RFC 642 (rfc642) - Page 2 of 4

Ready line philosophy and implementation

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

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




Burchfiel