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Network Working Group Jerry Burchfiel Request for Comments: 642 BBN NIC: 30872 July 5, 1974

              Ready Line Philosophy and Implementation

I. Introduction

 BBN Report #1822, Specifications for the Interconnection of a Host
 and an IMP, gives a complete specification of the Host-IMP interface.
 However, the authors of this document bent over backward to avoid
 issuing arbitrary dictatorial directives to host interface
 implementors.  They succeeded admirably in this goal by describing
 the IMP implementation, and suggesting similar behavior on the part
 of the host.

 ARPA has appointed a PDP-11 local host interface standardization
 committee composed of myself, Dave Retz of SCRL, and Yuval Peduel of
 MIT Lincoln Labs.  During our review of various interfaces designed
 by the ARPA community, we have found total chaos, confusion and
 misunderstanding about the recommended host interface implementation.

 This note is an attempt to make explicit the recommendations which
 are implicit in Report #1822.  It provides a cookbook for interface
 implementors, including a set of recommended do's and don't's in the
 common problem areas.  This document has been reviewed and approved
 by the BBN IMP group.

II. Ready-line Philosophy

 The following is an attempt to spell out in detail a consistent plan
 for operation of the IMP ready line and host ready line with the
 following objectives:

    1.  Reliably resynchronize and resume transmission after a
        temporary lapse of service and possible loss of state
        information by either system.

    2.  Make the programming of the host interface as simple as
        possible.  This will minimize bugs, and make it possible to
        create a small ROM network-bootstrap loader.

 First, consider the IMP ready line.  When it drops, the IMP has
 suffered a possible loss of state, so the message in transit from the
 IMP to the host (if any) is likely to be incomplete.  Similarly, the
 message in transit from the host to the IMP (if any) is likely to be
 incomplete.  Both the host and the IMP must recognize this explicitly

Burchfiel [Page 1]  RFC 642 Ready Line Philosophy and Implementation July 1974

 by sending a message intended to be thrown away* (which may he
 appended to the current message) and throw away the message currently
 being received.  (Both the host - IMP message and the IMP - host
 message).

 The simplest arrangement for the host's interface driver is a pair of
 processes, one sending messages and the other receiving messages.
 This drop of the IMP's ready line must be provided as an error status
 bit to each process.  However, the two processes will need to clear
 this condition independently: the simplest implementation is an Input
 Error flop and an Output Error flop.  Both flops are set by a drop of
 the IMP's ready line, and they are cleared independently under
 program control.

 When the IMP raises its ready line, each contact bounce will again
 set the Error flops in the host's interface.  To insure that messages
 are not flowing across the interface at this time, assertions of the
 signals "there's your IMP bit" and "ready for next host bit" have
 been delayed sufficiently in the IMP to guarantee that the IMP ready
 line has stabilized.

III. Programming

 The interface driver processes can be described simply:

 INPUT:  Wait until an input buffer is available
         Wait until IMP ready
         Start input
         Wait until input is complete
         IF Input Error
         THEN clear Input Error  // Flush smashed message.  Input
                                 // buffer will be reused.
         ELSE queue message on input queue
         GOTO INPUT

 OUTPUT: Wait until a message is present on output queue
         Wait until IMP ready
         Start output
         Wait until output is complete
         IF Output Error
         THEN clear Output Error  // smashed message is flushed
         ELSE deque message from output queue  // Free up
                                               // output buffer
         GOTO 0UTPUT

1. ———

• The standard convention uses the host-IMP NOP message.

Burchfiel [Page 2]  RFC 642 Ready Line Philosophy and Implementation July 1974

 The only initialization required for system startup or restart is
 clearing the host READY flop, waiting 1/2 second, and setting the
 host READY flop.  Simply starting (or restarting) the above processes
 will properly resynchronize host-IMP communication.  As explained in
 RFC #636, the IMP ready line (and error flops) should only affect the
 two processes above: this resynchronization should be invisible to
 the NCP, and should have no effect on the connection data base.  The
 NCP will be resynchronized or reinitialized by the type 10 IMP-to-
 host message "interface was reset."

 Actually, it is possible to share a single Error flop between the
 input and output processes by implementing Input Error and Output
 Error as software flags.  A process testing for error must test both
 the Error flop and its own flag.  An interlock is required (e.g.  a
 mutual exclusion  semaphore) to guarantee that only one process at a
 time is testing and modifying the flags.  If the Error flop is set,
 the process must copy it into the other process' flag before clearing
 the flop and its own flag.

IV. Host Ready Line Implementation

 When the host drops and raises its ready line, the IMP behaves in a
 fashion symmetric to that outlined above.  Of course, this drop
 indicates that the state of the host's interface driver, as well as
 the current incoming and outgoing messages, are likely to be lost.
 The appropriate action is triggered by setting the Error flop or
 flops in the host interface, and the processes specified above will
 correctly resynchronize message flow in both directions.  Of course,
 to guarantee that messages are not flowing across the interface while
 the host ready line is undergoing contact bounce, the host must delay
 transmission until its ready line has stabilized.  This may be done
 in two ways:

    Hardware: an integrating one-shot driven by the host ready line
         flop is ANDed with "there's your host bit" and "ready for
         next IMP bit" to guarantee that message transfer will not
         start until the ready flop has been on for 1/2 second.

    Software: the initialization program executes a 1/2 second wait
         after setting the host ready flop before permitting input or
         output to begin.

Burchfiel [Page 3]  RFC 642 Ready Line Philosophy and Implementation July 1974

V. Summary

 This determines the specification READY line controls for the host's
 interface to the IMP:

    1.  It contains a program settable/clearable host READY flop which
        drives a relay closure to the IMP.

    2.  It detects the IMP's ready signal as a program-readable status
        condition.  (But not an interrupt condition)

    3.  It contains one or two ERROR flops set when either the host
        READY flop is off or the IMP ready signal is off.  The flop
        (flops) is a program-readable and program-clearable status
        condition.  (But not an interrupt condition).  These status
        flops must not be cleared by system initialization.

    4.  If hardware stabilization of the host's READY line is
        provided, it is a 1/2 second integrating one-shot driven by
        the host READY flop.  This signal is ANDed with "there's your
        host bit" and "ready for next IMP bit".

     [ This RFC was put into machine readable form for entry ]
     [ into the online RFC archives by Alex McKenzie with    ]
     [ support from GTE, formerly BBN Corp.           2/2000 ]

Burchfiel [Page 4]