	DEVICES
	-------

1)	TT:		Terminal Driver.

Standard DEC device driver modified with minor changes.


a)	IO.RAL	If prl[3] is non zero then this is used as
		a terminating character in the manner of
		IO.RST

b)	IO.WBT	It is no longer necessary to be privileged
		to issue this function




2)	NT0: - NT7:	Network interprogram pipelines.


Purpose
-------
	Interprogram communication with no interpretation 
of data. Intended for use with NT.... the network comm-
unications manager.

Assumptions.
-----------
	The device is transparent, and record orientated,
It can accept however any length record. Any reader may
read any data packet written to this device, but only
one reader will actually read any data packet.

Functions
---------

	IO.KIL IO.RLB IO.WLB IO.WVB IO.RVB	Accepted
	IO.ATT IO.DET IO.CRE etc		NOP'd

Additional Functions
--------------------

	IO.STS	#17400	Returns the current pending i/o
			function and length in the i/o
			status block.

Additional Features of standard functions
-----------------------------------------

	IO.WLB	IO.WVB	If prl[6] is non zero it is used
			as the i/o status word to be
			delivered to the reader of the
			data packet.



3)	VT0: - VT3:	Virtual Terminal.

Purpose.
--------
	This device is intended to enable a program to
control the running of other programs and to monitor
their output, filing or displaying it as neccessary.

Assumptions.
-----------
	The device is a record orientated, carriage
control, terminal device with semi-transparent i/o.
A <CR>, <^Z> or <ESC> as the last character of a record
will be used as the terminator and a suitable i/o 
status word will be returned to the reader.

Functions.
---------

	IO.KIL IO.RLB IO.WLB IO.WVB IO.RVB	Accepted
	IO.ATT IO.DET IO.CRE etc		NOP'd

Additional Functions
--------------------

	IO.STS	#17400	returns the function code of the
			current i/o packet on the device
			as i/o status block word 2.

	IO.MCR	#17400	sends a command line to MCR over
			the given device.
			prl[1]=address prl[2]=length



4)	NG0:		Nimbus G Communications Device

Purpose.
-------
	To enable communication between a control program
and several other programs with no interleaving of parts
of varaible length records. Including the sending of 
messages which require a reply.

Assumptions.
-----------
	That of the communicating tasks one is always the
master and one the slave and that both tasks are aware of
their status.

Functions
---------

	The only standard functions recognised are IO.KIL
	and IO.WLB which function as expected.

Additional Functions
--------------------

	IO.WWR	#401	Write with reply. prl[1-3] are for
			write buffer as IO.WLB ( #400 ).
			prl[4-6] are read reply buffer.

	IO.RDT	#17400	Read data. This a function for the
			control program, and does  wait.

	IO.WDT	#17000	Write data. as read data.
			but does not wait.

NOTES
-----
a)	IO.RDT IO.WDT are for use by control. They act if
	a suitable request is present at the user end or
	give a response or IE.DNR if the request is of the
	wrong type and IE.EOF if there are no user requests.

b)	All the data buffers are word aligned though an odd
	number of bytes is permitted. The word alignment is
	to enable the sending tasks name to be inserted as 
	the first two words in the receive task's buffer.
	This occurs in both IO.WWR and IO.RDT.

c)	Any subfunction bits given for IO.WLB will cause it
	to be treated as IO.WWR and usually rejected on bad
	read buffer.

d)	NOTE the positioning of parameters is opposite in
	IO.WWR from that in IO.RPR.
