!!acro-SVC !.Q. im'T---- CHECK
DCB BPALI BLKSIZE BUPCB BUFL BUFIO DDNlftE DSOBG EODAD EXLST KEYLEN LIMCT LRECL ftlCRP OPTCD RECFft SYNAD IfCP All the options of POINT are supported. POINT causes
the control program to start processing the next read
or write operation at the specified item number. The
TTR field in the block address is used as an item
number.
All the options of CHECK are supported.
the I/O operation for errors and
conditions.
The following fields of a rCB may be
CHECK tests
exceptional
specified,
relative to the particular access method indicated: BDAM P,D F,D P,D F,D
n (number) n n n
a (address) a a a
n n n n
n n n n
s (symbol) s s s V1 PO PS PS a a a
a a a a
n n
n
n n n R,i R,i R, i, P G,P,L,M A,E,F,R F,', U F,V,U F,V,B,S,A,M,U F,V,B,U,A,M,S a a a a
n n ACCESS ftETHOD SUPPORT The manipulation of data is governed by an access method. To facilitate
the execution of OS Code under CMS, the processing program must see data
as OS would present it. For instance, when the processors expect an
access method to acquire input source cards sequentially, CMS invokes
specially written routines that simulate the as sequential access method
and pass data to the processors in the format that the OS access methods
would have produced. Therefore, data appears in storage as if it had
been manipulated using an OS access method. For example, block
descriptor words (BDi), buffer pool management, and variable records are
updated in storage as if an OS access method had processed the data.
The actual writing to and reading from the I/O device is handled by file management. Note that the character string X'61FFFF61' is
interpreted by CftS as an end of file indicator.
The essential work of the volume table of contents (VTOC) and the
data set control block (DSCB) is done in CMS by a master file directory (ftPD) which updates the disk contents, and a file status table (FSt) (one for each data file). All disks are formatted in physical blocks of 800 bytes. CftS continues to update the OS format, within its own format, on the auxiliary device, for files whose filemode number is 4. That is, the
block and record descriptor words (BDW and RDW) are written along with
the data. If a data set consists of blocked records, the data is
written to, and read from, the I/O device in physical blocks, rather
than logical records. CMS also simulates the specific methods of manipulating data sets. 2-42 IBft 'ft/370 System Logic and program Determination--Voluae 2

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