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	Open -- a request to open the parallel port

	error = OpenDevice("parallel.device", unit, ioExtPar, flags)
	D0                  A0                D0    A1        D1

	This function allows the requestor software access to the parallel
	device.  Unless the shared-access bit (bit 5 of io_ParFlags) is
	set, exclusive use is granted and no other access is allowed
	until the owner closes the device.

	A FAST_MODE, can be specified (bit 3 of io_Parflags) to speed up
	transfers to high-speed printers. Rather than waiting for the printer
	to acknowledge a character using the *ACK interrupt, this mode will
	send out data as long as the BUSY signal is low. The printer must be
	able to raise the BUSY signal within 3 micro-seconds on A2630s,
	otherwise data will be lost. Should be used only in an exclusive-
	access Open().

	A SLOWMODE mode can be specified (bit 4 of io_ParFlags) when very
	slow printers are used. If the printer acknowledges data at less
	than 5000 bytes per second, then this mode will actually save CPU
	time, although it consumes much more with high-speed printers.

	The PTermArray of the ioExtPar is initialized only if the EOFMODE
	bit (bit 1 of io_ParFlags) is set. The PTermArray can be further
	modified using the PDCMD_SETPARAMS command.

	"parallel.device" - a pointer to literal string "parallel.device"
	unit - Must be zero for future compatibility
	ioExtPar - pointer to an IO Request block of structure IOExtPar to
		   be initialized by the Open() function. The io_ParFlags
		   field must be set as desired.
	flags - Must be zero for future compatibility

	d0 -- same as io_Error
	io_Error -- if the Open succeded, then io_Error will be null.
		    If the Open failed, then io_Error will be non-zero.