<s>
The	O
Burroughs	B-General_Concept
B6x00-7x00	I-General_Concept
instruction	I-General_Concept
set	I-General_Concept
includes	O
the	O
set	O
of	O
valid	O
operations	O
for	O
the	O
Burroughs	O
B6500	O
,	O
</s>
<s>
B7500	O
and	O
later	O
Burroughs	B-Device
large	I-Device
systems	I-Device
,	O
including	O
the	O
current	O
(	O
as	O
of	O
2006	O
)	O
Unisys	O
Clearpath/MCP	O
systems	O
;	O
it	O
does	O
not	O
include	O
the	O
instruction	O
for	O
other	O
Burroughs	B-Device
large	I-Device
systems	I-Device
including	O
the	O
B5000	O
,	O
B5500	O
,	O
B5700	O
and	O
the	O
B8500	O
.	O
</s>
<s>
These	O
unique	O
machines	O
have	O
a	O
distinctive	O
design	O
and	O
instruction	B-General_Concept
set	I-General_Concept
.	O
</s>
<s>
Further	O
,	O
the	O
machines	O
are	O
stack	B-General_Concept
based	O
to	O
the	O
point	O
that	O
they	O
had	O
no	O
user-addressable	O
registers	B-General_Concept
.	O
</s>
<s>
As	O
you	O
would	O
expect	O
from	O
the	O
unique	O
architecture	O
used	O
in	O
these	O
systems	O
,	O
they	O
also	O
have	O
an	O
interesting	O
instruction	B-General_Concept
set	I-General_Concept
.	O
</s>
<s>
The	O
Name	O
Call	O
and	O
Value	O
Call	O
syllables	O
contain	O
address	O
couples	O
;	O
the	O
Operator	O
syllables	O
either	O
use	O
no	O
addresses	O
or	O
use	O
control	O
words	O
and	O
descriptors	O
on	O
the	O
stack	B-General_Concept
.	O
</s>
<s>
Since	O
there	O
are	O
no	O
programmer-addressable	O
registers	B-General_Concept
,	O
most	O
of	O
the	O
register	O
manipulating	O
operations	O
required	O
in	O
other	O
architectures	O
are	O
not	O
needed	O
,	O
nor	O
are	O
variants	O
for	O
performing	O
operations	O
between	O
pairs	O
of	O
registers	B-General_Concept
,	O
since	O
all	O
operations	O
are	O
applied	O
to	O
the	O
top	O
of	O
the	O
stack	B-General_Concept
.	O
</s>
<s>
This	O
also	O
makes	O
code	O
files	O
very	O
compact	O
,	O
since	O
operators	O
are	O
zero-address	O
and	O
do	O
not	O
need	O
to	O
include	O
the	O
address	O
of	O
registers	B-General_Concept
or	O
memory	O
locations	O
in	O
the	O
code	O
stream	O
.	O
</s>
<s>
Many	O
of	O
the	O
operators	O
are	O
generic	O
or	O
polymorphic	B-Application
depending	O
on	O
the	O
kind	O
of	O
data	O
being	O
acted	O
on	O
as	O
given	O
by	O
the	O
tag	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
instruction	B-General_Concept
set	I-General_Concept
has	O
only	O
one	O
ADD	O
operator	O
.	O
</s>
<s>
Two	O
operators	O
are	O
important	O
in	O
the	O
handling	O
of	O
on-stack	O
data	O
Value	O
Call	O
(	O
VALC	O
)	O
and	O
Name	O
Call	O
(	O
NAMC	O
)	O
.	O
</s>
<s>
VALC	O
is	O
another	O
polymorphic	B-Application
operator	O
.	O
</s>
<s>
If	O
it	O
hits	O
a	O
data	O
word	O
,	O
that	O
word	O
is	O
loaded	O
to	O
the	O
top	O
of	O
stack	B-General_Concept
.	O
</s>
<s>
NAMC	O
simply	O
loads	O
the	O
address	O
couple	O
onto	O
the	O
top	O
of	O
the	O
stack	B-General_Concept
as	O
an	O
IRW	O
(	O
with	O
the	O
tag	O
automatically	O
set	O
to	O
1	O
)	O
.	O
</s>
<s>
In	O
the	O
following	O
operator	O
explanations	O
remember	O
that	O
A	O
and	O
B	O
are	O
the	O
top	O
two	O
stack	B-General_Concept
registers	B-General_Concept
.	O
</s>
<s>
Double	O
precision	O
extensions	O
are	O
provided	O
by	O
the	O
X	O
and	O
Y	O
registers	B-General_Concept
;	O
thus	O
the	O
top	O
two	O
double	O
precision	O
operands	O
are	O
given	O
by	O
AX	O
and	O
BY	O
.	O
</s>
<s>
Since	O
these	O
words	O
are	O
created	O
and	O
consumed	O
by	O
specific	O
instructions	O
or	O
the	O
hardware	O
,	O
the	O
exact	O
format	O
of	O
these	O
words	O
can	O
change	O
between	O
hardware	O
implementation	O
and	O
user	O
programs	O
do	O
not	O
need	O
to	O
be	O
recompiled	B-Language
,	O
since	O
the	O
same	O
code	O
stream	O
will	O
produce	O
the	O
same	O
results	O
,	O
even	O
though	O
system	O
word	O
format	O
may	O
have	O
changed	O
.	O
</s>
<s>
Tag	O
1	O
words	O
represent	O
on-stack	O
data	O
addresses	O
.	O
</s>
<s>
The	O
normal	O
IRW	O
simply	O
stores	O
an	O
address	O
couple	O
to	O
data	O
on	O
the	O
current	O
stack	B-General_Concept
.	O
</s>
<s>
The	O
SIRW	O
references	O
data	O
on	O
any	O
stack	B-General_Concept
by	O
including	O
a	O
stack	B-General_Concept
number	O
in	O
the	O
address	O
.	O
</s>
<s>
Tag	O
5	O
words	O
represent	O
off-stack	O
data	O
addresses	O
.	O
</s>
<s>
The	O
D[1]	O
environment	O
is	O
not	O
stored	O
on	O
the	O
current	O
stack	B-General_Concept
because	O
it	O
can	O
be	O
referenced	O
by	O
all	O
processes	O
sharing	O
this	O
code	O
.	O
</s>
<s>
Tag	O
3	O
represents	O
code	O
words	O
themselves	O
,	O
which	O
wo	O
n't	O
occur	O
on	O
the	O
stack	B-General_Concept
.	O
</s>
<s>
Tag	O
3	O
is	O
also	O
used	O
for	O
the	O
stack	B-General_Concept
control	O
words	O
MSCW	O
,	O
RCW	O
,	O
TOSCW	O
.	O
</s>
<s>
A	O
stack	B-General_Concept
hardware	O
optimization	O
is	O
the	O
provision	O
of	O
D	O
(	O
or	O
"	O
display	O
"	O
)	O
registers	B-General_Concept
.	O
</s>
<s>
These	O
are	O
registers	B-General_Concept
that	O
point	O
to	O
the	O
start	O
of	O
each	O
called	O
stack	B-General_Concept
frame	O
.	O
</s>
<s>
These	O
registers	B-General_Concept
are	O
updated	O
automatically	O
as	O
procedures	O
are	O
entered	O
and	O
exited	O
and	O
are	O
not	O
accessible	O
by	O
any	O
software	O
.	O
</s>
<s>
There	O
are	O
32	O
D	O
registers	B-General_Concept
,	O
which	O
is	O
what	O
limits	O
to	O
32	O
levels	O
of	O
lexical	O
nesting	O
.	O
</s>
<s>
If	O
we	O
do	O
n't	O
have	O
D	O
registers	B-General_Concept
,	O
we	O
have	O
to	O
look	O
at	O
the	O
control	O
word	O
at	O
the	O
base	O
of	O
the	O
D[5]	O
frame	O
,	O
which	O
points	O
to	O
the	O
frame	O
containing	O
the	O
D[4]	O
environment	O
.	O
</s>
<s>
(	O
The	O
architecture	O
keeps	O
both	O
the	O
data	O
stack	B-General_Concept
and	O
the	O
call	B-General_Concept
stack	I-General_Concept
in	O
the	O
same	O
structure	O
,	O
but	O
uses	O
control	O
words	O
to	O
tell	O
them	O
apart	O
.	O
)	O
</s>
<s>
With	O
D	O
registers	B-General_Concept
,	O
the	O
D[2]	O
register	O
points	O
at	O
the	O
base	O
of	O
the	O
lexical	O
level	O
2	O
environment	O
,	O
and	O
all	O
we	O
need	O
to	O
do	O
to	O
generate	O
the	O
address	O
of	O
the	O
variable	O
is	O
to	O
add	O
its	O
offset	O
from	O
the	O
stack	B-General_Concept
frame	O
base	O
to	O
the	O
frame	O
base	O
address	O
in	O
the	O
D	O
register	O
.	O
</s>
<s>
(	O
There	O
is	O
an	O
efficient	O
linked	O
list	O
search	O
operator	O
LLLU	O
,	O
which	O
could	O
search	O
the	O
stack	B-General_Concept
in	O
the	O
above	O
fashion	O
,	O
but	O
the	O
D	O
register	O
approach	O
is	O
still	O
going	O
to	O
be	O
faster	O
.	O
)	O
</s>
<s>
With	O
D	O
registers	B-General_Concept
,	O
access	O
to	O
entities	O
in	O
outer	O
and	O
global	O
environments	O
is	O
just	O
as	O
efficient	O
as	O
local	O
variable	O
access	O
.	O
</s>
<s>
|	O
D[4]==>3	O
|	O
MSCW	O
|	O
(	O
4	O
,	O
0	O
)	O
The	O
Mark	O
Stack	B-General_Concept
Control	O
Word	O
containing	O
the	O
link	O
to	O
D[3]	O
.	O
</s>
<s>
|	O
D[3]==>3	O
|	O
MSCW	O
|	O
(	O
3	O
,	O
0	O
)	O
The	O
Mark	O
Stack	B-General_Concept
Control	O
Word	O
containing	O
the	O
link	O
to	O
D[2]	O
.	O
</s>
<s>
|	O
D[2]==>3	O
|	O
MSCW	O
|	O
(	O
2	O
,	O
0	O
)	O
The	O
Mark	O
Stack	B-General_Concept
Control	O
Word	O
containing	O
the	O
link	O
to	O
the	O
previous	O
stack	B-General_Concept
frame	O
.	O
</s>
<s>
If	O
we	O
had	O
invoked	O
the	O
procedure	O
p	O
as	O
a	O
coroutine	O
,	O
or	O
a	O
process	O
instruction	O
,	O
the	O
D[3]	O
environment	O
would	O
have	O
become	O
a	O
separate	O
D[3]-based	O
stack	B-General_Concept
.	O
</s>
<s>
Taking	O
this	O
one	O
step	O
further	O
,	O
a	O
totally	O
different	O
program	O
could	O
call	O
another	O
program	O
's	O
code	O
,	O
creating	O
a	O
D[3]	O
stack	B-General_Concept
frame	O
pointing	O
to	O
another	O
process	O
 '	O
D[2]	O
environment	O
on	O
top	O
of	O
its	O
own	O
process	B-General_Concept
stack	I-General_Concept
.	O
</s>
<s>
At	O
an	O
instant	O
the	O
whole	O
address	O
space	O
from	O
the	O
code	O
's	O
execution	O
environment	O
changes	O
,	O
making	O
the	O
D[2]	O
environment	O
on	O
the	O
own	O
process	B-General_Concept
stack	I-General_Concept
not	O
directly	O
addressable	O
and	O
instead	O
make	O
the	O
D[2]	O
environment	O
in	O
another	O
process	B-General_Concept
stack	I-General_Concept
directly	O
addressable	O
.	O
</s>
<s>
At	O
such	O
a	O
cross-stack	O
call	O
,	O
the	O
calling	O
code	O
and	O
called	O
code	O
could	O
even	O
originate	O
from	O
programs	O
written	O
in	O
different	O
source	O
languages	O
and	O
be	O
compiled	B-Language
by	O
different	O
compilers	B-Language
.	O
</s>
<s>
The	O
D[1]	O
and	O
D[0]	O
environments	O
do	O
not	O
occur	O
in	O
the	O
current	O
process	O
's	O
stack	B-General_Concept
.	O
</s>
<s>
Stack	B-General_Concept
frames	O
actually	O
do	O
n't	O
even	O
have	O
to	O
exist	O
in	O
a	O
process	B-General_Concept
stack	I-General_Concept
.	O
</s>
<s>
This	O
feature	O
was	O
used	O
early	O
on	O
for	O
file	O
I/O	O
optimization	O
,	O
the	O
FIB	O
(	O
file	O
information	O
block	O
)	O
was	O
linked	O
into	O
the	O
display	O
registers	B-General_Concept
at	O
D[1]	O
during	O
I/O	O
operations	O
.	O
</s>
<s>
On	O
other	O
systems	O
,	O
the	O
compiler	B-Language
might	O
build	O
its	O
symbol	O
table	O
in	O
a	O
similar	O
manner	O
,	O
but	O
eventually	O
the	O
storage	O
requirements	O
would	O
be	O
collated	O
and	O
the	O
machine	O
code	O
would	O
be	O
written	O
to	O
use	O
flat	O
memory	O
addresses	O
of	O
16-bits	O
or	O
32-bits	O
or	O
even	O
64-bits	O
.	O
</s>
<s>
At	O
each	O
lexical	O
level	O
,	O
variables	O
were	O
placed	O
at	O
displacements	O
up	O
from	O
the	O
base	O
of	O
the	O
level	O
's	O
stack	B-General_Concept
,	O
typically	O
occupying	O
one	O
word	O
-	O
double	O
precision	O
or	O
complex	O
variables	O
would	O
occupy	O
two	O
.	O
</s>
<s>
And	O
indeed	O
,	O
this	O
was	O
reflected	O
in	O
the	O
form	O
of	O
the	O
VALC	O
instruction	O
(	O
value	O
call	O
)	O
that	O
loaded	O
an	O
operand	O
onto	O
the	O
stack	B-General_Concept
.	O
</s>
<s>
Further	O
,	O
only	O
the	O
VALC	O
opcode	O
loaded	O
data	O
:	O
opcodes	O
for	O
ADD	O
,	O
MULT	O
and	O
so	O
forth	O
did	O
no	O
addressing	O
,	O
working	O
entirely	O
on	O
the	O
top	O
elements	O
of	O
the	O
stack	B-General_Concept
.	O
</s>
<s>
Offsets	O
from	O
a	O
specified	O
D-register	O
would	O
be	O
checked	O
by	O
the	O
hardware	O
against	O
the	O
stack	B-General_Concept
frame	O
bound	O
:	O
rogue	O
values	O
would	O
be	O
trapped	O
.	O
</s>
<s>
Mark	O
stack	B-General_Concept
(	O
build	O
a	O
new	O
stack	B-General_Concept
frame	O
resulting	O
in	O
an	O
MSCW	O
on	O
the	O
top	O
,	O
</s>
<s>
Convert	O
an	O
IRW	O
as	O
placed	O
by	O
NAMC	O
into	O
an	O
SIRW	O
which	O
references	O
data	O
in	O
another	O
stack	B-General_Concept
.	O
</s>
<s>
Delete	O
the	O
value	O
off	O
the	O
stack	B-General_Concept
.	O
</s>
<s>
The	O
Load	O
instruction	O
could	O
find	O
itself	O
tripping	O
on	O
an	O
indirect	B-Language
address	I-Language
,	O
or	O
worse	O
,	O
a	O
disguised	O
call	O
to	O
a	O
call-by-name	O
thunk	B-Application
routine	O
.	O
</s>
<s>
Load	O
the	O
value	O
given	O
by	O
the	O
address	O
(	O
tag	O
5	O
or	O
tag	O
1	O
word	O
)	O
on	O
the	O
top	O
of	O
stack	B-General_Concept
.	O
</s>
<s>
All	O
these	O
operators	O
are	O
protected	O
from	O
buffer	B-General_Concept
overflows	I-General_Concept
by	O
being	O
limited	O
by	O
the	O
bounds	O
in	O
the	O
descriptors	O
.	O
</s>
<s>
These	O
were	O
used	O
for	O
scanning	O
strings	O
useful	O
in	O
writing	O
compilers	B-Language
.	O
</s>
<s>
All	O
these	O
operators	O
are	O
protected	O
from	O
buffer	B-General_Concept
overflows	I-General_Concept
by	O
being	O
limited	O
by	O
the	O
bounds	O
in	O
the	O
descriptors	O
.	O
</s>
