Iii 11 I Reserve/Release 1 Virtual Reserve/ ICCW Comndl Type Alternate IExecutes in thelRelease RequestedlSent by 1 of Path IHardware (2-4 I (V Added to IVM/310 tol i
Device Support IChannel Switch) I Mode in MDISK) I Device fNotel I 1 1 1 I I" IDedicatedlNot defined 1 Not applicable I Not applicable I Reserve 1 1 1 IDASD or 1-----------1---------------1-----------------1---------1----1 ITape IDefined 1 Not applicable 1 Not applicable 1 Sense I 2 I I 1 I I 1 I I IMinidiskslNot defined 1 Yes No I Reserve I 1 I I 1-----------1--------------- -----------------1---------1----1 I INot defined I Yes Yes I Reserve I 1 I 1 1-----------1--------------- -----------------1---------1----1 1 INot defined 1 No No 1 Reserve 1 3 1 I 1-----------1--------------- -----------------1---------1----1 I INot defined 1 No Yes 1 Sense I 4 1 1 1-----------1--------------- -----------------1---------1----1 I IDefined 1 Not applicable Not applicable 1 Sense 1 5 1 I 1 lNormal Operation --The command is passed unChanged to the hardware. 1 I 2When the VM/370 system has been generated with alternate path support 1 for those devices, it prevents the devices from being reserved. This 1 action causes VM/370 to avoid a possible channel lockout. VM/370 does not return any indication of this action to the operating system
issuing the ccw command that the device was not reserved.
3without the two-channel switch special feature, VM/370 sends the
reserve/release CCW command unchanged to the hardware. However, the
hardware rejects the command and does not reserve the device. ·Before sending the command to the hardWare, VM/370 changes the
reserve ccw command to a sense CCW command and places a virtual
reserve on the minidisk. The real device is not reserved. The
virtual reserve prevents other operating systems running under the same VM/370 system from accessing the minidisk; however, these same virtual systems may reserve other minidisks located on the same real volu.e. Because the two-channel switch feature is not
installed on the channels, only one address path goes to the device
from the V8/370 processor. This path allows V8/370 virtual
reserve/release processing to send a sense CCW to the device,
although the reserve CCW command would be rejected by the hardware. 5When alternate paths to a device have been defined (by the ALTCU operand on the RDEVICE macro instruction and the ALTCB operand on the RCTUNIT macro instruction), VM/370 changes reserve/release CCW commands to CCW commands to prevent a possible channel lockout.
Figure 2. Summary of VM/370 Reserve/Release Support
section 1. General Considerations 23

ALTERNATING BETWEEN OPERATING A virtual machine user may require the facilities of more than one system during a single terminal session. When running an system from a terminal, use the CMS editor
to create and modify job streams and to analyze the results and output. programmers who normally use CftS to interactively create,
medify, and test programs, may require facilities for compilation or
execution that are not supported or available in The technique described in this topic uses multiple operating systems
consecutively. Job control cards, compiler or assembler source
programs, and test data streams are created and modified at the terminal
under control of the editor. The job stream is then executed, by
passing control to an appropriate operating system that has the
necessary facilities.
In this way, the programmer uses the terminal-oriented facilities of CMS to create and update source programs and JCL. When ready to compile or test, he can give control of his virtual machine to the operating
system. After execution is finished, he can give control back to CMS to
selectively scan and display printer and punch output at the terminal.
This approach assumes that the programmer has created source program files and data files under CMS. To execute under another operating system (in this example, OS), the programmer must also create JCL
records that specify the compilation, link editing, or execution, as
appropriate. These records are created under CftS and named with a
distinctive filename and filetype (for example, PLICOMP JCL). Job
control records, source program files, and data files can then be merged
together in the virtual card reader to form a single OS job stream. The
CP and commands (shown in Figure 3) create and transfer this job
stream.
The CP SPOOL com.and transfers subsequent (not currently existing) card images from the virtual card punch of one virtual machine to the virtual
card reader of that same or SOme other virtual machine. During this
time, no real cards are punched or read; V8/310 manages the transfer of card-image data files through disk spooling operations only.
Figure 3 shows how to punch files to the virtual machine's card
reader. The virtual machine is in the CMS environment at the start of
the example. The com.and "SPOOL OOc cont eof" specifies that reading be
continuous until all files spooled to the virtual machine are exhausted
and the virtual end-of-file button on the reader is pushed. NOBEAD!E specifies that no special control cards are to be inserted at the
teginning of each punched file. Virtual device 230 is an OS system voluae. Virtual device 231 contains the OS job queue, SYS1.SYSJOEQ!. All standard eMS and OS responses are omitted from the example; however,
the OS READY message is included to more fully illustrate the 1Ft sequence. Also, assuming that the user has a 2141, he must press the
attention key before entering each OS command. The attention
interruptions are not shown in Figure 3.
24 IBM VM/370 Operating Systems in a Virtual Machine