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			USR for FORTRAN IV			USR for FORTRAN IV			___ ___ _______ __

	This subroutine allows users of DEC's FORTRAN IV for the PDP-8
and PDP-12 to open and close files at execution time.	It can be used
to open files for reading or writing on any legal OS/8 device with
such files subject only to the standard OS/8 restrictions (e.g. only one
output file can be open on a given device at a given time, etc.).
A second output file can be opened on a device after the first one is
closed, and multiple input files can be simultaneously open, of course.
USR is called from FORTRAN by the following statement:

	CALL USR (UNIT, NAME, FUNCT, ERROR)	CALL USR (UNIT, NAME, FUNCT, ERROR)

Description of variables:

	UNIT  - Logical unit number.  Only numbers 5 thru 9 are
		allowed.  1000(8) words of core are allocated to each unit
		number for its buffer and handler but this core is used
		only if the corresponding unit number is used.	As the user's
		FORTRAN program gets larger, there eventually will not be enough
		core for the program and for all logical unit numbers.	When
		this happens, USR automatically eliminates first unit 5,
		then unit 6, and so forth from the allowable logical unit
		numbers, thus protecting the user's program.  If the user
		specifies a logical unit number outside the range 5 thru
		9, or a logical unit number that has been eliminated
		because of the size of the user's program, ERROR=3 will
		be returned (see below).

	NAME  - Device:filename.ext.  This variable must be an ASCII string
		of up to 18 characters.  It can be a Hollerith literal,
		(i.e., a character string enclosed within single quotes)
		or an array (of dimension 3) loaded with a DATA statement
		or a READ statement with format 3A6.  Inbeded spaces
		and null characters ('@') are ignored in this field.

	FUNCT - Function code:	2 - open file for input.
				3 - open file for output.
				4 - close output file.
			If FUNCT is negative, the first character of
			every line is treated as a FORTRAN IV carriage
			control character.  If FUNCT is positive, all
			characters are output as such.
		Note:  The output filename given for a close (FUNCT=4)
		request must agree with the corresponding open request
		for that unit.	If it does not, the file will be closed
		and given the name in the close request, but no check
		will be made for duplicate file names and it may
		therefore be difficult to reference the file at a
		later time.  Closing a file with zero blocks (i.e. one that
		has not been written to) or closing an input file (a file
		opened with FUNCT=2) will delete that filename from the
		directory.
USR for FORTRAN IV (continued)




	ERROR - Returns error condition:
			0 - No errors.
			1 - Illegal (non-existant?) device.
			2 - Illegal (non-existant input) filename.
			3 - Illegal unit number.  Unit number is not between
				5 and 9 or the requested unit number has been
				preempted by a large user program.
			4 - Illegal function code.  Function code not between
				2 and 4.

	None of the above errors is fatal.  If they occur, execution
will continue and it is the user's responsibility to check these errors and
handle them apropriately.  However, it is a fatal error if the user
defines a device with a non-resident handler external to USR and then
tries to open a file with USR.  In this case, the computer will reply:

	USER ERROR USR	  0002

and execution will be terminated.


EXAMPLES:EXAMPLES:

1. Define unit 9 to be the existing file NEW.DA on device DSK:.
	CALL USR (9, 'NEW.DA', 2, ERROR)	CALL USR (9, 'NEW.DA', 2, ERROR)

2. Define unit 8 to be the existing file JUNK.TX on device LTA0:.
	CALL USR (8, 'LTA0:JUNK.TX', 2, ERROR)	CALL USR (8, 'LTA0:JUNK.TX', 2, ERROR)

3. Read the name of a file from the keyboard terminal (unit 4).
   (If no device is included in the input statment, DSK: is assumed.)
   Open the file for output.  When the program is done, close the file.
	DIMENSION NAME(3)	DIMENSION NAME(3)
	READ (4, 100) NAME	READ (4, 100) NAME
   100	FORMAT (3A6)   100	FORMAT (3A6)
	CALL USR (7, NAME, 3, ERROR)	CALL USR (7, NAME, 3, ERROR)
	  <continue program>
	CALL USR (7, NAME, 4, ERROR)	CALL USR (7, NAME, 4, ERROR)


USR for FORTRAN IV (continued)




GETTING USR ON THE AIR:GETTING USR ON THE AIR:

	USR is written in RALF (FPP assembler) language and can be included
in the FORTRAN IV library.  The following commands will assemble the program
and include it in the user's FORTRAN IV library:

	.R RALF			/Run the RALF assembler
	*USR<USR		/Assemble USR; loader module to file USR.RL
	.R LIBRA		/Run the FORTRAN IV librarian program
	*FORLIB<USR		/Insert USR into the library.  If there is
				/  insufficient space in the library or its
				/  directory, the following command may be
				/  necessary:
				/*FORLIB[10]<FORLIB,USR/Z=50
	*^C			/Return to OS/8 moniter

	One patch must be made to FRTS so USR will know how much core
is available.	The patch deletes code which initializes FRTS when an
Analex line printer is present in the system.	The following commands
will insert this patch:

	.GET SYS:FRTS
	.ODT
	*12475/ xxxx 7300;1311;3010;1023;3410;3410;1025;3410;1026
	*12506/ xxxx 3410;7000;5766;0120
	*^C
	.SAVE SYS:FRTS

The following additinal patch must be made if an Analex line printer is
included in the system:

	*12507/ 7000 6652
	*240/ 7450 0
	*511/ 6663 6651
	*513/ 6667 6652