This summer a group of us worked together to resurrect the original ARPAnet IMP software, and I’m now happy to say that the IMP lives again in simulation. It’s possible to run the original IMP software on a modified version of the H316 simh and to set up a virtual network of simulated IMPs talking to each other. IMP to IMP connections, which would have originally been carried over leased telephone lines, are tunneled over IP. As far as we can tell, everything works pretty much as it did in the early 1970s. IMPs are able to exchange routing information, console to console communications, network statistics, and they would carry host traffic if there were hosts on the network. The hooks are in there to allow simh to support the IMP side of the 1822 host interface, and the next step would be to recover the OS for an ARPAnet era host and then extend the corresponding simulator to talk to the IMP simulation.
The logic here is based on the idea that a restore image contains the memory content for a running simulator, while the attached files contain the disk contents for that simulator. If the disk contents have changed since the memory image was created then the two data sets are likely out of sync and disk details cached in memory (i.e. file system information, storage allocation, etc.) will likely result in corrupted disk structures if they are used.
The default behavior is to fail the restore operation if these inconsistencies are noticed. This sanity check can be overridden if the restore command is invoked with the '-F' switch: sim> restore -F simulator-state.file
Also added logging of all erro messages produced during a restore operation to both stdout and a simulator log file if it is being used.
