<s>
A	O
Special	B-Device
Function	I-Device
Register	I-Device
(	O
or	O
Special	O
Purpose	O
Register	B-General_Concept
,	O
or	O
simply	O
Special	O
Register	B-General_Concept
)	O
is	O
a	O
register	B-General_Concept
within	O
a	O
microprocessor	B-Architecture
,	O
which	O
controls	O
or	O
monitors	O
various	O
aspects	O
of	O
the	O
microprocessor	B-Architecture
's	O
function	O
.	O
</s>
<s>
Depending	O
on	O
the	O
processor	B-Architecture
architecture	I-Architecture
,	O
this	O
can	O
include	O
,	O
but	O
is	O
not	O
limited	O
to	O
:	O
</s>
<s>
For	O
example	O
,	O
the	O
program	B-General_Concept
counter	I-General_Concept
is	O
not	O
directly	O
writeable	O
in	O
many	O
processor	O
architectures	O
.	O
</s>
<s>
Instead	O
,	O
the	O
programmer	O
uses	O
instructions	O
such	O
as	O
return	O
from	O
subroutine	O
,	O
jump	O
,	O
or	O
branch	O
to	O
modify	O
the	O
program	B-General_Concept
counter	I-General_Concept
.	O
</s>
<s>
For	O
another	O
example	O
,	O
the	O
condition	O
code	O
register	B-General_Concept
might	O
not	O
directly	O
writable	O
,	O
instead	O
being	O
updated	O
only	O
by	O
compare	O
instructions	O
.	O
</s>
<s>
Some	O
of	O
SFR	O
(	O
Special	B-Device
Function	I-Device
Register	I-Device
)	O
bits	O
may	O
be	O
set	O
directly	O
using	O
SETB/LDB	O
instructions	O
on	O
proper	O
address	O
,	O
whereas	O
others	O
may	O
require	O
usage	O
of	O
specific	O
instructions	O
.	O
</s>
<s>
The	O
Intel	O
80196	O
class	O
microcontroller	O
has	O
24	O
SFRs	O
,	O
each	O
1	O
Byte	O
in	O
size	O
;	O
standard	O
Intel	B-Architecture
8051	I-Architecture
chips	O
have	O
21	O
SFRs	O
.	O
</s>
<s>
The	O
Special	B-Device
Function	I-Device
Register	I-Device
(	O
SFR	O
)	O
is	O
the	O
upper	O
area	O
of	O
addressable	O
memory	O
,	O
from	O
address	O
0x80	O
to	O
0xFF	O
.	O
</s>
<s>
All	O
port	O
input	B-General_Concept
and	I-General_Concept
output	I-General_Concept
can	O
therefore	O
be	O
performed	O
by	O
memory	O
move	O
operations	O
on	O
specified	O
addresses	O
in	O
the	O
SFR	O
.	O
</s>
<s>
Also	O
,	O
different	O
status	O
registers	O
are	O
mapped	O
into	O
the	O
SFR	O
,	O
for	O
use	O
in	O
checking	O
the	O
status	O
of	O
the	O
8051	B-Architecture
,	O
and	O
changing	O
some	O
operational	O
parameters	O
of	O
the	O
8051	B-Architecture
.	O
</s>
