<s>
MAC	O
was	O
a	O
Macro	B-Language
assembler	I-Language
for	O
computers	O
of	O
the	O
NORD-1	B-Device
,	O
NORD-10	B-Device
,	O
and	O
ND-100	B-Device
lines	O
from	O
Norsk	O
Data	O
.	O
</s>
<s>
For	O
example	O
to	O
copy	O
the	O
contents	O
of	O
the	O
X	O
register	B-General_Concept
to	O
the	O
A	O
register	B-General_Concept
you	O
would	O
write	O
:	O
</s>
<s>
The	O
SX	O
+	O
SA	O
part	O
would	O
most	O
likely	O
result	O
in	O
either	O
the	O
value	O
of	O
some	O
third	O
register	B-General_Concept
or	O
would	O
overflow	O
so	O
that	O
the	O
operation	O
part	O
of	O
the	O
instruction	O
was	O
modified	O
changing	O
it	O
from	O
copy	O
to	O
some	O
other	O
unintended	O
operation	O
.	O
</s>
<s>
The	O
MON	O
instruction	O
is	O
equivalent	O
to	O
the	O
INT	O
instruction	O
found	O
in	O
Intel	B-Device
CPUs	I-Device
.	O
</s>
<s>
In	O
the	O
latter	O
days	O
of	O
SINTRAN	B-Operating_System
the	O
problem	O
then	O
was	O
to	O
find	O
available	O
codes	O
for	O
these	O
system	O
calls	O
as	O
all	O
256	O
of	O
them	O
had	O
already	O
been	O
taken	O
by	O
several	O
such	O
near	O
identical	O
functions	O
.	O
</s>
<s>
Thus	O
,	O
the	O
extended	O
multi-function	O
monitor	O
calls	O
entered	O
the	O
scene	O
where	O
one	O
monitor	O
call	O
could	O
do	O
a	O
number	O
of	O
functions	O
with	O
a	O
function	O
code	O
specified	O
in	O
a	O
register	B-General_Concept
designating	O
which	O
of	O
its	O
subroutines	O
would	O
be	O
executed	O
.	O
</s>
