full screen I/O operation. An ERASE/WRITE or ERASE/WRITE ALTER­
NATE instruction should be issued to reestablish full screen mode and refor­
mat the screen. If control of the PAl key interrupt had been transferred to the
virtual machine via the CONTROL option, it must be respecified to return PAl key control back to the virtual machine. Otherwise, depression of the PAl k.ey places the display in CP mode.
An X'8E' in the CSW unit status byte following an ERASE/WRITE or
ERASE/WRITE ALTERNATE instruction indicates that non-full screen data (CP mode) is waiting to be read. The application program should issue a
non-full screen READ and then reissue the ERASE/WRITE instruction.
6. Other non-full screen virtual machine messages are displayed immediately
when in full screen mode.
7. The application program must establish an environment to handle attention
interruptions. This could be done using the CMS macros HNDINT and
W AITD. There are two conditions when CP posts an attention interruption to
the application program:
a. When CP receives an attention interruption indicating that the virtual
machine console operator has caused an interruption. (For example,
depressed the ENTER or a PF key on the display keyboard).
b. When a CP priority message is to be displayed. In either case the applica­
tion program should respond by issuing a READ.
8. The application program must also establish an environment to handle I/O interruptions and must ensure that channel end and device end have been
received before processing continues.
9. If the test request key is depressed from a local 3270 when in full screen mode, X'604040' is returned to the application program in the read buffer. The test
request key is not supported for remote 3270 terminals. 10. If you press the PAl key in full screen mode, CP posts an attention interrupt to
your virtual machine. If the virtual machine does not respond with a READ
and you press the PAl key a second time, your virtual machine is put in CP mode and "CP READ" is displayed in the screen's status area. However, if
you set bit X'lO' of the control option on before the initial ERASE/WRITE or
ERASE/WRITE ALTERNATE, and press the PAl key, the interrupt is
reflected to your virtual machine for handling. If you have not set bit X'lO' of
the CONTROL option on and you press the PAl key, your virtual machine is
put in CP mode and "CP READ" is displayed in the screen's status area.
11. The application programmer must be aware that long data streams may result
in very high CP storage use and possible system degradation. In addition, long
data streams sent over BSC lines may cause degradation of response time on
other terminals on the same BSC line.
Full Screen Interactions (3270 SIO) Full screen console (3270 SIO) support enables a guest virtual machine and CP to
share a locally attached display terminal controlled by CP. The virtual machine can
use the display terminal as a graphics device in full screen mode; CP can use the
same terminal as a line device. When the terminal is in full screen mode, the screen
DIAGNOSE Instruction in a Virtual Machine 251

format, data checking, and error checking are under the complete control of the
application program running in the virtual machine. A guest virtual machine can
use either the DIAGNOSE X'58' or the SIO instruction to initiate full screen mode,
but not both.
Before the guest virtual machine can issue 3270 SIO commands, it must first issue
the CP TERMINAL command with the CONMODE 3270 option to be able to
issue 3270 SIO commands. In addition, the SCRNSAVE ON option of the CP TERMINAL command gives a virtual machine (that has also specified CONMODE 3270) the ability to save the full screen display when the screen
enters CP mode. If SCRNSA VE ON is specified, the screen is automatically redis­
played when the console returns to full screen mode. If SCRNSA VE OFF has
been specified by a virtual machine that has specified CONMODE 3270 and CP takes over a screen, CP presents a CLEAR attention interrupt to the virtual
machine when CP is ready to give up control of the screen. It is the responsibility
of the application program to issue an ERASE/WRITE to refresh the screen. If
the virtual machine issues only a WRITE that does not cover the entire screen,
information that CP displayed can remain on the screen. To use the CP TERMI­
NAL SCRNSA VE OFF: 1. Always issue a WRITE after a READ.
2. CP can break into a CCW chain containing WRITEs (with the WCC byte
making the keyboard locked) and take over the screen. Upon return to full
screen mode, the next CCW in the chain is processed as if it is the first CCW.
The guest system must provide a means to handle this situation.
3. Refresh the screen with an ERASE/WRITE when CP issues a CLEAR atten­
tion interrupt.
4. When ATTENTION from the console is received, the guest program must
issue a READ.
The TERMINAL BREAKIN GUESTCTL option allows a guest operator to
control break-ins (when CP takes over the full screen). Each time a CP request is received, it is put on a defer queue and an audible alarm sounds. The
guest operator can switch to CP mode by hitting the break-in key.
The TERMINAL BRKKEY option allows the user to specify a PF key as the
break-in key in full screen mode. The default break-in key is PAL PAl atten­
tions are sent to the virtual machine when PAl is not defined as the BRKKEY.
Some applications may interpret this PAl attention as a user request to enter
the CP environment.
Notes:
1. DIAGNOSE X'58' is a 3215 command and causes command rejects if exe­
cuted with CONMODE 3270.
2. DIAGNOSE X'58' can be used with BREAKIN and BRKKEY.
3. CONMODE must be 3215 to run CMS. If CMS sets CONMODE to 3270 while CMS is running, results are unpredictable.
4. SCRNSA VE ON must be specified if running a guest SCP such as MVS with CONMODE 3270. Otherwise results are unpredictable.
252 VM/SP System Programmer's Guide