• Identifying disk files. When executing programs in CMS/DOS, users
can read sequential disk files directly from DOS disks, but cannot
write on them. Instead, for testing purposes, write disk files
or output files to the virtual punch or printer, or to a tape. An exception to this rule is when executing COBOL and PL/I programs in CMS/DOS. eMS can be used to both read and write VSIM files located
on DOS disks. • Using the CMS and VM/370 debugging facilities to debug a program
under CMS. Users can set address stops (called breakpoints in the CMS debug environment) and temporarily halt the execution of a program to
examine or change the contents of registers or specific storage
locations. When a is
prepare a jcb stream
virtual machine.
tested and debugged in CMS/DOS, users
to catalog and execute the program in
can also
a DOS/VS For cOMplete details about how to use CMS/DOS, refer to User's Guide. For details about how to specify CMS commands, refer to Summary When a virtual machine user loads DOS/VS into his virtual machine, the
terminal beccmes the DOS/VS operator console, and the virtual machine
user becomes the operator responsible for entering all commands and
respcnses. The three basic techniques for using DOS/VS in a virtual machine are: running DOS/VS in batch mode, using the IPL command to
alternate between DOS/VS and eMS in a single virtual machine. and
running DOS/VS disconnected.
Before using cne of these techniques, an installation must understand
how to: • Generate DOS/VS to run in a virtual machine • create VM/370 directory entries for DOS/VS virtual machines • Access the DOS/VS system residence volume • Ensure that the proper I/O devices are attached to the DOS/VS virtual
machine • IPL and operate DOS/VS under VM/370 The primary objectives when generating DOS/VS to run in a virtual machine should be to avoid double ccw translation and to reduce the
number of SID instructions issued by DOS/VS. To meet these objectives,
an installaticn needs to consider how it generates both VM/370 and DOS/VS.. (DOS/VS can be generated under VM/370.) DOS/VS oFeration depends upon how DOS/VS was generated. There may be
additional cperator commands and control statements that must be entered
at the ccnscle before running jobs on the DOS/VS virtual machine. There are many that DOS/VS virtual machine users can be helped
by using the CMS component of VM/370. The CMS editor and EXEC facility
can be used to prepare jobs for execution in a DOS/VS virtual
machine. CMS commands can be used to develop and test programs on disks. The CMS/DOS environ.ent provides many commands that simulate DOS/VS functions. Section 3. DOS/VS in a Virtual Machine 105

106 IBM VM/370 Operating Systems in a Virtual Machine