In either situation, after a user logs onto VM/370 (by using the
device specified in the CONSOLE statement) and loads the OS/VS system into the virtual machine, a user must issue the CP DIAL command at the
lecal 3270 that is to be used in display mode. This action logically
cennects that 3270 and its real I/O address of lB8 to the operating
system. To drop the dialed connection, a user must issue the CP RESET command.
Note: A remote 3270 cannot be used in this manner because the DIAL command does not support remote 3270 terminals.
If the second console is a remote terminal such as a 2741 or 3767
cennected by either a 2702 or a 3704/3705 in 2702 emulation mode, the SPECIAL state.ent would appear as follows: SPECIAL 01F 2702 IBM The DEFINE command would be:
define line as 01f ibm VIRTUAL MACHINE I/O MANAGEMENT
A real disk device and a real or emulated 270% transmission control unit (TCU) can be shared among multiple virtual machines. Virtual disk
device sharing is specified in the VM/370 directory entry or by a user command. A specific virtual machine may be assigned read-only or
read/write access to a shared disk device. To gain access to the shared
virtual device, a user must supply the appropriate password. To ensure
device integrity, VM/370 checks each virtual machine I/O operation
against the parameters in the virtual machine configuration.
The virtual machine operating system is responsible for the operation
of all virtual devices associated with it. These virtual devices may be
defined in the VM/370 directory entry of the virtual machine, or they may be attached dynamically to (or detached from) the virtual machine's
configuraticn for the duration of the terminal session. Virtual devices may be in one of these three states or conditions: • -- when mapped to a fully equivalent real device • Shared -- when mapped to a minidisk or when specified as a shared vIrtual disk device • -- when VM/370 places the device output on intermediate
direct access storage
For example: In a real machine when running under control of the
operating system, the problem program requests the system to issue a SIC instruction to a specific device. The operating system normally
initiates the I/O operation and handles any device error recovery. In a
virtual machine, the operating system performs the same functions, but
the device address specified and the storage locations referenced are
both virtual. VM/370 has the responsibility for translating the virtual
specifications to real. When I/O interruptions occur, VM/370 reflects them to the virtual machine for its interpretation and processing. When I/O errors occur. VM/370 records the error, but it does not initiate error recovery
operations. The operating system must handle error recovery_ 16 IBM VM/370 operating Systems in a Virtual Machine

When VM/370 initiates I/O operations for its own paging and spooling,
the operaticn is not subject to translation, and VM/370 itself performs
the operation.
In most cases, many virtual machines share the I/O devices and control
units on a channel both as minidisks and dedicated devices and with VM/370 system functions such as paging and spooling. Because of this
sharing, VM/370 has to schedule all the I/O requests to achieve a
balance amcng virtual machines. In addition, VM/370 must reflect the
results of the subsequent I/O interruption to the appropriate storage
areas of each virtual machine.
By specifying a dedicated channel for a virtual machine (using the
class B ATTACH CHANNEL command), the virtual machine has the channel and
all the devices for its own exclusive use. For dedicated channels, VM/370 translates the virtual storage locations specified in channel co.mands to real locations and performs any necessary paging operations.
However, VM/370 does not translate a device address because the virtual
device addresses on·the dedicated channel must match the real device
addresses; thus, minidisks cannot be usede Dedicated devices should be considered as an alternative to dedicated
channels because then the only translation done by VM/370 is device
address translation. Dedicated devices should be specified on separate
channels so that VM/370 can handle them more efficiently for a virtual Ilachine. Virtual machines can DASD as either minidisks or dedicated volumes.
Areal disk volume can be shared by several virtual machine users 6 each
owning a number of contiguous cylinders. This logical subdividing of a
real disk volume is called physical pack sharing, and each subdivision
of cylinders is called a virtual disk or minidisk. A real disk voluae can also be dedicated to a specific virtual machine for its own private
use. When using dedicated disk volumes, the virtual machine's operating
system must perform all necessary interruption handling, error recovery,
and error recording.
By using either the LINK directory control statement or the CP LINK
command, a user can share the data on a minidisk or entire disk volume
with the owner of the virtual disk. The LINK statement or command
allows controlled, concurrent access to the data on the virtual disk.
This sharing is called logical data sharing.
If any virtual machine temporarily requires additional direct access space, the user can use the CP DEFINE command to obtain it dynamically from a pool of temporary (T-disk) space. To define a pool of T-disk space, an installation specifies the size of the T-disk pool when
allocating disk space with the stand-alone CP format/allocate program. Section 1. General Considerations 17