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Page States
A page may be in one of three states:
• Disconnected
• Connected
• Addressable
If
disconnected, the page does not have a page
frame assigned to it. Any attempt by the CPU to
access a disconnected page causes a page-access
exception or, when a CLEAR PAGE instruction is
being executed, a page-state exception. Any
attempt by a channel to access a disconnected
page, in order to fetch a CCW or to access a data
area designated during the execution of a CCW,
creates an I/O-interruption condition indicating
protection check. However, if a CCW is
prefetched, a protection check is not indicated until
the C CW is due to be executed.
If
connected, the page has a page frame assigned
to it. A connected page may be accessed, if the
protection mechanism permits:
1. By
I/O
channels
2. By the CPU as the operand of the CLEAR
PAGE instruction
Except when executing CLEAR PAGE, an
attempt by the CPU to access a connected page
causes a page-access exception.
If
addressable, the page has a page frame
assigned to it, and the CPU and
I/O
channels may
access the page if the protection mechanism
permits.
Although the addressable state implies that the
page is also connected, the term "connected state"
refers specifically to the state of a page that is
neither addressable nor disconnected.
The page state is checked for all storage accesses
to locations that are explicitly or implicitly- accessed
by the CPU or by a channel.
The page state is changed by instructions, which
may make the transition from the disconnected to
the connected state and from the connected to the
addressable state, or vice versa. The instructions
are CONNECT PAGE, DECONFIGURE PAGE,
DISCONNECT PAGE, MAKE ADDRESSABLE,
and MAKE UNADDRESSABLE. Most of these
instructions may also be applied to pages which are
already in the desired state. An attempt to change
directly from disconnected to addressable, or vice
versa, causes a page-transition exception. A
page-transition exception is also caused by
DECONFIGURE PAGE when applied to a
disconnected page.
The first page, page 0, containing byte locations
o
to 2047, is always addressable. It cannot be
placed in the connected or disconnected state.
The clear-reset function causes a number of
consecutive pages, starting with page 0, to be
assigned to page frames, cleared, and placed in the
addressable state. (See the section "Clear Reset"
in Chapter 4, "Control. ")
Programming Notes
1. The three page states permit programs to
distinguish pages in the disconnected state,
which cannot be accessed at all, from pages in
the connected state, which are undergoing
I/O
operations to or from auxiliary page storage,
and from pages in the addressable state, which
are ready for normal sto'rage access.
2. The storage-control instructions other than
CLEAR PAGE operate on page descriptions,
not pages. Instructions which operate on page
descriptions do not make storage accesses and
do not cause page-access or protection
exceptions.
3. All channel accesses to storage appear as if
they referred to pages and not to the associated
page frames.
If
a page that is being accessed
by a channel becomes disconnected and
another channel access is attempted, protection
check is indicated, and the
I/O
operation
terminates.
If
the page becomes disconnected
and then becomes reconnected before
protection check is indicated, protection check
may be indicated subsequently, or accesses may
continue using the newly assigned page frame.
Frame Index
A unique 16-bit unsigned binary integer is assigned
to each page frame existing in the machine. This
integer is the frame index of the page frame. The
value of the frame index ranges from zero up to,
but not including, the existing . . . frame-capacity count
(EFCe) (see the section "CapaCity Counts" in this
chapter).
When a CONNECT PAGE instruction connects
a page to a frame, the frame index of the
connected frame is returned by the instruction.
The frame index remains associated with that page
until the page is disconnected. When the same
page is connected again, the new frame index is, in
general, different and unpredictable unless there
was only one free frame remaining.
The frame index for an already connected or
addressable page may be displayed by LOAD
FRAME INDEX.
When DECONFIGURE PAGE makes a page
frame unavailable, the frame index of that frame
will not recur until a clear-reset operation is
performed.
Chapter 3. Storage
3-5
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