Simulate Non-eMS Operating Environments
The following contains descriptions for: access method support for non-eMS operating systems, eMS simulation of as functions, and implementation of DOS/VS functions.
Access Method Support for Non-eMS Operating Environments OS ACCESS METHOD SUPPORT An access method governs the manipulation of data. To make the
execution of OS generated code easier under eMS, the processing program
.ust see data as OS would present it. For instance, when the processors
expect an access method to acquire input source records sequentially, CMS invokes its sequential access method and passes data to the
processors in the format that the as access methods would have produced.
Therefore, data appears in storage as if it had been manipulated using
an as access method. For example, block descriptor words (BDW), buffer
pool management, and variable records are maintained 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 eMS file management.
The work of the volume table of contents (VTOC) and the data set
control tlock (DSCB) is done by a master file directory (MFD) to
maintain disk contents and a file status table (PST) for each data file.
All disks are formatted in physical blocks of 800 bytes. CMS continues to maintain the OS format, within its own format, on
the auxiliary device, for files whose filemode number is 4. That is, ihe block and record descriptor words (BDi and RDW) are written along
with the data. If a data set consists of blocked records, the data is
written to and read from tbe I/O device in pbysical blocks, rather than
logical records. CMS also simulates the specific methods of
manipulating data sets.
To accomplish this simulation, CMS supports certain essential macros
for the following access methods: BDAM (direct)--identifying a record by a key or by its relative
position within the data set. BPAM (partitioned)--seeking a named member within an entire data set. BDAM/QSAM (sequential)--accessing a record in a sequence relative to VSAM (direct or sequential)--accessing a record sequentially or
directly by key or address. CMS support of as VSAM files is
based on DOS/VS access method services and the virtual storage
access method (VSAM). Therefore, the OS user is restricted to
those services available under DOS/VS A85 and VSAM. CMS Method of Operation and Program Organization 2-113
eMS Support for the Virtual Storage
Access Method CMS simulation of OS and DOS includes support for the virtual storage
access method (VSAM). The description of this support is in three
parts: A description of the access method services program (AMSERV), which
allows you to create and update VSAM files. A description of support for VSAM functions under CMS/DOS. A description of support for VSAM functions fer the CMS OS si.ulation
routines.
The routines that support VSAM reside in three discontiguous shared
segments (DCSSs). The CMSAMS DCSS, which contains the DOS/VS AMS code to support AMSEHV processing.
The CMSVSAM DCSS, which contains actual DOS/VS VSAM code, and the CMS/VSAM as interface program for processing as VSAM requests.
The CMSDOS DCSS, which contains the cede that supports DOS requests under CMS. !2te: which performs processing for CMS/VSAM support,
resides in the CMS nucleus.
CREATING THE DOSCB CHAIN
The DLBL command creates a control block called a DOSCB in CMS free
storage. The ddname specified in this DLBL command is associated with
the ddname parameter in the program's ACB.
The DOSCB contains information defining the file for the system. The
information in the DOSCB parallels the information written on the label
information cylinder of a real DOS SYSRES unit, e.g. the name, and mode (volume serial number) of the data set, its logical unit specification,
and its data set type (SAM or VSAM). The ancher for this chain is at
location DOSFIRST in NUCON. Executing an AMSERV Function
The CMS AMSERV command invokes the module DMSAMS, which is the CMS interface to the DOS/VS access method services (AMS) program. Module DMSAMS loads DOS/VS AMS code contained in the CMSAMS ncss by means cf
the LOADSYS DIAGNOSE 64. The AMS code requires the services of DOS/VS code that resides in the CMSVSAM DCSS so that ncss is also loaded via LOADSYS DIAGNOSE 64 when the VSAM master catalog is opened. Figure 19
shows the relationship in storage between the fnterface module DMSAMS and the CMSAMS and CMSVSAM DCSSs. The following is a general description of the DMSAMS method of
operation.
2-114 IBM VM/370 System Logic and Program Determination--Volume 2
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