ND-500 Swapper Loading Mechanism

Complete Analysis of How the Swapper is Loaded and Initialized for ND-500 Systems

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Overview

The ND-500 swapper is NOT code that runs on the ND-500 processor. Instead, it is an ND-100 RT-program named "5SWAP" that handles page swapping for ND-500 processes. The swapper communicates with ND-500 via shared multiport memory (5MPM).

This document traces the complete loading and initialization sequence from SINTRAN III NPL source code.

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Key Insight: Swapper Architecture

┌─────────────────────────────────────────────────────────────────────────────┐
│                        ND-500 Swapper Architecture                          │
├─────────────────────────────────────────────────────────────────────────────┤
│                                                                             │
│  ND-100 Side                            ND-500 Side                         │
│  ────────────                           ───────────                         │
│                                                                             │
│  ┌──────────────────┐                   ┌──────────────────┐               │
│  │ RT-Program       │                   │ User Process     │               │
│  │ "5SWAP"          │◄──────────────────┤ (Page Fault)     │               │
│  │ (5SWRT entry)    │     5MPM          │                  │               │
│  └────────┬─────────┘   Message         └──────────────────┘               │
│           │             Buffer                   │                          │
│           │                                      │ TRAP                     │
│           ▼                                      ▼                          │
│  ┌──────────────────┐                   ┌──────────────────┐               │
│  │ SINTRAN III      │                   │ ND-500 Microcode │               │
│  │ Monitor Calls    │                   │ (Sets MICFU=3)   │               │
│  │ (MON 131 ABSLI)  │                   │                  │               │
│  └──────────────────┘                   └──────────────────┘               │
│                                                                             │
└─────────────────────────────────────────────────────────────────────────────┘

The swapper runs entirely on the ND-100, using SINTRAN III monitor calls to perform disk I/O for the ND-500 processes.

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1. System Initialization Sequence

1.1 INZ500 - Main ND-500 System Initialization

Source: 5P-P2-MON60.NPL lines 616-680

INZ500:
    A:=L=:"INZ5LREG"
    MLEV; *MST PIE
    IF 5MSINIT NBIT 5CHALIVE THEN
       CALL 5CONOMD                            % Detect ND-500 CPUs
       5MSINIT BONE 5CHALIVE=:5MSINIT
    FI
    IF N5CPU=0 THEN ENOCPU; GO FAR INZRET FI   % No ND-500 CPUs found
    CALL CHMEMDEF; GO FAR INZRET
    IF 5MSINIT BIT 5ALBUF GO FAR INZ2

    % ... allocate various buffers ...

INZ1:
    5MSINIT BONE 5ALBUF=:5MSINIT

INZ2:
    IF 5MSINIT NBIT 5INBUF THEN
       CALL MSINIT; 5MSINIT BONE 5INBUF=:5MSINIT  % SETUP MESSAGE BUFFERS
       "S5CPUDF"=:NCCPUDF
    FI
    IF 5MSINIT NBIT 5RTSTART THEN
       CALL IN5FUDMA                           % Initialize Fast-UDMA option
       "A5PIT"=:D; CALL DALTON
       "5SWAP"=:INZRTP; A:="INZPRT"; *MON 2RT  % <<<< CREATE 5SWAP RT-PROGRAM
       CALL ALTOFF
       5MSINIT BONE 5RTSTART=:5MSINIT
    FI

Key Steps: 1. 5CONOMD - Detect ND-500 CPUs connected to system 2. MSINIT - Set up message buffers including swapper message buffer (SWMSG) 3. MON 2RT - Create the "5SWAP" RT-program (swapper)

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1.2 MSINIT/XMSINIT - Message Buffer Initialization

Source: MP-P2-N500.NPL (via CC-P2-N500.NPL dispatch) lines 40533-40667

XMSINIT: *1BANK; COPY SD DA; STA I (SVDRE; 2BANK
    5FPMAILBOX=:D:=0; AD SH 12; A=:5MBBANK     % Memory bank for messages

    % ... initialize CPU datafields ...

MSIN0:
    A:=55MSNEGSIZE+D=:SWMSG                    % <<<< SWMSG = Swapper Message Buffer
    T:=5MBBANK; A=:X:=0 BONE 5SYSRES
    *AAX 5MSFL; STATX; AAX -5MSFL
    5SWPROC=:MSINPROCNO; X:="S500S"            % <<<< S500S = Swapper Process Descriptor
    FOR MSINPROCNO DO WHILE MSINPROCNO<<=MX5PROCS
       X=:MSPRDESCR
       % ... initialize each process message buffer ...
       A:=D+55MSNEGSIZE=:X.MESSBUFF            % Message buffer address into process descriptor
       T:=5MBBANK; X:=:A; *AAX XADPR; STATX
       MSINPROCNO; *SENDE@3 STATX
       X:=MSPRDESCR+5PRDSIZE; 55MESSIZE; D+A
    OD

Key Variables: | Variable | Octal Value | Purpose | |----------|:-----------:|---------| | SWMSG | 110054 | Swapper message buffer address in 5MPM | | S500S | 115542 | Swapper process descriptor (process #0) | | 5SWPROC | - | Swapper process number | | 5MBBANK | - | Memory bank for 5MPM buffers | | 55MESSIZE | 000200 | Size of each message buffer (128 decimal) |

Source: Values verified from SINTRAN L07 symbol files (../NPL-SOURCE/SYMBOLS/L07/SYMBOL-1-LIST.SYMB.TXT, SYMBOL-2-LIST.SYMB.TXT)

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1.3 Creating the 5SWAP RT-Program

Source: 5P-P2-MON60.NPL line 674

"5SWAP"=:INZRTP; A:="INZPRT"; *MON 2RT

This executes Monitor Call 2 (RT - Create RT-Program) with: - Name: "5SWAP" - Entry point: The 5SWRT routine

The RT-program is created as a system RT-program with special privileges.

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2. The 5SWRT RT-Program (Swapper Code)

Source: RP-P2-N500.NPL lines 12-58

%      ( R )    N 5 0 0 - A B S T R A N S   P R O G R A M
%
%      RT-program to read/write page from/to disk to/from memory
%
SUBR 5SWRT
5SWRT: *2BANK; IOF
       "S500S"=:B; X:=RTREF
       CALL BRESERVE; IF A<0 THEN CALL ERRFATAL FI  % RESERVE PROCESS #0
       0=:PSTAT
       X:=SWMSG; T:=5MBBANK; A:="S500S"; *AAX XADPR; STATX  % ADDR OF PROC.DESCR
       A:=SWMSG+"SWPINFO"=:D:=5MBBANK; AD=:DSWMSG           % PHYS.ADDR OF SWPINFO IN SWMSG
       GO INRT; *)FILL

       % - B-reg will be set by actswapper
       DO
          *ION
INRT:     X:="S500S"; CALL FAR SETIOWAIT                % WAIT FOR ACTIVATION
          *IOF
          "S500S".PSTAT BZERO F5BUFF=:X.PSTAT
          X:=SWMSG; CALL RN5STATUS
          IF A><PSW1WAIT THEN
             GO INRT                                      % Not ready, wait again
          FI
          *ION
          IF T:=XSDUNIT=0 THEN A:=-1; GO 5SWERR FI
          "ABSLI"; *MON 131                              % <<<< DISK I/O VIA MON 131
          IF X:="N500DF".SYSINITFLAG BIT B5STOP GO INRT
          IF A>=0 THEN
             T:=0=:A=:D                                   % TRANSFER OK.
          ELSE
5SWERR:      A=:SWEHSTAT
             % ... error handling ...
          FI
          *IOF
          X:=SWMSG; CALL MONICO                          % RESTART SWAPPER (on ND-500)
          % ... continue loop ...
       OD

Swapper Main Loop

flowchart TD
    A[5SWRT Entry] --> B[Reserve Process #0 S500S]
    B --> C[Initialize SWMSG pointers]
    C --> D[INRT: Set I/O Wait]
    D --> E{Swapper Status?}
    E -->|Not PSW1WAIT| D
    E -->|PSW1WAIT| F[Get XSDUNIT]
    F --> G{Unit valid?}
    G -->|No| H[5SWERR: Error]
    G -->|Yes| I[MON 131 ABSLI]
    I --> J{Transfer OK?}
    J -->|No| H
    J -->|Yes| K[Set result = 0]
    K --> L[MONICO: Restart ND-500]
    H --> L
    L --> M[TER500: Terminate ND-500]
    M --> N[ACTRDY: Activate ready]
    N --> O[LOWACT500: Low-level activate]
    O --> D

    style A fill:#2196F3,stroke:#1976D2,stroke-width:2px,color:#fff
    style D fill:#FFA726,stroke:#F57C00,stroke-width:2px,color:#fff
    style I fill:#4CAF50,stroke:#388E3C,stroke-width:2px,color:#fff
    style L fill:#9C27B0,stroke:#7B1FA2,stroke-width:2px,color:#fff

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3. Swapper Activation Mechanism

3.1 5ACTSWAPPER - Activate the Swapper

Source: MP-P2-N500.NPL lines 2857-2907

When an ND-500 process needs page swapping (page fault), the 5ACTSWAPPER routine is called:

5ACTSWAPPER: A:=L=:"LREG"
    CALL SLOCK; 0/\0                                % Lock semaphore
    X=:D=:MSGTOSW; A:=5MBBANK
    *NNC24,CNVWADR
    AD=:CMSGTOSW
    SWPWAIT; CALL WN5STATUS                         % Mark: waiting for swapper
    X:=SWMSG; CALL RN5STATUS
    IF A=PSWWAIT THEN                               % Swapper free?
       T:=5MBBANK; X:=SWMSG
       AD:=CMSGTOSW; *AAX HSWPI; STDTX
       SWACTIVE; *AAX SWPFU-HSWPI; STATX
       X:="N500DF".X500DF; *AAX X5SWO
       CMSGTOSW; T:=5MBBANK; *STDTX
       X:=MSGTOSW; SWPPING; CALL WN5STATUS          % Mark: using swapper
       T:=5MBBANK; *AAX 5MSFL; LDATX
       % ... set up message for swapper ...
       X:=SWMSG; *AAX SWPST; STATX                  % Save reason for activating
       A:=6; *AAX NUMPA-SWPST; STATX                % Par #2 & #3 will be written
       A:=0=:D; *AAX FUNCV-NUMPA; STDTX
       *AAX KFLIP-FUNCV; STZTX; AAX -KFLIP
       3MONCO; *MICFU@3 STATX
       CALL MCCO                                     % Restart swapper via mon.call
       CALL SUNLOCK
       CALL XACTRDY
       GO OUT
    ELSE
       % Swapper busy - insert in Swap-wait-fifo
       % ...
    FI
OUT:   X:=MSGTOSW; GO LREG

3.2 5SWACTRT - Start the Swapper RT-Program

Source: MP-P2-N500.NPL line 45

5SWACTRT: A:=L=:"LREG"; X:="S500S"; GO ACT5SWAP    % START 5SWAP

This is called to restart the swapper RT-program after it completes a page I/O operation.

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4. Data Structures

4.1 Swapper Process Descriptor (S500S)

The swapper uses process descriptor #0, located at address S500S:

Offset	Field	Description
0	PSTAT	Process status flags
...	MESSBUFF	Pointer to message buffer (SWMSG)
...	...	Other standard process descriptor fields

Symbol table entry: S500S=115542 (octal)

4.2 Swapper Message Buffer (SWMSG)

Source: MP-P2-N500.NPL line 40578

A:=55MSNEGSIZE+D=:SWMSG

The swapper message buffer is allocated as the first message buffer during MSINIT.

Offset	Field	Description
SWPINFO	Swap info	Page fault information
SWPST	Swap status	Reason for activation
SWPFU	Swap function	Operation to perform
HSWPI	...	Swap information pointer

4.3 Key Variables

Source: DP-P2-VARIABLES.NPL lines 116-118

INTEGER 5SWPROC                    % Swapper process number
INTEGER 5SSEGSIZE                  % Size of Swapper's Segment Table Entry
INTEGER BMMSIZE                    % Size of Swapper's Memory Map Element in bytes

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5. Complete Loading Sequence

sequenceDiagram
    participant SINTRAN as SINTRAN III
    participant INZ500 as INZ500
    participant MSINIT as MSINIT
    participant MON2RT as MON 2RT
    participant 5SWRT as 5SWRT
    participant ND500 as ND-500

    SINTRAN->>INZ500: System startup
    INZ500->>INZ500: 5CONOMD: Detect ND-500 CPUs
    INZ500->>MSINIT: Setup message buffers
    MSINIT->>MSINIT: Allocate SWMSG (swapper msg buffer)
    MSINIT->>MSINIT: Initialize S500S (process #0)
    MSINIT->>MSINIT: Initialize all process descriptors
    INZ500->>MON2RT: Create "5SWAP" RT-program
    MON2RT->>5SWRT: Entry point = 5SWRT
    5SWRT->>5SWRT: Reserve process #0
    5SWRT->>5SWRT: SETIOWAIT (wait for activation)

    Note over ND500: ND-500 process runs...
    ND500->>ND500: PAGE FAULT!
    ND500->>SINTRAN: 5ACTSWAPPER
    SINTRAN->>5SWRT: Wake up (MONICO)
    5SWRT->>5SWRT: MON 131 (disk I/O)
    5SWRT->>SINTRAN: MONICO (restart ND-500)
    SINTRAN->>ND500: Resume process

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6. Why Swapper Runs on ND-100

The swapper runs on the ND-100 because:

1. Disk I/O Controllers - Disk controllers are connected to the ND-100 I/O bus, not the ND-500
2. SINTRAN Ownership - All disk drivers and file system code are part of SINTRAN on ND-100
3. Monitor Calls - The swapper uses MON 131 (ABSLI - absolute disk I/O) which is an ND-100 monitor call
4. Simplicity - Only one copy of disk driver code needed (on ND-100)
5. Coordination - ND-100 coordinates all ND-500 processes and their memory needs

The ND-500 sends page fault requests via the 5MPM message buffer, and the ND-100 swapper services these requests.

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7. Related Documentation

• ND500-SWAPPER-ANALYSIS.md - Swapper event-driven behavior
• ND500-SCHEDULING-ANALYSIS.md - Process scheduling
• ND500-MONITOR-CALL-MECHANISM.md - Monitor call dispatch
• ../OS/06-MULTIPORT-MEMORY-AND-ND500-COMMUNICATION.md - 5MPM architecture

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8. Source Code References

File	Lines	Content
5P-P2-MON60.NPL	616-680	INZ500 initialization
MP-P2-N500.NPL	40533-40667	XMSINIT message buffer setup
MP-P2-N500.NPL	2857-2907	5ACTSWAPPER routine
MP-P2-N500.NPL	45	5SWACTRT entry point
RP-P2-N500.NPL	12-58	5SWRT RT-program
DP-P2-VARIABLES.NPL	116-118	Swapper variables

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9. Summary

The ND-500 swapper is loaded as follows:

1. INZ500 is called during SINTRAN system startup
2. MSINIT allocates the swapper message buffer (SWMSG) and process descriptor (S500S)
3. MON 2RT creates the "5SWAP" RT-program with entry point at 5SWRT
4. 5SWRT runs on ND-100, waiting for activation
5. When ND-500 process page faults, 5ACTSWAPPER wakes the swapper
6. Swapper performs disk I/O via MON 131 (ABSLI)
7. Swapper restarts ND-500 via MONICO

Critical Point: The swapper is NOT loaded into ND-500 memory. It runs entirely on the ND-100 as an RT-program, communicating with ND-500 via shared multiport memory (5MPM).

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10. Verification and Assumptions

Verified from Source Code

The following facts are directly verified from NPL source code:

Claim	Source	Line(s)
5SWAP RT-program created via MON 2RT	5P-P2-MON60.NPL	674
5SWRT is the swapper entry point	RP-P2-N500.NPL	16-17
SWMSG is swapper message buffer	MP-P2-N500.NPL	40578
S500S is process descriptor #0	MP-P2-N500.NPL	40581
5SWPROC is swapper process number	DP-P2-VARIABLES.NPL	116
Swapper uses MON 131 (ABSLI) for disk I/O	RP-P2-N500.NPL	37
5ACTSWAPPER activates the swapper	MP-P2-N500.NPL	2857
MSINIT initializes message buffers	5P-P2-MON60.NPL	668
INZ500 is called during system init	5P-P2-MON60.NPL	616
Swapper waits via SETIOWAIT	RP-P2-N500.NPL	28
MONICO restarts ND-500 process	RP-P2-N500.NPL	51

Interpretations (Low Uncertainty)

The following are interpretations based on code patterns but not explicitly stated in comments:

1. "Swapper runs on ND-100" - Interpretation based on:

   • 5SWRT uses ND-100 monitor calls (MON 131)
   • Code is in RP-P2-N500.NPL which contains ND-100 RT-programs
   • Uses *2BANK (ND-100 instruction)
   • Comments say "N500-ABSTRANS PROGRAM" but it performs disk I/O via ND-100

2. "Page fault triggers swapper" - Interpretation based on:

   • Comments mention "trap (pagefault)" at line 2879
   • MICFU field set to 3 (3SWMESS or trap number)
   • Source comment at line 2876: "% Message to swapper?"

3. "Process #0 is special swapper process" - Interpretation based on:

   • 5SWPROC=:MSINPROCNO; X:="S500S" suggests process #0 = swapper
   • Consistent use of S500S for swapper operations

Assumptions (Higher Uncertainty)

1. MON 2RT parameter format - Assumed that INZRTP contains "5SWAP" name and INZPRT contains entry point address. The exact MON 2RT parameter format is not fully documented in this code.

2. Exact message buffer layout - The exact byte-level layout of SWMSG fields (SWPINFO, SWPST, SWPFU, HSWPI) is inferred from field offsets in AAX instructions, not from explicit structure definitions.

3. Diagram accuracy - The sequence diagram shows logical flow; exact timing and intermediate steps may differ.

What This Document Does NOT Cover

• Swapper algorithm details - How the swapper decides which pages to swap
• Swap file management - How swap files are created and managed
• Page table management - How ND-500 page tables are updated
• Memory allocation - How physical memory is allocated for ND-500 processes

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Document Version: 1.0 Created: 2026-01-29 Source: SINTRAN III NPL Source Code Analysis