Network Debugging Protocol
Remote debugging protocol for PDP-11 systems over the network
Mader, E., "Network Debugging Protocol", RFC 643, DOI 10.17487/RFC0643, July 1974, <https://www.rfc-editor.org/info/rfc643>.
@misc{rfc643,
series = {Request for Comments},
number = 643,
howpublished = {RFC 643},
publisher = {RFC Editor},
doi = {10.17487/RFC0643},
url = {https://www.rfc-editor.org/info/rfc643},
author = {E. Mader},
title = {{Network Debugging Protocol}},
pagetotal = 7,
year = 1974,
month = jul,
} RFC text
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
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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)
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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
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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.
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Figure 2
FORMAT OF STATE VECTORS
------------
| R0 |
------------
| R1 |
------------
| R2 |
------------
| ... |
------------
| PC |
------------
| PS |
------------
AI ELI5
On this page
§1Remote Debugging Over the Network
RFC 643 defines a protocol for debugging PDP-11 processes running on remote hosts via network commands. The protocol is designed for use with PDP-11 systems running an operating system capable of performing low-level debugging tasks like setting and removing breakpoints and single-stepping instructions. A subset can be used on standalone programs with fewer debugging capabilities.
The protocol operates as a level-2 protocol, bypassing the standard HOST-HOST protocol. Messages are exchanged on a single link (377 octal) between a remote debugger and the PDP-11. Commands from the remote host request actions like stopping a process or examining memory, and responses confirm success or indicate that a process has trapped or hit a breakpoint.
A programmer sends commands over the network to a distant computer telling it to pause, check memory, or start running again. The distant computer sends back what it finds.
§2Command Structure
Each command consists of an 8-bit operation code, an 8-bit process identifier, two 16-bit arguments, and an optional byte string. The operation code from the PDP-11 response mirrors the command code for successful completion, or has the 200 bit set to indicate failure (CAN'T reply). An operation code with the 100 bit set indicates an asynchronous event—a process has stopped for a reason other than a command from the remote host.
Commands include NOP (no operation), DEBUG (begin debugging a process), STOP (halt a running process), DEPOSIT (write bytes to memory), EXAMINE (read bytes from memory), BREAK (set a breakpoint), UNBREAK (remove a breakpoint), and SINGLE STEP. Asynchronous replies include TRAP (process terminated abnormally), HALT (process terminated normally), BPT (hit breakpoint), and TTRAP (trace trap from single step).
Commands are short messages with an operation number, process number, and data. Replies say whether the command worked or what happened to the process.
§3Debugging Workflow
A debugging session begins with a DEBUG command to claim control of a process; the PDP-11 responds DEBUGGING if successful or CAN'T DEBUG if another remote host is already debugging it. Once debugging, the remote host can issue commands to examine memory (EXAMINE), modify memory (DEPOSIT), set breakpoints (BREAK), or single-step execution (SINGLE STEP). When a breakpoint is hit or a process traps, the PDP-11 sends an asynchronous reply with the complete state vector for the process—the values of all registers and program counter. Process creation and destruction are also supported, allowing the remote host to create a fresh process for debugging.
The protocol assumes the PDP-11 can manage its own breakpoint tables, remember which remote host is debugging each process, and report process state changes. The remote host is responsible for maintaining the state of the debugging session and issuing appropriate follow-up commands.
The programmer asks to debug a program, then sends commands to pause it, check values, set traps, or step through code one line at a time. The computer reports back what it finds.