Rapid
Context
Switching. When responding
to
a new
set
of
interrupt-initiated
circumstances, a computer system must
preserve
the
current
operating
environment, for
continuance
later,
whi Ie setting up
the
new environment. This
changing
of environments must be done
quickly,
with a minimum of
II
overhead
ll
time costs.
Anyone
of the four blocks of
general-purpose
arithmetic
registers
can,
if desired, be
as-
signed to a
specific
environment. All
relevant
information
about
the
current
environment (instruction address,
current
general
register
block,
memory-protection key,
etc.)
is
kept in
the
program status words. A single instruction
stores
the
current
program status words anywhere in memory
and loads new ones from memory
to
establish a new
en-
vironment, which includes information identifying a new
block of
general-purpose
registers. Thus, the system's
operating
environment
can
be preserved and
changed
com-
pletely
through
the
execution
of a single instruction.
Memory Protection. Both foreground
(real-time)
and
back-
ground
can
run
concurrently
in
the
system because a
fore-
ground program
is
protected
against
destruction
by
an
un-
checked
background program. Under operating system
control,
the memory
access-protection
feature
prevents
accessing
memory for
specified
combinations of
reading,
writing,
and instruction
acquisition.
Variable
Precision Arithmetic. Much of
the
data
encoun-
tered
in
real-time
systems
are
16 bits or less.
To
process
this
data
efficiently,
both halfword and fullword
arithmetic
operations
are
provided. For
extended
precision,
double-
word
arithmetic
operations
are
also
included.
Direct
Input/Output.
For handling asynchronous
I/O,
a
32-bitword
can
be transferred
direct!y
between
any
genera!-
purpose register and
external
devices.
Reliability,
Maintainability,
Availability.
The
capabil-
ities
described
in the
section,
II
Time-Sharing Features
ll
apply
equally
to
the
real-time
environment.
MULTIUSE
FEATURES
As
implemented in this system, IImultiuse
ll
combines two or
more
application
areas.
The
real-time
application
is
the
most
difficult
general
computing task because of its severe
requirements. Similarly,
another
difficult
multiuse task is
a time-sharing
application
that
includes one or more
real-
time processes. Because
the
system is designed on a
real-
time base,
it
is
qualified
for a mixture of
applications
in a
multiuse environment. Many hardware features
that
prove
valuable
for Certain
application
areas
are
equally
USeful
in
others, although in
different
ways. This multiple
capa-
bility
makes
the
system
particularly
effective
in
multi-
use
applications.
The major multiuse features
are
described in
the
follow-
i
ng
paragraphs.
6 Multiuse Features/Multiprocessor Features
Priority Interrupt System. In a multiuse environment, many
elements
operate
simulatneously and asynchronously. Thus,
an
efficient
priority interrupt system is essential.
It
allows
the
computer system
to
respond
quickly,
and
in proper
or-
der,
to the many demands made on
it,
with
attendant
im-
provements in resource
efficiency.
Quick
Response. The many features
that
combine to
pro-
duce
a
quick-response
system (multiple register blocks,
rapid
context
saving, multiple push-pull operations)
benefit
all users because more of the system's resources
are
readi
Iy
avai
lable
at
any
instant.
Memory Protection. The memory protection features
protect
each
user from every other user and
guarantee
the
integrity
of programs essential to
critical
real-time
applications.
Input/Output.
Because of
the
wide range of
capacities
and
speeds,
the
I/O
system simultaneously satisfies
the
needs of
many
different
application
areas
economically,
both in
terms of equipment and programming.
Instruction Set. The comprehensive instruction
set
provides
the
computational
and
data-handling
capabilities
required
for widely differing
application
areasi
therefore,
each
user's
program length
and
running time is minimized,
and
the
throughput is maximized.
MULTIPROCESSOR
FEATURES
System design
readily
permits expansion
to
shared memory
in a multiprocessor system. The system
can
contain
a
com-
bination
of functional clusters,
each
of which in turn may
contain
multiple processors. The total number of clusters
is restricted to the maximum port limitation of six. All
pro-
cessors
ina
system may share common memory.
The following paragraphs describe
the
major multiprocessor
features of
the
system,
MULTIPROCESSOR
INTERLOCK
In a multiprocessor system, the basic processors often need
exclusive
control of a system resource. This resource may
be a region of memory, a
particular
peripheral
device,
or,
in some
cases,
a
specific
software process. There
isa
special
instruction
to
provide this required multiprocessor interlock.
This special instruction,
LOAD
AND
SET,
unconditionally
sets a
11111
bit
inthe
sign position of
the
referenced
memory
location
during
the
restore
cycle
of the memory operation.
If
this
bit
had been previously set by
another
processor, the
interlock
is said
to
be
IIset" and
the
testing program
pro-
ceeds
to
another
task.
On
the
other hand, if the sign bit
of
the
tested
location
is a
zero,
the resource is
allocated
to
the testing processor,
and
simultaneously
the
interlock
is
set for
any
other processor.
To Next Page
Loading...
Need help?
General
