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TAD (Terminal Access Device) Analysis Summary

Overview

This document provides a summary of the comprehensive TAD protocol analysis for SINTRAN III X.25 communication.

Analysis Date: 2025-10-13

Analyst: AI-assisted analysis of SINTRAN NPL source code

Documentation Files Created

1. TAD-Protocol-Analysis.md

Purpose: Complete API and protocol reference

Contents: - Message Type Enumeration: 30+ message types with codes and purposes - Message Format Structure: Header format, alignment rules, parsing - TAD Driver Entry Points: 17 main entry points with line numbers - RPIT Entry Points: 31 routine entry points for message processing - Data Structure Fields: Complete input/output datafield specifications - Critical Flag Bits: DFLAG, FLAGB, TINFO bit definitions - Connection Lifecycle: Initialize → Send → Receive → Disconnect - Example Message Sequences: Connection, data transfer, break handling, disconnection, error recovery

Key Findings: - TAD uses XMSG for buffer management and port communication - Messages have even/odd byte alignment with automatic padding - Two priority levels: normal (queued) and high (immediate) - Buffer pool managed via POOLLI chain with MGETPOOL/MPUTPOOL routines

File Size: ~50 KB, comprehensive reference

2. TAD-Message-Formats.md

Purpose: Detailed message format specifications

Contents: - General Message Format: Pad byte, message type, byte count, data field - Message Header Creation: WRMHEAD routine (line 162-178) - Message Parsing: GETMES routine (line 224-242) - Data Messages: 7BDAT format with break handling - Control Messages: 7TMOD, 7TTYP, 7DESC, 78MOD, 7OPSV - Break and Echo Messages: 7BMMX (3 or 23 bytes), 7ECKM (1 or 21 bytes) - Request and Response Messages: 7RFI, 7REJE, 7ISRQ, 7ISRS, 7ERRS - System Control Messages: 7SYCN, 7USCN, 7CPCO - Connection Control Messages: 7RESE, 7RECO, 7DCON - Escape Messages: 7ESCA, 7CERS, 7RLOC - Nowait Mode Messages: 7NOWT, 7TNOW, 7NWRE - Special Messages: 7DUMM, 7TREP, 7UMOD - Message Priority Levels: Normal vs. high-priority processing - Buffer Management Integration: XMSG operations for buffers

Format Details: - Each message documented with: - Purpose and code - Binary format diagram - Data field structure - Source code references - Processing logic (send/receive) - Example messages

File Size: ~75 KB, complete format reference

3. TAD-Protocol-Flows.md

Purpose: Visual protocol flow diagrams

Contents (17 Mermaid Diagrams):

Connection Flows:

  1. Full Connection Sequence: INIBDR → INISND → BDRINP with XMSG calls
  2. Connection State Machine: 8 states (Disconnected → Opening → Connected)

Data Transfer Flows:

  1. Output Data Flow: User → BDPUT → Buffer → XMSG → Remote
  2. Input Data Flow: Remote → XMSG → BDRINP → BDGET → User
  3. 8-Byte Transfer: BB8OUT/BB8INP monitor call flows

Buffer Management:

  1. Buffer Pool Management: Initialize → Allocate → Get → Use → Return
  2. Buffer State Transitions: 5 states (Free → Active → Sending → Processed)

Escape and Break:

  1. Escape Sequence: Detect → Process → Response
  2. Break Message Flow: Set strategy → Detect → Notify
  3. Echo Strategy Flow: Configure → Apply → Echo

Error Handling:

  1. Message Rejection Flow: Parse → Validate → Reject → RFI
  2. Ready-For-Input (RFI) Flow: Empty buffer → Request → Receive
  3. Fatal Error Recovery: Detect → Cleanup → Logout → Disconnect

Disconnection:

  1. Graceful Disconnection: BDDSCN → Release → XFDCT
  2. Remote-Initiated Disconnection: 7DCON → DSTOTA → Cleanup
  3. Forced Disconnection: STOTAD → Logout → TDDSCN

Advanced Features:

  1. Nowait Mode: State machine and status reporting
  2. ISIZE Query: Request → Process → Response
  3. Complete TAD Session: 5-phase terminal session (Connect → Login → Command → Escape → Logout)
  4. Priority Message Handling: High vs. normal priority flowchart

File Size: ~45 KB, 17 comprehensive diagrams

Key Technical Insights

1. X.25 TAD Message Structure

Discovery: TAD messages are variable-length with automatic byte alignment

┌──────────┬────────────┬────────────┬─────────────┐
│ Pad (0-1)│ Type (1)   │ Count (1)  │ Data (0-255)│
└──────────┴────────────┴────────────┴─────────────┘
  • Pad byte inserted if starting on odd boundary
  • Message type = 7-bit ASCII string (e.g., "BDAT")
  • Byte count = data field size only (not including header)
  • Maximum data size = 255 bytes per message

2. XMSG Integration Architecture

Discovery: TAD is built on top of XMSG message-passing layer

XMSG Operations Used: - XFOPN - Open port - XFALM - Allocate message space - XFGET - Get buffer - XFSCM - Set current buffer - XFSND - Send buffer - XFRCV - Receive buffer - XFREL - Release buffer - XFDCT - Disconnect port - XFWHD - Write header - XFRHD - Read header - XFPST - Port status - XFWDF - Wake driver function

Buffer Flow: 1. Allocate pool during INIBDR 2. Get buffer from pool (MGETPOOL) 3. Fill with messages (CREMES, BYTPUT) 4. Send via XMSG (XFSND) 5. Return to pool (MPUTPOOL)

3. Two-Priority Message System

Discovery: TAD processes messages at two priority levels

High Priority (Immediate): - Escape signals (7ESCA, 7RLOC) - Connection control (7DCON) - Response messages (7CERS, 7RECO, 7ISRS, 7ERRS, 7TREP) - Nowait control (7NWRE)

Normal Priority (Queued): - Data messages (7BDAT) - Configuration (7TMOD, 7TTYP, 7DESC, 78MOD, 7OPSV) - Dummy messages (7DUMM)

Processing Difference: - High: Received immediately into TMPBUF, processed before returning - Normal: Received only when BUFFID=0, queued in input buffer

Source: MP-P2-TAD.NPL:445-496

4. Connection Establishment Protocol

Discovery: 3-phase connection with automatic configuration

Phase 1: Initialize (INIBDR) - Open XMSG port - Allocate buffer pool - Reserve N buffers (typically 4-8)

Phase 2: Send Initial (INISND) - Get buffer from pool - Write dummy message header - Send to partner - Configure wake-up for BDRINP

Phase 3: Configuration Exchange - Remote sends: 7TMOD, 7TTYP, 7DESC, 7OPSV - Local applies settings - Connection ready for data

Critical: Remote must send configuration messages before data

5. Break and Echo Strategy Tables

Discovery: Custom character tables for break/echo (20 bytes = 160 bits)

Table Format: - 8 words (16 bits each) - Each bit represents one character (0x00-0x7F) - Bit set = character is break/echo character

Example: Break on CR (0x0D) 

Word 0: bits 0-15   (chars 0x00-0x0F) - bit 13 set
Word 1: bits 0-15   (chars 0x10-0x1F)
...
Word 7: bits 0-15   (chars 0x70-0x7F)

Storage: PBRK7 array in input datafield, PECH7 in output datafield

Source: RP-P2-TAD.NPL:717-727 (CBRECTA)

6. Nowait Mode Operation

Discovery: Asynchronous I/O mode with status reporting

State Variable: ISTATE - 0 = Normal synchronous mode - <0 = Nowait mode active - 2 = Waiting to redo monitor call

Status Messages: - 7NOWT [00] = Success - 7TNOW [error] = Failure - 7NWRE = Restart request

Bit 17 in status = break character indicator

Source: RP-P2-TAD.NPL:1088-1102 (NOWTSTA)

7. RFI (Ready For Input) Mechanism

Discovery: Automatic flow control for input data

Triggers: - User reads from empty buffer - Reject data message - Nowait input request

Processing: 1. Check output buffer available 2. If yes: Create 7RFI message, send 3. If no: Move input buffer to output, send 7RFI 4. If no buffers: Set DFLAG 5WRQI (driver sends later) 5. Mark RFI sent: DFLAG 5RQI

Response: Remote sends 7BDAT with data

Source: RP-P2-TAD.NPL:1115-1147 (SNDRFI)

8. Escape Processing

Discovery: Two-stage escape handling with enable/disable

Escape Types: - 7ESCA = Escape character received - 7RLOC = Remote/local toggle (NORD-NET)

Processing: 1. Check DFLAG 5IESC (escape enabled?) 2. If enabled: - Extract escape char from CESCP - Call escape handler - Send 7CERS response 3. If disabled: - Send 7CERS (disabled response)

Delayed Escape: - FLAGB 5ESCLOFF set during delay - Input blocked until escape sent - Cleared after response

Source: MP-P2-TAD.NPL:602-625, RP-P2-TAD.NPL:514-528

9. Error Recovery Strategies

Discovery: Multi-level error handling

Message Errors: - Inconsistent size (A=3) → Send 7REJE - Unknown type → Send 7REJE - Rejected data message → Also send 7RFI

Buffer Errors: - Pool empty → Wait for buffer return or set 5WRQI - Buffer full → Send current, get new

Connection Errors: - XMSG error → Convert with CNVERR, disconnect - Fatal error → FAERR → DSTOTA → logout/disconnect

Source: RP-P2-TAD.NPL:1161-1201 (SNDREJ), MP-P2-TAD.NPL:843-845 (FAERR)

10. OPSYS Version and Protocol Negotiation

Discovery: Version exchange during connection

Format: 7OPSV [3 bytes] - Byte 0: SINTRAN OS version - Bytes 1-2: TAD protocol number

Protocol Features: | Protocol | Features | |----------|----------| | 1-2 | Basic TAD | | 3 | Break strategy 8,9; Screen control; Mail system | | 4+ | UMOD message support |

Processing: Stored in OSVTPN field, checked before protocol-specific features

Source: MP-P2-TAD.NPL:223-235 (BDOPSV)

Emulator Implementation Checklist

Required Message Support

Connection Phase: - [ ] 7TMOD - Terminal mode - [ ] 7TTYP - Terminal type - [ ] 7DESC - Escape character - [ ] 7OPSV - OS version (recommend protocol 3)

Data Transfer: - [ ] 7BDAT - Data messages - [ ] 7RFI - Ready for input - [ ] 7REJE - Message reject

Break/Echo: - [ ] 7BMMX - Break message (strategies 1-7) - [ ] 7ECKM - Echo message (strategies 1-7)

Control: - [ ] 7SYCN - System control - [ ] 7USCN - User control - [ ] 7CPCO - Completion code

Connection Control: - [ ] 7RESE - Reset - [ ] 7RECO - Reset confirm - [ ] 7DCON - Disconnect

Escape: - [ ] 7ESCA - Escape signal - [ ] 7CERS - Escape response

Query: - [ ] 7ISRQ - ISIZE request - [ ] 7ISRS - ISIZE response

Optional (Protocol 3+): - [ ] 7NOWT/7TNOW - Nowait status - [ ] 7NWRE - Nowait restart - [ ] 7TREP - TREP status - [ ] 78MOD - 8-bit mode

Optional (Protocol 4+): - [ ] 7UMOD - UMOD strategy

Buffer Management

  • [ ] Implement buffer pool (4-8 buffers)
  • [ ] Buffer size: Configurable (typically 512-2048 bytes)
  • [ ] Track buffer states (Free/Input/Output/Temp/Mail)
  • [ ] Implement POOLLI chain for free buffers

Message Processing

  • [ ] Parse message headers (handle even/odd alignment)
  • [ ] Support priority levels (high vs. normal)
  • [ ] Implement message queue for normal priority
  • [ ] Process high-priority messages immediately

State Management

  • [ ] Track connection state (Disconnected → Connected)
  • [ ] Track ISTATE for nowait mode
  • [ ] Maintain DFLAG, FLAGB, TINFO flags
  • [ ] Track RSPNUM for response waiting

XMSG Integration

  • [ ] Map to your transport layer (X.25, TCP, etc.)
  • [ ] Implement buffer exchange mechanism
  • [ ] Support wake-up notification for input
  • [ ] Handle partner addressing

Testing Recommendations

Phase 1: Connection Testing

  1. Verify INIBDR allocates buffers
  2. Verify INISND sends dummy message
  3. Verify remote sends 7TMOD, 7TTYP, 7DESC
  4. Verify settings applied correctly

Phase 2: Data Transfer Testing

  1. Send simple 7BDAT messages
  2. Verify byte alignment handling
  3. Test buffer full conditions
  4. Test RFI mechanism

Phase 3: Control Message Testing

  1. Test break strategies (1-6)
  2. Test echo strategies (1-6)
  3. Test escape processing
  4. Test system/user control

Phase 4: Error Handling Testing

  1. Send invalid messages (verify 7REJE)
  2. Test buffer exhaustion (verify 5WRQI)
  3. Test XMSG errors (verify CNVERR)
  4. Test fatal errors (verify FAERR → disconnect)

Phase 5: Disconnection Testing

  1. Test graceful disconnect (BDDSCN)
  2. Test remote disconnect (7DCON)
  3. Test forced disconnect (STOTAD)
  4. Verify cleanup (buffers released, port closed)

Phase 6: Advanced Feature Testing

  1. Test ISIZE query/response
  2. Test nowait mode operation
  3. Test custom break/echo tables (strategy 7)
  4. Test 8-bit mode (78MOD)
  5. Test completion codes (7CPCO)

Source Code Statistics

MP-P2-TAD.NPL (Driver)

  • Lines: 989
  • Entry Points: 17
  • Key Functions:
    • INIBDR (382): Initialize connection
    • INISND (398): Send initial buffer
    • BDRINP (439): Input driver
    • BDROUT (800): Output driver
    • BERESP (638): Escape response
    • STOTAD (660): Stop TAD

RP-P2-TAD.NPL (Routines)

  • Lines: 1,480
  • Entry Points: 31+
  • Key Functions:
    • GETPOOL (44): Get buffer from pool
    • PUTPOOL (20): Return buffer to pool
    • CREMES (198): Create message
    • GETMES (224): Get message
    • BYTPUT (365): Put byte
    • BYTGET (431): Get byte
    • SNDBUF (87): Send buffer
    • SNDRFI (1115): Send RFI
    • SNDREJ (1161): Send reject

Total Analysis

  • Total Lines Analyzed: 2,469
  • Message Types Documented: 30+
  • Entry Points Documented: 48
  • Mermaid Diagrams Created: 17
  • Documentation Pages: ~170 KB total

XMSG Analysis

  • Location: Source Code\Sintran L\Analysis\hdlc-analysis\XMSG_Metadata_Buffer_Analysis.md
  • Purpose: XMSG buffer and port management patterns
  • Relevance: TAD relies heavily on XMSG for all buffer operations

HDLC Analysis

  • Location: Source Code\Sintran L\Analysis\hdlc-analysis\
  • Purpose: Low-level X.25/HDLC protocol implementation
  • Relevance: TAD sits on top of XMSG which uses HDLC for transport

SCSI Analysis

  • Location: Source Code\Sintran L\NPL\
  • Files: SCSI-*.md
  • Purpose: Similar driver analysis methodology
  • Relevance: Demonstrates SINTRAN driver architecture patterns

Implementation Notes

Critical Implementation Details

  1. Byte Alignment: Always check if TDBTPT BIT "0" and insert pad byte (0x00) if starting on odd boundary

  2. Buffer IDs: Never reuse buffer IDs - always use XMSG-provided IDs

  3. Response Waiting: When RSPNUM != 0, connection is waiting for specific response message

  4. High Priority: Messages like 7ESCA, 7DCON must be processed immediately, not queued

  5. RFI Flow Control: When buffer empty, always send 7RFI to request more data

  6. POOLLI Chain: Maintain linked list of free buffers with proper head/tail management

  7. ISTATE Tracking: Critical for nowait mode - never mix synchronous and asynchronous operations

  8. Flag Bit Management: DFLAG, FLAGB, and TINFO bits must be maintained correctly for proper operation

  9. Error Conversion: XMSG errors must be converted using CNVERR before reporting to user

  10. Cleanup on Disconnect: Must release ALL buffers (TMPBUF, BUFFID, MBFID) and clear BXTADD before XFDCT

Conclusion

The TAD protocol analysis reveals a sophisticated terminal access system with:

  • Message-based communication over X.25 networks
  • Two-priority message system for responsive control
  • Automatic flow control with RFI mechanism
  • Flexible break/echo strategies with custom tables
  • Comprehensive error recovery at multiple levels
  • Efficient buffer management via XMSG integration

The protocol supports both synchronous and asynchronous (nowait) operation, escape handling, remote/local switching, and extensive terminal configuration.

For emulator implementation, the most critical aspects are: 1. Correct message format handling (alignment, padding) 2. Proper buffer pool management 3. High-priority message processing 4. RFI flow control 5. State machine management

All documentation is complete and ready for emulator development.

Document Path: Source Code\Sintran L\NPL\TAD-Analysis-Summary.md

Complete Documentation Set: 1. TAD-Protocol-Analysis.md - API and protocol reference 2. TAD-Message-Formats.md - Message format specifications 3. TAD-Protocol-Flows.md - Protocol flow diagrams 4. TAD-Analysis-Summary.md - This summary document

Total Documentation: ~170 KB, comprehensive coverage of TAD protocol