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Program Mask: Bits 36-39 are the four
program-mask bits. Each bit is associated with a
program exception, as follows: '
Program-
Mask Bit
Program Exception
36
Fixed-point overflow
37
Decimal overflow
38
Exponent underflow
39
Significance
When the mask bit is one, the exception results
in an interruption. When the mask bit is zero, no
interruption occurs. The setting of the exponent-
underflow-mask bit or the significance-mask bit
also determines the manner in which the operation
is completed when the corresponding exception
occurs.
Instruction Address: Bits 40-63 form the
instruction address. This address designates the
location of the leftmost byte of the next
instruction.
Control Registers
The control registers provide a means for
maintaining and manipulating control information
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that resides outside the PSW.There may be up to
sixteen 32-bit control registers.
One or more specific bit positions in control
registers are assigned to each facility requiring such
register space.
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The LOAD CONTROL instruction loads control
information from storage into control registers,
whereas the STORE CONTROL instruction
transfers information from control registers to
storage.
The instruction LOAD CONTROL causes all
register positions, within those registers designated
by the instruction, to be loaded. Information
loaded into the control registers becomes active
(that is, assumes control over the system) at the
completion of the instruction causing the
information to be loaded.
When STORE CONTROL is executed, it returns
the current value in each register position. Values
corresponding to unassigned register positions are
unpredictable.
Only the general structure of control registers is
described here; a definition of the register positions
appears with the description of the facility with
which the register position is associated. The figure
"Assignment of Control-Register Fields" shows the
control-register positions which are assigned and
the initial value of the field upon execution of
reset.
Programming Note
To ensure that existing programs run if and when
new facilities using additional control-register
positions are installed, the program should load
zeros in unassigned control-register positions.
Although STORE CONTROL may provide zeros in
the bit positions corresponding to unassigned
register positions, the program should not depend
on such zeros.
It
is permissible, however, for the
program to load into the control registers, by
LOAD CONTROL, any information previously
stored by means of STORE CONTROL.
Chapter 4. Control
4-7
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