Three commands alter the I/O configuration of a user's virtual aachine
after he has logged on. Two are user commands, while the third a systea operator command, because it affects the status of real devices attached
to the system. The ATTACH and DETACH co.mands are contained in D!KVDI, DMKYDC, DMKYDD, and DKKYDE and the DEFINE command in D!KDEF. The systea command scanner (DMKCFM) calls both page able modules after their foraat
and privilege classes have been validated. These commands access the
same control-block building subroutines in the m04-u1e--- Dft-KV-DS that DKKLOG, the LOGON processor, uses. § Real The system operator can dedicate any real
device to a single virtual machine by issuing the ATTICH comaand. The
device attached is available only to the given virtual machine, and all I/O requests to it are handled by CCW If the device is a DASD, cylinder relocation does not occur when SEEK addresses or home addresses are referenced. The I/O supervisor does not queue operations
on the device, nor does it autoaatically restart it or do ordered seek
queuing. Nonsharable devices such as tape drives must be attached to a
virtual machine to be accessed by the virtual machine. I virtual machine can also have a dedicated card reader/punch or printer.
However, this is usually not necessary because of the unit record
spooling facilities of CP. Unit record input or output on a dedicated
(attached) device is not spooled by CP. The unit attached =ay be given a virtual address different from its real address; however, the virtual machine may not already have a virtual device at the attached address.
A real device cannot be attached (1) if it is currently dedicated to
another virtual machine, (2) if it contains minidisks that are in use by other vitual machines, or (3) if it is a systea-owned volume that is in
use for spooling or paging. Yiri!!! Device: A system user can define a new virtual device
with the DEFINE command that does not require the dedication of a
corresponding real device. Devices that can be defined are consoles,
spooled readers, punches and printers, dialable TP lines, virtual
channel-to-channel adapters, pseudo timers, and teaporary disks. With
the DEFINE command, the user can change any existing virtual "device
address whether it corresponds to a shared or dedicated real device or
no real device unit.
The DEFINE command can also describe the virtual machine channel mode of operation, that is, either selector or block multiplexer. The
default mode, selector channel mode, reflects a channel busy to any SIO operation attempted on the same channel path that has not coapleted the
previous channel SID operation. Block multiplexer mode allows the
successful initiation of different devices on the same channel path.
Channel 0, a byte-multiplexer channel, is unaffected by the DEFINE command. Also, any channel with a channel-to-channel adapter (CTCI) defaults to selector mode of operation regardless of the channel mode selected. Use of the DEFINE command with the CHANNELS operand generates
a virtual machine reset; therefore, it should be invoked prior to the
virtual machine IPL operation.
Note: The channel mode selected has no bearing on the types of channels
that are attached to the real system. Temporary disks are dynamically obtained cylinders of DISD stoiage
space. They are available to the user for as long as they are part of
his virtual aachine configuration, but the data on thea is destroyed
after the user detaches the area. For all other purposes, however, they
appear to be a standard disk.
CP Introduction 1-125

g !!rtyg! A virtual device can be removed from a
virtual machine configuration prior to logging off with the DETACH
command. A user can detach any of his own devices, and the system operator can detach a real device from a virtual machine. If the
operator detaches the device, the user is informed of the operator's
action. A real device can be detached only if it is dedicated to a
single virtual machine or is attached to the system and is not in use
when the DETACH is issued.
A user may permanently or temporarily disconnect his terminal or virtual machine from the system by a console command, or the terminal or virtual machine may be forcibly disconnected by the operator. The system can
also log off the virtual machine. In any case, the routines that handle
the termination process are in the pageable module, DftKUSO. PERftANENT The user may voluntarily remove his virtual machine from the system via the LOGOFF command. This command terminates
all virtual machine operation, releases all storage occupied by control
blocks and virtual storage pages, and disconnects the teleprocessing
line connection to the user's terminal. If the user specifies the HOLD
option with LOGOFF, all of the above occurs, except that the
teleprocessing line remains enabled. This option is especially useful
for dialed connections that are reused immediately by another user.
The virtual machine can be forced off the system by the system operator via the FORCE command. This has the same effect as a
user-initiated logoff, except that the user is informed that the
operator has logged off his machine. A virtual machine may also be logged off the system: • If the time for a read of a system password expires (28 seconds). • If the user makes a connection to the system but does not logon
within a given period. • If the virtual machine is running disconnected (without an active
terminal) and the virtual machine attempts a ter.inal read or enters
a disabled wait state.
The DftKUSOLG and DMKUSOFF subroutines process the LOGOFF co.mand. DftKDSP calls DftKUSOFF directly by DMKDSP to force the logoff of a user as previously described. A user may temporarily disconnect his terminal from his virtual machine by using the DISCONN com.and, while allowing
the virtual machine to continue to run. This co.mand flags the virtual machine as being disconnected and releases the user's terminal and
teleprocessing line. If the HOLD option was specified in the DISCOBR command, CP allows the line to remain enabled, and another user can use
the terminal to log on. The disconnected virtual machine continues to
be dispatched until it either attempts to execute a terminal read to the
disconnected console or it enters a disabled wait state. At this time, the dispatcher (DMKDSP) calls the routine DftKUSOFF directly to force the machine out of the system. While the machine is disconnected from its
virtual console (real terminal) any terminal output is lost; in
addition, CP may apply a disconnected penalty to the machines scheduling
priority, to bias the system in favor of interactive users.
1-126 IBft VM/370 System Logic and Problem Deteraination--Volume 1