Loader Debugger Protocol



                                  RFC-909










                            Christopher Welles

                      BBN Communications Corporation


                              Walter Milliken

                             BBN Laboratories




                                 July 1984

Status of This Memo

   This RFC specifies a proposed protocol for the ARPA Internet
   community, and requests discussion and suggestions for
   improvements.  Distribution of this memo is unlimited.









                             Table of Contents





     1   Introduction.......................................... 1
     1.1   Purpose of This Document............................ 1
     1.2   Summary of Features................................. 2

     2   General Description................................... 3
     2.1   Motivation.......................................... 3
     2.2   Relation to Other Protocols......................... 4
     2.2.1   Transport Service Requirements.................... 5

     3   Protocol Operation.................................... 9
     3.1   Overview............................................ 9
     3.2   Session Management.................................. 9
     3.3   Command Sequencing................................. 10
     3.4   Data Packing and Transmission...................... 10
     3.5   Implementations.................................... 12

     4   Commands and Formats................................. 15
     4.1   Packet Format...................................... 15
     4.2   Command Format..................................... 16
     4.2.1   Command Header................................... 16
     4.3   Addressing......................................... 19
     4.3.1   Long Address Format.............................. 20
     4.3.2   Short Address Format............................. 25

     5   Protocol Commands.................................... 29
     5.1   HELLO Command...................................... 29
     5.2   HELLO_REPLY........................................ 29
     5.3   SYNCH Command...................................... 33
     5.4   SYNCH_REPLY........................................ 34
     5.5   ABORT Command...................................... 35
     5.6   ABORT_DONE Reply................................... 35
     5.7   ERROR Reply........................................ 36
     5.8   ERRACK Acknowledgement............................. 39

     6   Data Transfer Commands............................... 41
     6.1   WRITE Command...................................... 42
     6.2   READ Command....................................... 43
     6.3   READ_DATA Response................................. 45
     6.4   READ_DONE Reply.................................... 47
     6.5   MOVE Command....................................... 48
     6.6   MOVE_DATA Response................................. 50



                                                                Page i







     6.7   MOVE_DONE Reply.................................... 52
     6.8   REPEAT_DATA........................................ 53
     6.9   WRITE_MASK Command (Optional)...................... 54

     7   Control Commands..................................... 59
     7.1   START Command...................................... 59
     7.2   STOP Command....................................... 61
     7.3   CONTINUE Command................................... 62
     7.4   STEP Command....................................... 62
     7.5   REPORT Command..................................... 63
     7.6   STATUS Reply....................................... 64
     7.7   EXCEPTION Trap..................................... 66

     8   Management Commands.................................. 69
     8.1   CREATE Command..................................... 69
     8.2   CREATE_DONE Reply.................................. 74
     8.3   DELETE Command..................................... 75
     8.4   DELETE_DONE Reply.................................. 76
     8.5   LIST_ADDRESSES Command............................. 76
     8.6   ADDRESS_LIST Reply................................. 77
     8.7   LIST_BREAKPOINTS Command........................... 79
     8.8   BREAKPOINT_LIST Reply.............................. 80
     8.9   LIST_PROCESSES Command............................. 82
     8.10   PROCESS_LIST Reply................................ 83
     8.11   LIST_NAMES Command................................ 84
     8.12   NAME_LIST Reply................................... 85
     8.13   GET_PHYS_ADDR Command............................. 87
     8.14   GOT_PHYS_ADDR Reply............................... 88
     8.15   GET_OBJECT Command................................ 90
     8.16   GOT_OBJECT Reply.................................. 91

     9   Breakpoints and Watchpoints.......................... 93
     9.1   BREAKPOINT_DATA Command............................ 95

     10   Conditional Commands................................ 99
     10.1   Condition Command Format......................... 100
     10.2   COUNT Conditions................................. 101
     10.3   CHANGED Condition................................ 102
     10.4   COMPARE Condition................................ 103
     10.5   TEST Condition................................... 105

     11   Breakpoint Commands................................ 109
     11.1   INCREMENT Command................................ 109
     11.2   INC_COUNT Command................................ 110
     11.3   OR Command....................................... 111
     11.4   SET_PTR Command.................................. 112
     11.5   SET_STATE Command................................ 113



     Page ii







     A   Diagram Conventions................................. 115

     B   Command Summary..................................... 117

     C   Commands, Responses and Replies..................... 121

     D   Glossary............................................ 123











































                                                              Page iii







                                  FIGURES




     1  Relation to Other Protocols............................ 4
     2  Form of Data Exchange Between Layers................... 6
     3  Packing of 16-bit Words............................... 11
     4  Packing of 20-bit Words............................... 12
     5  Network Packet Format................................. 15
     6  LDP Command Header Format............................. 16
     7  Command Classes....................................... 17
     8  Command Types......................................... 18
     9  Long Address Format................................... 20
     10  Long Address Modes................................... 21
     11  Short Address Format................................. 26
     12  Short Address Modes.................................. 27
     13  HELLO Command Format................................. 29
     14  HELLO_REPLY Format................................... 30
     15  System Types......................................... 31
     16  Target Address Codes................................. 31
     17  Feature Levels....................................... 32
     18  Options.............................................. 33
     19  SYNCH Command Format................................. 33
     20  SYNCH_REPLY Format................................... 34
     21  ABORT Command Format................................. 35
     22  ABORT_DONE Reply Format.............................. 36
     23  ERROR Reply Format................................... 37
     24  ERROR Codes.......................................... 38
     25  ERRACK Command Format................................ 40
     26  WRITE Command Format................................. 42
     27  READ Command Format.................................. 44
     28  DATA Response Format................................. 46
     29  READ_DONE Reply Format............................... 47
     30  MOVE Command Format.................................. 49
     31  MOVE_DATA Response Format............................ 51
     32  MOVE_DONE Reply Format............................... 52
     33  REPEAT_DATA Command Format........................... 54
     34  WRITE_MASK Format.................................... 56
     35  START Command Format................................. 60
     36  STOP Command Format.................................. 61
     37  CONTINUE Command Format.............................. 62
     38  STEP Command Format.................................. 63
     39  REPORT Command Format................................ 64
     40  STATUS Reply Format.................................. 65
     41  EXCEPTION Format..................................... 66
     42  CREATE Command Format................................ 70



     Page iv







     43  Create Types......................................... 71
     44  CREATE BREAKPOINT Format............................. 71
     45  CREATE MEMORY_OBJECT Format.......................... 73
     46  CREATE_DONE Reply Format............................. 74
     47  DELETE Command Format................................ 75
     48  DELETE_DONE Reply Format............................. 76
     49  LIST_ADDRESSES Command Format........................ 77
     50  ADDRESS_LIST Reply Format............................ 78
     51  LIST_BREAKPOINTS Command Format...................... 80
     52  BREAKPOINT_LIST Reply Format......................... 81
     53  LIST_PROCESSES Command Format........................ 82
     54  PROCESS_LIST Reply Format............................ 84
     55  LIST_NAMES Command Format............................ 85
     56  NAME_LIST Reply Format............................... 86
     57  GET_PHYS_ADDR Command Format......................... 88
     58  GOT_PHYS_ADDR Reply Format........................... 89
     59  GET_OBJECT Command Format............................ 90
     60  GOT_OBJECT Reply Format.............................. 91
     61  Commands to Manipulate Breakpoints................... 93
     62  Breakpoint Conditional Command Lists................. 95
     63  BREAKPOINT_DATA Command Format....................... 96
     64  Breakpoint Data Stream Format........................ 97
     65  Conditional Command Summary.......................... 99
     66  Condition Command Header............................ 101
     67  COUNT Condition Format.............................. 101
     68  CHANGED Condition................................... 102
     69  COMPARE Condition................................... 104
     70  TEST Condition...................................... 106
     71  Breakpoint Command Summary.......................... 109
     72  INCREMENT Command Format............................ 110
     73  INC_COUNT Command Format............................ 111
     74  OR Command Format................................... 111
     75  SET_PTR Command Format.............................. 112
     76  SET_STATE Command Format............................ 113
     77  Sample Diagram...................................... 115
     78  Command Summary..................................... 118
     79  Commands, Responses and Replies..................... 122













                                                                Page v







                                 CHAPTER 1


                               Introduction



          The Loader-Debugger Protocol (LDP) is an  application  layer
     protocol   for  loading, dumping  and  debugging  target machines
     from hosts in a network environment.  This protocol  is  designed
     to  accommodate a variety  of  target  cpu  types.  It provides a
     powerful set of debugging services.  At  the  same  time,  it  is
     structured  so  that  a  simple  subset  may  be  implemented  in
     applications like boot  loading   where efficiency and space  are
     at a premium.


          The authors would like  to  thank  Dan  Franklin  and  Peter
     Cudhea  for providing many of the ideas on which this protocol is
     based.




     1.1  Purpose of This Document

          This is a technical specification for the LDP protocol.   It
     is intended to be comprehensive enough to be used by implementors
     of the  protocol.   It  contains  detailed  descriptions  of  the
     formats  and usage of over forty commands.  Readers interested in
     an overview of LDP should read the Summary  of  Features,  below,
     and  skim  Sections  2  through  3.1.   Also  see Appendix B, the
     Command Summary.  The remainder of the document reads  best  when
     accompanied by strong coffee or tea.

















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     RFC-909                                                 July 1984



     1.2  Summary of Features

          LDP has the following features:

          o   commands to perform loading, dumping and debugging

          o   support for multiple connections to a single target

          o   reliable performance in an internet environment

          o   a small protocol subset for target loaders

          o   addressing  modes  and  commands  to  support   multiple
              machine types

          o   breakpoints and watchpoints  which  run  in  the  target
              machine.

































     Page 2



     LDP Specification                             General Description



                                 CHAPTER 2


                            General Description



     2.1  Motivation

          LDP is an  application  protocol  that  provides  a  set  of
     commands  used  by  application programs for loading, dumping and
     debugging target machines across a network.

          The goals of this protocol are shown in the following list:


          o   The protocol should support various processor types  and
              operating  systems.   Overhead  and complexity should be
              minimized for simpler cases.


          o   The protocol should provide support for applications  in
              which  more  than  one  user  can  debug the same target
              machine.  This implies an underlying transport mechanism
              that supports multiple connections between a host-target
              pair.


          o   LDP should have a minimal subset of  commands  for  boot
              loading  and dumping.  Target machine implementations of
              these applications are often restricted in the amount of
              code-space  they  may  take.   The  services  needed for
              loading and dumping  should  be  provided  in  a  small,
              easily implemented set of commands.


          o   There should be a means for communicating exceptions and
              errors from the target LDP process to the host process.


          o   LDP should allow the application to implement a full set
              of debugging functions without crippling the performance
              of the target's application (i.e., PSN,  PAD,  gateway).
              For  example,  a  breakpoint  mechanism  that  halts the
              target machine while breakpoint commands are  sent  from
              the  host  to the target is of limited usefulness, since
              the target will  be  unable  to  service  the  real-time



                                                                Page 3



     RFC-909                                                 July 1984



              demands of its application.



     2.2  Relation to Other Protocols

          LDP is an application protocol that fits  into  the  layered
     internet  protocol environment. Figure 1 illustrates the place of
     LDP in the protocol hierarchy.





                  +------------------------------+
                  |              LDP             |        Application
                  +------------------------------+        Layer
                        |                  |
                        |                  |
                        |                  |
                  +---------+          +---------+
                  |   RDP   |    or    |  TCP    |        Transport Layer
                  +---------+          +---------+
                    | or |                |
                    |    |                |
                    |  +--------------------+
                    |  |  Internet Protocol |             Internetwork
                    |  +--------------------+             Layer
                    |              |
                  +------------------------------+
                  |    Network Access Protocol   |        Network Layer
                  +------------------------------+


                        Relation to Other Protocols
                                 Figure 1














     Page 4



     LDP Specification                             General Description



     2.2.1  Transport Service Requirements

          LDP requires that the underlying transport layer:



          o   allow connections to be opened by specifying  a  network
              (or  internet)  address.   Support  passive  and  active
              opens.

          o   for each connection, specify the maximum message size.

          o   provide a mechanism for sending and  receiving  messages
              over an open connection.

          o   deliver messages reliably and in sequence

          o   support multiple connections, and  distinguish  messages
              associated  with  different connections.  This is only a
              requirement where LDP is  expected  to  support  several
              users at the same time.

          o   explictly return the outcome (success/failure)  of  each
              request  (open,  send,  receive), and provide a means of
              querying the  status  of  a  connection  (unacknowledged
              message count, etc.).


          Data is passed from the application program to the LDP  user
     process  in  the  form of commands.  In the case of an LDP server
     process, command responses originate in LDP itself.  Below LDP is
     the  transport  protocol.  The  Reliable  Data  Protocol  (RDP --
     RFC 908) is the recommended transport procotol.  Data  is  passed
     across  the  LDP/RDP interface in the form of messages.  (TCP may
     be used in place of RDP, but it will be  less  efficient  and  it
     will  require  more  resources  to implement.)  An internet layer
     (IP) normally comes between RDP and the network  layer,  but  RDP
     may exchange data packets directly with the network layer.

          Figure  2  shows  the  flow  of  data  across  the  protocol
     interfaces:









                                                                Page 5



     RFC-909                                                 July 1984





                               +------+
                               |      |
                               |Appli-|
                               |cation|
                               |      |
                               +------+
                                  ^
                       Commands   |
                                  V
                               +------+
                               |      |
                               | LDP  |
                               |      |
                               +------+
                                  ^
                       Messages   |
                                  V
                               +-----+
                               |     |
                               | RDP |
                               |     |
                               +-----+
                                  ^
                       Segments   |
                                  V
                                +----+
                                |    |
                                | IP |
                                |    |
                                +----+
                                  ^
                       Datagrams  |
                                  V
                              ?  *     !
                           $  =      ^   +
                             *
                          >    Internet
                            ,            ?
                                 !    )
                              *   %     $


                   Form of Data Exchange Between Layers
                                 Figure 2




     Page 6



     LDP Specification                             General Description





















































                                                                Page 7



     RFC-909                                                 July 1984





















































     Page 8



     LDP Specification                              Protocol Operation



                                 CHAPTER 3


                            Protocol Operation



     3.1  Overview

          An LDP session consists  of  an  exchange  of  commands  and
     responses  between an LDP user process and an LDP server process.
     Normally,  the  user  process  resides  on  a  host  machine   (a
     timesharing  computer  used  for network monitoring and control),
     and the server process resides on a  target  machine  (PSN,  PAD,
     gateway,  etc.).   Throughout  this document, host and target are
     used  as  synonyms  for  user   process   and   server   process,
     respectively,  although  in  some implementations (the Butterfly,
     for example) this  correspondence  may  be  reversed.   The  host
     controls  the  session  by  sending commands to the target.  Some
     commands elicit responses, and all commands may elicit  an  error
     reply.

          The protocol contains five classes  of  commands:  protocol,
     data  transfer,  management,   control  and breakpoint.  Protocol
     commands are used to verify the command sequencing mechanism  and
     to handle erroneous commands.  Data transfer commands involve the
     transfer of data from one place to another, such  as  for  memory
     examine/deposit,  or  loading.  Management  commands are used for
     creating   and   deleting   objects   (processes,    breakpoints,
     watchpoints,  etc.)  in the target machine.  Control commands are
     used to control the execution of  target  code  and  breakpoints.
     Breakpoint commands are used to control the execution of commands
     inside breakpoints and watchpoints.



     3.2  Session Management

          An LDP session consists of a series of commands sent from  a
     host  LDP  to  a  target  LDP,  some  of which may be followed by
     responses from the target.  A session begins when a host opens  a
     transport  connection to a target listening on a well known port.
     LDP uses RDP port number zzz or TCP port number  yyy.   When  the
     connection  has been established, the host sends a HELLO command,
     and the target  replies  with  a  HELLO_REPLY.   The  HELLO_REPLY
     contains  parameters that describe the target's implementation of
     LDP, including protocol  version,  implementation  level,  system



                                                                Page 9



     RFC-909                                                 July 1984



     type,  and  address format.  The session terminates when the host
     closes the underlying  transport  connection.   When  the  target
     detects  that the transport connection has been closed, it should
     deallocate any resources dedicated to the session.

          The target process is the passive partner in an LDP session,
     and  it  waits for the host process to terminate the session.  As
     an implementation consideration, either LDP  or  the   underlying
     transport  protocol  in  the  target  should  have  a  method for
     detecting if the host process  has  died.   Otherwise,   an   LDP
     target  that  supported  only  one  connection  could be rendered
     useless by a host that crashed in the  middle of a session.   The
     problem  of  detecting  half-dead  connections  can be avoided by
     taking a different tack:  the target could allow new  connections
     to  usurp  inactive  connections.  A  connection with no activity
     could be declared 'dead', but would  not  be  usurped  until  the
     connection  resource  was  needed.   However,  this  would  still
     require the transport layer to support two  connection  channels:
     one  to  receive  connection  requests, and another to use for an
     active connection.




     3.3  Command Sequencing

          Each command sent from the host to the target has a sequence
     number.   The  sequence  number is used by the target to refer to
     the command in normal replies and error replies.  To save  space,
     these  numbers  are  not  actually  included  in  host  commands.
     Instead, each command sent from the host is assigned an  implicit
     sequence  number.   The  sequence  number  starts  at zero at the
     beginning of the LDP  session  and  increases  by  one  for  each
     command sent.  The host and target each keep track of the current
     number.  The SYNCH  command may be used  by  the
     host to synchronize the sequence number.





     3.4  Data Packing and Transmission

          The convention for the order of data packing was chosen  for
     its  simplicity:  data  are packed most significant bit first, in
     order of increasing target address, into  eight-bit  octets.  The
     octets of packed data are transmitted in sequential order.



     Page 10



     LDP Specification                              Protocol Operation



          Data are always packed according to the  address  format  of
     the  target  machine.   For  example, in an LDP session between a
     20-bit host and  a  16-bit  target,  16-bit  words  (packed  into
     octets)   are  transmitted  in  both  directions.   For  ease  of
     discussion, targets are treated here  as  if  they  have  uniform
     address  spaces.  In practice, the size of address units may vary
     within a target -- 16-bit macromemory, 32-bit micromemory, 10-bit
     dispatch  memory,  etc.   Data packing between host and target is
     tailored to the units of the current target address space.

          Figures showing the packing of data for targets with various
     address  unit  sizes  are given below.  The order of transmission
     with respect to the diagrams is top to bottom.  Bit numbering  in
     the  following diagrams refers to significance in the octet:  bit
     zero  is  the  least  significant  bit  in  an  octet.   For   an
     explanation  of  the bit numbering convention that applies in the
     rest of this document, please see Appendix A.

          The packing of data for targets with word lengths  that  are
     multiples   of  8  is  straightforward.   The  following  diagram
     illustrates 16-bit packing:



                          7                               0
                          ---------------------------------
               Octet 0    |      WORD 0 bits 15-08        |
                          ---------------------------------
               Octet 1    |      WORD 0 bits 07-00        |
                          ---------------------------------
               Octet 2    |      WORD 1 bits 15-08        |
                          ---------------------------------
               Octet 3    |      WORD 1 bits 07-00        |
                          ---------------------------------
                                        *
                                        *
                                        *
                          ---------------------------------
               Octet 2n-1 |      WORD n bits 07-00        |
                          ---------------------------------


                          Packing of 16-bit Words
                                 Figure 3






                                                               Page 11



     RFC-909                                                 July 1984



     Packing  for  targets  with  peculiar  word   lengths   is   more
     complicated.   For  20-bit  machines,  2 words of data are packed
     into  5  octets.   When  an  odd  number  of  20-bit  words   are
     transmitted,  the  partially used octet is included in the length
     of the command, and the octet is padded to the right with zeroes.



                          7                               0
                          ---------------------------------
               Octet 0    |      WORD 0 bits 19-12        |
                          ---------------------------------
               Octet 1    |      WORD 0 bits 11-04        |
                          ---------------------------------
               Octet 2    |  WORD 0 03-00 |  WORD 1 19-16 |
                          ---------------------------------
               Octet 3    |      WORD 1 bits 15-08        |
                          ---------------------------------
               Octet 4    |      WORD 1 bits 07-00        |
                          ---------------------------------


                          Packing of 20-bit Words
                                 Figure 4







     3.5  Implementations

          A subset of LDP commands may be implemented in targets where
     machine  resources  are  limited and the full capabilities of LDP
     are  not  needed.  There  are  three  basic  levels   of   target
     implementations:       LOADER_DUMPER,      BASIC_DEBUGGER     and
     FULL_DEBUGGER.  The target communicates  its  LDP  implementation
     level  to the host during session initiation.  The implementation
     levels are described below:










     Page 12



     LDP Specification                              Protocol Operation



     LOADER_DUMPER

          Used  for   loading/dumping    of   the   target    machine.
          Includes   all  protocol   class  commands and replies; data
          transfer commands READ, WRITE,  MOVE  and  their  responses;
          control   command   START  and  control  reply    EXCEPTION.
          Understands at least PHYS_MACRO and HOST  addressing  modes;
          others if desired.

     BASIC_DEBUGGER

          Implements  LOADER_DUMPER  commands,  all  control commands,
          all  addressing modes appropriate to the target machine, but
          does  not  have finite state machine  (FSM)  breakpoints  or
          watchpoints.   Default  breakpoints  are  implemented.   The
          target understands long addressing mode.

     FULL_DEBUGGER

          Implements all commands and addressing modes appropriate  to
          the   target  machine,  and  includes  breakpoint  commands,
          conditional commands and BREAKPOINT_DATA.   Watchpoints  are
          optional.



























                                                               Page 13



     RFC-909                                                 July 1984





















































     Page 14



     LDP Specification                            Commands and Formats



                                 CHAPTER 4


                           Commands and Formats



     4.1  Packet Format

          LDP commands are enclosed in RDP transport messages.  An RDP
     message  may contain more than one command, but each command must
     fit entirely within a single message.  Network packets containing
     LDP commands have the format shown in Figure 5.


                      +----------------+
                      |  Local Network |
                      |    Header(s)   |
                      +----------------+
                      |   IP Header    |
                      +----------------+
                      |   RDP Header   |
                      +----------------+     +-+
                      |   LDP Command  |      |
                      |   Header       |      |
                      +----------------+      |
                      |   Optional     |      |
                      .   LDP          .      | LDP Command
                      .   Data         .      | Format
                      |                |      |
                      +----------------+      |
                      |   LDP Padding  |      |
                      +----------------+     +-+
                      |   Additional   |
                      .   LDP          .
                      .   Commands     .
                      .                .
                      +----------------+


                           Network Packet Format
                                 Figure 5








                                                               Page 15



     RFC-909                                                 July 1984



     4.2  Command Format

          LDP commands consist of a standard two-word header  followed
     optionally  by  additional data.  To facilitate parsing of multi-
     command messages, all commands contain an even number of  octets.
     Commands that contain an odd number of data octets must be padded
     with a null octet.

          The commands defined by the LDP specification  are  intended
     to  be of universal application to provide a common basis for all
     implementations.  Command class and type codes from 0 to 63.  are
     reserved  by the protocol.  Codes above 63. are available for the
     implementation of target-specific commands.




     4.2.1  Command Header

          LDP commands begin with a fixed length header.   The  header
     specifies the type of command and its length in octets.


                       0             0 0   1         1
                       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
                      +---------------+---------------+
                    0 |     Command Length (octets)   |
                      +---------------+---------------+
                    1 | Command Class | Command Type  |
                      +---------------+---------------+


                         LDP Command Header Format
                                 Figure 6


     HEADER FIELDS:

     Command Length

          The command length gives the total number of octets  in  the
          command,  including the length field and data, and excluding
          padding.

     Command Class
     Command Type




     Page 16



     LDP Specification                            Commands and Formats



          The command class and type  together  specify  a  particular
          command.   The  class selects one of six command categories,
          and the type gives the command within  that  category.   All
          codes are decimal.  The symbols given in Figures 7 and 8 for
          command classes and types are used in the remainder of  this
          document for reference.

          The command classes that have been defined are:


                     Command Class  |  Symbol
                    ----------------+-----------
                            1       | PROTOCOL
                            2       | DATA_TRANSFER
                            3       | CONTROL
                            4       | MANAGEMENT
                            5       | BREAKPOINT
                            6       | CONDITION
                            7 - 63  | 


                              Command Classes
                                 Figure 7


          Command  type  codes  are  assigned  in  order  of  expected
          frequency  of use.  Commands and their responses/replies are
          numbered  sequentially.   The  command  types,  ordered   by
          command class, are:





















                                                               Page 17



     RFC-909                                                 July 1984





                     Command Class  |  Command Type | Symbol
                    ----------------+---------------+----------
                      PROTOCOL      |       1       | HELLO
                                    |       2       | HELLO_REPLY
                                    |       3       | SYNCH
                                    |       4       | SYNCH_REPLY
                                    |       5       | ERROR
                                    |       6       | ERRACK
                                    |       7       | ABORT
                                    |       8       | ABORT_DONE
                                    |       9 - 63  | 
                                    |               |
                      DATA_TRANSFER |       1       | WRITE
                                    |       2       | READ
                                    |       3       | READ_DONE
                                    |       4       | READ_DATA
                                    |       5       | MOVE
                                    |       6       | MOVE_DONE
                                    |       7       | MOVE_DATA
                                    |       8       | REPEAT_DATA
                                    |       9       | BREAKPOINT_DATA
                                    |       10      | WRITE_MASK
                                    |       11 - 63 | 
                                    |               |
                      CONTROL       |       1       | START
                                    |       2       | STOP
                                    |       3       | CONTINUE
                                    |       4       | STEP
                                    |       5       | REPORT
                                    |       6       | STATUS
                                    |       7       | EXCEPTION
                                    |       8 - 63  | 
                                    |               |
                      MANAGEMENT    |       1       | CREATE
                                    |       2       | CREATE_DONE
                                    |       3       | DELETE
                                    |       4       | DELETE_DONE
                                    |       5       | LIST_ADDRESSES
                                    |       6       | ADDRESS_LIST
                                    |       7       | GET_PHYS_ADDRESS
                                    |       8       | GOT_PHYS_ADDRESS
                                    |       9       | GET_OBJECT
                                    |       10      | GOT_OBJECT
                                    |       11      | LIST_BREAKPOINTS
                                    |       12      | BREAKPOINT_LIST



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     LDP Specification                            Commands and Formats



                                    |       13      | LIST_NAMES
                                    |       14      | NAME_LIST
                                    |       15      | LIST_PROCESSES
                                    |       16      | PROCESS_LIST
                                    |       17 - 63 | 
                                    |               |
                      BREAKPOINT    |       1       | INCREMENT
                                    |       2       | INC_COUNT
                                    |       3       | OR
                                    |       4       | SET_PTR
                                    |       5       | SET_STATE
                                    |       6 - 63  | 
                                    |               |
                      CONDITION     |       1       | CHANGED
                                    |       2       | COMPARE
                                    |       3       | COUNT_EQ
                                    |       4       | COUNT_GT
                                    |       5       | COUNT_LT
                                    |       6       | TEST
                                    |       7 - 63  | 


                               Command Types
                                 Figure 8





     4.3  Addressing

          Addresses are used  in  LDP  commands  to  refer  to  memory
     locations,  processes,  buffers,  breakpoints and other entities.
     Many of these entities are machine-dependent; some machines  have
     named  objects,  some  machines have multiple address spaces, the
     size of address spaces varies, etc.  The  format  for  specifying
     addresses  needs  to  be  general  enough  to handle all of these
     cases.   This  speaks  for  a  large,  hierarchically  structured
     address  format.   However, the disadvantage of a large format is
     that it imposes extra overhead on communication with targets that
     have simpler address schemes.

          LDP resolves this conflict by employing two address formats:
     a  short  three-word format for addressing simpler targets, and a
     long five-word format for others.  Each target LDP is required to
     implement  at least one of these formats.  At the start of an LDP
     session, the target specifies the address format(s)  it  uses  in



                                                               Page 19



     RFC-909                                                 July 1984



     the  Flag field of the HELLO_REPLY message.  In each address, the
     first bit of the mode octet is a format flag:  0  indicates  LONG
     address format, and 1 indicates SHORT format.




     4.3.1  Long Address Format

          The long address format is five words long and consists of a
     three-word  address  descriptor and a two-word offset (see Figure
     9). The descriptor specifies an address space to which the offset
     is applied.  The descriptor is subdivided into several fields, as
     described below.  The structuring of the descriptor  is  designed
     to  support  complex  addressing  modes.  For example, on targets
     with  multiple  processes,  descriptors  may  reference   virtual
     addresses,  registers,  and  other  entities  within a particular
     process.

          The addressing modes defined below are intended as a base to
     which  target-specific  modes  may be added.  Modes up to 63. are
     reserved by the protocol.  The range 64. to 127. may be used  for
     target-specific address modes.


               Long Format - Format bit is LONG=0

                0             0 0   1         1
                0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
               +-------------------------------+  +-+
               |0|       Mode  |  Mode Arg     |   |
               +-------------------------------+   |
               |                    (31-16)    |   | Descriptor
               +----          ID            ---+   |
               |                    (15-0)     |   |
               +-------------------------------+  +-+
               |                    (31-16)    |   |
               +----        Offset          ---+   | Offset
               |                    (15-0)     |   |
               +-------------------------------+  +-+


                            Long Address Format
                                 Figure 9


     LONG ADDRESS FIELDS:



     Page 20



     LDP Specification                            Commands and Formats



     Mode

          The address mode identifies the type of address space  being
          referenced.   The mode is qualified by the mode argument and
          the ID field.  Implementation of modes other  than  physical
          and  host is machine-dependent.  Currently defined modes and
          the address space they reference are shown in Figure 10.


          Mode | Symbol               | Address space
          -----+----------------------+---------------------------

            0    HOST                   Host
            1    PHYS_MACRO             Macromemory
            2    PHYS_MICRO             Micromemory
            3    PHYS_I/O               I/O space
            4    PHYS_MACRO_PTR         Macro contains a pointer
            5    PHYS_REG               Register
            6    PHYS_REG_OFFSET        Register plus offset
            7    PHYS_REG_INDIRECT      Register contains address
                                        of a pointer

            8    PROCESS_CODE           Process code space
            9    PROCESS_DATA           Process data space
           10    PROCESS_DATA_PTR       Process data contains a ptr
           11    PROCESS_REG            Process virtual register
           12    PROCESS_REG_OFFSET     Process register plus offset
           13    PROCESS_REG_INDIRECT   Process register contains
                                        address of a pointer

           14    OBJECT_OFFSET          Memory object (queue, pool)
           15    OBJECT_HEADER          System header for an object
           16    BREAKPOINT             Breakpoint
           17    WATCHPOINT             Watchpoint
           18    BPT_PTR_OFFSET         Breakpoint ptr plus offset
           19    BPT_PTR_INDIRECT       Breakpoint ptr plus offset
                                        gives address of a pointer
           20 -  
           63


                            Long Address Modes
                                 Figure 10



     Mode Argument



                                                               Page 21



     RFC-909                                                 July 1984



          Provides a numeric argument to the  mode  field.   Specifies
          the  register  in  physical  and  process REG and REG_OFFSET
          modes.

     ID Field

          Identifies a particular process, buffer or object.

     Offset

          The offset into the linear  address  space  defined  by  the
          mode.  The size of the machine word determines the number of
          significant bits in the offset.   Likewise,  the  addressing
          units of the target are the units of the offset.

     The interpretation of the mode argument, ID field and offset  for
     each address mode is given below:

     HOST

          The ID and offset fields are numbers assigned arbitrarily by
          the  host  side  of the debugger.  These numbers are used in
          MOVE and MOVE_DATA messages.  MOVE_DATA responses containing
          this  mode  as the destination are sent by the target to the
          host.  This may occur in debugging when data is sent to  the
          host from the target breakpoint.

     PHYS_MACRO

          The  offset  contains  the  32-bit  physical  address  of  a
          location in macromemory.  The mode argument and ID field are
          not used.   For  example,  mode=PHYS_MACRO  and  offset=1000
          specifies location 1000 in physical memory.

     PHYS_MICRO

          Like PHYS_MACRO, but the location is in micromemory.

     PHYS_I/O

          Like PHYS_MACRO, but the location is in I/O space.

     PHYS_MACRO_PTR

          The offset contains the address of a pointer in macromemory.
          The  location  pointed to (the effective address) is also in
          macromemory.  The mode argument and ID field are unused.



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     LDP Specification                            Commands and Formats



     PHYS_REG

          The mode argument  gives  the  physical  register.   If  the
          register  is  used by the LDP target process, then the saved
          copy from  the  previous  context  is  used.   This  comment
          applies  to  PHYS_REG_OFFSET  mode as well.  The ID field is
          not used.

     PHYS_REG_OFFSET

          The offset is added to the contents of a register  given  as
          the mode argument.  The result is used as a physical address
          in macromemory.  ID is unused.

     PHYS_REG_INDIRECT

          The register specified in the mode arg contains the  address
          of  a  pointer in macromemory.  The effective address is the
          macromemory location specified  in  the  pointer,  plus  the
          offset.  The ID field is unused.

     PROCESS_CODE

          The ID is a process ID, the offset is into  the  code  space
          for this process.  Mode argument is not used.

     PROCESS_DATA

          The ID is a process ID, the offset is into  the  data  space
          for  this  process.   Mode argument is not used.  On systems
          that do not distinguish between code and data  space,  these
          two  modes are equivalent, and reference the virtual address
          space of the process.

     PROCESS_DATA_PTR

          The offset contains the address of a  pointer  in  the  data
          space  of  the  process  specified  by the ID.  The location
          pointed to (the effective  address)  is  also  in  the  data
          space.  The mode argument is not used.

     PROCESS_REG

          Accesses the  registers  (and  other  system  data)  of  the
          process  given  by the ID field.  Mode argument 0 starts the
          registers.  After the registers, the  mode  argument  is  an
          offset into the system area for the process.



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     RFC-909                                                 July 1984



     PROCESS_REG_OFFSET

          The offset plus the contents of the register  given  in  the
          mode  argument specifies a location in the data space of the
          process specified by the ID.

     PROCESS_REG_INDIRECT

          The register specified in the mode arg contains the  address
          of  a  pointer in the data space of the process given by the
          ID.  The effective address is the location in  process  data
          space specified in the pointer, plus the offset.

     OBJECT_OFFSET (optional)

          The offset is into the memory space defined by the object ID
          in   ID.    Recommended  for  remote  control  of  parameter
          segments.

     OBJECT_HEADER (optional)

          The  offset  is  into  the  system  header  for  the  object
          specified by the ID.  Intended for use with the Butterfly.

     BREAKPOINT

          The descriptor specifies a breakpoint.  The offset is  never
          used,  this  type  is  only used in descriptors referring to
          breakpoints.  (See Breakpoints and Watchpoints,  below,  for
          an explanation of breakpoint descriptors.)

     WATCHPOINT

          The descriptor specifies a watchpoint.  The offset is  never
          used,  this  type  is  only used in descriptors referring to
          watchpoints.  (See Breakpoints and Watchpoints,  below,  for
          an explanation of watchpoint descriptors).

     BPT_PTR_OFFSET

          For  this  mode  and  BPT_PTR_INDIRECT,  the  mode  argument
          specifies  one  of two breakpoint pointer variables local to
          the breakpoint in which this address occurs.  These pointers
          and  the  SET_PTR command which manipulates them provide for
          an  arbitrary  amount  of  address  indirection.   They  are
          intended for use in traversing data structures: for example,
          chasing queues.  In BPT_PTR_OFFSET, the offset is  added  to



     Page 24



     LDP Specification                            Commands and Formats



          the  pointer  variable  to  give  the effective address.  In
          targets which support multiple processes, the location is in
          the  data  space of the process given by the ID.  Otherwise,
          the  location  is  a  physical  address   in   macro-memory.
          BPT_PTR.*   modes   are   valid   only  in  breakpoints  and
          watchpoints.

     BPT_PTR_INDIRECT

          Like BPT_PTR_OFFSET, except that it uses one more  level  of
          indirection.    The  pointer  variable  given  by  the  mode
          argument plus the offset specify an address which points  to
          the    effective    address.    See   the   description   of
          BPT_PTR_OFFSET for a discussion of  usage,  limitations  and
          address space.




     4.3.2  Short Address Format

          The  short  address  format   is   intended   for   use   in
     implementations  where protocol overhead must be minimized.  This
     format is a subset of the long address format:  it  contains  the
     same  fields  except  for  the  ID  field.   Therefore, the short
     addressing format supports only HOST and  PHYS_*  address  modes.
     Only  the LOADER_DUMPER implementation level commands may be used
     with the short addressing format.  The short  address  format  is
     three  words  long,  consisting  of  a 16-bit word describing the
     address space, and a 32-bit offset.




















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     RFC-909                                                 July 1984





                    Short Format - Format bit is SHORT=1

                     0             0 0   1         1
                     0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
                    +-------------------------------+
                    |1|       Mode  | Mode Argument |
                    +-------------------------------+  +-+
                    |                    (31-16)    |   |
                    +----        Offset          ---+   | Offset
                    |                    (15-0)     |   |
                    +-------------------------------+  +-+


                           Short Address Format
                                 Figure 11


     SHORT ADDRESS FIELDS:
     Mode

          The high-order  bit  is  1,  indicating  the  short  address
          format.   A  list  of  the  address modes supported is given
          below.  The interpretation of the  remaining  fields  is  as
          described above for the long addressing format.
























     Page 26



     LDP Specification                            Commands and Formats





          Mode | Symbol             | Address space
          -----+--------------------+---------------------------

            0    HOST                 Host
            1    PHYS_MACRO           Macro-memory
            2    PHYS_MICRO           Micro-memory
            3    PHYS_I/O             I/O space
            4    PHYS_MACRO_PTR       Macro contains a pointer
            5    PHYS_REG             Register
            6    PHYS_REG_OFFSET      Register plus offset
            7    PHYS_REG_INDIRECT    Register contains address
                                      of a pointer
            8 -
            32   


                            Short Address Modes
                                 Figure 12






























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     RFC-909                                                 July 1984





















































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     LDP Specification                               Protocol Commands



                                 CHAPTER 5


                             Protocol Commands



          Protocol  commands  are  used  for   error   handling,   for
     synchronizing  the command sequence number, and for communicating
     protocol implementation parameters.  Every protocol command has a
     corresponding  reply.   All  protocol  commands are sent from the
     host  to  the  target,  with  replies  flowing  in  the  opposite
     direction.




     5.1  HELLO Command

          The HELLO command is sent by the host to signal the start of
     an LDP session.  The target responds with HELLO_REPLY.


                       0             0 0   1         1
                       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
                      +---------------+---------------+
                    0 |               4               |
                      +---------------+---------------+
                    1 |   PROTOCOL    |    HELLO      |
                      +---------------+---------------+


                           HELLO Command Format
                                 Figure 13






     5.2  HELLO_REPLY

          A HELLO_REPLY is sent by the target in response to the HELLO
     command  at  the  start of an LDP session.  This reply is used to
     inform the host about the  target's implementation of LDP.





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     RFC-909                                                 July 1984





                       0             0 0   1         1
                       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
                      +---------------+---------------+
                    0 |               10              |
                      +---------------+---------------+
                    1 |    PROTOCOL   |   HELLO_REPLY |
                      +---------------+---------------+
                    2 |   LDP Version |  System Type  |
                      +---------------+---------------+
                    3 |   Options |W|S| Implementation|
                      +---------------+---------------+
                    4 | Address Code  |    Reserved   |
                      +---------------+---------------+


                            HELLO_REPLY Format
                                 Figure 14



     HELLO_REPLY FIELDS:

     LDP Version

          The  target's  LDP  protocol  version.    If   the   current
          host  protocol  version  does not agree  with  the  target's
          protocol  version,  the  host may terminate the session,  or
          may  continue it, at the discretion of the implementor.  The
          current version number is 2.

     System Type

          The type of system running on the target.  This is used as a
          check  against what the host thinks the target is.  The host
          is expected to have a table  of  target  system  types  with
          information  about  target  address  spaces, target-specific
          commands and addressing modes, and so forth.

          Currently defined system types are shown in Figure 15.  This
          list  includes  some  systems normally thought of as 'hosts'
          (e.g. C70, VAX), for implementations where targets  actively
          initiate and direct a load of themselves.






     Page 30



     LDP Specification                               Protocol Commands





             Code |  System       |  Description
          --------+---------------+---------------------------
               1     C30_16_BIT      BBN 16-bit C30
               2     C30_20_BIT      BBN 20-bit C30
               3     H316            Honeywell-316
               4     BUTTERFLY       BBN Butterfly
               5     PDP-11          DEC PDP-11
               6     C10             BBN C10
               7     C50             BBN C50
               8     PLURIBUS        BBN Pluribus
               9     C70             BBN C70
              10     VAX             DEC VAX
              11     MACINTOSH       Apple MacIntosh


                               System Types
                                 Figure 15


     Address Code

          The address code indicates which LDP address  format(s)  the
          target is prepared to use.  Address codes are show in Figure
          16.


           Address Code |  Symbol       | Description
          --------------+---------------+-----------------------------

                1         LONG_ADDRESS    Five word address format.
                                          Supports all address modes
                                          and commands.

                2         SHORT_ADDRESS   Three word address format.
                                          Supports only physical and
                                          host address modes.  Only
                                          the LOADER_DUMPER set of
                                          commands are supported.


                           Target Address Codes
                                 Figure 16


     Implementation



                                                               Page 31



     RFC-909                                                 July 1984



          The implementation level   specifies   which   features   of
          the   protocol   are  implemented  in the target.  There are
          three levels of protocol implementation.  These  levels  are
          intended to correspond to the three most likely applications
          of LDP:  simple loading and dumping,  basic  debugging,  and
          full  debugging.   (Please see Implementations, above, for a
          detailed description of implementation  levels.)  There  are
          are  also several optional features that are not included in
          any particular level.

          Implementation levels are cumulative, that is,  each  higher
          level  includes  the  features  of all previous levels.  The
          levels are shown in Figure 17.



          Feature Level |  Symbol       | Description
          --------------+---------------+-----------------------------
                 1        LOADER_DUMPER   Loader/dumper subset of LDP
                 2        BASIC_DEBUGGER  Control commands, CREATE
                 3        FULL_DEBUGGER   FSM breakpoints


                              Feature Levels
                                 Figure 17



     Options

          The options field (see  Figure  18)  is  an  eight-bit  flag
          field.   Bit  flags  are  used to indicate if the target has
          implemented particular optional commands.  Not all  optional
          commands  are  referenced  in  this  field.  Commands  whose
          implementation   depends  on  target  machine  features  are
          omitted.   The  LDP  application is expected to 'know' about
          target features that are  not  intrinsic  to  the  protocol.
          Examples  of  target-dependent  commands  are  commands that
          refer to named objects (CREATE, LIST_NAMES).











     Page 32



     LDP Specification                               Protocol Commands





                Mask |  Symbol     | Description
               ------+-------------+---------------+-----------------
                 1     STEP          The STEP command is implemented
                 2     WATCHPOINTS   Watchpoints are implemented


                                  Options
                                 Figure 18






     5.3  SYNCH Command

          The SYNCH command is sent by the host  to  the  target.  The
     target  responds  with  a  SYNCH_REPLY.   The SYNCH - SYNCH_REPLY
     exchange serves two functions: it synchronizes the host-to-target
     implicit sequence number and acts as a cumulative acknowledgement
     of the receipt and execution of  all  host  commands  up  to  the
     SYNCH.



                       0             0 0   1         1
                       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
                      +---------------+---------------+
                    0 |               6               |
                      +---------------+---------------+
                    1 |   PROTOCOL    |    SYNCH      |
                      +---------------+---------------+
                    2 |       Sequence Number         |
                      +---------------+---------------+


                           SYNCH Command Format
                                 Figure 19



     SYNCH FIELDS:

     Sequence Number




                                                               Page 33



     RFC-909                                                 July 1984



          The sequence number of this command.  If this  is  not  what
          the  target  is  expecting,  the target will reset to it and
          respond with an ERROR reply.




     5.4  SYNCH_REPLY

          A SYNCH_REPLY is sent by the target in reponse  to  a  valid
     SYNCH  command.   A SYNCH command is valid if its sequence number
     agrees  with  the  sequence  number  the  target  is   expecting.
     Otherwise, the target will reset its sequence number to the SYNCH
     command and send an ERROR reply.


                       0             0 0   1         1
                       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
                      +---------------+---------------+
                    0 |               6               |
                      +---------------+---------------+
                    1 |   PROTOCOL    |  SYNCH_REPLY  |
                      +---------------+---------------+
                    2 |       Sequence Number         |
                      +---------------+---------------+


                            SYNCH_REPLY Format
                                 Figure 20



     SYNCH_REPLY FIELDS:

     Sequence Number

          The sequence number of  the  SYNCH  command  to  which  this
          SYNCH_REPLY is the response.












     Page 34



     LDP Specification                               Protocol Commands



     5.5  ABORT Command

          The ABORT command is sent from the host to abort all pending
     operations  at  the target.  The target responds with ABORT_DONE.
     This is primarily intended to stop large data transfers from  the
     target.  A likely application would be during a debugging session
     when the user types an interrupt to abort  a  large  printout  of
     data  from  the  target.   The ABORT command has no effect on any
     breakpoints or watchpoints that may be enabled in the target.

          As a practical matter, the ABORT command may be difficult to
     implement  on  some  targets.   Its  ability to interrupt command
     processing on the target depends on the target being able to look
     ahead at incoming commands and receive an out-of-band signal from
     the host.  However, the effect of an ABORT  may  be  achieved  by
     simply closing and reopening the transport connection.


                       0             0 0   1         1
                       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
                      +---------------+---------------+
                    0 |               4               |
                      +---------------+---------------+
                    1 |   PROTOCOL    |    ABORT      |
                      +---------------+---------------+


                           ABORT Command Format
                                 Figure 21






     5.6  ABORT_DONE Reply

          The ABORT_DONE reply is sent from the target to the host  in
     response to an ABORT command.  This indicates that the target has
     terminated all  operations  that  were  pending  when  the  ABORT
     command  was  received.  The sequence number of the ABORT command
     is included in the reply.








                                                               Page 35



     RFC-909                                                 July 1984





                       0             0 0   1         1
                       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
                      +---------------+---------------+
                    0 |               4               |
                      +---------------+---------------+
                    1 |   PROTOCOL    |    ABORT_DONE |
                      +---------------+---------------+
                    2 |       Sequence Number         |
                      +---------------+---------------+


                          ABORT_DONE Reply Format
                                 Figure 22



     ABORT_DONE FIELDS:

     Sequence Number

          The sequence number of the ABORT command that elicited  this
          reply.   This  enables   the  host  to  distinguish  between
          replies to multiple aborts.





     5.7  ERROR Reply

          The ERROR reply is sent by the target in response to  a  bad
     command.   The  ERROR  reply  gives  the  sequence  number of the
     offending command and a reason code.  The target ignores  further
     commands   until  an  ERRACK command is received.  The reason for
     ignoring commands is that the  proper  operation  of  outstanding
     commands  may  be  predicated  on  the execution of the erroneous
     command.











     Page 36



     LDP Specification                               Protocol Commands





                       0             0 0   1         1
                       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
                      +---------------+---------------+
                    0 |        Command Length         |
                      +---------------+---------------+
                    1 |   PROTOCOL    |    ERROR      |
                      +---------------+---------------+
                    2 |   Command Sequence Number     |
                      +---------------+---------------+
                    3 |          Error code           |
                      +---------------+---------------+
                    4 |       Optional Data           |
                      +---------------+---------------+
                                      *
                                      *
                                      *
                      +---------------+---------------+
                    n |       Optional Data           |
                      +---------------+---------------+

                            ERROR Reply Format
                                 Figure 23


     ERROR Reply FIELDS:

     Command Sequence Number

          The implicit sequence number of the erroneous command.

     Error Code

          A code specifying what error has taken place.  The currently
          defined codes are shown in Figure 24.














                                                               Page 37



     RFC-909                                                 July 1984





                    Error Code |  Symbol
                    -----------+------------------------
                        1         BAD_COMMAND
                        2         BAD_ADDRESS_MODE
                        3         BAD_ADDRESS_ID
                        4         BAD_ADDRESS_OFFSET
                        5         BAD_CREATE_TYPE
                        6         NO_RESOURCES
                        7         NO_OBJECT
                        8         OUT_OF_SYNCH
                        9         IN_BREAKPOINT


                                ERROR Codes
                                 Figure 24


          An explanation of each of these error codes follows:
          BAD_COMMAND

               The command was not meaningful to the  target  machine.
               This includes commands that are valid but unimplemented
               in this target.  Also, the command  was  not  valid  in
               this context.  For example, a command given by the host
               that  is  only  legal  in  a   breakpoint   (e.g.   IF,
               SET_STATE).

          BAD_ADDRESS_MODE 

               The mode of an address given  in  the  command  is  not
               meaningful  to  this  target  system.   For  example, a
               PROCESS address mode on a target that does not  support
               multi-processing.

          BAD_ADDRESS_ID 

               The ID field of an  address  didn't  correspond  to  an
               appropriate  thing.  For example, for a PROCESS address
               mode, the ID of a non-existent process.

          BAD_ADDRESS_OFFSET 

               The offset field of the address was outside  the  legal
               range  for the thing addressed.  For example, an offset
               of 200,000 in PHYS_MACRO mode on a target with  64K  of



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     LDP Specification                               Protocol Commands



               macro-memory.

          BAD_CREATE_TYPE

               The object type in a CREATE command was unknown.

          NO_RESOURCES

               A CREATE  command  failed  due  to  lack  of  necessary
               resources.

          NO_OBJECT

               A GET_OBJECT command failed to find the named object.

          OUT_OF_SYNCH

               The sequence  number  of  the  SYNCH  command  was  not
               expected  by the target.  The target has resynchronized
               to it.

          IN_BREAKPOINT  
                []

               An error occurred within  a  breakpoint  command  list.
               The given 16-bit sequence-number refers to the sequence
               number  of  the  CREATE  command   that   created   the
               breakpoint,  while  breakpoint-sequence#  refers to the
               sequence number of the command  within  the  breakpoint
               given by .




     5.8  ERRACK Acknowledgement

          An  ERRACK  is sent by the host  in  response  to  an  ERROR
     reply  from  the  target.  The ERRACK is used to acknowledge that
     the host has received the ERROR reply.











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     RFC-909                                                 July 1984





                       0             0 0   1         1
                       0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5
                      +---------------+---------------+
                    0 |               4               |
                      +---------------+---------------+
                    1 |   PROTOCOL    |    ERRACK     |
                      +---------------+---------------+


                           ERRACK Command Format
                                 Figure 25





































     Page 40



     LDP Specification                          Data Transfer Commands



                                 CHAPTER 6


                          Data Transfer Commands



          Data transfer commands  transfer data between the  host  and
     the  target.  These commands are used for loading and dumping the
     target, and examining and depositing  locations  on  the  target.
     The  READ  command  reads  data from the target, the MOVE command
     moves data within the  target  or  from  the  target  to  another
     entity,  and  the  WRITE  command  writes  data  to  the  target.
     REPEAT_DATA makes copies of a pattern to  the  target  --  it  is
     useful  for  zeroing memory.  WRITE_MASK writes data with a mask,
     and is intended for modifying target  parameter tables.

          Data transmitted to  and from the target always  contains  a
     target  address.   In  writes  to the target, this is used as the
     destination of the data.  In reads from the  target,  the  target
     address  is  used by the host to identify where in the target the
     data came from.  In addition, the  MOVE  command  may  contain  a
     'host'  address  as  its  destination;  this  permits the host to
     further discriminate between possible sources of  data  from  the
     target -- from different breakpoints, debugging windows, etc.

          A read request to  the  target  may  generate  one  or  more
     response  messages.   In  particular,  responses  to requests for
     large amounts of data -- core  dumps,  for  example  --  must  be
     broken  up into multiple messages, if the block of data requested
     plus the LDP header exceeds the transport layer message size.

          In commands which contain data (WRITE, READ_DATA,  MOVE_DATA
     and REPEAT_DATA), if there are an odd number of data octets, then
     a  null octet is appended.  This is so that the next  command  in
     the  message,  if  any, will begin on an even octet.  The command
     length is the sum of the number of octets in the  command  header
     and  the  number  of octets of data, excluding the null octet, if
     any.

          The addressing formats which may be used with data  transfer
     commands  are  specified for each LDP session at the start of the
     session by the target  in  the  HELLO_REPLY  response.   See  the
     section  entitled  'Addressing',  above, for a description of LDP
     addressing formats and modes.   In  the  command  diagrams  given
     below,  the  short  addressing  format  is  illustrated.  For LDP
     sessions using long addressing, addresses are  five  words  long,



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     RFC-909                                                 July 1984