<s>
The	O
Commodore	B-Device
1551	I-Device
(	O
originally	O
introduced	O
as	O
the	O
SFS	B-Device
481	I-Device
)	O
is	O
a	O
floppy	B-Device
disk	I-Device
drive	I-Device
for	O
the	O
Commodore	B-Operating_System
Plus/4	I-Operating_System
home	O
computer	O
.	O
</s>
<s>
It	O
resembles	O
a	O
charcoal-colored	O
Commodore	B-Device
1541	I-Device
and	O
plugs	O
into	O
the	O
cartridge	O
port	O
,	O
providing	O
faster	O
access	O
than	O
the	O
C64/1541	O
combination	O
.	O
</s>
<s>
Like	O
the	O
1541	O
,	O
it	O
is	O
a	O
single-sided	O
170-kilobyte	O
drive	O
for	O
5¼	O
"	O
disks	O
,	O
with	O
each	O
disk	O
split	O
into	O
664	O
256-byte	O
blocks	O
available	O
for	O
user	O
data	O
plus	O
19	O
blocks	O
for	O
DOS	O
data	O
and	O
directory	O
;	O
the	O
file	B-Application
system	I-Application
makes	O
each	O
block	O
its	O
own	O
cluster	O
.	O
</s>
<s>
The	O
disk	O
drive	O
uses	O
group	B-Device
coded	I-Device
recording	I-Device
and	O
contains	O
an	O
MOS	B-General_Concept
Technology	I-General_Concept
6510T	I-General_Concept
processor	I-General_Concept
as	O
a	O
disk	B-General_Concept
controller	I-General_Concept
.	O
</s>
<s>
The	O
6510T	O
is	O
a	O
specialized	O
version	O
of	O
the	O
6510	B-General_Concept
processor	O
used	O
in	O
the	O
C64	O
,	O
and	O
it	O
is	O
only	O
used	O
in	O
the	O
1551	O
.	O
</s>
<s>
The	O
DOS	O
limits	O
the	O
number	O
of	O
files	O
per	O
disk	O
to	O
144	O
regardless	O
of	O
the	O
number	O
of	O
free	O
blocks	O
on	O
the	O
disk	O
because	O
the	O
directory	O
is	O
of	O
a	O
fixed	O
size	O
,	O
and	O
the	O
file	B-Application
system	I-Application
does	O
not	O
allow	O
for	O
subdirectories	O
.	O
</s>
