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Bolt Beranek and ~ewman
Inc.
Tables. Tables in core are used for the storage of queue
pointers, for trace blocks, for reassembly information, for
messages waiting for allocation, for routing, allocation,
sequence control, and for statistics and trace data. Figure 5
summarizes the IMP table storage. All IMPs have identical tables.
The program has twelve words of tables for each of th~ sixty-four
IMPs now possible in the network. The program has ninety-one
words of tables for each of the eight Hosts (four real and four
fake) that can be connected; additionally, twelve words of code
are replicated for each real Host that can be connected. The
program has fifty-five words of tables for each of the five lines
that can be connected; additionally, thirty-seven words of code
are replicated for each line that can be connected.
The size of the initialization code and the associated
tables deserves mention. This was originally quite small.
However, as the network has grown and the IMP's capabilities have
been expanded, the amount of memory dedicated to initialization
has steadily grown. This is mainly due to the fact that the
IMPs are no longer identically configured. An IMP may be required
to handle a Very Distant Host, or TIP hardware, or five
lines and two Hosts, or four Hosts and three lines, or a very
high speed line, or a satellite link. As the physical permutations
of the IMP have continued to increase, the criterion followed has
been that the program should be identical in all IMPs, allowing an
IMP to reload its program from a neighboring IMP and providing
other considerable advantages. However, maintaining only one
version of the program means that the program must rebuild itself
during initialization to be the proper program to handle the particular
physical configuration of the IMP. Furthermore, it must be
able to turn itself back into its nominal form when it is reloaded
into a neighbor. All of this takes tables and code. Unfortunately,
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