Page of GC20-1807-7 As Updated April 1, 1981 by TNL GN25-0829
The four methods of accessing BDAM records are:
1 • 2.
3 ..
Relative Block Relative Track TTR
Relative Track and .'-J. -=-J. 4. Actual Address MBBCCHHR
The restrictions on these access methods are as follows: • Only the BDAM identifiers above can be used to refer to
records, since eMS files have a two-byte record identifier. • CMS BDAM files are always created with 255 records on the first
logical track, and 256 records on all other logical tracks,
regardless of the block size. If BDAM methods 2, 3, or 4 are used
and the RECFM is U or V, the BDAM user must either write 255 records
on the first track and 256 records on every track thereafter, or he
must not update the track indicator until a NO SPACE FOUND message is
returned on a write. For method 3 (WRITE ADD), this message occurs
when no more dummy records can be found on a WRITE request. For
methods 2 and 4, this will not occur, and the track indicator will be
updated only when the record indicator reaches 256 and overflows into
the track indicator. I • The must create variable length BDAM files (in PL/1, they are I Regional 3 files) entirely under CMS. He must also specify, on the , XTENT option of the FILEDEF command, the exact number of records to I be written. When reading variable length BDAM files, the XTENT and I KEYLEN information specified for the file must duplicate the I information specified when the file was created. eMS does not I support WRITE ADD of variable length BDAM files; that is, the user , cannot add additional records to the end of an already existing I variable length BDAM file. • Two files of the same filetype, both of which use keys, cannot be
open at the same time. If a program that is updating keys does not
close the file it is updating for some reason, such as a system
failure or another IPL operation, the original keys for files that
are not fixed format are saved in a temporary file with the same
filetype and a filename of $KEYSAVE. To finish the update, run the
program again. • Once a file is created using keys, additions to the file must not be
made without using keys and specifying the original length. , • Note that there is limited support from the CMS file system for , BDAM-created files (sparse). Sparsed files will be manipulated with , CMS commands but will not be treated as sparsed files by most eMS I commands. The number of records in the FST will be treated as a I valid record number. • The number of records in the data set extent must be specified using
the FILEDEF command. The default size is 50 records. • The minimum LRECL for a CMS BDAM file with keys is eight bytes. 280 IBM VM/370 System Programmer's Guide

Page of GC20-1807-7 As Updated April 1, 1981 by TNt GN25-0829 READING OS DATA SETS AND DOS FILES USING as MACROS CMS useLS can read OS sequential and partitioned data sets that reside
on OS disks. The MOVEFILE command can be used to manipulate those
data sets, and the OS QSAM, BPAM, and BSAM macros can be executed under CMS to read them.
The CMS MOVEFILE command and the same OS macros can also be used to
manipulate and read DOS sequential files that reside on DOS The OS macros handle the DOS data as if it were OS data.
The followinq OS Release 20.0 BSAM, BPAM, and QSAM macros can be used with CMS to read as data sets and DOS files:
BLOL BSP CHECK CLOSE ENQ
FIND
GET NOTE RDJFCB
READ SYNADAF SYNADRLS Part 3. Conversational Monitor System (eMS) 280.1