<s>
In	O
computing	O
base	B-General_Concept
and	I-General_Concept
bounds	I-General_Concept
refers	O
to	O
a	O
simple	O
form	O
of	O
virtual	B-Architecture
memory	I-Architecture
where	O
access	O
to	O
computer	B-General_Concept
memory	I-General_Concept
is	O
controlled	O
by	O
one	O
or	O
a	O
small	O
number	O
of	O
sets	O
of	O
processor	B-General_Concept
registers	I-General_Concept
called	O
base	B-General_Concept
and	I-General_Concept
bounds	I-General_Concept
registers	O
.	O
</s>
<s>
In	O
its	O
simplest	O
form	O
each	O
user	O
process	B-Operating_System
is	O
assigned	O
a	O
single	O
contiguous	O
segment	O
of	O
main	O
memory	O
.	O
</s>
<s>
The	O
operating	B-General_Concept
system	I-General_Concept
loads	O
the	O
physical	B-General_Concept
address	I-General_Concept
of	O
this	O
segment	O
into	O
a	O
base	O
register	O
and	O
its	O
size	O
into	O
a	O
bound	O
register	O
.	O
</s>
<s>
Virtual	O
addresses	O
seen	O
by	O
the	O
program	O
are	O
added	O
to	O
the	O
contents	O
of	O
the	O
base	O
register	O
to	O
generate	O
the	O
physical	B-General_Concept
address	I-General_Concept
.	O
</s>
<s>
The	O
address	O
is	O
checked	O
against	O
the	O
contents	O
of	O
the	O
bounds	O
register	O
to	O
prevent	O
a	O
process	B-Operating_System
from	O
accessing	O
memory	O
beyond	O
its	O
assigned	O
segment	O
.	O
</s>
<s>
The	O
operating	B-General_Concept
system	I-General_Concept
is	O
not	O
constrained	O
by	O
the	O
hardware	O
and	O
can	O
access	O
all	O
of	O
physical	O
memory	O
.	O
</s>
<s>
This	O
technique	O
protects	O
memory	O
used	O
by	O
one	O
process	B-Operating_System
against	O
access	O
or	O
modification	O
by	O
another	O
.	O
</s>
<s>
By	O
itself	O
it	O
does	O
not	O
protect	O
memory	O
from	O
erroneous	O
access	O
by	O
the	O
owning	O
process	B-Operating_System
.	O
</s>
<s>
It	O
also	O
allows	O
programs	O
to	O
be	O
easily	O
relocated	B-Library
in	O
memory	O
,	O
since	O
only	O
the	O
base	B-General_Concept
and	I-General_Concept
bounds	I-General_Concept
registers	O
have	O
to	O
be	O
modified	O
when	O
the	O
program	O
is	O
moved	O
.	O
</s>
<s>
Some	O
computer	O
systems	O
extended	O
this	O
mechanism	O
to	O
multiple	O
segments	O
,	O
such	O
as	O
the	O
i	O
bank	O
and	O
d	O
bank	O
for	O
instructions	O
and	O
data	O
on	O
the	O
UNIVAC	B-Device
1100	I-Device
series	I-Device
computers	O
or	O
the	O
separation	O
of	O
memory	O
on	O
the	O
DEC	B-Device
PDP-10	I-Device
system	O
into	O
a	O
read/write	O
"	O
low	O
"	O
segment	O
for	O
the	O
user	O
process	B-Operating_System
and	O
a	O
read-only	O
"	O
high	O
"	O
segment	O
for	O
sharable	O
code	O
.	O
</s>
<s>
Segmented	O
virtual	B-Architecture
memory	I-Architecture
is	O
a	O
further	O
generalization	O
of	O
this	O
mechanism	O
to	O
a	O
large	O
number	O
of	O
segments	O
.	O
</s>
