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00
meaning
for
windows
2-4,
which suWOrt
random
transfers
to
432
system
memory
only;
the
random
transfer
mode
is
described
in
section
3-2.
Attempting
to
set
the
transfer
mode
of
windows
2-4
will
cause
a
fault.
OVERIAY
Sane
Peripheral
Subsystems
(e.g.,
those
based on
processors
with
limi
ted
address
spaces) may
not
be
able
to
dedicate
a block
of
memory
space
for
exclusive
use
as
IP
window
subranges. Such systems
may
elect
to
co-Iocate
all
or
part
of
the
IP'
s range
with
real
PS
memory.
If
a
window
is
then
opened
with
the
over
lay
attr
ibute,
the
IP
will
inhibit
the
co-located
memory
fran
responding
to
memory
references
in
the
subrange. Closing a
window
that
overlaid
menory
re-enables
the
memory
to
respond
to
subsequent
address
references
in
that
subrange. Thus,
when
the
IP
and
PS
merrory
both
occupy
the
same
addresses,
memory
will
respond
to
all
references
except
those
that
fall
in
the
subrange
of
a
window
open
with
the
overlay
attribute.
Figure
3-1
illustrates
a
hypothetical
configuration
in
which a bank
of
memory
and an
Interface
Processor both occupy a
64K
byte
block
of
addresses
in
the
Peripheral
Subsystem
memory
space. A
window
with
a
subrange base address
of
32K
and a subrange
size
of
4K
has been
opened
with
the
overlay
attribute
set.
Any
address
reference
falling
in
the
subrange
will
cause
the
IP
to
respond
rather
than
the
co-Iocated
memory.
Any
address
reference
outside
the
subrange
will
select
the
memory
rather
than
the
IP.
The
overlay
facility
is
~lemented
by
an
inhibit
signal
that
the
IP
asserts
when
it
recognizes an
address
reference
that
falls
in
an
overlaid
subrange. (See
the
iAPX
43203
Interface
Processor
Data
Sheet,
Order
No.
171874,
for
a
description
of
this
signal).
Use
of
the
overlay
facility
slows IP response time somewhat.
Note
that
opening a
window
with
the
overlay
attribute
set
when
there
is
00
co-located
memory
is
safe,
but
it
slows IP response
unnecessarily.
On
the
other
hand, opening a
window
without
specifying
overlay
when
there
is
co-Iocated
memory
will
produce an
undefined
result
when
both
components attempt
to
respond
to
a
subsequent address
reference
that
falls
in
the
overlaid
subrange.
3-7