To accomplish this simulation, CMS supports certain essential macros
for the following access methods: • BDAM @ BPAM (direct) -- identifying a record by a key or by its
relative position within the data set.
(partitioned) --seeking a named member within data set. • BSAM/QSAM (sequential) -- accessing a record in a sequence in
relation to preceding or following records. • VSAM (direct or sequential) --accessing a record sequentially
or directly by key or address. !Qte: CMS support of OS VSAM files is based on DOS/iS Access Method Services and Virtal Storage Access Method (VSAM). Therefore, the as user is restricted to those
functions a¥ailable under "DOS/VS Access Method Services." See the section "CMS Support for as and Des VSAM Functions" for details. eMS also updates those portions of the as control blocks that are
needed by the as simulation routines to support a program during
execution. Most of the simulated supervisory as control blocks are
contained in the following two eMS control blocks: CMSCVT simulates the communication vector table. Location 16 contains
the address of the CVT centrol section. CMSCB is allocated from system free storage whenever a FILEDEF com.and 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 (nCB) and the data event control block (DECB)
are used by the access method simulation routines of CMS. !Qte: The results may be unpredictable if two DCBs access the same data
set at the sa.e time. The GET and PUT macros are not supported for use with spanned
records. READ and WRITE are supported for spanned records, provided the
file.ode number is 4, and the data set is physical sequential (BSAM) format. GET (QSAM) All the QSAM options of GET
handled the same as move mode. number is 4, and the last block (X'61FFFF61') must be present
record.
GET (QISAM) QISAM is net supported in eMS. PUT (QSA8) are supported. Substitute mode is
If the DCBRECFM is FB, the file.ode
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 .ove mode. If the DCBRECFM is FB, the file.ode number is 4, and the last block is a short bleck, an EOF indicator is
written in the last block after the last record. CMS Introduction 2-43

PUT (QISAM) QISAK 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 When an input or output error occurs, do not depend on OS sense
hytes. 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 the under DMS message 120S. The four methods of accessing BDAM records are:
1. Relative Block 2. Relative Track TIft 3. Relative Track and Key 4. Actual Address MBBCCHfiR The restrictions on these access methods are as follows: • Only the BDAM identifiers underlined above can be used to refer to
records, since CMS 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 .ust 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. • 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.
2-44 IBM VM/370 System Logic and Program Determination--Volume 2