208 V8/370 Operator's Guide

Section 5. Operator Spooling Functions Spooling Input and output files for use and access by virtual machines through
unit record devices are maintained by CP as disk data files using a mechanism called spooling. Individual files can be identified and
manipulated using various console functions. The disk records are
chained to form a logical file from dynamically assigned areas on
specially formatted CP disk areas. Data records from disk are read into
available page space obtained through the CP paging mechanism. The data
records contain the actual data to be used and the CCWs to properly
control the format. These CCWs are directly executed to perform the
actual unit record oFerations on the real hardware. CP can support any number of virtual and real unit record devices given sufficient system resources.
The data is placed in the spool buffers through the virtual machine
unit record simulation routines in CP. Certain spool files have a
special data format (system dumps, for example) and are accessed using a
special interface. SPOOLING CONSIDERATIONS VM/310 spooling facilities allow several virtual machines to share one
or more unit record devices. Since virtual machines controlled by CMS ordinarily have modest requirements for unit record I/O, such device
sharing is quite advantageous, and it is the standard mode of system operation.
Each user has, as a general rule, a virtual reader, a virtual punch,
and a virtual printer as his spooling devices; In addition, the virtual
console can also be classified as a spool file generator as all input
and output to the console can be logged on a sFool file. This console
log, and the files created by the user's virtual spooling devices, can
be processed by the real unit record devices that attach to the system. CP controls and schedules the operation of the real unit record
devices via spooling techniques. Virtual machine SIO instructions
directed to those unit record devices designated as spool devices in the
user directory entry are intercepted and '.odified by CP. CP generates
another I/O operation, transparent to the virtual machine, which
replaces the one specified. The new operation is directed to a CP spooling disk area which acts as intermediate storage between the real
unit record device and the virtual machine. The data transfer operation
between a spooled unit record device and the virtual machine is, in
reality, between a CP spool file and the virtual machine. spool file
records are page size (that is, 4096 byte blocks), and are transferred
between storage media via the CP paging mechanism. Section 5. Operator Spooling Functions 209