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;;; -*- Mode:MIDAS -*- 
;;; Copyright (c) 1999 Massachusetts Institute of Technology
;;;
;;; This program is free software; you can redistribute it and/or
;;; modify it under the terms of the GNU General Public License as
;;; published by the Free Software Foundation; either version 3 of the
;;; License, or (at your option) any later version.
;;;
;;; This program is distributed in the hope that it will be useful,
;;; but WITHOUT ANY WARRANTY; without even the implied warranty of
;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
;;; General Public License for more details.
;;;
;;; You should have received a copy of the GNU General Public License
;;; along with this program; if not, write to the Free Software
;;; Foundation, Inc., 675 Mass Ave, Cambridge, MA 02139, USA.


IMPVRS==.IFNM2  ; Version of IMP code

IFN NCPP,.FATAL IMP Code doesn't support NCP any more!
IFE KSIMP,.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.
;
; All data from the IMP interface ends up in the -10 as left-justified
; 32-bit words. Objects of less than 32 bits length, such as IP octets,
; are stored in PDP10 ILDB byte order.
;
;------------------------------------------------------------------------
;1:  4.9-4.6 not used (0)
;    4.5-4.2 all 1's for new format, else old msg type (4=old nop)
;    4.1-3.3 network number (0)
;    3.2-2.8 not used (0)
;        2.7 trace (ignored)
;    2.6-2.4 leader flags (2.6 is to be ignored, 2.5-2.4 are not used!)
;    2.3-1.5 message type
;
;2:  4.9-4.2 Handling type (7 for big buffers, 4 for small buffers,
;                           0 for the control link)
;    4.1-3.3 Host number on IMP
;    3.2-1.5 IMP number
;
;3:  4.9-4.2 Link Number (High 8 bits of Message ID)
;    4.1-3.7 Low 4 bits of Message ID (0) 
;    3.6-3.3 Sub-type
;    3.2-1.5 Message length
;------------------------------------------------------------------------
;4:  4.9-1.5 First word of IP datagram
;    ....
;------------------------------------------------------------------------
;
;In message types 2 and 6, the going-down status 16-bit word is
;in word 3 bits 4.9-3.3.

;3.6-3.3 of word 3 are the padding count for type 4 (nop) from host.
;This is currently 0 (none).  Padding is only put on type-0 messages.
\f
IMPLDS==3                       ;IMP local leader size

                                ;Byte pointers to fields of input leader
IMOTBP: 340400,,IMPILB+0        ;Message format type
IMTBP:  041000,,IMPILB+0        ;Message type field
IMSABP: 043000,,IMPILB+1        ;Source address field (host+IMP)
IMSHBP: 241000,,IMPILB+1        ;Source host field
IMSIBP: 042000,,IMPILB+1        ;Source IMP field
IMLNBP: 341000,,IMPILB+2        ;Link number field
IMSTBP: 240400,,IMPILB+2        ;Subtype field
IMMLBP: 042000,,IMPILB+2        ;Message length field
\f


SUBTTL  ARPANET VARIABLES AND TABLES

EBLK

%IMXLN==:<<8159.-96.>+31.>/32.  ; Max # of 32-bit words in IMP regular msg,
                ; exclusive of leader and leader padding.  = 252.

%IMMTU==:251.*4 ; Used by IP/TCP. Driver currently rounds to PDP10 words,
                ; forcing this to be 1004 instead of 1007.

; ACTIVE HOST TABLE.  Entries herein are allocated as needed, using garbage
; collection.  Most "host number" fields are really indices into this table.
;
LIMPHT==<XBL+10.>       ; TCP conns plus a few extra

IMPHTF: -1      ;Host table free list, threaded through IMPHTB, end with -1

IMPHTS:                 ;Start of table area
IMPHTN: BLOCK LIMPHT    ; Host number.  1.1-1.8 HOST, 2.1-3.7 IMP
IMPHTB: BLOCK LIMPHT    ;Bits:
                        ;4.9            UNUSED
                        ;4.8            GC MARK BIT
                        ;4.7-4.3        UNUSED
                        ;4.2-4.1        STATUS OF HOST 0 DOWN, 1 RST SENT, 2 UP
                        ;3.9-3.1        UNUSED
        .SEE IMPHDS     ;RH   Last message from IMP about "host dead status"
IMPHTC: BLOCK LIMPHT    ; # active messages outstanding for host (8 max)
IMPHTT: BLOCK LIMPHT    ; Time of last RFNM received
IMPHTE==.-1             ;Last location in table

;Status variables
;
IMPUP:  -1      ;0 => IMP up  ;-1 => down  ;-2 => coming up, PI  still off
                ;1 => down for good, until next time IMP ready line changes
IMPTCU: 0       ;0 IMP up/down status not changing
                ;>0 Trying to reinitialize, SYSJOB hasn't done so yet
                ;-1 Has been reinitialized, haven't exchanged NOPs yet
IMPUCT: 0       ;IMP coming up timeout, if 4 NOPs don't go through promptly.
IMPDWN: BLOCK 3 ;Last message from IMP that it is going down
                ;WD0: "Reason" claimed by IMP (see ch 3 of BBN report 1822)
                ;WD1: Time when expected down
                ;WD2: Time when expected up (SYS time=1/30 sec since up)

IMERCN: -1      ;CONI into here when net goes down

BBLK
\f
EBLK

;Input side variables
;
IMPILB: BLOCK IMPLDS    ;Input leader buffer
IMPCSH: -1      ;Current source host (IMPHTB index).  -1 when idle.
IMPCLN: 0       ;Current link number
IMIFLS: 0       ;Flushing output at interrupt handler
IMPIEC: 0       ;Count of input errors while down.
IMPIBC: 0       ;Count of words available in DMA input buffer
IMPIBP: 0       ;Pointer into DMA input buffer

;Output side variables
;
                ;Output leader buffer
IMOLDR: <17_10.>,,0     ;Regular new-format message
IMOLAD: 0               ;Filled in with destination IMP address
        <233_10.>,,0    ;IP link field

IMNOPC: 0       ;< 0 => Send NOPs
IMPOAC: -1      ;>= 0 => Output active, don't restart
IMPBZY: 0       ;-1 == Waiting for output interrupt
IMPODP: 0       ;Pointer to IP datagram being output at PI level
IMPOS:  0       ;Output state
 %ISODL==:0     ; Not expecting output done (i.e. between messages)
 %ISONP==:1     ; Sending NOP
 %ISOID==:2     ; Sending IP Datagram

;Meters

;IP meters
IMNIPI: 0       ; # of IP datagrams input (rcvd)
IMNIPF: 0       ; # of IP datagrams flushed (input threw away)
IMNIPO: 0       ; # of IP datagrams output (sent)
IMNIPR: 0       ; # of IP RFNMs received
IMNIP7: 0       ; # of IP Type 7 (Dest Host Dead) messages received
IMNIP8: 0       ; # of IP Type 8 (Error) msgs rcvd
IMNIP9: 0       ; # of IP Type 9 (Incomplete Transmission) msgs rcvd
IMNWIG: 0       ; # words ignored by "Ignore" state (%ISIGN)
IMNWIF: 0       ; # words flushed by IMPRM5

;IMP meters
IMNSRF: 0       ;Number of spurious RFNMs on non-IP links
IMNBLK: 0       ;Number of times blockage avoided (output held up by ITS)

IMPMSR: BLOCK 20;Count of IMP messages rcvd
IMPM1S: BLOCK 4 ; # Type 1 (Error in Leader) subtype msgs
IMPM9S: BLOCK 2 ; # Type 9 (Incomplete Transmission) subtype msgs
IMPMSS: BLOCK 1 ;Count of IMP msg sent (we only send regular msgs)
IMCT1:  0       ;# Unvectored interrupts
IMCT2:  0       ;# Valid input interrupts
IMCT3:  0       ;# Valid output interrupts
BBLK
\f
SUBTTL  ARPANET MAIN-PROGRAM LEVEL
;IMPIBF, IMPOBF defined as low-memory buffer page in ITS
IF2,IFN IMPIBF&777,.FATAL IMPIBF not on DEC page boundary

;(Re)Start IMP. Called from SYSJOB to start IMP, or on error or
; user requested cycle through LOCK
;
IMPINI: SETOM IMPUP             ;Not up yet,
        SETOM IMPTCU            ; but thinking about it.
        MOVEI A,IMPIBF_-9.      ;DEC page # of IMP buffer page
        TRO A,%UQ16B\%UQVAL     ;Valid mapping, 16 bit device
        IOWRI A,UBAPAG+IUIMPG_1 ;Set up 1 DEC page of UBA mapping. Note that
                                ; the second half of IUIMPG isn't mapped at all
        CONO PI,NETOFF          ;Freeze things while IMP bashing occurs
        MOVEI A,%LHRST
        IOWRI A,%LHOCS          ;Reset output side
        IOWRI A,%LHICS          ;Reset Input side

        ;IMP now shut down. Reset variables
        SKIPE A,IMPODP          ;Have an output datagram ?
         PUSHJ P,IPIODN         ;Yep, release it.
        SETZM IMPODP            ;No output datagram
        SETZM IMPOS             ;Output idle
        SETOM IMPOAC            ;Output interrupt level inactive
        SETZM IMPBZY            ;No pending output interrupt

        SETZM IMPIBC            ;No input available
        SETZM IMPIEC            ;No input errors yet
        SETZM IMIFLS            ;Not flushing input at interrupt level
        SETOM IMPCSH            ;No current host table index
        SETOM IMPDWN+1          ;Time for IMP to go down, not known
        CONO PI,NETON           ;Allow interrupts again

        ;Wait one sec for IMP to notice rdy line drop
        MOVE T,TIME
        ADDI T,30.
        CAMLE T,TIME
         PUSHJ P,UFLS

        MOVNI A,30.             ;Allow 15 seconds to come up
        MOVEM A,IMPUCT
        MOVE T,TIME             ;Note when we last started IMP
        MOVEM T,LNETIM
        PUSHJ P,IMPHRS          ;Set host ready
        PUSHJ P,IMPIST          ;Start input

        MOVE T,TIME
        ADDI T,15.              ;Wait 1/2 sec before we try to output
        CAMLE T,TIME
         PUSHJ P,UFLS
        MOVNI A,4
        MOVEM A,IMNOPC          ;Send 4 NOPs to start
        ;Falls through to start output
\f
;Start IMP output
;
IMPOST:
IMPIOS: CONO PI,PIOFF           ;Freeze.
        AOSE IMPOAC             ;Do nothing if output already active.
         JRST PIONJ
        SETOM IMPBZY            ;Note we want an IMP interrupt
        CONO PI,NETRQ           ;Force nonvectored IMP interrupt
         JRST PIONJ

;Check if IMP ready line is set
; Called from SYSJOB.
; Return +1 if IMP not ready, +2 if so
;
IMPCKR: IORDI A,%LHICS          ;Get input CSR
        TRNN A,%LHINR           ;Skip if IMP not ready
         AOS (P)                ;Return +2 if ready
        POPJ P,                 ;That's all

;Set HOST READY. From SYS job only, please, loops waiting.
;
IMPHRS: IORDI T,%LHICS
        TRNN T,%LHRDY           ;Can we mung?
         BUG
        IORI T,%LHHRC\%LHSE     ;Turn on HR. SE prevents dropping messages
        IOWRI T,%LHICS
        MOVEI A,777777          ;I don't know why this takes so long.
IMPHR1: IORDI T,%LHICS          ;Get the bits back
        TRNE T,%LHHR            ;LHDH thinks host ready is ready
         RET                    ;HR line set
        SOJG A,IMPHR1           ;Timed out yet?
         BUG CHECK,[IMP: Timed out setting Host Ready]
        RET
\f
SUBTTL  HOST-TABLE MANAGEMENT

;IMPHTI - Initialize host table
; Performed whenever IMP announces that it has been reset
;
IMPHTI: SETOM IMPHTF            ;Force GC of IMPHTB on first reference
        SETZM IMPHTS            ;Clear table of old information
        MOVE A,[IMPHTS,,IMPHTS+1]
        BLT A,IMPHTE
        POPJ P,

; FNDHST - Look up host-table index for a given IMP host address.
;       Call with NETOFF or NETCHN PI in progress.
;       T/ IMP host address (maybe someday other nets?)
; Returns .+1 if failed (no room in table)
; Returns .+2
;       H/ host-table index
; Smashes W.

FNDHST: MOVEI H,LIMPHT-1        ;Search for an entry for this host
        CAME T,IMPHTN(H)
         SOJGE H,.-1
        JUMPGE H,POPJ1          ;Found
        SKIPGE H,IMPHTF         ;Not found, cons one off free list
         JRST FNDHS1            ;Oops, must garbage collect
        MOVE W,IMPHTB(H)
        CAIGE H,LIMPHT          ;Make sure H is valid idx
         CAIL W,LIMPHT          ;ditto W
          BUG HALT,[NET: FNDHST idx clobbered!!!]
        MOVEM W,IMPHTF
        MOVEM T,IMPHTN(H)
        SETZM IMPHTB(H)         ;Nothing is known about this host
        SETZM IMPHTC(H)         ;Assume no RFNMs outstanding
        SETZM IMPHTT(H)         ;Clear out time of last RFNM.
        JRST POPJ1

; Host-Table full, attempt to GC it and flush unused entries, by
; scanning all possible pointers into table.
;       IMP pointers are IMPCSH and IMPHTC(H)
;       TCP pointers are XBNADR(I)

; GC mark phase - mark entries in use
FNDHS1: PUSH P,I
        MOVSI W,200000          ;Mark bit
        MOVEI H,LIMPHT-1        ;Clear all mark bits
        ANDCAM W,IMPHTB(H)
        SOJGE H,.-1
        SKIPL H,IMPCSH          ;Mark from IMPCSH
         IORM W,IMPHTB(H)
IFN TCPP,[
        MOVEI I,XBL-1
        SKIPL H,XBNADR(I)       ; See if TCP conn has a net addr specified
         IORM W,IMPHTB(H)       ; Yes, set the mark bit.
        SOJGE I,.-2
] ;IFN TCPP
\f
; GC sweep phase - free all unmarked entries
        SETO I,                 ;Free pointer
        MOVEI H,LIMPHT-1
        MOVSI W,601000          ;Protect if RFNM-WAIT, RST-WAIT, or marked
FNDHS4:
        SKIPG IMPHTC(H)         ;Also protect if any outstanding RFNMs
         TDNE W,IMPHTB(H)
          SOJGE H,FNDHS4
        JUMPL H,FNDHS5
        SETZM IMPHTN(H)         ;Don't belong to any host
        MOVEM I,IMPHTB(H)       ;Cons onto free list
        MOVE I,H
        SOJGE H,FNDHS4
FNDHS5: MOVEM I,IMPHTF          ;Free list
        POP P,I
        SKIPGE IMPHTF
         POPJ P,                ;GC-overflow
        JRST FNDHST             ;Try again, should win

;See if IMP code is willing to handle a particular datagram right now.
; A/ IP DGM pointer
; C/ Immediate destination address
; Returns +1, can't send right now, +2, OK to send
;
IMPCTS: MOVE T,C                ;Set up for FNDHST
        AND T,[<377_16.>+377]   ;Mask out all but host and IMP field
        CALL FNDHST             ;Get host index in H
         POPJ P,                ;No host entry, don't send
        JSP T,IMPBLI            ;See if OK to send
         POPJ P,                ;IMP wants to block, no send
        JRST POPJ1              ;Skip return if OK to send.

\f
;;; IMP Blockage avoidance
;       The current IMP software will not accept more than 8 active
; messages to a single host; attempting to send a 9th message will block
; ALL output to the interface, until the first message has been ack'd
; by means of one of the following message types:
;       Type 5, RFNM - Message delivered OK
;       Type 7, Host dead - transmit failed ("permanent")
;       Type 8, Error in data - interface spazzed
;       Type 9, Incomplete Transmission - temporary failure
; If for some reason the first message simply becomes lost, the IMP timeout
; (and blockage) can last for up to 30-45 seconds.
; More details in BBN Report 1822.
;       ITS attempts to fix this by keeping a count of active un-ACKed
; messages for each host it is communicating with.  A timeout is also
; associated with each host; if output to a given host is blocked by ITS
; because there are 8 active messages, trying to send a 9th message
; will check the last-RFNM-received time and if this was more than
; 30 or so seconds then the IMP is probably not giving us what it should
; and we should reset things for that host.

%IMPMA==:8.     ; # of maximum active IMP messages allowed

; IMPBLI, IMPBLD - routines to hack active-message counts, called via JSP T,
;       IMPBLD decrements count.
;       IMPBLI increments count and skips if successful (else failed,
;               and must NOT output another message to this host!)
;               Also clobbers Q.
;
IMPBLI: AOS Q,IMPHTC(H)
        CAIGE Q,%IMPMA          ;Trying to send max or more messages?
         JRST 1(T)              ;No, can return safely.
        CAIG Q,%IMPMA           ;Is this the maximum # allowed?
         JRST [ MOVE Q,TIME     ;Yes, set up blockage timeout
                ADDI Q,60.*30.  ; for one minute.
                MOVEM Q,IMPHTT(H)
                JRST 1(T)]      ;And allow this one to be sent

        ; Trying to send too many messages, block it (check for timeout though)
        SOS IMPHTC(H)           ;Restore original count
        AOS IMNBLK              ;Increment # of times softwarily blocked.
        MOVE Q,IMPHTT(H)
        CAML Q,TIME             ;See if timeout still in the future
         JRST (T)               ;Yes, just take failure-return to block.
        BUG INFO,[IMP: RFNM-wait timeout! Hst=],OCT,IMPHTN(H)
        SETZM IMPHTC(H)         ;This may be dangerous... oh well.
        SETZM IMPHTT(H)
        JRST (T)                ;Block one last time, next try will win.

;Decrement block count on receipt of any kind of ACK
;
IMPBLD: SOSL Q,IMPHTC(H)
         JRST IMPBL2
        BUG INFO,[IMP: negative RFNM-wait cnt, Hst=],OCT,IMPHTN(H)
        SETZB Q,IMPHTC(H)
IMPBL2: CAIL Q,%IMPMA-1         ;If we were blocking on this host,
         PUSHJ P,IMPIOS         ;Ensure IMP output started up so blocked stuff
        JRST (T)                ; gets sent promptly.

\f
SUBTTL  ARPANET INPUT INTERRUPT LEVEL

;First level interrupt handling for input side. Here from UBA vector hardware.

IFN NETCHN-UTCCHN,.ERR NETCHN assumed == UTCCHN at IMPIBK

        EBLK
IMPIBK: 0
        BBLK
        JSR UTCSAV              ;Save AC's, get a stack
        AOS IMCT2               ;Count input interrupts
        IORDI TT,%LHICS         ;Get CS register
        TRNE TT,%LHERR\%LHNXM
         JRST IMPRST            ;Try resetting the IMP
        TRNE TT,%LHMRE          ;Ready line flapped
         JRST IMPIER            ;Go directly to error routine
        TRNN TT,%LHRDY          ;Device ready for new operation?
         JRST IMPRST            ;Try resetting. 
        TRNN TT,%LHEOM          ;Saw EOM from IMP?
         JRST IMPIB1            ;No, word count ran out before message
        SKIPGE IMIFLS           ;Flushing output?
         JRST [ SETZM IMIFLS    ;Not any more!
                JRST IMPIRT ]   ;But flush last piece by queueing new request
        IORDI A,%LHIWC          ;End of message. Get remaining UB word count
        SKIPE A                 ;This would be a surprise, really
         TDO A,[-1,,600000]     ;36bit number of UBA words remaining in bfr
        IDIVI A,2               ;Number of PDP10 words (cleverly rounded)
        ADDI A,IMPBFS           ;Number of PDP10 words of message
        MOVEM A,IMPIBC          ;Set current count of available data words
        MOVEI A,IMPIBF
        MOVEM A,IMPIBP          ;Set pointer to available data words
        JRST IMPLDD             ;Go process input message

;Here when finished processing current message.
IMPIRT: SETOM IMPCSH            ;Note no current host
        PUSHJ P,IMPIST          ;Restart input listener
        JRST IMPEX

;Message didn't fit in input buffer. Shouldn't ever get here, but
;might if messages concatenated due to ready line randomness
;
IMPIB1: BUG INFO,[IMP: Huge message]
        SETOM IMIFLS            ;Say we are flushing output
        JRST IMPIRT             ;And go queue up another read

;Here if error during IMP message transfer
; Currently the only error handled is the IMP going non-ready

IMPIER: SKIPE IMPUP             ;Is the IMP supposed to be up?
         JRST IMPIE1            ;No, handle errors differently
        BUG INFO,[IMP: Input Ready Error]
        JRST IMPRST             ;Ready line flapped while up, cycle interface

IMPIE1: AOS T,IMPIEC
        CAIG T,10.              ;Huge number of errors while down?
         JRST IMPIRT            ;No, just ignore this input and start another
        SETZM IMPIEC            ;Reset
        BUG INFO,[IMP: Excessive input errors while down]
        JRST IMPRST             ;Cycle the interface
\f
IMPRST: IORDI T,%LHICS          ;Record IMP status for sysjob
        HRLZM T,IMERCN
        IORDI T,%LHOCS
        HRRM T,IMERCN
        MOVEI A,%LHRST          ;Avoid randomness by resetting HW now.
        IOWRI A,%LHOCS          ;Reset output side
        IOWRI A,%LHICS          ;Reset Input side
        SETOM IMPUP             ;IMP is down
        MOVEI T,1
        MOVEM T,IMPTCU          ;But trying to come up (sysjob poked)
        MOVSI J,SCLNET          ;Ask SYSJOB to cycle the IMP
        IORM J,SUPCOR
        JRST IMPEX
\f
        SUBTTL IMP leader dispatch handling

IMPLDD: SKIPG B,IMPIBC          ;Get count of available data
         POPJ P,                ;None, nothing to do.
        CAILE B,IMPLDS          ;More than a leader worth?
         MOVEI B,IMPLDS         ;Yep, only want leader now
        MOVS A,IMPIBP           ;Copy and reformat leader to IMPILB
        HRRI A,IMPILB
        BLTUB A,IMPILB-1(B)
        ADDM B,IMPIBP           ;Increment buffer pointer,
        MOVN C,B
        ADDM C,IMPIBC           ; and decrement count

        LDB T,IMOTBP            ;Examine new-format flag bits of leader
        CAIL B,IMPLDS           ;Large enough to be a valid leader?
         CAIE T,17              ;Verify that leader is "new" 96-bit fmt.
          JRST IMPLDE           ;Go process error in leader

        LDB T,IMLNBP            ;Extract link number (high 8 bits of msg-id)
        MOVEM T,IMPCLN          ;Save link message arrived on
        LDB T,IMSABP            ;Get arpanet address (source host+imp)
IFN 0,[ 
        LDB T,IMSHBP            ;Source host
        LDB A,IMSIBP            ;Source imp
        DPB A,[112000,,T]       ;Form host address
];IFN 0
        PUSHJ P,FNDHST          ;H gets host table index
         JRST IMPLHE            ;Host table full
        MOVEM H,IMPCSH          ;Save current host
        LDB A,IMTBP             ;Get message type in A
        CAIL A,IMTDTS
         JRST IMPUN             ;Unknown type?
        AOS IMPMSR(A)           ;Count IMP msgs rcvd
        JRST @IMTDT(A)          ;Dispatch

IMTDT:  IMPRM   ; 0 Regular Message
        IMPBE1  ; 1 Error in Leader (no msg-id)
        IMPGD   ; 2 IMP Going Down
        IMPUN   ; 3  -
        IMPIN   ; 4 NOP
        IMPRFN  ; 5 RFNM - Ready For Next Message (transmit succeeded)
        IMPHDS  ; 6 Host Dead Status (general info)
        IMPDHD  ; 7 Destination Host Dead (transmit failed)
        IMPBE8  ; 8 Error in Data (has msg-id)
        IMPINC  ; 9 Incomplete Transmission (transmit failed temporarily)
        IMPIRS  ;10 Interface Reset - IMP dropped its ready line
IMTDTS==.-IMTDT

IMPLHE: BUG INFO,[IMP: Message discarded due to host table full],OCT,IMPILB,OCT,IMPILB+1,OCT,IMPILB+2
        JRST IMPIRT
\f
;Here if leader is too short or of wrong format.
; B/ Leader length
IMPLDE: SKIPE IMPUP             ;If IMP is not up, we'll take anything.
         JRST IMPIRT
        CAIGE B,IMPLDS          ;Announce short leader if so.
         BUG INFO,[IMP: Short leader, ],DEC,B,[wds. WD1=],OCT,IMPILB,[WD2=],OCT,IMPILB+1
        LDB A,IMOTBP            ;Get message format type
        CAIN A,4                ;Old-type NOP?
         JRST IMPIRT            ; Just ignore it.
IFN 0,[ ;;For the moment this is a problem because the IMP is marked
        ;;up too soon, before all incoming NOPS are received.
        CAIN A,16               ;Is it 1822L format?
         BUG INFO,[IMP: 1822L leader],OCT,IMPILB
        CAIE A,17               ;Is it not the long-leader format?
         BUG INFO,[IMP: Old-type leader],OCT,IMPILB
]
        JRST IMPIRT             ;Ignore rest of message, if any

;;; IMP->Host Type X (e.g. 3, 11-255) - bad type

IMPUN:  BUG INFO,[IMP: Unknown msg type ],OCT,A,[ leader ],OCT,IMPILB,OCT,IMPILB+1,OCT,IMPILB+2
        JRST IMPIRT

;;; IMP->Host Type 1 - Error in leader (msg-id not given)
;;; IMP->Host Type 8 - Error in data  (msg-id given)

IMPBE1: LDB T,IMSTBP            ;Get subtype (4 bits)
        ANDI T,3                ;Only 2 bits should be used
        AOS IMPM1S(T)           ;Increment count of Type 1 subtype messages
IMPBE8: SKIPN IMPUP             ;Ignore error during initial syncronization
         BUG INFO,[IMP: Type ],DEC,A,[err msg, leader],OCT,IMPILB,OCT,IMPILB+1,OCT,IMPILB+2
        MOVE B,IMPCLN           ;Get link msg came in on
        CAIN B,233              ;Internet link?
         AOS IMNIP8             ;Yes, count IP meter
        CAIN A,8.               ;Error identified with a particular message?
         JSP T,IMPBLD           ;Decrement count of active messages
        JRST IMPIRT

;;; IMP->Host Type 2 - IMP going down

IMPGD:  LDB B,[420200,,IMPILB+2]        ;Reason (see 1822)
        MOVEM B,IMPDWN
        LDB B,[360400,,IMPILB+2]        ;How soon going down * 5 mins
        MOVE H,B
        IMULI B,5*60.*30.       ;Ticks in 5 mins
        ADD B,TIME
        MOVEM B,IMPDWN+1
        LDB C,[241200,,IMPILB+2]        ;How long to be down * 5 minutes
        MOVE Q,C
        IMULI C,5*60.*30.       ;Downtime in ticks
        ADD C,B                 ;Add to time down
        MOVEM C,IMPDWN+2        ;Store time when will be up
        IMULI H,5               ;Minutes
        IMULI Q,5
        BUG INFO,[IMP: Going down in ],DEC,H,[mins for ],DEC,Q,[mins, reason],DEC,IMPDWN
        JRST IMPIRT

;;; IMP->Host Type 4 - NOP

IMPIN:  JRST IMPIRT             ;One more NOP from IMP
\f
;;; IMP->Host Type 5 - RFNM (Ready For Next Message)

IMPRFN: JSP T,IMPBLD            ;Decrement outstanding message count for host
        MOVE A,IMPCLN           ;Get link #
        CAIE A,233              ;IP link number?
         JRST IMRFNX            ;No, skip IP code
        AOS IMNIPR              ;Bump count of IP RFNMs received
        JRST IMPIRT             ; and do nothing else about it, ugh.

IMRFNX: BUG INFO,[IMP: Spurious RFNM from ],OCT,IMPHTN(H),[link],OCT,IMPCLN
        AOS IMNSRF
        JRST IMPIRT

;;; IMP->Host Type 6 - Host Down Status
;       H/ host index

IMPHDS: LDB A,[301400,,IMPILB+2];Bits 65-76 of leader, 4.9-3.7 3rd word
        HRRM A,IMPHTB(H)        ;Store, hope user read RFC 611
        JRST IMPIRT

;;; IMP->Host Type 7 - Destination Host Dead

IMPDHD: MOVEI E,%NCDED
        JRST IMPHNR

;;; IMP->Host Type 9 - Incomplete Transmission

IMPINC: LDB T,IMSTBP            ;Get subtype field (4 bit reason for failure)
        AOS IMPM9S(T)           ;Bump count of subtypes
        MOVEI E,%NCINC          ;This is an incomplete msg response

IMPHNR: JSP T,IMPBLD            ;Decrement active IMP msg count for this host
        MOVE A,IMPCLN           ;Link for this message?
        CAIE A,233              ;IP Link?
         JRST IMPHN1
        CAIN E,%NCINC           ;Yes, count IP meters
         AOS IMNIP9
        CAIN E,%NCDED
         AOS IMNIP7
        JRST IMPIRT

IMPHN1: BUG INFO,[IMP: DHD or IT msg rcvd on non-IP link]
        JRST IMPIRT

;;; IMP->Host Type 10 - Interface Reset

IMPIRS: PUSHJ P,IMPHTI          ;Initialize host table
        BUG INFO,[IMP: Interface reset]
        JRST IMPIRT

\f
;;; IMP->Host Type 0 - Regular Host-Host message
; Unless the source host screwed up and sent a dataless message,
; there is at least one word waiting to be read in the buffer.
;
IMPRM:  SKIPG A,IMPIBC          ;Get available input word count
         JRST IMPIRT            ;None left, just ignore message
        MOVE B,IMPCLN           ;Is link number the magic cookie for IP?
        CAIE B,233
         JRST IMPIRT            ;No, ignore it

;IP datagram. Copy into IP buffer and pass it up.
;
        AOS IMNIPI              ;Bump count of IP datagrams received
        PUSHJ P,IPGIPT          ;Ask IP for buffer of size (A)
         JRST [ AOS IMNIPF      ;Punted, bump cnt of datagrams lost
                JRST IMPIRT ]   ;Flush this message (err msg already printed)
        MOVE B,PK.BUF(A)        ;Destination
        HRL B,IMPIBP            ;Source
        MOVE C,IMPIBC           ;Input words available
        ADDI C,-1(B)            ;Last destination address
        BLTUB B,(C)             ;Copy and reformat data to IP buffer
        MOVE B,IMPIBC           ;Get word count back to B
        SETZB C,IMPIBC          ;Say zero offset to IP header, and clear count.
        MOVE J,IMPCSH           ;Set idx to host-table entry dgm received from.
        PUSHJ P,IPRDGM          ;Hand off rcvd datagram to IP
        JRST IMPIRT             ;Return from PI level, setting up for next msg

;Start listening for new input from IMP
;
IMPIST: HRREI T,-IMPBFS*2
        IOWRI T,%LHIWC          ;Read up to a buffer full of data
        MOVEI T,<IUIMPG_12.>+<4*<IMPIBF&777>>
        IOWRI T,%LHICA          ;Read data to here
        MOVEI T,%LHIE\%LHHRC\%LHSE\%LHGO ;Interrupt, store data, go
        IOWRI T,%LHICS          ;Start read
        RET

\f
SUBTTL  ARPANET OUTPUT INTERRUPT LEVEL

;Here on unvectored interrupt caused by call to IMPIOS

IMPINT: AOS IMCT1               ;Count unvectored interrupts
        JRST IMPOBZ             ;Go try to start output
                                ;Returns directly to interrupt dispatcher

;First-level interrupt handling, from hardware dispatch.
; Crash on severe interface errors
; Restart IMP on ready line flappage.
; Else, go look for more output to send.

IFN NETCHN-UTCCHN,.ERR NETCHN assumed == UTCCHN at IMPOBK

        EBLK
IMPOBK: 0
        BBLK
        JSR UTCSAV              ;Save AC's, get a stack
        AOS IMCT3               ;Count output interrupts
        IORDI TT,%LHOCS         ;Get CS register
        TRNE TT,%LHERR\%LHNXM   ;Interface lost?
         JRST IMPOEC            ;Try cycling
        TRNE TT,%LHMRE          ;Somebody bounce a ready line?
         JRST IMPOER
IMPOBE: TRNN TT,%LHRDY          ;Device ready for new operation?
         JRST IMPOEC            ;Try cycling
        PUSHJ P,IMPOBZ          ;No IMP error. Perform planned action.
        JRST IMPEX              ;Dismiss interrupt.

;Error while outputting datagram. Currently, just means ready line flapped.
; If IMP was up, finish up IP output if necessary to free buffer, then
;  cycle the interface.
; If trying to send NOPS to come up, just send a few more to resynch.
;
IMPOER: SKIPE IMPUP             ;IMP up?
         JRST IMPOE1            ;Not running normally, maybe OK.
        BUG INFO,[IMP: Output RDY error]
IMPOEC: SKIPE A,IMPODP          ;Have an output datagram ?
         PUSHJ P,IPIODN         ;Yep, release it.
        SETZM IMPOS             ;Doing nothing useful
        JRST IMPRST             ;Go poke SYSJOB to cycle the IMP

;Here if interface wasn't fully up. 
IMPOE1: MOVE T,IMPOS
        CAIE T,%ISONP           ;Were we sending a NOP?
         BUG CHECK,[IMP: Confusing output error]
        SOS IMNOPC              ;Add another NOP to make up for this one
        SETZM IMPOS             ;NOt doing anything anymore
        JRST IMPOBE             ;Go continue processing.

;IMPOBZ - Toplevel output processing routing.
; Called from output done interrupt handler or from MP/CLOCK level with
; NETOFF to start output.

IMPOBZ: SKIPL B,IMPOS
         CAIL B,IMPODL
          BUG HALT,[IMP: Bad output state]
        JRST @IMPODT(B)

IMPODT: OFFSET -.
%ISODL::        IMPOB0  ; 0 Idle, look for something to send
%ISONP::        IMPOB1  ; 1 Finished NOP
%ISOID::        IMPOB2  ; 2 Finished IP datagram messge
IMPODL::OFFSET 0
\f
;Here when interrupt has finished processing a complete message.
; Attempt to find something else to do.
;
IMORET: 
        ;JRST IMPOB0            ;Fall through to try for more

; Idle - Look for output to send.  First ensure we can send stuff,
;       then try things in the order:
;       (1) Send NOP if net coming up
;       (2) Check IP datagram queue
;
IMPOB0: HRRZ T,IMPUP            ;Get current IMP state.
        CAIE T,-2               ;Don't say it's up when it's still going down
         CAIN T,1               ;or when it is broken
          JRST IMPOBN

        ; First check to see if NOP needs to be sent.
        AOSG IMNOPC             ;Check to see if sending NOPs
         JRST IMONOP            ;Output a NOP
        SETZM IMPUP             ;Say IMP is up
        SETZM IMPTCU            ;Say no longer trying to come up

        ; Now see if there is any real traffic to send
        PUSHJ P,IPGIOQ          ;Check IP. Get IP IMP output queue entry if any
         JRST IMPOBN            ;Nothing there, we're done.

        ; Returns A/ ptr to IP dgm struct
        ;         B/ BLKO pointer to 32-bit words (unused)
        ;         C/ Arpanet address
        ;
        ;Build an IMP-format datagram at IMPOBF.
        ; Set correct address in prototype leader.
        ; BLT and reformat leader to IMPOBF.
        ; BLT and reformat datagram to IMPOBF + leader size.

        MOVEM A,IMPODP          ;Save ptr to datagram being output
        AOS IMNIPO              ;# of IP datagrams sent

        ;Put together the IMP leader in IMOLDR.
IFN 0,[ ;First and third word initialized at assembly time
        MOVE B,[17_10.,,0]      ;Regular message
        MOVEM B,IMOLDR
        MOVSI B,233_10.         ;IP link # in left 8 bits
        MOVEM B,IMOLDR+2        ;Set up third word
]
        LSH C,4.                ;Move net address to correct field
        MOVEM C,IMOLAD          ;set up second word of leader
        
        ;Copy leader and datagram to IMPOBF
        MOVE B,[IMOLDR,,IMPOBF]
        BLTBU B,IMPOBF+IMPLDS-1 ;Move IMP leader into place
        MOVS B,PK.BUF(A)        ;SOURCE,,COUNT
        MOVEI A,IMPLDS(B)       ;Size including leader to A
        HRRI B,IMPOBF+IMPLDS    ;Put d'gram immediately after leader.
        BLTBU B,IMPOBF-1(A)
        PUSHJ P,IMPSOH          ;Go start hardware
        MOVEI C,%ISOID          ;Remember we are outputting IP leader
        MOVEM C,IMPOS
        POPJ P,

\f

;Here if nothing to output.
; Flag output inactive and disable interface
;
IMPOBN: SETOM IMPOAC            ;No more output
        IORDI TT,%LHOCS
        TRZ TT,%LHIE\%LHGO      ;Deactivate interface for a bit
        IOWRI TT,%LHOCS
        POPJ P,

;IMONOP - Send a NOP, here from IMPOBZ only.
;
IMONOP: MOVEI A,IMPNOS          ;Length of NOP message
        MOVE B,[IMPNOP,,IMPOBF] ;BLT (and reformat) NOP to output buffer
        BLTBU B,IMPOBF-1(A)
        PUSHJ P,IMPSOH          ;Start output of (A) words from IMPOBF to IMP
        MOVEI C,%ISONP          ;Set correct output FSM state
        MOVEM C,IMPOS
        POPJ P,

;Prefabricated NOP Host-IMP leader
IMPNOP: 17_10.,,4_4             ;New format, type 4 = NOP
        0
        0                       ;No padding required on regular messages
IMPNOS==.-IMPNOP

;Finished sending NOP, from IMPOBZ
;
IMPOB1: SETZM IMPOS             ;Reset state
        JRST IMORET             ;Go look for something else to do.

;Was sending IP datagram, from IMPOBZ
;
IMPOB2: SETZB A,IMPOS           ;Reset output state.
        EXCH A,IMPODP           ;Get active datagram and reset pointer
        PUSHJ P,IPIODN          ;Tell IP level that datagram was output
        JRST IMORET             ;Go see if there is anything else to do.

;Start output hardware.
; A/ Count of PDP10 words to write to net.
; Data to output in IMPOBF.
; Must be NETOFF or at NETCHN interrupt level. Bashes A only.

IMPSOH: ASH A,1                 ;Convert PDP10 word count to...
        MOVNS A                 ; negative unibus word count.
        IOWRI A,%LHOWC          ;Tell the interface
        MOVEI A,<IUIMPG_12.>+<4*<IMPOBF&777>>
        IOWRI A,%LHOCA          ;Unibus address of output buffer
        MOVEI A,%LHIE\%LHELB\%LHGO ;Enable interrupts, send EOM, GO
        IOWRI A,%LHOCS          ;Start DMA transfer
        POPJ P,                 ;And forget it.
\f
SUBTTL  ARPANET CLOCK LEVEL

IMRSTO: RET                     ;Nothing to do any more?
\f
OVHMTR UUO

;NETHST (HOST INFO)
; ARG 1 - HOST => VAL 1 - STATUS, VAL 2 - HOST NUMBER
; ARG 1 - -1 => VAL 1 - (STATUS), VAL 2 - OUR HOST NUMBER
;NOT CURRENTLY IMPLEMENTED- ARG 1 - -1, ARG 2 - OUR GOING-DOWN REASON
;
ANETHST:HRRE T,A                ;Let immediate -1 win (777777 not a valid host)
        AOJE T,ANETH2           ;Jump if want local status and host number
        MOVE T,A
        JSP J,STDHST            ;Standardize and error-check host number
        MOVE B,T                ;Return new format
        TLO B,(NW%ARP)
        MOVEI H,LIMPHT-1
        CONO PI,NETOFF          ;Do we have status for this host?
        CAME T,IMPHTN(H)        ;Scan table
         SOJGE H,.-1
        JUMPGE H,ANETH1         ;Yes, return it
        CONO PI,NETON           ;No, have to go get it
        MOVEM T,SRN3(U)
        POPJ P,                 ;Oh, too bad

;Here to return status of foreign ARPAnet host
ANETH1: MOVE A,IMPHTB(H)        ;Get status
        CONO PI,NETON
        EXCH A,B
        CALL CVH2NA             ;Convert to HOSTS2 for compat
        EXCH A,B
        JRST LSWCJ1             ;Return IMSOC, NETLST if not done already

;Here to return our status, host
ANETH2:
REPEAT 0,[
        CAIL W,2                ;(This is a crock)
         MOVEM B,NTHDSW         ;If 2 args, set our reason for going down.
]
        SKIPE IMPUP             ;Fake up our status
         TDZA A,A               ;We're down
          MOVSI A,2000          ;We are up
        MOVEI B,IMPUS           ;And our host umber
        JRST POPJ1


;NETIMP (REASON,TIMEDOWN,TIMEUP)  READ/SET
;
ANETIM: JUMPLE W,ANETM1         ;No args, return current data
        CAIGE W,3               ;Must have 3 args if any
         JRST OPNL30
        MOVEM A,IMPDWN          ;Set data
        MOVEM B,IMPDWN+1
        MOVEM C,IMPDWN+2
        JRST POPJ1

ANETM1: MOVE A,IMPDWN           ;Get data to return
        MOVE B,IMPDWN+1
        MOVE C,IMPDWN+2
        SKIPE IMPUP             ;Note current condition of IMP, too
         TLO A,400000
        JRST POPJ1