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access to the page state and frame index
following the sequence rules for
storage-operand store references.
10. During the execution of the instructions
DECONFIGURE PAGE and DISCONNECT
P AGE, the accesses to set the reference bit and
the change bit to zeros occur concurrently with
or after the access to set the page state to
disconnected.
11. The instructions MAKE ADDRESSABLE and
MAKE UNADDRESSABLE modify only the
page state.
12. The instruction LOAD FRAME INDEX
inspects but does not modify the page state and
frame index. The page state and frame index
may only be modified explicitly by other
instructions.
The record of references provided by the
reference bit is not necessarily accurate, and the
handling of the reference bit is not subject to the
concurrency rules. However, in the majority of
situations, reference recording approximately
coincides with the storage reference.
In certain situations, the change bit may be set
when no storing has actually taken place.
Storage-Operand. References
A storage-operand reference is the fetching or
storing of the explicit operand or operands in the
storage locations specified by the instruction.
During the execution of an instruction, all or
some of the storage operands for that instruction
may be fetched, intermediate results may be
maintained for subsequent modification, and final
results may be temporarily held prior to placing
them in storage. Stores caused by channels do not
necessarily affect these intermediate results.
Storage-operand references are of three
types:
fetches, stores, and updates.
Storage-Operand Fetch References
When the bytes of a storage operand participate in
the instruction execution only as a source, the
operand is called a fetch-type operand, and the
reference to the location is called a storage-operand
fetch reference. A fetch-type operand is identified
in individual instruction definitions by indicating
that the access exception is for fetch.
All bits within a single byte of a fetch reference
are accessed concurrently. When an operand
consists of more than one byte, the bytes may be
fetched from storage piecemeal, one byte at a time.
Unless otherwise specified, the bytes are not
necessarily fetched in any particular sequence.
5-10
IBM 4300 Processors Principles of Operation
Storage-Operand Store References
When the bytes of a storage operand participate in
the instruction execution only as a destination, to
the extent of being replaced by the result, the
operand is called a store-type operand, and the
reference to the location is called a storage-operand
store reference. A store-type operand is identified
in individual instruction definitions by indicating
that the access exception is for store.
All bits within a single byte of a store reference
are accessed concurrently. When an operand
I
consists of more than one byte, the bytes may be
placed in storage piecemeal, one byte at a time.
Unless otherwise specified, the bytes are not
necessarily stored in any particular sequence.
The CPU may delay storing results into storage.
There is no defined limit on the length of time that
results may remain pending before they are stored.
This delay does not affect the sequence in which
results are placed in storage. The results of one
instruction are placed in storage after the results of
all preceding instructions have been placed in
storage and before any results of the succeeding
instructions are stored as observed by channels.
The results of anyone instruction are stored in the
sequence specified for that instruction.
The CPU does not fetch operands from a storage
location until all information destined for that
location by the CPU has been stored. Prefetched
instructions may appear to be updated before the
information appears in storage.
The stores are necessarily completed only as a
result of a serializing operation and before the CPU
enters the stopped state.
Storage-Operand Update References
In some instructions, the storage-operand location
participates both as a source and as a destination.
In these cases, the reference to the location consists
first of a fetch and subsequently of a store. Such
an operand is called an update-type operand, and
the combination of the two accesses is referred to
as an update reference. Instructions such as
MOVE ZONES, TRANSLATE, OR (OC,
01),
and
ADD DECIMAL cause an update to the
first-operand location. No special interlock is
provided between the fetch and store, and accesses
by channels are permitted. An update-type
operand is identified in the individual instruction
definition by indicating that the access exception is
for both fetch and store. The fetch and store
accesses associated with an update reference do not
necessarily occur one immediately after the other,
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