• CHKSENSE Routine CHKSENSE checks sense bits to determine the
recoverability of a unit check error if one occurs. • CHKITNT Routine CHKITNT checks to determine if the end of split
cylinder or the end of extent occurred, and, if so, updates to the
next split cylinder or extent. • DISKIO Routine -- DISKIO starts I/O operation on a CCW string via a DIAGNOSE X'20'. • GETALT Routine --GETALT switches reading from alternate track to
prime track, and from prime track to alternate track. • RDCNT Routine --RDCNT reads count fields on the track to deter.ine
the last record number on the track. • SETXTNT Routine -- SETITNT sets OSFSTEND to the value of the end of the extent and, if a new extent is specified, sets CCHHR to the value
of the start of the extent. Simulating a DOS Environment under eMS CMS/DOS is a functional enhancement to CMS that provides Des
installations with the interactive capabilities of a VM/370 virtual machine. CMS/DOS operates as the background DOS partition; the other
four partitions are unnecessary, since the CMS/DOS virtual machine is a
one-user machine. CMS/DOS provides read access to real DOS data sets, but not write or
update access. Real DOS private and system relccatable, source
statement, and core-image libraries can be read. This read capability is
supported to the extent required to support the CMS/DOS linkage editor,
the DOS/PLI and DOS/VS COBOL compilers, the FETCH routine, and the RSERV, SSERV, and ESERV commands. No read or write capatility exists fer
the DOS procedure library, except for copying procedures from the
procedure library (via the PSERV command) or displaying the procedure
library (via the DSERV command) • CMS/DOS does not sUPFort the standard label cylinder.
INITIALIZING DOS AND PROCESSING DOS SYSTEM CONTROL COMMANDS Initialization of the CMS/DOS operating environment requires the setting
of flags and the creation of certain data areas in storage. Once initialized, these flags and data areas may then be changed by routines
invoked by the system control commands.
Five modules are described in this section: • DMSSET Activates the CMS/DOS environment control blocks to be used
during CMS/DOS processing. • DMSOPT Sets or resets compiler execution-time options. • DMSASN Relates logical units to physical units. • DMSLLU Lists the assignments of CMS/DOS physical units. • DMSDLB Associates a DTF with a logical unit for CMS/DOS processing. eMS Method of Operation and Program Organization 2-137

DMSSET initializes the C"S/DOS operating environment as follows: • Verifies that the mode, if specified, is for a DOS formatted disk. • Stores appropriate data in the SYSRES LUB and PUB. • Locates and loads the CMS/DOS discontiguous shared segment. Saves (in NUCON) the addresses of the two major CMS/DOS data blocks, SYSCOM, BGCOM,and the address of the CMS/DOS discontiguous shared
segment (C"SDOS). • Sets the DOSMODE and DOSSVC bits in DOSFLAGS in NueON. • Assigns (via ASSGN) the SYSLOG logical unit as the CMS virtual
console.
The CMS/DOS operating environ.ent is entered when the CMS SET DOS eN i& invoking the Several data areas are prepared for processing during initialization.
The .ain CMS data area, NUCOH, is modified to contain the addresses of
two DOS data areas, SYSCOM and BGCOM. The SYSCOM DSECT is the DOS system communications region. It
consists mainly of address constants, including the addresses of the AB
option table, the PUB ownership table, and the FETCH table. It also
includes such information as the number of partitions (always one for CMS/DOS) and the length of the PUB table.
The BGCOM DSECT is the partition communication region. It includes
such information as the date, the location of the end of supervisor
storage, the end address of the last phase loaded, the end address of
the longest phase loaded, bytes used to set the language translator and
supervisor options, and the addresses of many other DOS data areas such
as the LUB, PUB, NICL, FICL, PIB, PIB2TAB, and the PCTIE. The LUB and PUB tables are also made available during initialization.
The LUB is the logical unit block table. It acts as an interface
between the user's program and the CMS/DOS physical units. It contains
an entry for each sy.bolic device available in the system.
Each of the symbolic names in the LUB is mapped into an element in
the PUB, the physical unit block table. The PUB table contains an entry
for each channel and device address for all devices physically available
to the system and also contains such information as device type cede, eMS disk mode, tape mode setting, and 7-track indicator.
Two bits are set in DOSFLAGS in NUCOH, DOSHODE and DOSSVC. DOSMODE specifies that this virtual machine is running in the eMS/DOS operating
environment. DOSSVC indicates Whether OS or DOS SVCs are operative in
the operating environment. If DOSSVC is set, DOS SVCs are used;
otherwise, as SVCs are operative.
2-138 IBM VM/370 System Logic and Program Determination--Volume 2