<s>
The	O
IBM	B-Operating_System
Personal	I-Operating_System
Computer	I-Operating_System
AT	I-Operating_System
(	O
model	O
5170	O
,	O
abbreviated	O
as	O
IBM	B-Operating_System
AT	I-Operating_System
or	O
PC/AT	B-Operating_System
)	O
was	O
released	O
in	O
1984	O
as	O
the	O
fourth	O
model	O
in	O
the	O
IBM	B-Device
Personal	I-Device
Computer	I-Device
line	O
,	O
following	O
the	O
IBM	B-Device
PC/XT	I-Device
and	O
its	O
IBM	B-Device
Portable	I-Device
PC	I-Device
variant	O
.	O
</s>
<s>
It	O
was	O
designed	O
around	O
the	O
Intel	B-General_Concept
80286	I-General_Concept
microprocessor	B-Architecture
.	O
</s>
<s>
The	O
AT	O
is	O
IBM	B-Device
PC	I-Device
compatible	O
,	O
with	O
the	O
most	O
significant	O
difference	O
being	O
a	O
move	O
to	O
the	O
80286	B-General_Concept
processor	O
from	O
the	O
8088	O
processor	O
of	O
prior	O
models	O
.	O
</s>
<s>
Like	O
the	O
IBM	B-Device
PC	I-Device
,	O
the	O
AT	O
supported	O
an	O
optional	O
math	O
co-processor	O
chip	O
,	O
the	O
Intel	O
80287	O
,	O
for	O
faster	O
execution	O
of	O
floating	B-Algorithm
point	I-Algorithm
operations	I-Algorithm
.	O
</s>
<s>
In	O
addition	O
,	O
it	O
introduces	O
the	O
AT	B-Architecture
bus	I-Architecture
,	O
later	O
known	O
as	O
the	O
ISA	B-Architecture
bus	I-Architecture
,	O
a	O
16-bit	O
bus	O
with	O
backwards	O
compatibility	O
with	O
8-bit	O
PC-compatible	O
expansion	O
cards	O
.	O
</s>
<s>
The	O
bus	O
also	O
offered	O
fifteen	O
IRQs	B-General_Concept
and	O
seven	O
DMA	B-General_Concept
channels	O
,	O
expanded	O
from	O
eight	O
IRQs	B-General_Concept
and	O
four	O
DMA	B-General_Concept
channels	O
for	O
the	O
PC	O
,	O
achieved	O
by	O
adding	O
another	O
8259A	B-Device
IRQ	B-General_Concept
controller	O
and	O
another	O
8237A	B-Device
DMA	B-General_Concept
controller	I-General_Concept
.	O
</s>
<s>
Some	O
IRQ	B-General_Concept
and	O
DMA	B-General_Concept
channels	O
are	O
used	O
by	O
the	O
motherboard	O
and	O
not	O
exposed	O
on	O
the	O
expansion	O
bus	O
.	O
</s>
<s>
Both	O
dual	O
IRQ	B-General_Concept
and	O
DMA	B-General_Concept
chipsets	O
are	O
cascading	O
which	O
shares	O
the	O
primary	O
pair	O
.	O
</s>
<s>
In	O
addition	O
to	O
these	O
chipsets	O
,	O
Intel	B-General_Concept
82284	I-General_Concept
Clock	O
Driver	O
and	O
Ready	O
Interface	O
and	O
Intel	O
82288	O
Bus	O
Controller	O
are	O
to	O
support	O
the	O
microprocessor	B-Architecture
.	O
</s>
<s>
The	O
24-bit	O
address	O
bus	O
of	O
the	O
286	B-General_Concept
expands	O
RAM	O
capacity	O
to	O
16MB	O
.	O
</s>
<s>
PC	B-Operating_System
DOS	I-Operating_System
3.0	O
was	O
included	O
with	O
support	O
for	O
the	O
new	O
AT	O
features	O
,	O
including	O
preliminary	O
kernel	O
support	O
for	O
networking	O
(	O
which	O
was	O
fully	O
supported	O
in	O
a	O
later	O
version	O
3.x	O
release	O
)	O
.	O
</s>
<s>
The	O
RTC	O
also	O
included	O
a	O
1024Hz	O
timer	O
(	O
on	O
IRQ	B-General_Concept
8	O
)	O
,	O
a	O
much	O
finer	O
resolution	O
than	O
the	O
18Hz	O
timer	O
on	O
the	O
PC	O
.	O
</s>
<s>
In	O
addition	O
to	O
keeping	O
the	O
time	O
,	O
the	O
RTC	O
includes	O
50	O
bytes	O
of	O
CMOS	B-Device
memory	I-Device
which	O
is	O
used	O
to	O
store	O
software-adjustable	O
BIOS	B-Operating_System
parameters	O
.	O
</s>
<s>
A	O
disk-based	O
BIOS	B-Operating_System
setup	O
program	O
which	O
saved	O
to	O
this	O
memory	O
took	O
the	O
place	O
of	O
the	O
DIP	O
switches	O
used	O
to	O
set	O
system	O
settings	O
on	O
PCs	B-Device
.	O
</s>
<s>
Most	O
AT	O
clones	O
have	O
the	O
setup	O
program	O
in	O
ROM	B-Device
rather	O
than	O
on	O
disk	O
.	O
</s>
<s>
The	O
standard	O
floppy	B-Device
drive	I-Device
was	O
upgraded	O
to	O
a	O
1.2MB	O
inch	O
floppy	B-Device
disk	I-Device
drive	I-Device
(	O
15	O
sectors	B-Application
of	O
512	B-Application
bytes	I-Application
,	O
80	O
tracks	O
,	O
two	O
sides	O
)	O
,	O
which	O
stored	O
over	O
three	O
times	O
as	O
much	O
data	O
as	O
the	O
360	O
KB	O
PC	O
floppy	B-Device
disk	I-Device
,	O
but	O
had	O
compatibility	O
problems	O
with	O
360k	B-Device
disks	O
(	O
see	O
Problems	O
below	O
)	O
.	O
</s>
<s>
inch	O
floppy	B-Device
drives	I-Device
became	O
available	O
in	O
later	O
ATs	O
.	O
</s>
<s>
A	O
20MB	O
hard	B-Device
disk	I-Device
drive	I-Device
was	O
included	O
as	O
standard	O
.	O
</s>
<s>
The	O
AT	O
included	O
the	O
AT	B-Device
keyboard	I-Device
,	O
initially	O
a	O
new	O
84-key	O
layout	O
(	O
the	O
84th	O
key	O
being	O
SysRq	B-Device
)	O
.	O
</s>
<s>
The	O
AT	B-Device
keyboard	I-Device
uses	O
the	O
same	O
5-pin	O
DIN	B-Architecture
connector	I-Architecture
as	O
the	O
PC	O
keyboard	O
,	O
but	O
a	O
different	O
,	O
bidirectional	O
electrical	O
interface	O
with	O
different	O
keyboard	O
scan	O
codes	O
.	O
</s>
<s>
The	O
AT	O
is	O
also	O
equipped	O
with	O
a	O
physical	B-General_Concept
lock	I-General_Concept
that	O
prevents	O
access	O
to	O
the	O
computer	O
by	O
disabling	O
the	O
keyboard	O
.	O
</s>
<s>
ATs	O
could	O
be	O
equipped	O
with	O
CGA	B-Device
,	O
MDA	B-Device
,	O
EGA	B-Device
,	O
or	O
PGA	B-Device
video	O
cards	O
.	O
</s>
<s>
The	O
8250	B-Device
UART	I-Device
from	O
the	O
PC	O
was	O
upgraded	O
to	O
the	O
16450	O
,	O
but	O
since	O
both	O
chips	O
had	O
single-byte	O
buffers	O
,	O
high-speed	O
serial	O
communication	O
was	O
problematic	O
as	O
with	O
the	O
XT	O
.	O
</s>
<s>
The	O
IBM	B-Operating_System
PC	I-Operating_System
AT	I-Operating_System
came	O
with	O
a	O
192-watt	O
switching	O
power	O
supply	O
,	O
significantly	O
higher	O
than	O
the	O
130-watt	O
XT	O
power	O
supply	O
.	O
</s>
<s>
To	O
solve	O
this	O
problem	O
,	O
entry-level	O
IBM	B-Operating_System
AT	I-Operating_System
models	O
that	O
did	O
not	O
have	O
a	O
hard	B-Device
drive	I-Device
were	O
shipped	O
with	O
a	O
5-ohm	O
,	O
50-watt	O
resistor	O
connected	O
on	O
the	O
+12V	O
line	O
of	O
the	O
hard	B-Device
disk	I-Device
power	O
connector	O
.	O
</s>
<s>
In	O
addition	O
to	O
the	O
unreliable	O
hard	B-Device
disk	I-Device
drive	I-Device
,	O
the	O
high-density	O
floppy	B-Device
disk	I-Device
drives	I-Device
turned	O
out	O
to	O
be	O
problematic	O
.	O
</s>
<s>
High-density	O
floppy	B-Device
diskette	I-Device
media	O
were	O
compatible	O
only	O
with	O
high-density	O
drives	O
.	O
</s>
<s>
There	O
was	O
no	O
way	O
for	O
the	O
disk	O
drive	O
to	O
detect	O
what	O
kind	O
of	O
floppy	B-Device
disk	I-Device
was	O
inserted	O
,	O
and	O
the	O
drives	O
were	O
not	O
distinguished	O
except	O
by	O
an	O
asterisk	O
molded	O
into	O
the	O
360kB	O
disk	O
drive	O
faceplate	O
.	O
</s>
<s>
If	O
the	O
user	O
accidentally	O
used	O
a	O
high-density	O
diskette	B-Device
in	O
the	O
360kB	O
drive	O
,	O
it	O
would	O
sometimes	O
work	O
,	O
for	O
a	O
while	O
,	O
but	O
the	O
high-coercivity	O
oxide	O
would	O
take	O
a	O
very	O
weak	O
magnetization	O
from	O
the	O
360kB	O
write	O
heads	O
,	O
so	O
reading	O
the	O
diskette	B-Device
would	O
be	O
problematic	O
.	O
</s>
<s>
Due	O
to	O
a	O
US	O
antitrust	O
consent	O
decree	O
with	O
IBM	O
,	O
the	O
PC	B-Operating_System
AT	I-Operating_System
architecture	O
was	O
functionally	O
an	O
open	O
design	O
,	O
and	O
IBM	O
's	O
efforts	O
to	O
trademark	O
the	O
AT	O
name	O
largely	O
failed	O
.	O
</s>
<s>
Many	O
286-based	O
PCs	B-Device
were	O
modeled	O
after	O
it	O
and	O
marketed	O
as	O
"	O
AT-compatible	O
"	O
.	O
</s>
<s>
The	O
label	O
also	O
became	O
a	O
standard	O
term	O
in	O
reference	O
to	O
PCs	B-Device
that	O
used	O
the	O
same	O
type	O
of	O
power	O
supply	O
,	O
case	O
,	O
and	O
motherboard	O
layout	O
as	O
the	O
5170	O
.	O
</s>
<s>
"	O
AT-class	O
"	O
became	O
a	O
term	O
describing	O
any	O
machine	O
which	O
supported	O
the	O
same	O
BIOS	B-Operating_System
functions	O
,	O
80286	B-General_Concept
or	O
greater	O
processor	O
,	O
16-bit	O
expansion	O
slots	O
,	O
keyboard	O
interface	O
,	O
1.2MB	O
inch	O
floppy	B-Device
disk	I-Device
drives	I-Device
and	O
other	O
defining	O
technical	O
features	O
of	O
the	O
IBM	B-Operating_System
PC	I-Operating_System
AT	I-Operating_System
.	O
</s>
<s>
In	O
the	O
United	O
States	O
,	O
popular	O
brands	O
of	O
AT	O
clones	O
included	O
the	O
Tandy	B-Device
3000	I-Device
,	O
Compaq	B-Device
Deskpro	I-Device
286	B-General_Concept
,	O
HP	O
Vectra	O
,	O
Zenith	O
Z-286	O
,	O
Epson	B-Device
Equity	I-Device
Models	O
II+	O
and	O
III	O
,	O
and	O
Commodore	O
PC-30	O
and	O
PC-40	O
.	O
</s>
<s>
The	O
AT	B-Architecture
bus	I-Architecture
became	O
the	O
de	O
facto	O
"	O
ISA	O
"	O
(	O
Industry	B-Architecture
Standard	I-Architecture
Architecture	I-Architecture
)	O
,	O
while	O
PC	B-Device
XT	I-Device
slots	O
were	O
retroactively	O
named	O
"	O
8-bit	O
ISA	O
"	O
.	O
</s>
<s>
The	O
disk	O
interface	O
was	O
standardized	O
as	O
"	O
ATA	O
"	O
which	O
evolved	O
and	O
was	O
later	O
renamed	O
"	O
PATA	B-Protocol
"	O
(	O
Parallel	B-Protocol
AT	I-Protocol
Attachment	I-Protocol
)	O
.	O
</s>
<s>
Further	O
,	O
the	O
same	O
interface	O
was	O
originally	O
named	O
"	O
IDE	O
"	O
after	O
the	O
fact	O
that	O
the	O
drive	O
controller	O
was	O
on	O
the	O
drive	O
(	O
Integrated	B-Protocol
Drive	I-Protocol
Electronics	I-Protocol
)	O
and	O
not	O
on	O
the	O
interface	O
card	O
.	O
</s>
<s>
Creative	B-General_Concept
Computing	I-General_Concept
chose	O
the	O
AT	O
as	O
the	O
best	O
desktop	O
computer	O
when	O
"	O
price	O
is	O
no	O
object	O
"	O
for	O
1984	O
,	O
describing	O
it	O
as	O
"	O
an	O
innovative	O
,	O
state-of-the-art	O
computer	O
that	O
has	O
the	O
competition	O
gasping	O
for	O
breath	O
"	O
.	O
</s>
<s>
An	O
industry	O
analyst	O
wrote	O
in	O
Computerworld	O
in	O
1985	O
that	O
the	O
AT	O
's	O
power	O
was	O
evidence	O
of	O
IBM	O
's	O
belief	O
that	O
personal	B-Device
computers	I-Device
were	O
more	O
important	O
for	O
the	O
company	O
than	O
minicomputers	O
.	O
</s>
