Page of GC20-1807-7 As Updated April 1, 1981 by TNL GN25-0829 CMSCB B" __ # ______ 6 __ ____ 6 _____ _______ ______ a or an OPEN (SVC 19) is issued for a data set. The CMS Control
Block consists of a file control block (FCB) for the data file,
and partial simulation of the job file control block (JFCB),
input/output block (lOB), and data extent block (DEB).
The data control block (DCB) and the data event control block (DECB)
are used by the access method simUlation routines of CMS. MQte: The results may be unpredictable if two DCBs access the same data
set at the same time.
The GET and PUT macros are not supported for use with spanned records
except in GET locate mode. READ, WRITE, and GET (in locate mode) are
supported for spanned records, provided the filemode number is 4, and
the data set is physical sequential format.
GET (QSAM) All the QSAM options of GET
handled the same as move mode.
number is 4, and the last block
(X' 61 FFFF61 • ) must be pr esent
record.
GET (QISAM) QISAM is not supported in CMS. PUT (QSAM) are supported. Substitute mode is
If the DCBRECFM is FB, the filemode
is a short block, an EOF indicator
in the last block after the last
All the QSAM options of PUT are supported. Substitute mode is
handled the same as move mode. If the DCBRECFM is FB,. the filemode
number is 4, and the last block is a short block, an EOF indicator is
written in the last block after the last record. When LOCATE mode is used with PUT, issue an explicit CLOSE prior to
returning to CMS to obtain the last record. PUT (QISAM) QISAM is not supported in CMS. PUTX PUTX support is provided only for data sets opened for QSAM-UPDATE with simple buffering. READ/WRITE (BISAM) BISAM is not supported in CMS. READ/WRITE (BSAM and BPAM)
All the BSAM and BPAM options of READ and WRITE are supported except
for the SE option (read backwards).
READ (Offset Read of Keyed BDAM dataset)
This type of READ is not supported because it is used only for
spanned records.
READ/WRITE (BDAM)
All the BDAM and BSAM (create) options of READ and WRITE are
supported except for the Rand RU options. When an input or output error occurs, do not depend on as sense
bytes. An error code is supplied by CMS in the ECB in place of the
sense bytes. These error codes differ for various types of devices and
their meaning can be found in under DMS message 120S. Part 3. Conversational Monitor System (CMS) 279

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