By TNL: GN22-0498
tion as part of the execution of the current instruc
tion, and such loading can occur when the wait state
is spedfied. Similarly, information from a segment
table or page-table entry having a format error may
be
More than one copy of a table entry may exist in
the TLB. For example, some implementations may
cause a copy of a valid table entry to be placed in
the TLB for each segment-table address by which
the entry becomes attached, and some implementa
tions may cause a valid page-table entry to be placed
in the TLB for each attached and valid segment
table entry by which the page-table entry is desig
nated.
Modification of Translation Tables
When an inactive, attached, and invalid table entry is
made valid, the change takes effect immediately.
Similarly, when an inactive, unattached, and valid
page-table entry is made attached by making an
inactive, attached, and invalid segment-table entry
valid, the change takes effect immediately.
However, since the exceptions associated with
dynamic address translation may be established by a
pretest for operand accessibility that is performed as
part of the initiation of the execution of the instruc
tion,
ception may be indicated on the basis of the state of
the table entry at the start of the execution of an
instruction. Consequently, a segment-translation or
page-translation exception may be indicated when a
table entry is invalid at the start of execution even if
the instruction would have validated the table entry
it
if the instruction were considered to process the
operands one byte at a time. See the section
"Recognition of Access Exceptions" in the chapter
"Interruptions" for the recognition of dynamic
addre:ss-translation exceptions for the interruptible
instructions.
A
for implicit translation any time from the instant of
the change through the completion of the following
purging of the TLB.
another value suitable for translation, or to a value
that prohibits its use for translation, any subsequent
attempt to use the entry for implicit address transla
tion before the TLB is purged may yield unpredicta
ble
tween instructions or during execution of an instruc
tion, including the instruction that caused the
change. Moreover, until the TLB is purged, the TLB
may
is unpredictable whether the old or new value is se-
66
lected for a particular access. If the use of the new
value of the entry causes an exception, the exception
mayor may not cause an interruption to occur, and
if an interruption does occur, the instruction execu
tion may be terminated even though the exception
would normally cause suppression or nullification.
Manipulation of attached table entries may cause
spurious table-entry values to be recorded in a TLB.
For example, if changes are made piecemeal, modifi
cation of a valid attached entry may cause a partially
updated entry to be recorded, and, if an intermediate
value is introduced in the process of the change, a
supposedly invalid entry may temporarily appear
valid and may be recorded in the TLB. Such an in
termediate value may be introduced if the change is
made by an
intermediate value is introduced during the execution
of a single instruction.
When
size, page size, segment-table address, or segment
table length, the values of these fields at the start of
the operation are in effect for the duration of the
operation.
The relation between the states of table entries
and their use is summarized in the table
Translation
changes an attached table entry, including a change
that makes the entry invalid, and subsequently uses
the entry for implicit translation, a changed entry is
being used without a prior purging of the TLB, and
the associated unpredictability of result values and
of exception indication applies.
All modifications to translation tables by the pro
gram should consider the effect of the TLB on the
use of the tables in main storage and the possible
effects of intermediate result values and of piece
meal changes. The following rules are recommended
for changing translation tables. If these rules are
observed, translation is performed as if the table
entries from main storage were always used for the
translation process.
1. An entry must not be changed, other than
changing the low-order bit of a page-table en
try, while it is being used by any
other than a change to the low-order bit of a
page-table entry, each
is active must issue
change occurs and prior to the use of the entry
for implicit address translation by that