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     TOPS20.DOC	January 1981
     TOPS-20 Release 4 with RP20 support

     COPYRIGHT (C) 1976,1977,1978,1979,1980,1981 BY

     The information in this document is subject to change without notice and
     should   not   be   construed  as  a  commitment  by  Digital  Equipment

     The software described in this document is furnished under a license and
     may only be used or copied in accordance with the terms of such license.

     No responsibility is assumed for the use or reliability of  software  on
     equipment that is not supplied by DIGITAL or its affiliated companies.
                                                                       Page 2

     The TOPS-20 RP20 release is a release of the TOPS-20 V4 operating system
     which  includes  support  for  the  RP20  disk  subsystem.  There are no
     additional enhancements to the operating  system  other  than  the  RP20
     The following documents are important sources of information about  this
          1.  Software installation Guide
              Describes  the  software  contents  of  the  release  and   the
              procedures for installing it.
          2.  TOPS20.BWR
              This file contains information which was  generated  after  the
              printed  documents  were  "frozen"  but  which  is  vital  to a
              successful installation and use of the software.
          3.  RP20-FUNCTIONAL-SPEC.MEM
              This file describes the basic changes which were  made  to  the
              operating system and utilities in order to support the RP20.
          4.  RP20-OPERATIONS.MEM
              This file contains information relating to the operation of the
              RP20, the 8000 controller and the DX20.
                                                                       Page 3

	Internal changes have been made to the monitor's RP20-driver code
	and to the utilities DX20LD, WATCH, and MOUNTR.  These changes
	do not affect the user environment.  Changes have been made to BOOT
	which are observable by the operator and by systems programmers.
	During a manual startup of the system, BOOT will reload all DX20's
	on the system, identifying (on the CTY) a TU7x-DX20 as "DX20A",
	and identifying an RP20-DX20 as "DX20B".  As before, appropriate
	unit and channel numbers are also displayed.  During an auto-reload
	of the system, BOOT will reload all RP20-DX20's.  Because of problems
	relating to a DX20-driven tape subsystem shared between systems,
	BOOT will not reload any TU7x-DX20's during an auto-reload.  (Note
	that any DX20 can be manually reloaded with DX20LD.)

	BOOT has the microcode for DX20's embedded in its own code.  Previously,
	BOOT would write these microcode pages into the dump file.  This
	has been changed - BOOT no longer writes those pages to the file
	and the file now contains a hole where those pages would have been.
	At the current BOOT load address of 40000, the hole in the dump
	file extends from page 22 to page 56.  Anyone using FILDDT to examine
	the dump will see a question mark displayed if they attempt to
	open a location within the hole.

	In summary, the changes to BOOT are as follows:

	1.  BOOT now reloads RP20-DX20's on an auto-reload of the system
	2.  The DX20 type is displayed on the CTY anytime any DX20 is loaded
	3.  Any dump created by BOOT will be missing pages 22 - 56.