h316_imp.md

ARPANET IMP and TIP Extensions

Revision of 2-Dec-2013

Copyright Notice

The SIMH source code and documentation is made available under a X11-style open source license; the precise terms are available at:

https://github.com/open-simh/simh/blob/master/LICENSE.txt

These ARPANET IMP/TIP extensions for the H316 simulator were written by Robert Armstrong. This work is based on the SIMH H316 simulator package written by Robert M Supnik.

Table of Contents

1 Introduction 2 Simulator Files 3 Additional Features and Devices 3.1.1 IMP Pseudo Device 3.1.2 Real Time Clock 3.1.3 Watch Dog Timer 3.2 Communications Devices 3.2.1 Modem Interface 3.2.2 Host Interface 4 Summary 4.1 Additional Devices 4.2 Modem Interface I/O Instructions 4.3 Host Interface I/O Instructions 4.4 Other I/O Instructions

Introduction

This memorandum documents the extensions made to the SIMH Honeywell H316 simulator to allow it to run the ARPANET Interface Message Processor (aka IMP) and Terminal Interface Processor (TIP) software. A single IMP or TIP instance isn’t very useful, however these extensions allow multiple SIMH instances, each running a copy of the IMP or TIP code, to communicate using virtual modem connections implemented with physical serial ports on the host computer. Alternatively, virtual modem connections may be tunneled over TCP/IP to a remote SIMH IMP/TIP instance anywhere in the world. Moreover, each SIMH IMP instance can then be connected via virtual host interface cards to other local PDP-10 or PDP-11 SIMH instances running ARPANET host software.

NOTE

Presently TIP support is not implemented, and IMP host card support is only partially implemented. The current implementation is sufficient to allow multiple IMPs to talk via virtual modem connections, and that’s all.

Simulator Files

These additional IMP/TIP specific files are added to SIMH:

Subdirectory	File	Contains
sim/h316/	h316_imp.h	IMP/TIP and ARPAnet specific definitions
	h316_imp.c	the IMP pseudo device
	h316_rtc.c	real time clock and watch dog timer
	h316_hi.c	host interfaces
	h316_mi.c	modem interfaces
	h316_udp.c	UDP support for MI and HI modules

Additional Features and Devices

The IMP/TIP adds the following devices to the H316 configuration:

Device name	Simulates
MI	modem interface (up to 5)
HI	host interface (up to 4)
WDT	watchdog timer
RTC	real time clock (replaces H316 CLK device)
IMP	TASK, IMPN and MLC functions (addresses 418 and 428)

IMP Pseudo Device

The IMP pseudo device implements two sets of miscellaneous I/O instructions in the original IMP/TIP hardware. This device responds both to I/O address 41₈, which implements the TASK switching interrupt and the IMP identification number, and to I/O address 42₈, which implements the MLC test. The IMP device may be enabled with the standard command

SET IMP ENABLED

The IMP address, which is returned by the RDIMPN (INA 1041) instruction, may be set with the command:

Command	Action
SET IMP NUM=n	set IMP station address to n

This value was apparently hardwired into each original IMP/TIP station. The IMP device also implements the AMIMLC (SKS 0042) instruction, which skips if the current hardware is a multi-line controller (aka a TIP). Since TIP emulation is not currently supported, this instruction is presently hard wired as a NOP (i.e. it never skips).

The IMP device implements the following registers:

Name	Size	Comments
MLC	1	always zero (TIP flag)
IEN	1	task interrupt enabled
IRQ	1	task interrupt pending

These registers can be viewed with the command

EXAMINE IMP STATE

The IMP device implements these debugging flags:

Command	Action
SET IMP DEBUG=WARN	print warnings for unusual conditions
SET IMP DEBUG=IO	trace all IMP device I/O instructions

Remember that you must enable debugging output first before these settings will be effective; refer to the SIMH User’s Guide, “Controlling Debugging,” for more information.

Real Time Clock

The IMP/TIP hardware contained a custom real time clock which was unique to that implementation, and further, the IMP/TIP did not have the standard H316 power line clock or real time clock. The IMP/TIP clock is in some ways similar to the H316 real time clock option, but it is not the same. Normally the CLK device would be disabled when emulating the IMP, however there is no actual conflict between the CLK and the RTC devices and both can be enabled simultaneously.

The RTC device supports the following SET commands:

Command	Action
SET RTC ENABLED	enable RTC emulation
SET RTC INTERVAL=i	set the RTC tick interval to i microseconds
SET RTC QUANTUM=q	set the RTC resolution to q ticks

The INTERVAL parameter sets the interval between RTC clock ticks, in microseconds and the QUANTUM parameter sets how often, as a number of clock ticks, SIMH updates the RTC count. To give an example, if INTERVAL was set to 20 and QUANTUM was 1, SIMH would attempt to increment the RTC count by one at a rate of 50,000 times per second. Modern PCs are very fast and it would probably be possible for SIMH to do this, but the amount of overhead would be huge and it would be impractical.

Instead, for example, if INTERVAL was still set to 20 but QUANTUM was now set to 50, SIMH would add 50 to the RTC count at a rate of 1000 times per second. This represents much less overhead and yet gives the same net counting rate as before. However, now the clock count jumps in increments of 50 rather than 1. Most software wouldn’t notice, but it’s always possible this could cause problems.

It’s important to note that the QUANTUM does not affect the overall clock frequency – as long as the INTERVAL is set to 20 the clock would count at an effective rate of 50 kHz regardless of the QUANTUM value.

The RTC device implements the following registers:

Name	Size	Comments
ENA	1	RTC is enabled
COUNT	16	current count
IEN	1	RTC interrupt enabled
IRQ	1	RTC interrupt pending
TPS	1	effective ticks per second
WAIT	24	simulator time until the next tick

These registers can be viewed with the command

EXAMINE RTC STATE

The RTC device implements these debugging flags:

Command	Action
SET RTC DEBUG=WARN	print warnings for unusual conditions
SET RTC DEBUG=IO	trace all RTC device I/O instructions

Please refer to the SIMH User’s Guide, “Controlling Debugging,” for more information and remember that you must enable debugging output first before these settings will be effective.

Watch Dog Timer

The IMP/TIP hardware also had a custom watch dog timer implementation which would force a non-maskable interrupt if it was ever allowed to expire. The WDT device in SIMH implements this feature. The IMP/TIP watch dog timer also implemented a couple of other unrelated features – these include the status display panel, which you can see featured prominently in some IMP photos, and a flag to indicate whether the CPU was a H316 or DDP-516.

The WDT device supports the following SET commands

Command	Action
SET WDT ENABLED	enable WDT emulation
SET WDT DELAY=n	set the WDT delay to n milliseconds

The DELAY parameter sets the WDT timeout, in milliseconds. This is a 16 bit unsigned value, so the maximum WDT delay is just over a minute. Note that there are no H316 instructions that disable the WDT – only one to reset it – so if the WDT device is enabled the WDT will run anytime simulation is active. If the code fails to reset the WDT in a timely fashion, the WDT interrupt will occur regardless of the enabled or disabled state of the interrupt system.

Setting the WDT delay to zero

SET WDT DELAY=0

prevents the WDT from ever generating a timeout. This special feature allows the WDT device to be enabled, so that code which executes WDT, status light or AMI512 instructions can be executed and debugged, but without inadvertently triggering a WDT timeout.

The WDT device implements the following registers

Name	Size	Comments
COUNT	16	current countdown
TMO	1	WDT timed out
LIGHTS	16	last "set status lights"
WAIT	24	calculated time until the next tick

These registers can be viewed with the command

EXAMINE WDT STATE

The WDT device implements these debugging flags:

Command	Action
SET WDT DEBUG=WARN	print warnings for unusual conditions
SET WDT DEBUG=IO	trace all WDT device I/O instructions
SET WDT DEBUG=LIGHTS	trace IMP status light changes

Remember that you must enable debugging output first before these settings will be effective; refer to the SIMH User’s Guide, “Controlling Debugging,” for more information.

Communications Devices

Modem Interface

IMPs and TIPs communicated with each other via leased telephone lines and synchronous modem links. These modem links were point to point – each IMP/TIP modem communicated with exactly one other modem attached to exactly one other IMP/TIP. SIMH simulates these modem interfaces and allows them to be attached to either a real, physical, serial port on the host computer or to a virtual UDP/IP port. Either can then be connected to another SIMH instance running the IMP/TIP software, or even a real IMP or TIP if someone gets one working again, and a network can be assembled.

Simh implements five modem interfaces, MI1 thru MI5. Initially MI1 thru 3 are enabled and MI4 and 5 are not, however this can be changed with the commands:

Command	Action
SET MIn ENABLED	enable modem line n
SET MIn DISABLED	disable modem line n

A limitation of the original IMP hardware is that the DMC channels used by modem lines 4 and 5 conflict with those used by host interfaces 3 and 4. Thus it is not possible to enable all modem lines and all host interfaces at the same time (see 3.2.1.2).

Modem interfaces implement one parameter which may be explicitly set:

SET MIn BPS=56000

This parameter sets the simulated line speed, in bits per second, for UDP/IP virtual modem connections.

Modem interfaces also implement an interface (local) and a line (remote) loopback feature. This causes the modem to receive its own transmitted messages, and are analogous to features in the IMP modem hardware. Loopback may be enabled or disabled with these commands

Command	Action
SET MIn LOOPINTERFACE	enable interface loopback on line n
SET MIn NOLOOPINTERFACE	disable interface loopback
SET MIn LOOPLINE	enable line loopback on line n
SET MIn NOLOOPLINE	disable line loopback

Modem interfaces support the following registers:

Name	Size	Comments
RXPOLL	32	receiver polling interval
RXPEND	1	receiver waiting for input
RXERR	1	receiver error flag
RXIEN	1	receiver interrupt enable
RXIRQ	1	receiver interrupt request
RXTOT	32	count of total messages received
TXDLY	32	calculated delay before transmitter done
TXIEN	1	transmitter interrupt enable
TXIRQ	1	transmitter interrupt request
TXTOT	32	count of total messages transmitted
LINK	32	link number for h316_udp module
BPS	32	simulated line speed for UDP connections
ILOOP	1	interface (local) loopback enabled
LLOOP	1	line (remote) loopback enabled

These registers can be viewed with the command

EXAMINE MIn STATE

The modem interface device implements these debugging flags:

Command	Action
SET MIn DEBUG=WARN	print warnings for unusual conditions
SET MIn DEBUG=IO	trace all modem interface I/O instructions
SET MIn DEBUG=UDP	trace all UDP packets and connections
SET MIn DEBUG=MSG	trace all IMP messages sent or received

Remember that you must enable debugging output first before these settings will be effective; refer to the SIMH User’s Guide, “Controlling Debugging,” for more information.

UDP/IP Tunnels

A virtual modem may also be tunneled using UDP/IP to another SIMH instance. UDP connections are symmetrical – each virtual modem listens for incoming packets on a port which you define, and transmit outgoing packets to another host and port which you also define. It’s your responsibility to define the ports appropriately so that the input of one modem is logically connected to the output of another modem. These connections must be one to one. Connecting more than one modem output to the same modem input will not cause any problems for SIMH, but the IMP software will become hopelessly confused.

The general form of a virtual modem ATTACH command for UDP connections is like this

ATTACH MIn llll:w.x.y.z:rrrr

This will listen for incoming packets on port “llll”, which should be a decimal number, and will transmit outgoing packets to port “rrrr” on the host with IP address “w.x.y.z”. The actual port numbers you use are arbitrary; however some care must be used to avoid conflicts with any other network applications.

The UDP attach command also has a few alternative forms; for example

ATTACH MIn 4431:imp.jfcl.com:4432

will listen on port 4431 of the current host, do a DNS lookup to determine the IP of the host "imp.jfcl.com", and then transmit to port 4432 on that host. In another example

ATTACH MIn 1201::1202

will listen to port 1201 on the current, local, host and transmit to port 1202 also on the local host. This is useful when both SIMH instances are running on the same PC. In this case, you must use different ports for transmitting and receiving. If they are the same, the modem will transmit to itself!

Either end of the UDP connection may be disconnected with the command

DETACH MIn

Physical Serial Tunnels

The command

ATTACH -p MIn COMnn

is reserved for a possible future option to attach a physical serial port to a virtual modem. This functionality is not currently implemented.

Host Interface

NOTE

The current Host Interface implementation is presently only a skeleton – just enough code exists to allow the IMP software to run. In this version, the IMP effectively sees all attached hosts as permanently powered down. Hopefully the host interface implementation can be completed at some point, just as soon as there is a suitable host emulator for it to talk to.

The IMP used the host interface to connect the H316 to an Arpanet host mainframe. This could be a PDP-10, an SDS Sigma 7, a CDC 6600, an IBM 360, or any one of many other machines. Each IMP could support up to four host interface cards; in SIMH these are the devices HI1 thru HI4. Initially HI1 and 2 are enabled and HI3 and 4 are not, however this can be changed with the commands:

Command	Action
SET HIn ENABLED	enable host interface n
SET HIn DISABLED	disable host interface n

Host interfaces support the following registers:

Name	Size	Comments
POLL	32	host polling interval
RXIEN	1	receiver interrupt enable
RXIRQ	1	receiver interrupt request
RXTOT	32	count of total messages received
TXIEN	1	transmitter interrupt enable
TXIRQ	1	transmitter interrupt request
TXTOT	32	count of total messages transmitted
READY	1	host ready
FULL	1	host buffer full
ERROR	1	host error
LLOOP	1	local loopback enabled

These registers can be viewed with the command

EXAMINE HIn STATE

The host interface device implements these debugging flags:

Command	Action
SET HIn DEBUG=IO	trace all host interface I/O instructions

Remember that you must enable debugging output first before these settings will be effective; refer to the SIMH User’s Guide, “Controlling Debugging,” for more information.

UDP/IP Tunnels

In SIMH, host interfaces are simulated using UDP connections to other SIMH instances that simulate the host mainframe and run the Arpanet host operating system. The ATTACH command is used to connect a host interface to this other SIMH instance

ATTACH HIn tba

The host connection may be broken with the command

DETACH HIn

Summary

Additional Devices

Table 1 lists the IMP/TIP specific devices added to SIMH along with their I/O address, interrupt number and vector, and DMC channel assignment. Note that all the IMP/TIP devices use the H316 extended interrupts and so interrupt 1, for example, corresponds to A bit 1 in the SMK 120 instruction. Table 2 shows the extended interrupt mask associated with these devices.

Name	Address8	Number10	Vector8	DMC10	Description
WDT	26		000062		Watch Dog Timer
RTC	40	15	000102		Real Time Clock
IMP	41	16	000103		Task Switch
IMP	42				MLC support
HI4	50	9	000074	10	Host Interface #4 (RX)
		4	000067	5	Host Interface #4 (TX)
HI3	51	10	000075	16	Host Interface #3 (RX)
		5	000070	15	Host Interface #3 (TX)
HI2	60	14	000101	14	Host Interface #2 (RX)
		12	000077	12	Host Interface #2 (TX)
HI1	70	13	000100	13	Host Interface #1 (RX)
		11	000076	11	Host Interface #1 (TX)
MI1	71	1	000064	1	Modem Interface #1 (RX)
		6	000071	6	Modem Interface #1 (TX)
MI2	72	2	000065	2	Modem Interface #2 (RX)
		7	000072	7	Modem Interface #2 (TX)
MI3	73	3	000066	3	Modem Interface #3 (RX)
		8	000073	8	Modem Interface #3 (TX)
MI4	74	4	000067	4	Modem Interface #4 (RX)
		9	000074	9	Modem Interface #4 (TX)
MI5	75	5	000070	5	Modem Interface #5 (RX)
		10	000075	10	Modem Interface #5 (TX)

1	2	3	4	5	6	7	8	9	10	11	12	13	14	15	16
M1RX	M2RX	M3RX	M4RX	M5RX	M1TX	M2TX	M3TX	M4TX	M5TX	H1TX	H2TX	H1RX	H2RX	RTC	TASK
			H4TX	H3TX				H4RX	H3RX

Table 2 - Extended Interrupt Mask

Modem Interface I/O Instructions

Table 3 summarizes the modem interface I/O instructions. In this table, “n” represents the modem number, (1 thru 5), and “dd” is the device I/O address assigned to that modem (71₈ thru 75₈).

Mnemonic	Opcode	Operation
MnOUT	0300dd	start modem output
MnUNXP	0301dd	un-cross patch modem
MnLXP	0302dd	enable line cross patch
MnIXP	0303dd	enable interface cross patch
MnIN	0304dd	start modem input
MnERR	0704dd	skip on modem error

Table 3 – Modem Instructions

Host Interface I/O Instructions

Table 4 summarizes the host interface I/O instructions. In this table, “n” represents the host number, (1 thru 4), and “dd” is the device I/O address assigned to that host (50₈, 51₈, 60₈, or 70₈).

Mnemonic	Opcode	Operation
HnROUT	0300dd	start regular output to host
HnIN	0301dd	start host input
HnFOUT	0302dd	start host final output
HnXP	0303dd	cross patch host
HnUNXP	0304dd	un-cross patch host
HnENAB	0305dd	enable host interface
HnERR	0700dd	skip on host error
HnRDY	0701dd	skip on host ready
HnEOM	0702dd	skip on end of host message
HnFULL	0705dd	skip on host buffer full

Table 4 - Host Instructions

Other I/O Instructions

Table 5 summarizes the additional miscellaneous I/O instructions.

Mnemonic	Opcode	Device	Operation
SMK 120	170120	CPU	set extended interrupt mask
	030026	WDT	reset watch dog timer
	170026	WDT	set status lights
AMI512	070026	WDT	skip if this machine is a DDP-516
CLKON	030040	RTC	enable RTC
CLKOFF	031040	RTC	disable RTC
RDCLOK	131040	RTC	read RTC count and always skip
TASK	030041	IMP	cause task switch interrupt
RDIMPN	131041	IMP	read IMP number and skip
AMIMLC	070042	IMP	skip if this machine is a multi-line controller

Table 5 - Other Instructions

Notes:

• On SIMH, AMI512 is a NOP and never skips, because SIMH simulates an H316, not the DDP-516.
• “Multi-line controller” was the official name for the TIP. MLC support is not implemented, and AMIMLC is currently a NOP and never skips.