<s>
The	O
PDP-11	B-Device
architecture	I-Device
is	O
a	O
CISC	B-Architecture
instruction	B-General_Concept
set	I-General_Concept
architecture	I-General_Concept
(	O
ISA	O
)	O
developed	O
by	O
Digital	O
Equipment	O
Corporation	O
(	O
DEC	O
)	O
.	O
</s>
<s>
It	O
is	O
implemented	O
by	O
central	B-General_Concept
processing	I-General_Concept
units	I-General_Concept
(	O
CPUs	O
)	O
and	O
microprocessors	B-Architecture
used	O
in	O
PDP-11	B-Device
minicomputers	O
.	O
</s>
<s>
It	O
was	O
in	O
wide	O
use	O
during	O
the	O
1970s	O
,	O
but	O
was	O
eventually	O
overshadowed	O
by	O
the	O
more	O
powerful	O
VAX	B-Device
architecture	O
in	O
the	O
1980s	O
.	O
</s>
<s>
Sixteen-bit	O
words	O
are	O
stored	O
little-endian	O
(	O
with	O
least	O
significant	O
bytes	B-Application
first	O
)	O
.	O
</s>
<s>
Thirty-two-bit	O
data	O
—	O
supported	O
as	B-Device
extensions	O
to	O
the	O
basic	O
architecture	O
,	O
e.g.	O
,	O
floating	B-Algorithm
point	I-Algorithm
in	O
the	O
FPU	O
Instruction	B-General_Concept
Set	I-General_Concept
,	O
double-words	O
in	O
the	O
Extended	O
Instruction	B-General_Concept
Set	I-General_Concept
or	O
long	O
data	O
in	O
the	O
Commercial	O
Instruction	B-General_Concept
Set	I-General_Concept
—	O
are	O
stored	O
in	O
more	O
than	O
one	O
format	O
,	O
including	O
an	O
unusual	O
middle-endian	O
format	O
sometimes	O
referred	O
to	O
as	B-Device
"	O
PDP-endian	O
"	O
.	O
</s>
<s>
The	O
PDP-11	B-Device
'	O
s	O
16-bit	O
addresses	O
can	O
address	O
64KB	O
.	O
</s>
<s>
By	O
the	O
time	O
the	O
PDP-11	B-Device
yielded	O
to	O
the	O
VAX	B-Device
,	O
8-bit	O
bytes	B-Application
and	O
hexadecimal	O
notation	O
were	O
becoming	O
standard	O
in	O
the	O
industry	O
;	O
however	O
,	O
numeric	B-Algorithm
values	O
on	O
the	O
PDP-11	B-Device
always	O
use	O
octal	O
notation	O
,	O
and	O
the	O
amount	O
of	O
memory	O
attached	O
to	O
a	O
PDP-11	B-Device
is	O
always	O
stated	O
as	B-Device
a	O
number	O
of	O
words	O
.	O
</s>
<s>
The	O
basic	O
logical	B-General_Concept
address	I-General_Concept
space	O
is	O
32K	O
words	O
,	O
but	O
the	O
high	O
4K	O
of	O
physical	O
address	O
space	O
(	O
addresses	O
160000	O
through	O
177777	O
in	O
the	O
absence	O
of	O
memory	O
management	O
)	O
are	O
not	O
populated	O
because	O
input/output	O
registers	B-General_Concept
on	O
the	O
bus	O
respond	O
to	O
addresses	O
in	O
that	O
range	O
.	O
</s>
<s>
So	O
originally	O
,	O
a	O
fully	O
expanded	O
PDP-11	B-Device
had	O
28K	O
words	O
,	O
or	O
56	O
kbytes	O
in	O
modern	O
terms	O
.	O
</s>
<s>
The	O
processor	O
reserves	O
low	O
memory	O
addresses	O
for	O
two-word	O
vectors	O
that	O
give	O
a	O
program	B-General_Concept
counter	I-General_Concept
and	O
processor	O
status	O
word	O
with	O
which	O
to	O
begin	O
a	O
service	O
routine	O
.	O
</s>
<s>
When	O
an	O
I/O	O
device	O
interrupts	O
a	O
program	O
,	O
it	O
places	O
the	O
address	O
of	O
its	O
vector	O
on	O
the	O
bus	O
to	O
indicate	O
which	O
service	O
routine	O
should	O
take	O
control	O
.	O
</s>
<s>
Traps	O
occur	O
on	O
some	O
program	O
errors	O
,	O
such	O
as	B-Device
an	O
attempt	O
to	O
execute	O
an	O
undefined	O
instruction	O
;	O
and	O
also	O
when	O
the	O
program	O
executes	O
an	O
instruction	O
such	O
as	B-Device
BPT	O
,	O
EMT	O
,	O
IOT	O
,	O
or	O
TRAP	O
to	O
request	O
service	O
from	O
the	O
operating	O
system	O
.	O
</s>
<s>
During	O
the	O
life	O
of	O
the	O
PDP-11	B-Device
,	O
the	O
16-bit	O
logical	B-General_Concept
address	I-General_Concept
space	O
became	O
an	O
increasing	O
limitation	O
.	O
</s>
<s>
Later-model	O
PDP-11	B-Device
processors	O
include	O
memory	O
management	O
to	O
support	O
virtual	B-General_Concept
addressing	I-General_Concept
.	O
</s>
<s>
The	O
logical	B-General_Concept
address	I-General_Concept
space	O
(	O
that	O
is	O
,	O
the	O
address	O
space	O
available	O
at	O
any	O
moment	O
without	O
changing	O
the	O
memory	O
mapping	O
table	O
)	O
remains	O
limited	O
to	O
16bits	O
.	O
</s>
<s>
Some	O
models	O
,	O
beginning	O
with	O
the	O
PDP-11/45	B-Device
,	O
can	O
be	O
set	O
to	O
use	O
32K	O
words	O
(	O
64KB	O
)	O
as	B-Device
the	O
"	O
instruction	O
space	O
"	O
for	O
program	O
code	O
and	O
a	O
separate	O
32K	O
words	O
of	O
"	O
data	O
space	O
"	O
.	O
</s>
<s>
Some	O
operating	O
systems	O
—	O
notably	O
Unix	B-Application
since	O
edition	O
V7	O
,	O
and	O
RSX11-M	O
+	O
—	O
rely	O
on	O
this	O
feature	O
.	O
</s>
<s>
Programming	O
techniques	O
,	O
such	O
as	B-Device
overlaying	B-General_Concept
a	O
block	O
of	O
stored	O
instructions	O
or	O
data	O
with	O
another	O
as	B-Device
needed	O
,	O
can	O
conceal	O
paging	O
issues	O
from	O
the	O
application	O
programmer	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
Modula-2	B-Language
compiler	O
produces	O
code	O
under	O
which	O
the	O
run-time	O
system	O
swaps	O
8Kb	O
pages	O
into	O
memory	O
as	B-Device
individual	O
procedures	O
receive	O
control	O
.	O
</s>
<s>
The	O
CPU	O
contains	O
eight	O
general-purpose	O
16-bit	O
registers	B-General_Concept
(	O
R0	O
to	O
R7	O
)	O
.	O
</s>
<s>
Register	O
R7	O
is	O
the	O
program	B-General_Concept
counter	I-General_Concept
(	O
PC	O
)	O
.	O
</s>
<s>
Although	O
any	O
register	O
can	O
be	O
used	O
as	B-Device
a	O
stack	O
pointer	O
,	O
R6	O
is	O
the	O
stack	O
pointer	O
(	O
SP	O
)	O
used	O
for	O
hardware	O
interrupts	O
and	O
traps	O
.	O
</s>
<s>
To	O
speed	O
up	O
context	O
switching	O
,	O
some	O
PDP-11	B-Device
models	O
provide	O
dual	O
R1-R5	O
register	O
sets	O
.	O
</s>
<s>
Kernel	O
,	O
Supervisor	O
(	O
where	O
present	O
)	O
,	O
and	O
User	O
modes	O
have	O
separate	O
memory	O
maps	O
,	O
and	O
also	O
separate	O
stack	O
pointers	O
(	O
so	O
that	O
a	O
user	O
program	O
cannot	O
cause	O
the	O
system	O
to	O
malfunction	O
by	O
storing	O
an	O
invalid	O
value	O
in	O
the	O
stack	O
pointer	B-General_Concept
register	I-General_Concept
)	O
.	O
</s>
<s>
The	O
encoding	O
of	O
the	O
sixbit	O
operand	O
addressing	O
mode	O
is	O
as	B-Device
follows	O
:	O
</s>
<s>
In	O
the	O
following	O
sections	O
,	O
each	O
item	O
includes	O
an	O
example	O
of	O
how	O
the	O
operand	O
would	O
be	O
written	O
in	O
assembly	B-Language
language	I-Language
.	O
</s>
<s>
Rn	O
means	O
one	O
of	O
the	O
eight	O
registers	B-General_Concept
,	O
written	O
R0	O
through	O
R7	O
.	O
</s>
<s>
Their	O
effects	O
when	O
applied	O
to	O
R6	O
(	O
the	O
stack	O
pointer	O
,	O
SP	O
)	O
and	O
R7	O
(	O
the	O
program	B-General_Concept
counter	I-General_Concept
,	O
PC	O
)	O
are	O
set	O
out	O
separately	O
in	O
the	O
following	O
sections	O
.	O
</s>
<s>
Autoincrement	O
and	O
autodecrement	O
operations	O
on	O
a	O
register	O
are	O
by	O
1	O
in	O
byte	B-Application
instructions	O
,	O
by	O
2	O
in	O
word	O
instructions	O
,	O
and	O
by	O
2	O
whenever	O
a	O
deferred	O
mode	O
is	O
used	O
,	O
since	O
the	O
quantity	O
the	O
register	O
addresses	O
is	O
a	O
(	O
word	O
)	O
pointer	O
.	O
</s>
<s>
When	O
R7	O
(	O
the	O
program	B-General_Concept
counter	I-General_Concept
)	O
is	O
specified	O
,	O
four	O
of	O
the	O
addressing	O
modes	O
naturally	O
yield	O
useful	O
effects	O
:	O
</s>
<s>
The	O
only	O
common	O
use	O
of	O
absolute	O
mode	O
,	O
whose	O
syntax	O
combines	O
immediate	O
and	O
deferred	O
mode	O
,	O
is	O
to	O
specify	O
input/output	O
registers	B-General_Concept
,	O
as	B-Device
the	O
registers	B-General_Concept
for	O
each	O
device	O
have	O
specific	O
memory	O
addresses	O
.	O
</s>
<s>
A	O
program	O
that	O
uses	O
relative	O
mode	O
(	O
and	O
relative	O
deferred	O
mode	O
)	O
exclusively	O
for	O
internal	O
references	O
is	O
position-independent	B-Operating_System
;	O
it	O
contains	O
no	O
assumptions	O
about	O
its	O
own	O
location	O
,	O
so	O
it	O
can	O
be	O
loaded	O
into	O
an	O
arbitrary	O
memory	O
location	O
,	O
or	O
even	O
moved	O
,	O
with	O
no	O
need	O
for	O
its	O
addresses	O
to	O
be	O
adjusted	O
to	O
reflect	O
its	O
location	O
(	O
relocated	B-Library
)	O
.	O
</s>
<s>
In	O
computing	O
such	O
addresses	O
relative	O
to	O
the	O
current	O
location	O
,	O
the	O
processor	O
performed	O
relocation	B-Library
on	O
the	O
fly	O
.	O
</s>
<s>
When	O
the	O
auxiliary	O
word	O
is	O
"	O
in	O
the	O
instruction	O
"	O
as	B-Device
the	O
above	O
table	O
says	O
,	O
the	O
PC	O
for	O
the	O
next	B-General_Concept
instruction	I-General_Concept
is	O
automatically	O
incremented	O
past	O
the	O
auxiliary	O
word	O
.	O
</s>
<s>
As	B-Device
PC	O
always	O
points	O
to	O
words	O
,	O
the	O
autoincrement	O
operation	O
is	O
always	O
by	O
2	O
.	O
</s>
<s>
R6	O
,	O
also	O
written	O
SP	O
,	O
is	O
used	O
as	B-Device
a	O
hardware	O
stack	O
for	O
traps	O
and	O
interrupts	O
.	O
</s>
<s>
A	O
convention	O
enforced	O
by	O
the	O
set	O
of	O
modes	O
the	O
PDP-11	B-Device
provides	O
is	O
that	O
a	O
stack	O
grows	O
downward	O
—	O
toward	O
lower	O
addresses	O
—	O
as	B-Device
items	O
are	O
pushed	O
onto	O
it	O
.	O
</s>
<s>
When	O
a	O
mode	O
is	O
applied	O
to	O
SP	O
,	O
or	O
to	O
any	O
register	O
the	O
programmer	O
elects	O
to	O
use	O
as	B-Device
a	O
software	O
stack	O
,	O
the	O
addressing	O
modes	O
have	O
the	O
following	O
effects	O
:	O
</s>
<s>
Although	O
software	O
stacks	O
can	O
contain	O
bytes	B-Application
,	O
SP	O
is	O
always	O
a	O
stack	O
of	O
words	O
.	O
</s>
<s>
The	O
PDP-11	B-Device
operates	O
on	O
bytes	B-Application
and	O
words	O
.	O
</s>
<s>
Bytes	B-Application
are	O
specified	O
by	O
a	O
register	O
number	O
—	O
identifying	O
the	O
register	O
's	O
low-order	O
byte	B-Application
—	O
or	O
by	O
a	O
memory	O
location	O
.	O
</s>
<s>
Words	O
are	O
specified	O
by	O
a	O
register	O
number	O
or	O
by	O
the	O
memory	O
location	O
of	O
the	O
low-order	O
byte	B-Application
,	O
which	O
must	O
be	O
an	O
even	O
number	O
.	O
</s>
<s>
In	O
most	O
instructions	O
that	O
take	O
operands	O
,	O
bit	O
15	O
is	O
set	O
to	O
specify	O
byte	B-Application
addressing	O
,	O
or	O
clear	O
to	O
specify	O
word	O
addressing	O
.	O
</s>
<s>
In	O
the	O
lists	O
in	O
the	O
following	O
two	O
sections	O
,	O
the	O
assembly-language	O
programmer	O
appended	O
B	O
to	O
the	O
instruction	O
symbol	O
to	O
specify	O
a	O
byte	B-Application
operation	O
;	O
for	O
example	O
,	O
MOV	O
became	O
MOVB	O
.	O
</s>
<s>
A	O
few	O
instructions	O
,	O
for	O
example	O
MARK	O
and	O
SOB	O
,	O
were	O
not	O
implemented	O
on	O
some	O
PDP-11	B-Device
models	O
.	O
</s>
<s>
The	O
high-order	O
four	O
bits	O
specify	O
the	O
operation	O
to	O
be	O
performed	O
(	O
with	O
bit	O
15	O
generally	O
selecting	O
word	O
versus	O
byte	B-Application
addressing	O
)	O
.	O
</s>
<s>
Two	O
groups	O
of	O
six	O
bits	O
specify	O
the	O
source	O
operand	O
addressing	O
mode	O
and	O
the	O
destination	O
operand	O
addressing	O
mode	O
,	O
as	B-Device
defined	O
above	O
.	O
</s>
<s>
The	O
ADD	O
and	O
SUB	O
instructions	O
use	O
word	O
addressing	O
,	O
and	O
have	O
no	O
byte-oriented	O
variations	O
.	O
</s>
<s>
Where	O
a	O
register	O
pair	O
is	O
used	O
(	O
written	O
below	O
as	B-Device
"	O
(	O
Reg	O
,	O
Reg+1	O
)	O
"	O
,	O
the	O
first	O
register	O
contains	O
the	O
low-order	O
portion	O
of	O
the	O
operand	O
and	O
must	O
be	O
an	O
even	O
numbered	O
register	O
.	O
</s>
<s>
The	O
high-order	O
ten	O
bits	O
specify	O
the	O
operation	O
to	O
be	O
performed	O
,	O
with	O
bit	O
15	O
generally	O
selecting	O
byte	B-Application
versus	O
word	O
addressing	O
.	O
</s>
<s>
A	O
single	O
group	O
of	O
six	O
bits	O
specifies	O
the	O
operand	O
as	B-Device
defined	O
above	O
.	O
</s>
<s>
In	O
contrast	O
to	O
Intel	O
processors	O
in	O
the	O
x86	B-Operating_System
architecture	I-Operating_System
,	O
MOV	O
instructions	O
set	O
them	O
too	O
,	O
so	O
a	O
branch	O
instruction	O
could	O
be	O
used	O
to	O
branch	O
depending	O
on	O
whether	O
the	O
value	O
moved	O
was	O
zero	O
or	O
negative	O
.	O
</s>
<s>
The	O
high-order	O
byte	B-Application
of	O
the	O
instruction	O
specifies	O
the	O
operation	O
.	O
</s>
<s>
Bits	O
9	O
through	O
15	O
are	O
the	O
op-code	O
,	O
and	O
bit	O
8	O
is	O
the	O
value	O
of	O
the	O
condition	B-General_Concept
code	I-General_Concept
calculation	O
which	O
results	O
in	O
the	O
branch	O
being	O
taken	O
.	O
</s>
<s>
The	O
low-order	O
byte	B-Application
is	O
a	O
signed	O
word	O
offset	O
relative	O
to	O
the	O
current	O
location	O
of	O
the	O
program	B-General_Concept
counter	I-General_Concept
.	O
</s>
<s>
The	O
limited	O
range	O
of	O
the	O
branch	O
instructions	O
meant	O
that	O
,	O
as	B-Device
code	O
grew	O
,	O
the	O
target	O
addresses	O
of	O
some	O
branches	O
would	O
become	O
unreachable	O
.	O
</s>
<s>
As	B-Device
JMP	O
has	O
no	O
conditional	O
forms	O
,	O
the	O
programmer	O
would	O
change	O
BEQ	O
to	O
a	O
BNE	O
that	O
branched	O
around	O
a	O
JMP	O
.	O
</s>
<s>
Programs	O
that	O
did	O
not	O
need	O
this	O
feature	O
specified	O
PC	O
as	B-Device
the	O
register	O
(	O
JSR	O
PC	O
,	O
address	O
)	O
and	O
the	O
routine	O
returned	O
using	O
RTS	O
PC	O
.	O
</s>
<s>
C	B-Algorithm
indicating	O
a	O
carry	O
condition	O
.	O
</s>
<s>
Instructions	O
in	O
this	O
group	O
were	O
what	O
Digital	O
called	O
"	O
micro-programmed	O
"	O
:	O
A	O
single	O
bit	O
in	O
the	O
instruction	O
word	O
referenced	O
a	O
single	O
condition	B-General_Concept
code	I-General_Concept
.	O
</s>
<s>
The	O
assembler	B-Language
did	O
not	O
define	O
syntax	O
to	O
specify	O
every	O
combination	O
,	O
but	O
the	O
symbols	O
SCC	O
and	O
CCC	O
assembled	O
an	O
instruction	O
that	O
set	O
or	O
cleared	O
,	O
respectively	O
,	O
all	O
four	O
condition	O
codes	O
.	O
</s>
<s>
Clearing	O
or	O
setting	O
none	O
of	O
the	O
condition	O
codes	O
(	O
opcodes	O
000240	O
and	O
000260	O
,	O
respectively	O
)	O
could	O
effectively	O
be	O
considered	O
as	B-Device
no-operation	O
instructions	O
.	O
</s>
<s>
Over	O
the	O
life	O
of	O
the	O
PDP-11	B-Device
,	O
subtle	O
differences	O
arose	O
in	O
the	O
implementation	O
of	O
instructions	O
and	O
combinations	O
of	O
addressing	O
modes	O
,	O
though	O
no	O
implementation	O
was	O
regarded	O
as	B-Device
correct	O
.	O
</s>
<s>
The	O
inconsistencies	O
did	O
not	O
affect	O
ordinary	O
use	O
of	O
the	O
PDP-11	B-Device
.	O
</s>
<s>
The	O
FIS	O
instruction	B-General_Concept
set	I-General_Concept
is	O
an	O
option	O
for	O
the	O
PDP-11/35/40	O
and	O
11/03	O
.	O
</s>
<s>
A	O
three	O
bit	O
field	O
specifies	O
which	O
register	O
is	O
used	O
as	B-Device
the	O
floating	B-Algorithm
point	I-Algorithm
operand	O
stack	O
pointer	O
.	O
</s>
<s>
Each	O
float	B-Algorithm
is	O
two	O
words	O
and	O
each	O
floating	B-Algorithm
point	I-Algorithm
instruction	O
operates	O
on	O
two	O
floats	B-Algorithm
,	O
returning	O
one	O
float	B-Algorithm
as	B-Device
a	O
result	O
.	O
</s>
<s>
This	O
was	O
the	O
optional	O
floating	B-Algorithm
point	I-Algorithm
processor	O
option	O
for	O
11/45	O
and	O
most	O
subsequent	O
models	O
.	O
</s>
<s>
The	O
Commercial	O
Instruction	B-General_Concept
Set	I-General_Concept
,	O
known	O
as	B-Device
CIS	O
or	O
CIS11	O
,	O
adds	O
string	O
and	O
binary	O
coded	O
decimal	O
(	O
BCD	O
)	O
instructions	O
used	O
by	O
COBOL	B-Application
and	O
DIBOL	B-Language
.	O
</s>
<s>
Strings	O
are	O
represented	O
by	O
two	O
16-bit	O
integers	O
stored	O
in	O
any	O
two	O
of	O
the	O
general-purpose	O
registers	B-General_Concept
,	O
or	O
as	B-Device
two	O
16-bit	O
values	O
in	O
subsequent	O
locations	O
in	O
memory	O
.	O
</s>
<s>
One	O
is	O
designated	O
"	O
n	B-Algorithm
"	O
,	O
which	O
is	O
the	O
length	O
up	O
to	O
64kB	O
,	O
and	O
the	O
other	O
"	O
A	O
"	O
,	O
which	O
is	O
a	O
pointer	O
to	O
the	O
start	O
of	O
the	O
character	O
data	O
in	O
memory	O
.	O
</s>
<s>
Together	O
,	O
an	O
n/A	O
pair	O
indicates	O
the	O
location	O
and	O
length	O
of	O
the	O
string	O
.	O
</s>
<s>
The	O
basic	O
operations	O
are	O
MOVEC	O
,	O
MOVTC	O
and	O
MOVRC	O
,	O
which	O
move	O
the	O
character	O
data	O
in	O
memory	O
from	O
the	O
location	O
indicated	O
in	O
one	O
n/A	O
pair	O
to	O
the	O
location	O
in	O
a	O
second	O
n/A	O
.	O
</s>
<s>
MOVECI	O
,	O
MOVTCI	O
and	O
MOVRCI	O
did	O
the	O
same	O
but	O
with	O
the	O
locations	O
indicated	O
by	O
n/A	O
pairs	O
in	O
memory	O
instead	O
of	O
registers	B-General_Concept
.	O
</s>
<s>
MOVTC/MOVTCI	O
translates	O
characters	O
during	O
copy	O
using	O
a	O
256-byte	O
lookup	O
table	O
held	O
in	O
a	O
third	O
n/A	O
pair	O
,	O
with	O
the	O
A	O
pointing	O
to	O
the	O
start	O
of	O
the	O
table	O
and	O
the	O
lower	O
eight	O
bits	O
of	O
n	B-Algorithm
being	O
an	O
ASCII	O
code	O
used	O
to	O
fill	O
the	O
destination	O
string	O
if	O
the	O
source	O
string	O
is	O
shorter	O
.	O
</s>
<s>
Translations	O
use	O
the	O
source	O
string	O
's	O
ASCII	O
value	O
as	B-Device
an	O
index	O
number	O
and	O
copy	O
the	O
value	O
in	O
the	O
translation	O
table	O
at	O
that	O
index	O
into	O
the	O
destination	O
string	O
.	O
</s>
<s>
This	O
can	O
be	O
used	O
for	O
EBCDIC	B-Language
conversions	O
,	O
for	O
instance	O
,	O
by	O
placing	O
the	O
corresponding	O
EBCDIC	B-Language
character	O
code	O
for	O
the	O
mapped	O
ASCII	O
codes	O
in	O
the	O
table	O
.	O
</s>
<s>
The	O
character	O
"	O
E	O
"	O
is	O
character	O
69	O
in	O
ASCII	O
and	O
197	O
in	O
EBCDIC	B-Language
,	O
so	O
to	O
convert	O
EBCDIC	B-Language
to	O
ASCII	O
one	O
would	O
make	O
a	O
table	O
of	O
256	O
bytes	B-Application
with	O
a	O
69	O
in	O
location	O
197	O
.	O
</s>
<s>
Character	O
sets	O
are	O
a	O
table	O
of	O
256	O
bytes	B-Application
,	O
split	O
into	O
subsets	O
.	O
</s>
<s>
This	O
data	O
is	O
also	O
represented	O
by	O
two	O
16-bit	O
registers	B-General_Concept
or	O
memory	O
locations	O
,	O
with	O
the	O
second	O
number	O
being	O
the	O
A	O
identical	O
to	O
the	O
string	O
case	O
.	O
</s>
<s>
DEC	O
referred	O
to	O
unpacked	O
data	O
,	O
with	O
one	O
digit	O
per	O
byte	B-Application
,	O
as	B-Device
"	O
numeric	B-Algorithm
strings	O
"	O
.	O
</s>
<s>
Using	O
packed	O
data	O
,	O
with	O
two	O
BCD	O
digits	O
per	O
byte	B-Application
,	O
a	O
16-word	O
string	O
held	O
BDC	O
numbers	O
up	O
to	O
32	O
digits	O
long	O
.	O
</s>
<s>
It	O
also	O
includes	O
a	O
set	O
of	O
instructions	O
to	O
convert	O
BCD	O
numbers	O
between	O
packed	O
and	O
unpacked	O
formats	O
,	O
as	B-Device
well	O
as	B-Device
to	O
and	O
from	O
binary	O
values	O
.	O
</s>
<s>
A	O
final	O
set	O
of	O
instructions	O
is	O
used	O
to	O
load	O
string	O
and	O
BCD	O
data	O
to	O
and	O
from	O
the	O
internal	O
registers	B-General_Concept
,	O
avoiding	O
the	O
need	O
for	O
bit-fiddling	O
in	O
code	O
.	O
</s>
<s>
Instructions	O
found	O
on	O
all	O
but	O
the	O
earliest	O
PDP-11s	B-Device
give	O
programs	O
more	O
direct	O
access	O
to	O
the	O
register	O
.	O
</s>
<s>
On	O
PDP-11s	B-Device
that	O
provide	O
multiple	O
instruction	O
spaces	O
and	O
data	O
spaces	O
,	O
a	O
set	O
of	O
non-orthogonal	O
Move	O
instructions	O
give	O
access	O
to	O
other	O
spaces	O
.	O
</s>
<s>
PDP-11	B-Device
processor	O
speed	O
varies	O
by	O
model	O
,	O
memory	O
configuration	O
,	O
op	O
code	O
,	O
and	O
addressing	O
modes	O
.	O
</s>
<s>
For	O
example	O
,	O
on	O
the	O
PDP-11/70	B-Device
(	O
circa	O
1975	O
)	O
,	O
an	O
instruction	O
of	O
the	O
form	O
ADDx(Rm )	O
,	O
y(Rn )	O
had	O
a	O
fetch/execute	O
time	O
of	O
1.35	O
microseconds	O
plus	O
source	O
and	O
destination	O
times	O
of	O
0.6	O
microseconds	O
each	O
,	O
for	O
a	O
total	O
instruction	O
time	O
of	O
2.55	O
microseconds	O
.	O
</s>
<s>
Floating	B-Algorithm
point	I-Algorithm
is	O
even	O
more	O
complex	O
,	O
since	O
there	O
is	O
some	O
overlap	O
between	O
the	O
CPU	O
and	O
the	O
floating-point	B-Algorithm
processor	O
,	O
but	O
in	O
general	O
,	O
floating	B-Algorithm
point	I-Algorithm
is	O
significantly	O
slower	O
.	O
</s>
<s>
The	O
PDP-11	B-Device
operates	O
at	O
a	O
priority	O
level	O
from	O
0	O
through	O
7	O
,	O
specified	O
by	O
three	O
bits	O
in	O
the	O
Processor	O
Status	O
Word	O
(	O
PSW	O
)	O
,	O
and	O
high-end	O
models	O
can	O
operate	O
in	O
a	O
choice	O
of	O
modes	O
,	O
Kernel	O
(	O
privileged	O
)	O
,	O
User	O
(	O
application	O
)	O
,	O
and	O
sometimes	O
Supervisor	O
,	O
according	O
to	O
two	O
bits	O
in	O
the	O
PSW	O
.	O
</s>
<s>
The	O
printer	O
's	O
readiness	O
for	O
another	O
character	O
is	O
the	O
lowest	O
priority	O
(	O
BR4	O
)	O
,	O
as	B-Device
it	O
can	O
remain	O
ready	O
indefinitely	O
.	O
</s>
<s>
The	O
bus-grant	O
lines	O
are	O
not	O
common	O
lines	O
but	O
are	O
a	O
daisy	B-Application
chain	I-Application
:	O
The	O
input	O
of	O
each	O
gate	O
is	O
the	O
output	O
of	O
the	O
previous	O
gate	O
in	O
the	O
chain	O
.	O
</s>
<s>
A	O
gate	O
is	O
on	O
each	O
bus	O
device	O
,	O
and	O
a	O
device	O
physically	O
closer	O
to	O
the	O
processor	O
is	O
earlier	O
in	O
the	O
daisy	B-Application
chain	I-Application
.	O
</s>
<s>
The	O
processor	O
saves	O
the	O
program	B-General_Concept
counter	I-General_Concept
(	O
PC	O
)	O
and	O
PSW	O
,	O
enters	O
Kernel	O
mode	O
,	O
and	O
loads	O
new	O
values	O
from	O
the	O
specified	O
vector	O
.	O
</s>
<s>
For	O
a	O
device	O
at	O
BR6	O
,	O
the	O
new	O
PSW	O
in	O
its	O
vector	O
typically	O
specifies	O
6	O
as	B-Device
the	O
new	O
processor	O
priority	O
,	O
so	O
the	O
processor	O
will	O
honor	O
more	O
urgent	O
requests	O
(	O
BR7	O
)	O
during	O
the	O
service	O
routine	O
,	O
but	O
defer	O
requests	O
of	O
the	O
same	O
or	O
lower	O
priority	O
.	O
</s>
<s>
The	O
routine	O
ends	O
with	O
the	O
RTI	O
(	O
ReTurn	O
from	O
Interrupt	O
)	O
instruction	O
,	O
which	O
restores	O
PC	O
and	O
PSW	O
as	B-Device
of	O
just	O
before	O
the	O
processor	O
granted	O
the	O
interrupt	O
.	O
</s>
<s>
MACRO-11	B-Device
is	O
the	O
assembly	B-Language
language	I-Language
for	O
the	O
PDP-11	B-Device
.	O
</s>
<s>
It	O
is	O
the	O
successor	O
to	O
PAL-11	B-Device
(	O
Program	O
Assembler	B-Language
Loader	O
)	O
,	O
an	O
earlier	O
version	O
of	O
the	O
PDP-11	B-Device
assembly	B-Language
language	I-Language
without	O
macro	O
facilities	O
.	O
</s>
<s>
MACRO-11	B-Device
is	O
supported	O
on	O
all	O
DEC	B-Device
PDP-11	I-Device
operating	O
systems	O
.	O
</s>
<s>
PDP-11	B-Device
Unix	B-Application
systems	I-Application
also	O
include	O
an	O
assembler	B-Language
(	O
called	O
"	O
as	B-Device
"	O
)	O
,	O
structurally	O
similar	O
to	O
MACRO-11	B-Device
,	O
but	O
with	O
different	O
syntax	O
and	O
fewer	O
features	O
.	O
</s>
