TAD (Terminal Access Device) Protocol Analysis¶

Overview¶

This document analyzes SINTRAN III's TAD protocol implementation for X.25 network communication. TAD enables terminal access over X.25 networks, allowing remote terminals to connect to SINTRAN systems.

Source Files: - MP-P2-TAD.NPL (989 lines) - TAD driver with XMSG interface and interrupt handling - RP-P2-TAD.NPL (1480 lines) - TAD protocol routines with message creation/processing

Message Types¶

Core Message Type Constants¶

Defined in MP-P2-TAD.NPL:328-338:

Symbol	Octal Code	Hex	Message Type	Purpose
EBUFF	7DUMM	0x7D554D	Empty Buffer/Dummy	Padding or empty message
BDESC	7ESCA	0x7E5343	Escape	Escape character received
RLOCA	7RLOC	0x7D4C4F	Remote Local	Remote/local mode switch (NORD-NET)
BDDIS	7DCON	0x7D434F	Disconnect	TAD disconnection request
CESCR	7CERS	0x7D455253	CESC Response	Escape response
RESCF	7RECO	0x7D45434F	Reset Confirm	Reset confirmation
NWREM	7NWRE	0x7D5752	Nowait Restart	Restart nowait mode
ISZRS	7ISRS\2	0x7D495253	ISIZE Response	Buffer size response (2 bytes data)
ERRSP	7ERRS\2	0x7D455252	Error Response	Error code response (2 bytes data)
TREPS	7TREP\2	0x7D545245	TREP Status	Transmission status (2 bytes data)

Additional Message Types¶

Found in RP-P2-TAD.NPL:

Symbol	Octal Code	Message Type	Purpose	Data Size
7BDAT	7BDAT	Data Message	User data transmission	Variable
7TMOD	7TMOD	Terminal Mode	Terminal mode settings	1 byte
7TTYP	7TTYP	Terminal Type	Terminal type code	2 bytes
7DESC	7DESC	Define Escape	Set escape character	1 byte
78MOD	78MOD	8-bit Mode	Enable 8-bit UMOD	2 bytes
7OPSV	7OPSV	OPSYS Version	OS version and protocol	3 bytes
7REJE	7REJE	Reject	Reject invalid message	1 byte (rejected type)
7RFI	7RFI	Ready For Input	Request more input data	0 bytes
7RESE	7RESE	Reset	Reset connection	0 bytes
7BMMX	7BMMX	Break Message	Break strategy	3 or 23 bytes
7ECKM	7ECKM	Echo Message	Echo strategy	1 or 21 bytes
7SYCN	7SYCN	System Control	System control command	2 bytes
7USCN	7USCN	User Control	User control command	2 bytes
7CPCO	7CPCO	Completion Code	Operation completion	4 bytes
7NOWT	7NOWT	Nowait Status	Nowait operation status	1 byte
7TNOW	7TNOW	Terminate Nowait	End nowait mode	1 byte
7ISRQ	7ISRQ	ISIZE Request	Query buffer size	0 bytes
7UMOD	7UMOD	UMOD Strategy	User mode strategy	2 bytes

Message Format Structure¶

General Message Format¶

+-------------------+-------------------+-------------------+
| Message Type      | Byte Count        | Data Field        |
| (1 byte)          | (1 byte)          | (0-255 bytes)     |
+-------------------+-------------------+-------------------+

Alignment Rules: - Messages can start on even or odd byte boundaries - If message starts on odd byte, a pad byte (0x00) is inserted first - Byte count includes only the data field, not the header

Message Header Creation¶

Source: RP-P2-TAD.NPL:162-178 (WRMHEAD)

WRMHEAD: A=:D; T:=TDTAFI; X:=TDTALA
         IF TDBTPT BIT "0" THEN                    % ODD START, CREATE PAD BYTE
            A SHZ -1; X+A; *LDATX
            A/\177400; *STATX
            MIN TDBTPT; REMSIZ-1=:REMSIZ
         FI
         X:=TDTALA
         TDBTPT SHZ -1; X+A; D=:A; *STATX          % WRITE IN BUFFER
         TDBTPT+2=:TDBTPT; REMSIZ-2=:REMSIZ
         EXIT

Message Parsing¶

Source: RP-P2-TAD.NPL:224-242 (GETMES)

GETMES: IF BUFFID=0 THEN A:=1; EXIT FI             % NO BUFFER
TSTSP:  IF REMSIZ<2 THEN T=:A; EXIT FI
        T:=TDTAFI; X:=TDTALA; TDBTPT=:D SHZ -1
        X+A; *LDATX
        MIN TDBTPT
        IF D BIT "0" THEN                          % ODD BYTE
           IF A/\377=0 THEN REMSIZ-1=:REMSIZ; GO TSTSP FI  % PAD BYTE
           A=:CURMES; X+1; *LDATX
           A SHZ -10; MIN TDBTPT; GO RCHK
        FI
        A=:T SHZ -10
        IF A=0 THEN
           REMSIZ-1=:REMSIZ; GO TSTSP              % PAD BYTE
        FI
        A=:CURMES:=T/\377; MIN TDBTPT
RCHK:   IF A>REMSIZ-2 THEN A:=3; EXIT FI           % INCONSISTENT MESSAGE
        A=:T+1-=:REMBYT; X:=REMSIZ-2=:REMSIZ
        A:=CURMES; EXITA                           % MESSAGE FOUND

Returns: - Skip return (EXITA): Message found - A-REG = Message type - T-REG = Number of bytes in data field - CURMES = Message type - REMBYT = Bytes remaining in message - Error returns: - A=1: No input buffer - A=2: Buffer empty - A=3: Inconsistent message (bigger than buffer)

TAD Driver Entry Points¶

Main Entry Points¶

Source: MP-P2-TAD.NPL:363-367

Entry Point	Line	Purpose	Called From
INIBDR	382	Initialize TAD connection	TADADM (connection phase)
INISND	398	Send initial buffer	TADADM (after INIBDR)
BDRINP	439	Input driver (receive)	XMSG when buffer queued
TDRINP	440	Input driver entry (internal)	BDRINP, BDROUT
BDROUT	800	Output driver (transmit)	Output monitor calls
BERESP	638	Send escape response	Escape handling
STOTAD	660	Stop TAD connection	User logout/disconnect
DSTOTA	661	Disconnect TAD (internal)	STOTAD, FAERR
BDLOUT	695	Logout TAD user	STOTAD when TLREP disabled
BDDSCN	713	Disconnect without logout	Explicit disconnect request
TDDSCN	714	Disconnect (internal)	BDDSCN, DSTOTA
BDRWT	847	Leave driver level	All driver routines
BMAIRES	741	Reserve mail buffer	Mail system (TWMESS)
MSNDBUF	769	Send mail buffer	Mail timeout routine
MRELBUF	789	Release mail buffer	Mail timeout (partner not responding)
FAERR	843	Fatal error handler	Error conditions

RPIT Entry Points¶

Source: RP-P2-TAD.NPL

Entry Point	Line	Purpose	Context
PUTPOOL	20	Return buffer to free pool	IOF (interrupt off)
GETPOOL	44	Get buffer from free pool	IOF (interrupt off)
MOVITO	63	Move input buffer to output	Buffer relocation
SNDBUF	87	Send output buffer	Output operations
SNDWT	84	Send and wait for response	Operations requiring response
CBRERSP	106	Send escape response callback	Escape handling
BDTOU	121	Normal output timeout	Timeout timer
MTDTOU	135	Mail output timeout	Mail system timeout
WRMHEAD	168	Write message header	Message creation
CREMES	198	Create message	Output operations
CRHEEV	263	Create header (even byte)	Complete messages
CRHEOD	293	Create header (odd byte)	Complete messages
UPDMBC	315	Update message byte count	Byte operations
GETMBC	343	Get message byte count	Query operations
BYTPUT	365	Put byte in buffer	Output operations
WORDPUT	398	Put word in buffer	Output operations
BYTGET	431	Get byte from buffer	Input operations
LOADBYT	452	Load byte (no buffer update)	Scanning operations
STORBYT	469	Store byte in buffer	Output operations
GETMES	224	Get next message	Input processing
REDOM	495	Redo monitor call	Buffer wait conditions
IEDCHK	514	Check delayed escape	Input validation
CNVERR	536	Convert XMSG error	Error handling

Data Structure Fields¶

TAD Input Datafield¶

Field	Purpose	Type
BUFFID	Current input buffer ID	INTEGER
TDTADD	Buffer address (TDTAFI:TDTALA)	TRIPLE
TDTAFI	Buffer address (file index)	INTEGER
TDTALA	Buffer address (local address)	INTEGER
TDBTPT	Byte pointer in buffer	INTEGER
REMSIZ	Remaining size in buffer	INTEGER
CURMES	Current message type	INTEGER
REMBYT	Remaining bytes in message	INTEGER
PORTNO	XMSG port number	INTEGER
PARTNER	Partner port number	TRIPLE
RPORT	Remote port number	INTEGER
DFLAG	Datafield flags	INTEGER
FLAGB	Additional flags	INTEGER
TINFO	Terminal information	INTEGER
CESCP	Escape character	INTEGER
CTTYP	Terminal type	INTEGER
FBSIZ	Frame buffer size	INTEGER
NOBUFF	Number of buffers	INTEGER
LAST	Last character processed	INTEGER
DERROR	Error code	INTEGER
BRECST	Break/echo strategy	INTEGER
OSVTPN	OS version + TAD protocol	INTEGER
TDRADDR	TAD resident datafield address	ADDRESS
ISTATE	Input state (0=normal, <0=nowait)	INTEGER

TAD Output Datafield¶

All input fields plus:

Field	Purpose	Type
POOLLI	Free buffer pool list	TRIPLE
BUDIS	Buffer displacement	INTEGER
NOBDIS	Message start displacement	INTEGER
RSPNUM	Response number waiting	INTEGER
CURBRST	Current break strategy	INTEGER
CURECST	Current echo strategy	INTEGER
SCREEN	Screen/page control	INTEGER
MBFID	Mail buffer ID	INTEGER
BRCOUNT	Buffer rotate count	INTEGER

Critical Flag Bits¶

DFLAG bits: | Bit | Name | Purpose | |-----|------|---------| | 5CAPITAL | Capital letters mode | Convert to uppercase | | 5IESC | Escape disabled | Escape processing disabled | | 5RQI | RFI sent | Ready-for-input message sent | | 5WRQI | Wait RFI | Waiting to send RFI |

FLAGB bits: | Bit | Name | Purpose | |-----|------|---------| | 5LBLOG | Logout on carrier loss | Auto-logout on disconnect | | 5LSTA | Line status | Line active/dead | | 5LOGOUT | Logout pending | User logout requested | | 5TLREP | Terminal reply | Wait for terminal response | | 5LCHAR | Local character | Local character mode | | 5ESCLOFF | Escape delayed | Delayed escape action | | 5WESC | Wait escape | Escape waiting | | 5WLOC | Wait local | Local mode waiting |

TINFO bits: | Bit | Name | Purpose | |-----|------|---------| | 5CRDLY | CR delay | Carriage return delay | | 58BIT | 8-bit mode | Allow 8-bit characters | | 5UMOD | User mode | User mode active | | 5BFUL | Buffer overrun | Buffer overflow occurred | | 5PAER | Parity error | Parity error detected | | 5FRER | Framing error | Framing error detected |

TAD Connection Lifecycle¶

1. Connection Establishment¶

Sequence: INIBDR → INISND → BDRINP

Phase 1: Initialize Connection (`INIBDR` - Line 382)¶

INIBDR: CALL STADIWINDOW                           % SET WINDOW
        IF PORTNO=0 THEN                           % NO PORT OPENED
          0=:TDRADDR.BXTADD; T:=XFOPN; CALL MXMSG  % OPEN XMSG PORT
          A=:PORTNO                                % SET PORTNO IN DATAFIELD
          A:=0=:D; AD=:OTAD.POOLLI                 % POOL EMPTY
          T:=XFALM; FBSIZ; X:=NOBUFF; CALL MXMSG   % ALLOCATE MESSAGE SPACE
          FOR X:=1 TO NOBUFF DO
             T:=XFGET; A:=FBSIZ; CALL MXMSG        % RESERVE BUFFERS
             % ... add to pool ...
          OD
        FI
        GO BDRWT

Operations: 1. Open XMSG port (XFOPN) 2. Allocate message space (XFALM) 3. Reserve NOBUFF buffers of size FBSIZ 4. Build free buffer pool (POOLLI)

Phase 2: Send Initial Buffer (`INISND` - Line 398)¶

INISND: CALL STADIWINDOW
        OTAD=:B; CALL MGETPOOL                     % GET BUFFER FROM POOL
        T:=ITAD=:B; T:=PORTNO=:D
        T:=XFSCM; CALL MXMSG                       % SET CURRENT BUFFER
        TAD:=DMMES; T=:X:=XFWHD; CALL MXMSG        % WRITE DUMMY MESSAGE
        AD:=OTAD.PARTNER; X:=PORTNO
        T:=XFSND; CALL MXMSG                       % SEND BUFFER
        T:=XFDBK; A:=DPITBANK; CALL MXMSG          % SET BANK FOR R/W
        T:=XFWDF; A:="BDRINP"; CALL MXMSG          % SET WAKE-UP MODE
        FLAGB BZERO 5LSTA=:FLAGB                   % SET LINE OK BIT
        GO TDRINP                                  % ENTER RECEIVE MODE

Operations: 1. Get buffer from free pool 2. Set current buffer (XFSCM) 3. Write dummy message header 4. Send initial buffer to partner (XFSND) 5. Configure XMSG wake-up for BDRINP 6. Enter receive mode

2. Data Transfer¶

Sending Data¶

Entry: BDPUT (Line 1217) or BM8OUT/BB8OUT (Line 1307)

Flow:

User Data → BDPUT → BYTPUT → CREMES (if needed) → Buffer → SNDBUF → XMSG

Key Steps: 1. Check if current message is data message (7BDAT) 2. If not, create new data message (CREMES) 3. Add bytes to buffer (BYTPUT) 4. If buffer full or timeout, send buffer (SNDBUF)

Receiving Data¶

Entry: BDGET (Line 1257) or BB4INW/BB8INP (Line 1354, 1419)

Flow:

XMSG → BDRINP → GETMES → BYTGET → User Data

Key Steps: 1. Driver receives buffer from XMSG 2. Parse messages with GETMES 3. If data message (7BDAT), extract bytes 4. If buffer empty, send RFI (SNDRFI)

3. Disconnection¶

Graceful Disconnect (`BDDSCN` - Line 713)¶

BDDSCN: CALL STADIWINDOW
TDDSCN: FLAGB BONE 5LSTA=:FLAGB                    % SET LINE DEAD BIT
        IF PORTNO><0 AND TDRADDR.BXTADD><0 THEN
          IF TMPBUF><0 THEN
             0=:TMPBUF; T:=XFREL; CALL MXMSG       % RELEASE CURRENT BUFFER
          FI
          0=:PORTNO=:BUFFID=:OTAD.BUFFID           % CLEAR DATAFIELD
          TDRADDR=:B; BXTADD=:L; 0=:BXTADD
          T:=XFDCT; *MON 2XMSG                     % DISCONNECT PORT
        FI

Operations: 1. Set line dead flag (5LSTA) 2. Release current buffer if any 3. Clear all buffer IDs 4. Disconnect XMSG port (XFDCT)

Forced Disconnect with Logout (`STOTAD` - Line 660)¶

STOTAD: CALL STADIWINDOW
        IF DBPROG><0 THEN
          IF DBPROG=TDRADDR.RTRES AND FLAGB NBIT 5LOGOUT THEN
             IF FLAGB BIT 5TLREP THEN
                A BONE 5LOGOUT=:FLAGB              % MARK LOGOUT
                "IORESTART"; CALL CXRTACT          % RESTART PROGRAM
             ELSE
                CALL BDLOUT                        % LOGOUT USER
             FI
          FI
        FI
        GO TDDSCN                                  % DISCONNECT

Example Message Sequences¶

Connection Establishment¶

Local TAD                    Remote TAD
    |                            |
    |------ XFOPN (open) ------->|
    |<----- Port Allocated ------|
    |                            |
    |------ XFALM (alloc) ------>|
    |<----- Space Reserved ------|
    |                            |
    |------ DUMMY Message ------>|
    |                            |
    |<----- 7TMOD (mode) --------|
    |<----- 7TTYP (type) --------|
    |<----- 7DESC (escape) ------|
    |                            |
    |------ Ready ------------->|

Data Transfer¶

Terminal                     TAD                      Remote TAD
    |                         |                            |
    |--- User types "ABC" --->|                            |
    |                         |                            |
    |                         |--- 7BDAT [ABC] ----------->|
    |                         |                            |
    |                         |<-- 7RFI (ready) -----------|
    |                         |                            |
    |                         |<-- 7BDAT [response] -------|
    |                         |                            |
    |<-- Display response ----|                            |

Break Handling¶

Terminal                     TAD                      Remote TAD
    |                         |                            |
    |--- Break key pressed -->|                            |
    |                         |                            |
    |                         |--- 7BMMX [strategy] ------>|
    |                         |                            |
    |                         |<-- 7ECKM [echo] -----------|
    |                         |                            |
    |<-- Echo response -------|                            |

Disconnection¶

Terminal                     TAD                      Remote TAD
    |                         |                            |
    |--- Logout command ----->|                            |
    |                         |                            |
    |                         |--- 7DCON (disconnect) ---->|
    |                         |                            |
    |                         |--- XFREL (release) ------->|
    |                         |--- XFDCT (disconnect) ---->|
    |                         |                            |
    |<-- Connection closed ---|                            |

Error Recovery¶

Local TAD                    Remote TAD
    |                            |
    |------ Invalid Message ---->|
    |                            |
    |<----- 7REJE (reject) ------|
    |                            |
    |------ 7RFI (request) ----->|
    |                            |
    |<----- 7BDAT (data) --------|

Continue in Part 2...¶

This document continues in TAD-Protocol-Analysis-Part2.md with: - Detailed message format for each message type - XMSG integration details - State machine diagrams - C# emulator implementation guide