Dedicated Channels
Spooling Functions
In most cases, the I/O devices and control units on a channel are shared among
many virtual machines as minidisks and dedicated devices, and shared with CP sys­
tem functions such as paging and spooling. Because of this sharing, CP has to
schedule all the I/O requests to achieve a balance between virtual machines. In
addition, CP must reflect the results of the subsequent I/O interruption to the
appropriate storage areas of each virtual machine.
By specifying a dedicated channel (or channels) for a virtual machine via the Class
B ATTACH CHANNEL command, the CP channel scheduling function is
bypassed for that virtual machine. A virtual machine assigned a dedicated channel
has that channel and all of its devices for its own exclusive use. CP translates the
virtual storage locations specified in channel commands to reallocations and per­
forms any necessary paging operations, but does not perform any device address
translations. The virtual device addresses on the dedicated channel must match the
real device addresses; thus, a minidisk cannot be used.
A virtual unit record device, which is mapped directly to a rea] unit record device, is
said to be dedicated. The real device is then controlled completely by the virtual
machine's operating system. CP facilities allow multiple virtual machines to share unit record devices. Since vir­
tual machines controlled by CMS ordinarily have modest requirements for unit
record input/output devices, such device sharing is advantageous, and it is the
standard mode of system operation.
Spooling operations cease if the direct access storage space assigned to spooling is
depleted, and the virtual unit record devices appear in a not-ready status. The sys­
tem operator or the spooling operator may make additional spooling space available
by purging existing spool files or by assigning additional direct access storage space
to the spooling function. The spooling operator can use the class D SPT APE com­
mand to retrieve spool files from tape for output processing when spooling space
requirements are not critical. See the description of the SPT APE command in the
VM / SP Operator's Guide for further information.
Specific files can be transferred from the spooled card punch or printer of a virtual
machine to the card reader of the same or another virtual machine. Files trans­
ferred between virtual unit record devices by the spooling routines are not phys­
ically punched or printed. With this method, files can be made available to multiple
virtual machines, or to different operating systems executing at different times in
the same virtual machine.
Files may also be spooled to remote stations via the Remote Spooling Communi­
cations Subsystem (RSCS), a program product of VM/SP. CP spooling includes many desirable options for the virtual machine user and the
real machine operator. These options include printing multiple copies of a single
spool file, backspacing any number of printer pages, and defining spooling ,classes
for the scheduling of real output. Each output spool file has, associated with it, a
136-byte area known as the spool file tag. The information contained in this area
and its syntax are determined by the originator and receiver of the file. For exam­
ple, whenever an output spool file is destined for transmission to a remote location
VM/SP 11

Spool File Recovery
Warm Start
Checkpoint Start
via the Remote Spooling Communications Subsystem, RSCS expects to find the
destination identification in the file tag. Tag data is set, changed, and queried using
the CP TAG command.
It is possible to spool terminal input and output. All data sent to the terminal,
whether it be from the virtual machine, the control program or the virtual machine
operator, can be spooled. Spooling is particularly desirable when a virtual machine· is run with its console disconnected. Console spooling is usually started via the
command SPOOL CONSOLE START An exception to this is when a system operator logs on using a graphics device. In
this instance, console spooling is automatically started and continues in effect even
if the system operator should disconnect from the graphics device and log on to a
nongraphic device. In order to stop automatic console spooling, the system opera­
tor must issue the command SPOOL CONSOLE STOP If the system should suffer an abnormal termination, there are three degrees of
recovery for the system spool files; warm start (WARM), checkpoint start (CKPT),
and force start (FORCE). Warm start is automatically invoked if SET DUMP AUTO is in effect. Otherwise, the choice of recovery method is selected when the
following message is issued:
hh:rnm:ss START ((COLDIWARMICKPTIFORCE) (DRAIN)) I (SHUTDOWN): Note that a cold (COLD) start does not recover any spool files.
After a system failure, the warm start procedure copies spool file, accounting, and
system message data to the warm start area on the IPLed system residence volume.
When the system is reloaded, this information is retrieved and the spool file chains
and other system data are restored to their original status. If the warm start proce­
dure cannot be implemented because certain required areas of storage are invalid,
the operator is notified to take other recovery procedures.
Any new or revised status of spool file blocks, spooling devices, and spool hold
queue blocks is dynamically copied to the checkpoint area on the IPLed system res­
idence volume as it occurs. When a checkpoint (CKPT) start is requested, this is
the information that is used to recreate the spool file chains. It differs from warm
start data in that only spool file data is restored; accounting and system messages
information is not recovered. Also, the order of spool files on any particular
restored chain is not the original sequence but a random one.
12 VM/SP System Programmer's Guide