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NWG RFC 103
NIC 5764
IMPLEMENTATION OF INTERRUPT KEYS
R B Kalin
MIT Lincoln Laboratory
24 Feb 1971
The current protocol specifications contain a serious logical
error in the implementation of the program interrupt function. This
paper discusses the problem and offers a solution that is simple to
implement.
THE PROBLEM
As found on most time-sharing systems the program interrupt key,
elsewhere known as the break key, or help request button, has two
functions. It suspends temporarily the user process being run, and it
switches the keyboard input stream to a dormant supervisory process.
Unaccepted input typed prior to the interrupt request remains buffered
for the suspended user process. Subsequent typing is sent to a
supervisory routine.
The current NCP protocol implements only half this function. It
pprovides, through use of INS and INR control messages, for the
suspension of a remote process, but it offers no mechanism for
notifying the remote host at what time the data stream should be
switched. INR and INS messages are sent via the control link and
because messages on this link travel concurrently with those on the
user's keyboard input link, the receiving host can not rely on
relative arrival times as a source of synchronizing information.
Without such information the remote NCP can not know which input
characters are meant for the user process and which are meant for the
supervisory routine.
A solution found on some systems to this problem is that of
mapping the interrupt signal into some code from the character set --
typically an ASCII control-C. Unfortunately, this is not general
enough to be used within the ARPA network. Some systems, eg. MULTICS,
make use of all available ASCII codes for other purposes, none are
available for such an assignment. Even if such an assignment could be
made, there is the problem of getting the interrupt character to be
recognized by the remote host. Buffers on that user link may be full
and the sending host may be unable to transmit the message containing
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