[Simh] Resurrecting the ARPAnet IMP

[Phil Budne]

> .... 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.

I wonder if ITS NCP code could be resurected.  The KLH KS10 simulation
talks TCP thru an emulated ACC LHDH-11, but of course the modern (KS)
IMP support file doesn't do NCP (but the old KA/KL IMP support file
still exists);

    ;;; -*- Mode:MIDAS -*- 

    IMPVRS==.IFNM2	; Version of IMP code

    IFN NCPP,.FATAL IMP Code doesn't support NCP any more!
    IFE KSIMP\SSIMP,.FATAL Wrong IMP driver file included!

    OVHMTR IMP	;Charge all this stuff to the IMP

    $INSRT LHDH

    ; IMP 1822 PROTOCOL INFORMATION (EXTENDED-LEADER VERSION)

    ; The IMP leader is 96 bits long, usually organized as 3 words of 32 bits.
    ; For further details, these documents are available from the Network
    ; Information Center:
    ;	IMP-HOST protocol: BBN Report No. 1822
    ;	NCP protocol: NIC 8246, Host-to-Host Protocol for the ARPANET
    ;	IP, TCP: Internet Protocol Transition Workbook, and
    ;		 Internet Protocol Implementor's Guide
    ;
    ; Here is the leader format used by the IMP code. This format uses
    ; no IMP padding, assumes IP only, and expects all IMP<->HOST data
    ; transfers in 32-bit mode.
    ;
    ; Previous versions of this code which supported NCP used a much more
    ; complicated leader formatting scheme based on 36 bit transfers and
    ; IMP padding. That scheme is dead, see SYSTEM;IMPOLD WTHNCP for details.