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RFC 0643

Network Debugging Protocol                          Eric Mader
RFC: 643                                             July 1974
NIC #30873


     This document describes a proposed protocol  to  be  used  in  an
implementation   of   a   PDP-11   network  Bootstrap  service  and  a
cross-network  debugger.   The  protocol  is  designed  for  debugging
processes  running under an operating system which can perform some of
the "lower level" debugging  tasks,  such  as  planting  and  removing
breakpoints  and  single  stepping.  A subset of the protocol could be
used if such a capability does not exist (a  stand-alone  program  for
example).

     The protocol is a level 2 protocol, which  bypasses  the  ARPANET
HOST-HOST  protocol.   (This  is  implemented  on  TENEX using special
privileged system calls which allow messages to be  sent  directly  to
and  received  directly  from the IMP).  Messages are sent between the
PDP-11 and the remote debugger on  one  link  (currently  377  octal).
Each  message from the remote debugger to the PDP-ll is a request that
the PDP-11 perform some action, and each message from  the  PDP-11  to
the remote debugger is either a reply to that request or an indication
that  a  process  has  stopped  running  (i.e.   has  trapped,  hit  a
breakpoint, etc).  The exact format of the messages is shown in Figure
1 below.

     Each  command  consists  of  an  8-bit  op-code,  and  an   8-bit
process-id,  two  16-bit  arguments,  and  an optional string of 8-bit
bytes.  The op-code field from the PDP-11 should be the same  as  that
sent  by  the  remote  host  to  indicate successful completion of the
request or be the same as that set by the remote host with the 200 bit
set  to  indicate  failure to complete the request.  Op-codes from the
PDP-11 which have the 100 bit set are asynchronous indication  that  a
process  has stopped for a reason other than a request from the remote
host.  (See description of asynchronous replies  below).   An  op-code
from  the  PDP-11  with  both  the 100 and 200 bit set is meaningless.
Thus, the 8-bit op-code field sent by the PDP-11 can be thought of  as
a CAN'T Flag, an Asynchronous STOP Flag, and a 6-bit op-code.

     In the description that follows the commands will be given as

   NAME (Process-ID, Argument 1, Argument 2, BYTE STRING)
with only as many of the fields present as are used.

Op-Code 0 - NOP_______ _ _ ___

     This command is intended to be used to determine if the PDP-11 is
operational.   It  has no effect on any process running in the PDP-11.
The response is NOP.

Op-Code 1 - DEBUG (Process)_______ _ _ _____ _________

     This command requests the ability to  debug  the  given  process.
The PDP-11 should respond with

Page 2 DEBUGGING (process) (op-code 1) if no other remote host is currently doing so, and CAN'T DEBUG (process) (op-code 201) if another remote host has been given permission to debug the process, or the process doesn't exist. Op-Code 2 - END DEBUG (Process)_______ _ _ ___ _____ _________ This command relinquishes the ability to debug a process. The PDP-11 should reply END DEBUG (Process) (op-code 2) unless the remote host isn't debugging the process, in which case it should respond CAN'T END DEBUG (Process) (op-code 202). If the process isn't running when the END DEBUG is done, then the effect should be the same as RESUME (PROCESS) END DEBUG (PROCESS) Op-Code 3 - STOP (Process)_______ _ _ ____ _________ This command requests the PDP-11 to stop the given process from running. The PDP-11 should respond STOPPED (process) (op-code 3) unless the process isn't being debugged by the remote host. In this case it should reply CAN'T STOP (process) (op-code 203). Op-Code 4 - DEPOSIT (Process, Address, Count, Byte String)_______ _ _ _______ _________ ________ ______ ____ _______ This instructs the PDP-11 to deposit the Count bytes in Byte String into the given process's address space beginning at Address. The PDP-11 should reply DEPOSITED (Process, Address, Count) if successful or CAN'T DEPOSIT (Process, Address, Count)

Page 3 if the deposit doesn't succeed, or the remote host isn't debugging the process. Op-Code 5 - RESUME (Process)_______ _ _ ______ _________ This instructs the PDP-11 to allow the given process to resume running. The PDP-11 should reply RESUMED (Process) before it starts the process (in case it traps right away), or CAN'T RESUME (Process) if the process wasn't stopped, or the remote host isn't debugging the process. Op-Code 6 - EXAMINE (Process, Address, Count)_______ _ _ _______ _________ ________ ______ This instructs the PDP-11 to return Count bytes from the given process address space starting at Address. The PDP-11 should reply CONTENTS (Process, Address, Count, Byte String) if it can supply the bytes, or CAN'T EXAMINE (Process, Address, Count) if all the bytes don't exist in the address space, or the remote host isn't debugging the process. Op-Code 7 - DEPOSIT STATE VECTOR (Process, Index, Byte Count,_______ _ _ _______ _____ ______ _________ ______ ____ ______ Byte String) ____ _______ This instructs the PDP-11 to deposit the Byte Count bytes in Byte String into the state vector for the given process starting with byte Index. See Figure 2 for the format of the state vector. The PDP-11 should reply DEPOSITED STATE VECTOR (Process, Index, Byte Count) unless the bytes won't all fit in the state vector or the remote host isn't debugging the process. In this case the reply should be CAN'T DEPOSIT STATE VECTOR (Process, Index, Byte Count) Op-Code 10 - BREAK (Process, Address, Proceed Count)_______ __ _ _____ _________ ________ _______ ______ This instructs the PDP-11 to place a breakpoint at Address in the given process's address space, with a proceed count of Proceed Count. The proceed count can be changed by issuing another BREAK with a different count. The PDP-11 should reply

Page 4 BROKE (Process, Address, Proceed Count) if it was able to plant the breakpoint and CAN'T BREAK (Proceed, Address, Proceed Count) if it was unable to plant the breakpoint because the breakpoint tables for the process are full, or because the remote host isn't debugging the process. Note that the PDP-11 must keep track of where the breakpoints are, the old contents of these locations etc. Op-Code 11 - UNBREAK (Process, Address)_______ __ _ _______ _________ ________ This instructs the PDP-11 to remove the breakpoint from Address in the process's address space and release all storage associated with it. The PDP-11 should reply UNBROKE (Process, Address) when the breakpoint is removed, or CAN'T UNBREAK (Process, Address) if the process doens't have a breakpoint at Address or the remote host isn't debugging the process. Op-Code 12 - SINGLE STEP (Process)_______ __ _ ______ ____ _________ This instructs the PDP-11 to let the given process execute one instruction, using the Trace Trap feature. The PDP-11 should reply SINGLE STEPPING (Process) before the process is started, or CAN'T SINGLE STEP (Process) if the process is already running or the remote host isn't debugging the process. Op-Code 13 - PROCEED BPT (Process)_______ __ _ _______ ___ _________ This instructs the PDP-11 to allow the process to proceed from a break point trap (BPT, see below). The PDP-11 should reply PROCEEDING BPT (Process) before allowing the process to proceed, or CAN'T PROCEED BPT (Process) if the process isn't stopped at a breakpoint or the remote host isn't debugging the process.

Page 5 Op-Code 14 - CREATE PROCESS_______ __ _ ______ _______ This instructs the PDP-11 to create a process (with its own virtual address space if the system will support such a feature). The PDP-11 should reply CREATED PROCESS (Process) if it could create a process, with the process-id in process. If the process creation failed or the system cannot create processes, the PDP-11 should reply CAN'T CREATE PROCESS. Once a remote host has created a process it is debugging it, that is a DEBUG (process) need not be done. Op-Code 15 - DESTROY PROCESS (Process)_______ __ _ _______ _______ _________ This instructs the PDP-11 to destroy the given process. After the process is destroyed the remote host cannot, of course, debug it so there is no need to do an END DEBUG (Process). The PDP-11 should reply DESTROYED PROCESS (Process) if the process has been destroyed. If the system doesn't support a multi-process structure or the remote host isn't debugging the process the PDP-11 should reply CAN'T DESTROY PROCESS (Process). Asynchronous Stop Replies ____________ ____ _______ These replies (op-code between 100 and 177 octal) are sent by the PDP-11 wherever a process being debugged stops running for any reason other than in response to the STOP (Process) command. Each of these replies includes the entire state vector for the process.

Page 6 Op-Code 100 - TRAP (Process, Reason, 0, STATE VECTOR)_______ ___ _ ____ _________ _______ __ _____ _______ This informs the remote host that a process has "trapped", or terminated abnormally. The reason is a system error code, or could be a trap vector address in the stand-alone case. Op-Code 101 - HALT (Process, 0,0, STATE VECTOR)_______ ___ _ ____ _________ ____ _____ _______ This informs the remote host that the given process has terminated normally. Op-Code 102 - BPT (Process, 0, 0, STATE VECTOR)_______ ___ _ ___ _________ __ __ _____ _______ This informs the remote host that the given process has hit a breakpoint trap. Op-Code 103 - TTRAP (Process, 0, 0, STATE VECTOR)_______ ___ _ _____ _________ __ __ _____ _______ This informs the remote host that the given process has hit a trace trap, i.e. has just executed one instruction in response to the SINGLE STEP (Process) command.

Page 7 Figure 1 FORMAT OF NETWORK DEBUGGER MESSAGES ---------------- | TYPE | HOST | ---------------- | LINK | STYPE | ---------------- | CMD | PID | ---------------- | ARG1 | ---------------- | ARG2 | ---------------- TYPE -- The HOST-IMP/IMP-HOST message type, should always be zero. HOST -- The source host ID. LINK -- The debugger link. STYPE -- The message subtype, always zero. CMD -- The debugger command op-code, excess 100 for asynchronous reply's, excess 200 for "can't" reply's. PID -- Process ID of process being debugged. ARG1 -- The first argument of the command. ARG2 -- The second argument of the command.

Page 8 Figure 2 FORMAT OF STATE VECTORS ------------ | R0 | ------------ | R1 | ------------ | R2 | ------------ | ... | ------------ | PC | ------------ | PS | ------------

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