This is extracted from Phil Cannon's documentation.

	DABIO   Direct access I/O for virtual blocks
	13-Sept-76  P. Cannon
	This module provides the FORTRAN programmer with access
	to the  READ$ and WRITE$ macros.  The user can directly
	access any virtual block (256 words) within his file.

	The entry points DABRD and DABWT return when the I/O
	operation is complete. The entry points IDABRD and IDABWT
	initiate I/O transfers and the module WAIT should be
	used to test for the completion of the I/O operation.

	CALL IDABRD(FDB,BUF,LB,IBLK)
	CALL IDABWT(FDB,BUF,NB,IBLK)
	CALL  DABRD(FDB,BUF,LB,IBLK,/NBT/,/IER/)
	CALL  DABWT(FDB,BUF,NB,IBLK,/NBT/,/IER/)

	FDB     FILE DESCRIPTOR BLOCK
	BUF     DATA BUFFER
	LB      LENGTH OF BUFFER IN BYTES
	NB      NUMBER OF BYTES TO WRITE
	IBLK    INTEGER*2 VARIABLE CONTAINING THE VIRTUAL BLOCK NUMBER
	        TO BE READ/WRITTEN.
	NBT     NUMBER OF BYTES TRANSFERRED
	IER     ERROR FLAG
	        POSITIVE IMPLIES NO ERROR
	        ZERO IMPLIES END OF FILE
	        NEGATIVE IMPLIES HARDWARE I/O ERROR

FQIO    This is an ordinary QIO function which makes use of a special set of
QIO function codes for the files control processor to manipulate the file
headers of a volume to read and write attributes.

	SUBROUTINE TO USE THE FILES 11 QIO MACRO TO READ/WRITE FILE ATTRIBUTES
	CALL FQIO(FDB(67),LUN,IAT,N,IBUF,IDSW)   !TO READ ATTRIBUTE
	SIX  PARAMETERS
      1 FDB(67)= ADDRESS OF FILE ID=FILE NAME BLOCK (67=OFFSET OF FNB IN FDB)
      2 LUN    = ADDRESS OF LOGICAL UNIT TO USE
      3 IAT=ADDRESS OF ATTRIBUTE TYPE TO BE READ/WRITTEN ;SEE FILES 11 QIO NOTES
	    IF IAT <0 THEN ATTRIBUTE IS TO BE READ, IF IAT>0 THE WRITE ATTRIBUTE
      4 N      = NUMBER OF CHARACTERS TO READ OR WRITE  ;SEE FILES 11 QIO NOTES
      5 IBUF   = ADDRESS OF BUFFER CONTAINING DATA TO READ/WRITE IN FILE HEADER
      6 IDSW   = ADDRESS TO RETURN DIRECTIVE STATUS WORD

INIT    Initializes the FDB. Must be called before ANY reference to the
FDB is made.

	CALL INIT(FDB)

OFNB    Opens a file using the information stored in file name block of the file
descriptor block.  This subroutine calls the PARSE subroutine to fill in the
file name block.

	CALL OFNB(FDB,HOC,LUN,FNM,IER)
	CALLS SUBROUTINE PARSE TO FILL IN FILE NAME IN FDB FROM FNM.

	FDB     FILE DESCRIPTOR BLOCK ARRAY INTO WHICH FDB IS BUILT.
	HOC     HOW OPEN CODE (RACC,FACC)
	        1 IMPLIES INPUT
	        16 IMPLIES OUTPUT
	LUN     LOGICAL UNIT NUMBER
	FNM     FULL FILE DESCRIPTOR NAME IE. DK0:[100,100]NAME.INF;1
	IER     ERROR FLAG
	        POSITIVE IMPLIES NO ERROR
	        ZERO IMPLIES AN ERROR
	        NEGATIVE IMPLIES AN ERROR