP1; LDDTX                 % GET PIO DATA (parameter 2)
       A:=D=:UPIO
       *AAX 5AP3-5AP2; LDDTX                 % GET LOG DEV (parameter 3)
       IF A >< 0 GO UERR; A:=D=:UNI
       *AAX 5AP4-5AP3; LDDTX                 % GET IPAR1 (parameter 4)
       AD=:IPAR1

Pattern: - T:=5MBBANK - Set bank to 5MPM message buffer bank - *AAX offset; LDDTX - Read double word from offset into AD register - Parameters are double words (32 bits) even for 16-bit values

3. Monitor Call Number Handling¶

3.1 Standard Monitor Calls (0-377 octal = 0-255 decimal)¶

Source: MP-P2-N500.NPL lines 1290-1291

T:=5MBBANK; *AAX MCNO; LDATX              % YES, LOG
IF A<<1000 THEN                           % MON.CALL NUMBER 777B IS THE HIGHEST MON.CALL TO LOG

Monitor call numbers are stored at offset MCNO (000013 octal = 11 decimal).

3.2 Extended Monitor Calls (>255 decimal = >377 octal)¶

The ND-500 supports monitor calls beyond 255. These are handled specially on level 12.

Source: MP-P2-N500.NPL lines 1269-1271, 1382-1393

SYMBOL L12MIN=               500       % FIRST MON CALL THAT REQUIRES SPECIAL
                                       % TREATMENT ON LEVEL 12
SYMBOL L12MAX=               523       % LAST
...
IF A >= L12MIN AND A <= L12MAX THEN    % SPECIAL HANDLING ON LEVEL 12?
   A=:5CMNO; CALL MBSUSPROC
   5CMNO-L12MIN GOSW
      STAPROC,    NSTOPROC,   SWITPROC,   NINSTR,
      NOUTSTR,    GERRC,      5SIBMO,     SPRIO,
      SWMC,       DVIO,       A5XMSG,     B5XMSG,
      M5TMOUT,    5MTRANS,    M516,       M517,
      M520,       M521,       M522,       M523;
FI
GO NORMMC         % MONITOR CALL SHOULD BE HANDLED BY THE SYSTEM MONITOR.

3.3 Extended Monitor Call Dispatch Table¶

Mon Call	Octal	Handler	Purpose
500	764	STAPROC	Start process
501	765	NSTOPROC	Stop process
502	766	SWITPROC	Switch process
503	767	NINSTR	Input string (DVINST)
504	770	NOUTSTR	Output string
505	771	GERRC	Get error code
506	772	5SIBMO	SIB monitor call
507	773	SPRIO	Set priority
510	774	SWMC	Swapper monitor call
511	777	DVIO	Direct virtual I/O
512	1000	A5XMSG	XMSG A function
513	1001	B5XMSG	XMSG B function
514	1002	M5TMOUT	Timeout handling
515	1003	5MTRANS	Transfer
516-523	1004-1013	M516-M523	Patchable entries

3.4 Special Monitor Calls Outside Range¶

Mon Call	Octal	Symbol	Handler	Purpose
255	377	N5SWAP	SWPDECODER	Swapper internal
254	376	CERN	(special)	CERN code execution
231	347	-	5SERVER	Nucleus call
219	333	-	N5FUD	UDMA fast call

4. Response Value Write-Back Mechanism¶

4.1 The NUMPA Write-Back Mask¶

When a monitor call completes, only certain parameters need to be written back to the ND-500 address space. The NUMPA field controls which parameters are written.

Offset: NUMPA = 000012 octal = 10 decimal

Bit Meaning:

Bit	Mask (Oct)	Mask (Dec)	Parameter Written Back
0	000001	1	Parameter 1 (5AP1/5DP1)
1	000002	2	Parameter 2 (5AP2/5DP2)
2	000004	4	Parameter 3 (5AP3/5DP3)
3	000010	8	Parameter 4 (5AP4/5DP4)
15	100000	32768	Extended write-back (DVIO)

4.2 Setting Write-Back Mask Examples¶

Source: MP-P2-N500.NPL line 3705-3706 (INSMONCO)

IF A-511=0 THEN A:=100000 ELSE A:=4 FI   % 511=DVIO gets bit 15, others get bit 2
*AAX NUMPA-SMCNO; STATX                   % STORE WRITE BACK MASK

Source: MP-P2-N500.NPL lines 1901-1904 (DVIO/DVINST handling)

IF X.SMCNO=511 THEN                    % WHICH MONITOR CALL IS IT
   100000=:X.NUMPAR                    % MON DVIO; SET MONITOR CALL WRITE-BACK MASK
ELSE
   4=:X.NUMPAR                         % MON DVINST; SET MONITOR CALL WRITE-BACK MASK
FI

4.3 Writing Response Values¶

Source: MP-P2-N500.NPL lines 2300-2325

*AAX 5AP2; STZTX; AAX 5DP2-5AP2; STATX; AAX -5DP2   % Write parameter 2 result
...
*AAX 5AP3; STZTX; AAX 5DP3-5AP3; STATX             % Write parameter 3 result
...
*AAX 5AP4; STZTX; AAX 5DP4-5AP4; STATX             % Write parameter 4 result

5. Monitor Call Restart Mechanism¶

5.1 The MONICO Family of Routines¶

Source: CC-P2-N500.NPL lines 359-372

SUBR MONICO,EMONICO,OKMONICO,MCCO
INTEGER KKFLIP
EMONICO:  A=:D:=0; T:=1; GO MONICO          % Error restart (K=1)
OKMONICO: T:=0; A:=0; D:=0                  % OK restart (K=0, FUNCV=0)
MONICO:   T=:KKFLIP:=5MBBANK; *AAX FUNCV; STDTX      % SAVE FUNCTION VALUE
          A:=KKFLIP; *AAX KFLIP-FUNCV; STATX         % SET ERROR FLAG ON/OFF
          *AAX NUMPA-KFLIP; STZTX
          3MONCO; *AAX -NUMPA; STATX XMICF           % RESTART AFTER MONITOR CALL
MCCO:     T:=5MBBANK; 140300; *AAX H500A; STATX; AAX -H500A
          L=:D; MSGN500; CALL WN5STATUS; L:=D
          T:=5MBBANK; X=:D; *AAX XADPR; LDXTX
          X.PSTAT/\5CLRUNSTATUS+5ACTIVE=:X.PSTAT    % SET PROC. ACTIVE
          X:=D; EXIT

5.2 Restart Entry Points¶

Routine	KFLIP	FUNCV	Purpose
OKMONICO	0	0	Successful return, no error
EMONICO	1	(error code)	Error return with code in AD
MONICO	(param)	(param)	General restart with parameters
5MONICO	(param)	(param)	Extended with cache control
5EMONICO	1	(param)	Extended error restart

5.3 Microfunction Codes for Restart¶

Source: N500-SYMBOLS.SYMB.TXT

Octal	Decimal	Symbol	Meaning
000023	19	3STAR(T)	Start execution
000024	20	3MONC(O)	Monitor call complete - resume
000025	21	3TRAC(O)	Trap complete - resume
000026	22	3WMON(CO)	Write monitor call complete

5.4 The Restart Sequence¶

flowchart TD
    A[Monitor Call Processing Complete] --> B{Success or Error?}
    B -->|Success| C[OKMONICO]
    B -->|Error| D[EMONICO]

    C --> E[Set FUNCV = 0]
    C --> F[Set KFLIP = 0]
    D --> G[Set FUNCV = error code]
    D --> H[Set KFLIP = 1]

    E --> I[MONICO Common Path]
    F --> I
    G --> I
    H --> I

    I --> J[Write FUNCV to message buffer]
    J --> K[Write KFLIP to message buffer]
    K --> L[Clear NUMPA write-back mask]
    L --> M[Write 3MONCO to MICFU]
    M --> N[Set H500A = 140300]
    N --> O[Set N5STATUS = MSGN500]
    O --> P[Set PSTAT = 5ACTIVE]
    P --> Q[Return - process ready to restart]

    style A fill:#2196F3,stroke:#1976D2,stroke-width:2px,color:#fff
    style C fill:#4CAF50,stroke:#388E3C,stroke-width:2px,color:#fff
    style D fill:#F44336,stroke:#D32F2F,stroke-width:2px,color:#fff
    style M fill:#E91E63,stroke:#C2185B,stroke-width:2px,color:#fff
    style Q fill:#4CAF50,stroke:#388E3C,stroke-width:2px,color:#fff

6. What Happens on ND-500 During Monitor Call Processing¶

6.1 ND-500 CPU State: STOPPED¶

Critical: When an ND-500 process executes a monitor call, the ND-500 CPU stops. It does NOT continue executing another domain during the wait.

Evidence: The STOPREASON field (status register bits 10-14) indicates the CPU has stopped: - MOCALL (1) = Monitor call (MOCAL=000001, verified from SYMBOL-1-LIST.SYMB.TXT) - TRAPCODE (2) = Trap (page fault, etc.) (TRAPC=000002, verified) - 5FMOCALL (3) = File transfer monitor call (5FMOC=000003, verified)

Source: MP-P2-N500.NPL lines 808-814

*MICFU@3 LDATX                                       % MIC.FUNC
IF A=3MONCO OR A=3TRACO OR A=3START OR A=3WMONCO THEN
   T:=5MBBANK; *AAX STOPR; LDATX; AAX -STOPR
   IF A=MOCALL THEN CALL MCHANDLE                    % STOP-REASON IS MON.CALL
   ELSE IF A=5FMOCALL THEN CALL MCHANDLE             % STOP-REASON IS FILE-TRANSFER MONCALL
   ELSE IF A=TRAPCODE THEN CALL TRAPDECODER          % STOP-REASON IS TRAP
   ELSE CALL 5RRTWT                                  % RESTART ND-100 PROCESS
   FI FI FI

6.2 Execution Queue Processing¶

While one ND-500 process is stopped waiting for a monitor call, the ND-100 can: 1. Process the monitor call 2. Process other messages in the execution queue 3. Restart other waiting ND-500 processes

Source: MP-P2-N500.NPL line 818

GO NXTMSG                                            % HANDLE NEXT MESS. IN EX-QUEUE

6.3 Process Status During Wait¶

Status Value	Symbol	Meaning
5INMCALL	000010 (oct)	Process executing monitor call
5ACTIVE	-	Process active on ND-500
N5IOWAIT	-	Process waiting for I/O
MSGN500	000001 (oct)	ND-500 message pending

Source: MP-P2-N500.NPL line 1372

PROCAD.PSTAT/\5CLRUNSTATUS+5INMCALL=:X.PSTAT         % MARK PROC. IN MON.CALL.

6.4 ND-500 CPU Reactivation¶

When ready to restart an ND-500 process, XACT500 is called:

Source: MP-P2-N500.NPL lines 3084-3091

ACT50:           5MBBANK; T:=HDEV+LMAR5; *IOXT       % Load message buffer address
                 A:=X; *IOXT
                 A:=5; T+"LCON5-LMAR5"; *IOXT        % Operation code 5 = ACTIVATE
ELSE
                 % Enable for interrupt (no process waiting)
                 A:=10; T:=HDEV+LCON5;   *IOXT
                 A:=0;  T+"LSTA5-LCON5"; *IOXT
                 A:=1;  T+"LCON5-LSTA5"; *IOXT
                        T+"SLOC5-LCON5"; *IOXT

7. The Shadow RT-Program Mechanism¶

7.1 Why Shadow Programs?¶

Complex monitor calls that require significant processing (file I/O, terminal I/O, etc.) cannot be handled entirely on driver level 12. Instead: 1. The ND-500 driver removes the message from the execution queue 2. The shadow RT-program (running on ND-100) is restarted 3. The RT-program processes the request 4. When complete, the RT-program calls MONICO/5MONICO to restart the ND-500 process

7.2 5RRTWT: Remove and Restart¶

Source: MP-P2-N500.NPL lines 21-42

SUBR 5RRTWT,5XACTRT,5SWACTRT
...
5RRTWT: A:=L=:"LREG"=:"LRG"
       T:=5MBBANK; *AAX 5MSFL; LDATX; AAX -5MSFL
       IF A BIT 5SYSRES THEN
          ...
       FI
       CALL SLOCK; 0/\0
       CALL IFM500XQ                                 % REMOVE MESSAGE FROM EX-QUEUE
       CALL SUNLOCK
ACTRT: IF X><-1 THEN
          T:=5MBBANK; *SENDE@3 LDATX
          IF A+1=0 GO LREG                           % HISTOGRAM/WATCHDOG MESSAGE
          X=:CMESS; *AAX XADPR; LDXTX                % X=PROCESS DESCRIPTION
ACT5SWAP: X.PSTAT BONE F5BUFF BZERO T5BUFF=:X.PSTAT  % MARK THAT PROCESS IS RESTARTED BY THE DRIVER
          X:=:B
          *TRA STS
          IF A NBIT 17 THEN CALL XRTACT ELSE CALL RTACT FI  % INTERRUPT OFF OR ON?

7.3 Complete Monitor Call Flow with Shadow Program¶

flowchart TD
    A[ND-500 executes MON instruction] --> B[Microcode stops CPU]
    B --> C[Sets STOPREASON in status register]
    C --> D[Interface interrupts ND-100]
    D --> E[Driver level 12 receives interrupt]
    E --> F[NXTMSG: Get next message from ex-queue]
    F --> G[DECOMESS: Decode message type]
    G --> H{STOPREASON?}

    H -->|MOCALL=1| I[MCHANDEL: Handle monitor call]
    H -->|TRAPCODE=2| J[TRAPDECODER: Handle trap]
    H -->|5FMOCALL=3| I

    I --> K{Can handle on Level 12?}
    K -->|Yes - simple call| L[Process directly]
    L --> M[MONICO: Set restart params]

    K -->|No - complex call| N[5RRTWT: Remove from ex-queue]
    N --> O[RTACT: Restart shadow RT-program]
    O --> P[RT-program runs on ND-100]
    P --> Q[RT-program processes request]
    Q --> R[RT-program calls MONICO/5MONICO]
    R --> S[Message reinserted to ex-queue]

    M --> T[XACT500: Activate ND-500]
    S --> T
    T --> U[IOX LMAR5: Set message buffer]
    U --> V[IOX LCON5=5: ACTIVATE]
    V --> W[ND-500 microcode reads restart info]
    W --> X[Resume execution after MON]

    style A fill:#2196F3,stroke:#1976D2,stroke-width:2px,color:#fff
    style B fill:#F44336,stroke:#D32F2F,stroke-width:2px,color:#fff
    style E fill:#3F51B5,stroke:#303F9F,stroke-width:2px,color:#fff
    style I fill:#9C27B0,stroke:#7B1FA2,stroke-width:2px,color:#fff
    style L fill:#4CAF50,stroke:#388E3C,stroke-width:2px,color:#fff
    style O fill:#FFA726,stroke:#F57C00,stroke-width:2px,color:#fff
    style T fill:#E91E63,stroke:#C2185B,stroke-width:2px,color:#fff
    style X fill:#4CAF50,stroke:#388E3C,stroke-width:2px,color:#fff

8. Summary Tables¶

8.1 Key Offsets in Message Buffer¶

Symbol	Offset (Oct)	Purpose	Direction
N5STA	000002	Process status	R/W
MICFU	000006	Microfunction for restart	W
STOPR	000011	Stop reason code	R
KFLIP	000011	Error flag (K flip-flop)	W
NUMPA	000012	Write-back mask	W
FUNCV	000013	Return value	W
MCNO	000013	Monitor call number	R
SMCNO	000037	Saved mon call number	R/W
5AP1-5AP4	000100-000106	Input parameters	R
5DP1-5DP4	000101-000107	Output parameters	W

8.2 Extended Monitor Call Summary¶

Range	Handling
0-255 (0-377 oct)	Standard: System monitor or driver level
500-523 (764-1013 oct)	Driver level dispatch table (GOSW)
254 (376 oct)	CERN special code execution
255 (377 oct)	N5SWAP swapper internal
219 (333 oct)	UDMA fast call
231 (347 oct)	Nucleus call

8.3 ND-500 State During Monitor Call¶

Phase	ND-500 CPU	ND-100	Message Status
MON executed	Stopped	Running	In ex-queue
Driver processing	Stopped	Level 12	5INMCALL
RT-program active	Stopped	Level 1	Removed from ex-queue
Completion	Starting	Returns	MSGN500
After XACT500	Running	Available	5ACTIVE

ND500-MONITOR-CALL-MECHANISM.md - High-level monitor call mechanism
ND500-IF-USAGE-DEEP-ANALYSIS.md - Complete IOX command reference
ND500-SCHEDULING-ANALYSIS.md - Process scheduling details
SINTRAN-DOMAIN-SETUP-DEEP-DIVE.md - Domain and message buffer setup

Document Version: 1.0 Created: 2026-01-29 Sources: NPL source code (MP-P2-N500.NPL, CC-P2-N500.NPL, XC-P2-N500.NPL), N500-SYMBOLS.SYMB.TXT