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Bolt Beranek and Newman Inc.
(store and forward traffic, Host traffic, etc.) is always guaranteed
some buffers. For the sake of program speed and simplicity, no
attempt is made to retrieve the space wasted by partially filled
buffers.
In handling store and forward traffic, all processing is on
a per-packet basis. Further, although traffic to and from Hosts
is composed of messages, the IMP converts to dealing with packets;
the Host transmits a message as a single unit but the IMP takes
it one buffer at a time. As each buffer is filled, the program
selects another buffer for input until the entire message has
been provided for. These successive buffers will, in general,
be scattered throughout the IMP's memory. An equivalent inverse
process occurs on output to the Host after all packets of the
message have arrived at the destination IMP. No attempt is made
to collect the packets of a message into a contiguous portion
of IMP memory. The allocation of buffer space in core storage
is shown in Figure 4, as mentioned previously. Notice that
code is generally centered within a page, and there is code
on every page of core.
The IMP program uses the following set of rules to allocate
the approximately 80 available buffers to the various tasks
requiring them:
· Each line must be able to get its share of buffers
for input and output. In particular, one buffer is
always allocated for output on each line, guaranteeing
that output is always possible for each line. Double
buffering is provided for input on each line, which
permits all input traffic to be examined by the program.
Thus, acknowledgments can always be processed, which
frees buffers.
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