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KEY
CCNCEPTS
Since
a
windCM
is
referenced
like
memory, any
individual
transfer
may
be
between
an
obj
ect
and
PS
memory,
an
object
and a
PS
processor
register,
or
an
object
and
an
I/O
device.
The
latter
may
be
appealing
from
the
standpoint
of
"efficiency,"
rut
it
should
be
used
with
caution.
Using a
windCM
to
directly
"connect"
an
I/O
device
and an
object
in
432
memory
has
the
undesirable
effect
of
propagating
the
real-time
constraints
imposed by
the
device
beyond
the
subsystem
boundary
into
the
432
system.
It
may
seriously
complicate
error
recovery
as
well.
Finally,
since
there
is
a
finite
number
of
windCMs,
JOOst
applications
will
need
to
manage them
as
scarce
resources
which
will
not
always
be
instantly
available.
This
means
that
at
least
some
I/O
device
transfers
will
have
to
be
buffered
in
PS
memory
until
a windoo becomes
available.
It
may
be
simplest
to
buffer
all
I/O
device
transfers
in
memory, and
use
the
windows
to
transfer.
data
between
PS
memory
and 432
system
memory.
There
are
four
IP
windows which
may
be
mapped
onto
four
different
objects.
The
I/O
controller
may
alter
the
windCMS
during
execution
to
obtain
access
to
different
objects.
References
to
windowed
subranges
may
be
interleaved
and
may
be
driven
by
different
agents
in
the
Peripheral
Subsystem.
For
example,
the
Attached
Processor
and a
J:I.1A
controller
may
be
driving
transfers
concurrently,
subject
to
the
same
bus
arbitration
constraints
that
would
apply
if
they
were
accessing
memory.
FUNCrICNS
A
fifth
window,
the
control
windCM,
provides
the
IP
controller
with
access
to
the
Interface
Processor's
_function
request
facility.
The
IP
oontroller
requests
the
execution
of
an
IP
function
by
wr i
ting
operands
and
an
opcode
into
predefined
locations
in
the
control
windCM's
subrange.
This
procedure
is
very
similar
to
writing
comnands and
data
to
a memory-roapped
peripheral
controller
(e.g.,
floppy
disk
controller).
Up:>n
CC>11pletion
of
the
function,
the
IP
interrupts
the
AP
and
provides
status
information
which
the
IP
controller
can
read
through
the
control
window. The
IP
can
respond
to
transfer
requests
to
the
other
four
windows
while
it
is
executing
a
function.
In
addition,
data
transfers
through
windows 0
through
3
may
be
interleaved
wi.th
function
request
sequences
through
the
oontrol
window.
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