Accen' Method Support
CHECK
DCB Operand B ALN
BLKSIZE
BUFCB
BUFL BUFNO DDNAME DSORG EODAD EXLST KEYLEN8
LIMCT
LRECL
MACRF OPTCD RECFM SYNAD NCP All the options of CHECK are supported. CHECK tests the 110 operation for errors and exceptional conditions.
The following fields of a DCB may be specified, relative to the
particular access method indicated:
BDAM BPAM BSAM F,D F,D F,D ,D
n(number) n n n
a(address) a a a
n n n n
n n n n
s(symbol) s s s
DA PO PS PS a a a
a a a a
n n
n
n n n
R,W R,W R,W,P G,P,L,M A,E,F,R J J F,V,U F,V,U F,V,B,S,A,M,U F,V,B,U,A,M,S
a a a a
n n
The manipulation of data is governed by an access method. To facilitate the exe­
cution of as Code under CMS, the processing program must see data as as would
present it. For instance, when the processors expect an access method to acquire
input source cards sequentially, CMS invokes specially written routines that simu­
late the OS sequential access method and pass data to the processors in the format
that the as access methods would have produced. Therefore, data appears in stor­
age as if it had been manipulated using an as access method. For example, block
descriptor words (BDW), buffer pool management, and variable records are
updated in storage as if an as access method had processed the data. The actual
writing to and reading from the 110 device is handled by CMS file management.
Note that the character string X'61FFFF61' is interpreted by CMS as an end of
file indicator.
The essential work of the volume table of contents (VTOC) and the data set con­
trol block (DSCB) is done in CMS by a master file directory (MFD) 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. CMS continues to update the as format, within its own format, on the auxiliary
device, for files whose filemode number is 4. That is, the block and record descrip­
tor 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 110 device in physical
blocks, rather than logical records. CMS also simulates the specific methods of
manipulating data sets.
8
If an input data set is not a BDAM data set, zero is the only value that should be specified for
KEYLEN. This applies to the user exit lists as well as to the DCB macro instruction. OS Macro Simulation Under CMS 379

When the OPEN macro instruction is executed, the CMS simulation of the OS OPEN routine initializes the Data Control Block (DCB). The DCB fields are filled
in with information from the DCB macro instruction, the information specified on
the FILEDEF command, or, if the data set already exists, the data set label. How­
ever, if more than one source specifies information for a particular field, only one
source is used.
The DCB fields are filled in in this order:
1. The DCB macro instruction in your program.
2. The fields you had specified on the FILEDEF command.
3. The data set label if the data set already exists.
The DCB macro instruction takes precedence over the FILEDEF and the data set
label. Data set label information from an existing CMS file is used only when the OPEN is for input or update, otherwise the OPEN routine erases the existing file.
You can modify any DCB field either before the data set is opened or through a
Data Control Block open exit. CMS supports only the Data Control Block exit of
the EXIT LIST (EXLST) options.
When the data set is closed, the Data Control Block is restored to its original condi­
tion. Fields that were merged in at OPEN time from the FILEDEF and the data
set label are cleared.
To accomplish this simulation, CMS supports certain essential macros for the fol­
lowing access methods: • BDAM (direct) --identifying a record by a key or by its relative posi­
tion within the data set. • BP AM (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.
Note: CMS support of OS VSAM files is based on
VSE/VSAM. Therefore, the OS user is restricted to those
functions available under VSE/VSAM. See the section "CMS Support for OS and VSE/VSAM Functions" for details.
CMS also updates those portions of the OS control blocks that are needed by the OS simulation routines to support a program during execution. Most of the simu­
lated supervisory OS control blocks are contained in the following two CMS con­
trol blocks: 380 VM/SP System Programmer's Guide
CMSCVT
simulates the communication vector table. Location 16 contains the
address of the CVT control section.