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When a store access is prohibited because of
key-controlled protection, the contents of the
protected location remain unchanged. When a
fetch access is prohibited, the protected information
is not loaded into a register, moved to another
storage location, or provided to an I/O device. For
a prohibited instruction fetch, the instruction is
suppressed and an arbitrary instruction-length code
is indicated.
Key-controlled protection is always active,
regardless of whether the CPU is in the problem or
supervisor state, and regardless of the type of CPU
instruction or channel-command word being
executed.
All accesses to storage locations that are
explicitly designated by the program and that are
used by the CPU to store or fetch information are
subject to key-controlled protection.
All storage accesses by a channel to fetch a
CCW or to access a data area designated during
the execution of a CCW are subject to
key-controlled protection. However, if a CCW or
output data is prefetched, a protection check is not
indicated until the CCW is due to be executed or
the data is due to be written.
Key-controlled protection is not applied to
accesses that are implicitly made by the CPU or
channel for such sequences as:
• Interruptions,
• Updating the interval timer,
• Fetching the CAW during the execution of an
1/
°
instruction,
• Storing the CSW by an I/O instruction or
interruption,
• Storing channel identification during the
execution of STORE CHANNEL ID,
• Limited channel logout, or
• Initial program loading.
Similarly, protection does not apply to accesses
initiated via the operator facilities for altering or
displaying information. However, when the
program explicitly designates these locations, they
are subject to protection.
Reference Recording
Reference recording provides information for use in
selecting pages for replacement. Reference
recording uses the reference bit, bit 5 of the storage
key. The reference bit is set to one each time a
location in the corresponding page is referred to
either for fetching or storing information.
Reference recording is always active and takes
place for all storage accesses, including those made
by any CPU, I/O, or operator facility.
It
takes
3-8
IBM 4300 Processors Principles of Operation
place for implicit accesses made by thy machine,
such as those which are part of interruptions and
I/O-instruction execution.
Reference recording does not occur for operand
accesses of the following instructions since they
directly refer t<) a page description without
accessing a storage location:
CONNECT PAGE
DECONFIGURE PAGE (reference bit is set to zero)
DISCONNECT PAGE (reference bit is set to zero)
INSERT PAGE BITS
INSERT STORAGE KEY
LOAD FRAME INDEX
MAKE ADDRESSABLE
MAKE UNADDRESSABLE
RESET REFERENCE BIT (reference bit is set to zero)
SET PAGE BITS (reference bit is set to a specified value)
SET STORAGE KEY (reference bit is set to a specified
value)
The record provided by the reference bit is
substantially accurate. The reference bit may be
set to one by fetching data or instructions that are
neither designated nor used by the program, and,
under certain conditions, a reference may be made
without the reference bit being set to one. Under
certain unusual circumstances, a reference bit may
be set to zero by other than explicit program
action.
Change Recording
Change recording provides information as to which
pages have to be saved in auxiliary storage when
they are replaced in main storage. Change
recording uses the change bit, bit 6 of the storage
key.
The change bit is set to one each time a store
access causes the contents in the corresponding
page to be changed. A store access that does not
change the contents of storage mayor may not set
the change bit to one.
The change bit is not set to one for an attempt
to store if the access is prohibited. In particular:
1. For the CPU, a store access is prohibited
whenever an access exception exists for that
access, or whenever an exception exists which
is of higher priority than the priority of an
access exception for that access.
2. For I/O, a store access is prohibited whenever
a key-controlled-protection condition exists for
that access.
Change recording is always active and takes
place for all store accesses to storage, including
those made by any CPU, I/O, or operator facility.
It takes place for implicit references made by the
machine, such as those which are part of
interruptions.
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