[Simh] PDP-10 DECtapes

Timothe Litt litt at ieee.org
Thu Mar 28 16:09:56 EDT 2013 

I guess I'm not allowed a convenient memory lapse.

DOS-11 predates RSX & Files-11.  It has a MFD/UFD directory structure, 
vaguely echoing TOPS-10.  I think that the 14-bye tape file header is 
most of the directory entry from the on-disk DOS-11 structure.  This 
caused some grief on tape, as the ANSI standard (and some 
drives/drivers) require a minimum record length of 18 bytes; thus the 
directory entries on MM could be skipped as 'noise records'.

The DECtape file structure for DOS-11 is documented in 
http://bitsavers.trailing-edge.com/pdf/dec/pdp11/dos-batch/DOS_CourseHandouts.pdf, 
mostly in 
http://bitsavers.trailing-edge.com/pdf/dec/pdp11/dos-batch/DosBatchHandbook_v9_Apr74/02.pdf, 
with other random bits in the other chapters.

It's not the same as RSX or RT.  Different design goals - and different 
engineering groups :-)

DOS was serially multi-user.  So it had permissions, contiguous and 
linked files.  RT single user; no permissions, contiguous only.  RSX got 
Files-11 - the everything to everyone format - at the cost of ACPs, 
complexity and memory.

I think - but I'd have to dissect a tape - that KLDCP used the DOS file 
structure on -11 DECtape for simplicity, and the fact that there were 
interchange programs (flx,etc) that could read/write them on the other 
OSs.  It was the least common denominator file structure.

Of course, DTAs were so slow and expensive that you wanted to run from 
disk instead, even when floppies replaced DECtape as the KL's front-end 
device.

In any case, the simh problem appears to be a low-level format 
emulation; my remarks on the file structure were only provided to assist 
in identifying the data.

This communication may not represent my employer's views,
if any, on the matters discussed.

On 28-Mar-13 15:19, Johnny Billquist wrote:
> On 2013-03-28 19:23, Timothe Litt wrote:
>> Good information, but this thread is about DECtape, not 9-Track 
>> magtapes...
>
> Right.
>
>> The format looks about right for 9-track DOS-11 magtapes; I remember
>> writing code to extract files from them on the -10.
>>
>> It's not right (or at least, not complete) for the block-addressable
>> DECtapes.
>
> DECtapes (atleast on RSX) are Files-11 ODS-1 volumes, and not ANSI 
> labelled tapes. They have nothing in common with each other, apart 
> from both being tapes. The closest relative to a DECtape is a floppy.
>
> Also, the DOS-11 tape format is not the same as ANSI labelled tapes. 
> They are not compatible.
>
> In RSX, ANSI labelled tapes are accessed through MTAACP, and the tapes 
> are mounted as a known format. You then access them with the normal 
> tools you'd use for any normal work in RSX. (Such as PIP.)
> DOS-11 tapes are instead mounted foreign, and you need FLX to 
> read/write to them.
>
>     Johnny
>
>> I don't think DOS-11 would have been documented in any of the references
>> cited.
>>
>> There's now a SIMH repo on github; your utility could go into the tools
>> section.
>>
>> This communication may not represent my employer's views,
>> if any, on the matters discussed.
>>
>> On 28-Mar-13 14:09, Larry Baker wrote:
>>> On 28 Mar 2013, at 5:24 AM, <simh-request at trailing-edge.com
>>> <mailto:simh-request at trailing-edge.com>>
>>> <simh-request at trailing-edge.com
>>> <mailto:simh-request at trailing-edge.com>> wrote:
>>>
>>>> Google 'AA-JS16A-TC' for some Files-11 format information; I'd
>>>> have to think a bit on the DOS-11 format.
>>>
>>> RSX/VMS Files-11 tapes are ANSI labeled tapes.  I know OpenVMS also
>>> used HDR3/4 labels for RMS information.  There was a MOUNT option to
>>> suppress writing those labels.
>>>
>>> I wrote an RSX/VMS  program that will scan an unknown tape and decode
>>> it for you.  (If anyone wants it, let me know where I should upload
>>> it.)  The DOS format decoder looks for a 14 byte (physical) record at
>>> the start of the file.  It is decoded by the following code:
>>>
>>> C
>>> C...  DEC DOS labels
>>> C
>>>       Call R50ASC ( 6, buffer( 1), ascbuf( 1) )
>>>       Call R50ASC ( 3, buffer(13), ascbuf( 7) )
>>>       ascbuf(10) = '.'
>>>       Call R50ASC ( 3, buffer( 5), ascbuf(11) )
>>>       ltemp(1) = buffer(7)
>>>       ltemp(2) = 0
>>>       mem = itemp
>>>       ltemp(1) = buffer(8)
>>>       grp = itemp
>>>       ltemp(1) = buffer(11)
>>>       ltemp(2) = buffer(12)
>>>       jday = MOD(itemp,1000)
>>>       year = itemp/1000 + 70
>>>       Call JDCONV ( jday, mon, day, year )
>>>       ltemp(1) = buffer( 9)
>>>       ltemp(2) = buffer(10)
>>>       Write (STDOUT,619) ifile, grp, mem, ascbuf, day, month(mon),
>>>      1                   year, itemp
>>>   619 Format (//' File ', I4, ':', T34, '"', '[', O3.3, ',', O3.3, ']',
>>>      1        13A1, 2X, I2, '-', A, '-', I2.2, '  <', O3.3, '>', '"')
>>>
>>> The DOS label contents are:
>>>
>>> Words 1-2 RAD50 characters 1-6 of the file name part (before the
>>> implied period)
>>> Word 3 RAD50 characters 1-3 of the file type (after the implied period)
>>> Word 4 Octal File owner's User Identification Code (group code in
>>> high-order byte, member code in low-order byte)
>>> Word 5 Binary File creation date ( 1000 * ( year - 1970 ) + Julian 
>>> day )
>>> Word 6 Octal File protection (low-order to high-order RWED bits: read,
>>> write, extend, delete; grouped low-order to high-order for system,
>>> owner, group, world)
>>> Word 7 RAD50 characters 7-9 of the file name part (before the implied
>>> period)
>>>
>>> The references I used to write the program (back in the 1980's) are
>>> below.  Which reference had the DOS label format I don't remember (if
>>> any of them did).
>>>
>>> 6  References
>>>
>>> [1]  American National Standards Institute, 1978, Magnetic Tape Labels
>>>      and File Structure for Information Interchange (ANSI X3.27-1978).
>>>
>>> [2]  Digital Equipment  Corp.,  1985,  RSX-11M/M-Plus  MCR Operations
>>>      Manual (Order no. AA-FD10A-TC).
>>>
>>> [3]  Digital Equipment  Corp.,  1985,  RSX-11M-Plus  Command Language
>>>      Manual (Order no. AA-FD04A-TC).
>>>
>>> [4]  Digital Equipment Corp., 1985, RSX-11M/M-Plus and Micro/RSX  I/O
>>>      Operations Reference Manual (Order no. AA-FD14A-TC).
>>>
>>> [5]  Digital  Equipment  Corp.,  1985,  RSX-11M/M-Plus I/O Drivers
>>>      Reference Manual (Order no. AA-FD09A-TC).
>>>
>>> [6]  Digital Equipment Corp., 1984, VAX/VMS Command Definition Utility
>>>      Reference Manual (Order no. AA-Z408A-TE).
>>>
>>> [7]  Digital Equipment Corp., 1986, VAX/VMS DCL Dictionary (Order  no.
>>>      AA-Z200C-TE).
>>>
>>> [8]  Digital Equipment Corp., 1984, VAX/VMS  Mount  Utility Reference
>>>      Manual (Order no. AA-Z424C-TE).
>>>
>>> [9]  Digital Equipment Corp., 1986, Guide to VAX/VMS Disk and Magnetic
>>>      Tape Operations (Order no. AI-Y506B-TE).
>>>
>>> [10] Digital Equipment  Corp.,  1986,  VAX/VMS  I/O  User's Reference
>>>      Manual:  Part I (Order no. AA-Z600C-TE).
>>>
>>> [11] International Business Machines Corp., 1978,  OS/VS Tape  Labels
>>>      (Order No. GC26-3795-1).
>>>
>>> Larry Baker
>>> US Geological Survey
>>> 650-329-5608
>>> baker at usgs.gov <mailto:baker at usgs.gov>
>>>
>>>
>>> _______________________________________________
>>> Simh mailing list
>>> Simh at trailing-edge.com
>>> http://mailman.trailing-edge.com/mailman/listinfo/simh
>>
>>
>>
>> _______________________________________________
>> Simh mailing list
>> Simh at trailing-edge.com
>> http://mailman.trailing-edge.com/mailman/listinfo/simh
>>
>
>


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