IBM CMS dumpfile idiocy

Dave Wade dave.g4ugm at
Wed Dec 16 13:11:36 CST 2015

> -----Original Message-----
> From: cctalk [mailto:cctalk-bounces at] On Behalf Of Chuck
> Guzis
> Sent: 16 December 2015 18:08
> To: General Discussion: On-Topic and Off-Topic Posts
> <cctalk at>
> Subject: Re: IBM CMS dumpfile idiocy
> On 12/16/2015 07:57 AM, Paul Koning wrote:
> > IBM standard labels are older than ANSI.  Then again, IBM (in OS/360
> > at least) had something they called "ANSI label" that were not
> > actually ANSI at all.  They used "8 bit ASCII" which was a bizarre
> > code created from standard 7 bit ASCII by moving one or two of the
> > bits (bit 6 to bit 7?  I don't remember).
> Then there's 6 bit ASCII/USASCII.  I was trying to avoid using jargon that
> would not understand.  I'm quite familiar with IBM EBCDIC "SL"
> tape labels.
> My point was that the CMS dumpfile tape isn't even a standard labeled
> It just starts out with the first block of data and ends with a double
> No standard labels anywhere.  Any non-CMS system would not know what
> the heck the thing was.
> --Chuck

That's for the same reason MS DOS does not put standard labels on DDS or DAT
tape. CMS is the original personal computing environment. Each user gets an
individual isolated virtual machine and the IPL's CMS into their machine.
CMS is a single-user operating system and within that virtual machine they
can do what they want. A normal user can't see much outside their virtual
environment, but inside it they are god. They can even single step through
CMS.  As there are many VMs in a VM/370 environment Users can put whatever
they want on their tapes. It is up to the operators or other privileged user
to "attach" the physical tapes to the users' virtual machine and mount the
tapes with or without a ring. Once its attached the user can put what they
want on the tape, provided it has a write ring in it. Trying to insist on
standard labels would be pretty pointless. 

CMS TAPE files are not intended to be used by other systems and the format
is undocumented (well except a little in the source code) and has changed
several times as disk formats have changed. They contain physical dumps of
the DASD (disk) followed by the matching directory block. They may contain
multiple files. You know a file is complete when you hit is directory block.
So unlike OS tapes you don't need to specify a block factor and record type.
You just do "TAPE DUMP <filename> <filemode> <disk letter> where the names
may contain the usual wild cards and they get dumped. To see what is on a
tape do "TAPE SCAN" and to load them do "TAPE LOAD". No JCL, no DD cards no
knowledge of the underlying record formats are required.

CMS includes a "MOVEFILE" command that can produce standard label tapes in
the usual OS/DOS formats but normally users don't bother unless they are
sending tapes to other sites...


More information about the cctalk mailing list