Logical Device Support Facility
The Logical Device Support Facility allows an application running in a virtual
machine to create within CP one or more non-extended (i.e. no extended color,
extended highlighting, or programmed symbols) logical 3270 display devices.
Except for the logical device facility, CP is unaware of the fact that this terminal
has no real existence and is driven by the application program. In particular, the CP display terminal support sees it as a local 3270. Any output directed to a log­
ical device is redirected to the virtual machine for which the device was created.
The virtual machine can also transfer data to CP to be entered as input from a spe­
cific logical device, as if it were interactively produced on a real terminal.
The Logical Device Support Facility is made up of two data transfer subfunctions,
three control subfunctions, a special external interrupt (code X'2402'), and an
external control word for passing control information with the external interrupt.
To implement this facility, subfunctions are invoked using the DIAGNOSE instruc­
tion (code X'7C'). Registers Rx, Rx+ 1, Ry, and Ry+ 1 are used to indicate the
subfunction, logical device identification, and other subfunction-dependent infor­
mation.
A special interrupt code (X'2402') is used by module DMKHPS to notify a virtual
machine of pending logical device status for a logical device created for that virtual
machine. Along with this interrupt, the virtual machine receives a control word at a
virtual storage location indicating the ID of the associated logical device and the
reason for the interrupt.
Figure 27 on page 204 is a summary of Logical Device Support Facility subf­
unctions. More complete information about each of these sub functions is included
under "Descriptions of Logical Device Support Facility Subfunctions."
Data is directed to a logical device using the logical device ID. This ID is assigned
by CP during execution of the INITIATE subfunction. Data transfer takes place
within CP at a channel command level. I/O directed to a logical device proceeds
within CP via the normal path for a local 3270 terminal up to the point that DMKIOS is normally called to start I/O. At that point, control passes to DMKHPS to process the CCW string. Channel commands requiring interaction
cause external interrupts to the virtual machine for which the associated logical
device was created.
The format of data from the virtual machine must conform to 3270 architecture for
local display stations. Extended data streams are not supported. Up to eight virtual machines may simultaneously create logical devices, and each
virtual machine can create up to 512 of these devices.
Logical Device Support Facility 203
Subfunction Description
INITIATE Initiate logical device communications ACCEPT Transfer data written to logical device to virtual machine
storage. PRESENT Transfer data from virtual machine to CP as input from
logical device.
TERMINATE Drop a specific logical device.
TERMINATE ALL Drop all logical devices created for this virtual machine.
Figure 27. Summary of Logical Device Support Facility Subfunctions
VM!Pass-Through Facility program product is an example of an application using
the Logical Device Support Facility. Through the combined support of these two
facilities, a VM/SP user attached to system A via a 3270 Display Station can
access VM/SP system B as though the display station were locally attached to sys­
temB.
204 VM/SP System Programmer's Guide
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