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

AMSERV MODULE BALRIDCAMS CMSVSAM DCSS CMSAMS DCSS ---I IDCAMS: I AMS Root I B-disk
for OS or DOS I- -l J Uses _____ ---,I F CMS A-disk
Figure 19. Relationship in Storage between the efts Interface ftodule DMSAMS and the CMSAMS and CftSVS!ft Desss DMSAMS first determines whether the user is in the CMS/DOS environment. If not, a SET DOS ON (VSAM) command is issued to load the CMSDOS segment and initialize the eMS/DOS environment. In this case, DMSAMS must also issue ASSGB co.mands for the disk modes in the DOSCB chain created by the as user's DLBL commands. An ASSGB is also issued
for SYSCAT, the iSAK master catalog. DKS!MS then issues the ASSGN co.mand for the SYSIPT and SYSLST files,
assigning them to the user's A-disk. DLBL commands are then issued
associating these units with files on the user's A-disk. Input to the AKSERV processor is the SYSIPT file, which has the filetype AftSERV. Output from AMSERV processing is placed in the SYSLST file, which has a
filetype of LISTING. DIAGNOSE 64 (LOADSYS) is then issued to load the CftSAftS DCSS, which
contains the DOS/VS AMS code. A DOS/VS SVC 65 is issued to find the
address of the DOS/VS AKS root phase, IDCAftS. When the SVC returns with
the address of IDCAMS, a branch is made to IDCAftS, giving control to
"live" DOS/VS routines. rnCAKS expects parameters to be passed to it when it receives
control. DMSAKS passes duma, parameters in the list labeled AftSP1BMS. After the rcot phase IDCAKS receives control, the functions in the
file specified by the filename on the AftSERV coamand are executed. efts Method of operation and Program Organization 2-115