they set condition code o.
The fetch and store accesses associated with an
interlocked-update reference are not necessarily
made contiguously, but restrictions are made on
accesses to the location. The fetch access of an inter
locked update by another
es by another
between the fetch and the store accesses of
interlocked update, including the fetches of a CS or
CDS instruction which results in condition code 1,
may be made from the location during the interlock
period.
cur during the interlock period.
Within the limitations of the above requirements,
the fetch and store accesses associated with an up
date follow the same rules as the fetches and stores
described in the previous sections.
Programming Notes
When two
common main-storage location by an instruction that
causes fetching and subsequently storing of the up
dated information, it is possible for both
fetch
store access. The change made by the first
store the result in such a case is lost. Similarly, if
one
the fetch and store parts of the update reference, the
effect of the store is lost. If, instead of a store ac
cess, a
to the common storage field between the fetch and
store portions of an update due to another
change in the contents produced by the interlocked
update is lost.
field of both operations are considered to be part of
the common main-storage location. However, all bits
within a
even if the bits modified by the two operations do
not overlap. As an example, if one
instruction
immediate field and the other
(NI) with an immediate operand of FE (hex) on the
same
When the store access is part of an update refer
ence by the
contingent on whether the information to be stored
is different from the original contents of the loca
tion. In particular, the contents of all designated
byte
field, the entire contents of the byte are replaced.
26 System/370 Principles of Operation
An access to store information is performed, for
example, in the following cases:
a. Execution of the
b. Execution of
second-operand fields coinciding.
c. For those locations of the first operand of
TRANSLATE where the argument and func
tion values are the same.
The instructions TEST AND SET,
SW
by two
providing an interlocked update. It is possible, how
ever, for a channel to make an access to the same
storage location between the fetch and store por
tions of an interlocked update.
Storage-Operand Consistency
Single-Access References
mal data, storage-operand references are single
access references. A fetch reference is said to be a
single-access reference if the result of the operation
comprises a value fetched in a single access to each
byte of the data field. In the case of overlapping
operands, the location may be accessed once for
each operand. A store-type reference is said to be a
single-access reference if a single store access occurs
to each byte location within the data field. An up
date reference is said to be single-access if the fetch
and store accesses are each single-access.
The storage references associated with the follow
ing instructions are not necessarily single-access
references: the decimal-feature instructions and the
instructions
not a single-access reference, the contents placed at
a byte location are not necessarily the same for each
store access; thus, intermediate results in a single
byte location may be observed by channels or other
When multiple fetch accesses are made to a single
byte that is being changed by a channel or another
which could be obtained by fetching the bits individ
ually. For example, the process used in