9. If the virtual PSW is in EC mode, the following
conditions are verified for byte 0 of the virtual PSW and the ope,rand byte that is to replace
it (3):
a. TheDAT mode bit is unchanged.
b.. The PER mask is unchanged.
c. Bits 0, 2, 3, and 4 of the new PSW are z.eros. d. Neither the I/O mask (bit 6) nor the
external mask (bit 7) of the PSW is to be
changed from zero to one if a virtual
interruption is pending (that is, if bit 0 of
the word containing the virtual PSW address is one l. In the absence of any of these conditions,
execution ends with the invoking of the set­
system-mask function of the expanded-virtual­
machine assist if that facility is installed. If
that facility is not installed, execution ends with
a program interruption for a privileged­
operation exception.
When the virtual PSW is in BC mode,
execution ends if any channel mask, I/O mask,
or external mask is to be changed from zero to
one and a virtual interruption is pending.
Execution ends with a program interruption for
a privileged-operation exception. 10. The operand byte fetched is stored with a key
of zero as byte 0 of the virtual PSW. Execution of this function ends with completion
of the SET SYSTEM MASK instruction (4).
Figure 15 summarizes the fields used.
Programming Note
The value in bit position 1 of the real control
register 0 has no effect on the execution of the
set-system-mask function of the virtual-machine
assist.
Field Control Address Offset No. of STORE CONTROL The STORE CONTROL instruction is executed for
a virtual machine if the virtual-machine assist is
activated for System/370 instructions, unless (1) a
virtual-machine exception is recognized, or (2) the
second operand or some pertinent VM/370 control
field cannot be fetched.
The store-control function of the virtual-machine
assist is performed each time the CPU attempts to
execute a STORE CONTROL instruction when the
problem-state bit of the real PSW is one.
Execution of this function consists in performing
the following steps:
1. If bits 0-3 of control register 6 are not 10XO binary, execution of this function ends, and a
program interruption takes place for a
privileged-operation exception (1.A.1).
2. The word MICCREG, containing the address
of the control block ECBLOK, which in turn
contains the virtual control registers, is fetched
with a key of zero. Execution of this function
ends if an addressing condition is encountered
(1.A.2).
3. If an access condition is encountered in
fetching the second halfword of the STORE CONTROL instruction, execution of this
function ends, and a program interruption takes
place for the access exception encountered
(1.8).
4. If the second-operand effective address does
not specify a word boundary, execution of this
function ends, and a program interruption takes
place for a privileged-operation exception
(2.A).
5. The virtual control-register values specified by
the R 1 and R 3 fields of the instruction are
fetched from the ECBLOK with a key of zero
and stored with the PSW key in ascending
Name Block Type (Hex) Bytes Contents MICCREG MICBLOK Real 4 4 Address of ECBLOK MICVPSW MICBLOK Real 8 4 Address of VMPSWl EXTCRO ECBLOK Real 0 4 Virtual control
register 0 VMPSW VMBLOK Real 0 2 Virtual PSW bits 0-15 Operand 2 -Logical -1 New system mask
1 A virtual interruption is pending if bit 0 of MICVPSW is one.
Figure 15. Fields Used in SET SYSTEM MASK 20 Assist and Shadow·Table-Bypass Assist

locations starting with the location specified in
the second-operand address. Execution ends if
an addressing condition is encountered in
fetching control-register values. On some m-odeIs,--acrdfesSingcondllollsmayOe--------- --
recognized for all 16 virtual control registers.
If an access condition is encountered on the
second operand, a program interruption takes
place for the exception recognized (2.B).
If no exceptions are recognized, the execution of
this function and the instruction are complete.
Figure 16 summarizes the fields used. STORE THEN AND SYSTEM lVIASK The STORE AND SYSTEM MASK instruction is executed for a virtual machine if the
virtual-machine assist is activated for System/370 instructions, unless (1) a virtual-machine exception
is recognized, (2) the first operand or some
pertinent VM/370 control field cannot be accessed,
or (3) the PER mask or the DAT bit of the virtual PSW would be changed.
If the shadow-table-bypass assist is not installed,
the store-then-AND-system-mask function of the
virtual-machine assist is invoked each time a CPU attempts to execute a STORE THEN AND SYSTEM MASK instruction when the problem­
state bit of the real PSW is one. If the shadow­
table-bypass assist is installed, the store-then­
AND-system-mask function of the virtual-machine
assist may be invoked only from the STNSM function of the shadow-table-bypass assist. IField Control Address Offset No. of
Execution of this function consists in performing
the following steps:
1. If bits 0-3 of control register 6 are not lOXO binary, execution of this function ends, a
plugl am intett aption -takes-place--for--a----------­ privileged-operation exception, and execution
of the STORE THEN AND SYSTEM MASK instruction is suppressed (1.A.1).
2. The word MICVPSW, containing the address of
the virtual PSW, is fetched with a key of zero.
Execution ends if an addressing condition is
encountered (1.A.2).
3. The first halfword of VMPSW, which contains
the virtual PSW, is fetched with a key of zero.
Execution ends if an addressing condition is
encountered (I.A. 3).
4. A new first byte for the virtual PSW is
computed by taking the logical AND of the
immediate field (12) of the instruction and byte
o of the virtual PSW fetched. If the virtual PS\V is in the Ee mode (bit 12 is one) and if
the replacement of byte 0 of the virtual PSW by the new byte-O value would change bit 1 or
bit 5 from one to zero, execution of this
function ends. Ending of execution invokes the
store-then-AND-system-mask function of the
expanded virtual-machine assist if that assist is
installed. Otherwise, ending of execution
causes a program interruption to be taken for a
privileged-operation exception (l.A.4).
5. The second halfword of the instruction is
fetched. If an access condition is encountered,
execution of this function ends, and a program: IName Block Type (Hex) Bytes Contents MICCREG MICBLOK Real 4 4 Address of ECBLOK (virtual CRs) EXTCRO ECBLOK Real 0 4 Virtual CRO EXTCR, ,ECBLOK ,Real , 4 , 4 ,Virtual CRl , EXTCRl IECBLOK IReal 4 4 Virtual CRl EXTCRl ECBLOK Real 4 4 Virtual CRl EXTCRl ECBLOK Real 4 4 Virtual CRl IEXTCR15 IECBLOK jReal I 3C 4 IVirtual CR15 I 4n
1 IOperand 21 I Loa i ca 1 , 0 I I I Ln is the number of registers specified by the R 1 , R3 fields.
Figure 16. Fields Used in STORE CONTROL Virtual-Machine Assist 21