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the proliferation of IMP configurations which has taken place was
not foreseen; therefore, the program differences currently cannot
be conveniently computed from a simple configuration key. Instead,
the configuration irregularities must be explicitly tabled.
3. 1.2 Packet Flow Through Major IMP Routines
Figure 3-3 is a schematic drawing of packet processing.
The processing programs are described below. Packet flow may
be followed by referring to Figure 3-3.
The Host-to-IMP routine (H-I) handles messages being
transmitted into the IMP from a local Host. The routine first
accepts the leader to construct a header that is prefixed to
each packet of the message. It then accepts the first packet
and, if no allocation of space exists for the destination IMP,
constructs a request for buffer allocation, which it places on
the task queue. Single-packet messages are placed directly on
the task queue regardless of allocation status and are held via
the oendin~
oacket table until either a RFNM or allocation is
returned. A returned RFNM releases the packet. A returned
allocation for the single-packet message will cause retransmission
from the background loop. Requests for multipacket
allocation are sent without actual message data. The request
is recorded at the destination IMP and an allocation message
is returned via the background loop when space is available.
A returned allocation causes H-I to release the first packet
with header to the task queue via the programmable task interrupt.
Subsequent input is then accepted from the Host until
end of message (EOM) occurs. The routine also tests a hardware
trouble indicator and verifies the message format. The routine
is serially reentrant and services all Hosts connected to the
IMP.
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