<s>
In	O
computing	O
,	O
booting	B-Operating_System
is	O
the	O
process	O
of	O
starting	O
a	O
computer	O
as	O
initiated	O
via	O
hardware	B-Architecture
such	O
as	O
a	O
button	O
or	O
by	O
a	O
software	O
command	O
.	O
</s>
<s>
After	O
it	O
is	O
switched	O
on	O
,	O
a	O
computer	O
's	O
central	B-General_Concept
processing	I-General_Concept
unit	I-General_Concept
(	O
CPU	O
)	O
has	O
no	O
software	O
in	O
its	O
main	B-General_Concept
memory	I-General_Concept
,	O
so	O
some	O
process	O
must	O
load	O
software	O
into	O
memory	O
before	O
it	O
can	O
be	O
executed	O
.	O
</s>
<s>
This	O
may	O
be	O
done	O
by	O
hardware	B-Architecture
or	O
firmware	B-Application
in	O
the	O
CPU	O
,	O
or	O
by	O
a	O
separate	O
processor	O
in	O
the	O
computer	O
system	O
.	O
</s>
<s>
Restarting	O
a	O
computer	O
also	O
is	O
called	O
rebooting	B-Device
,	O
which	O
can	O
be	O
"	O
hard	O
"	O
,	O
e.g.	O
</s>
<s>
On	O
some	O
systems	O
,	O
a	O
soft	B-Operating_System
boot	I-Operating_System
may	O
optionally	O
clear	O
RAM	B-Architecture
to	O
zero	O
.	O
</s>
<s>
Both	O
hard	O
and	O
soft	O
booting	B-Operating_System
can	O
be	O
initiated	O
by	O
hardware	B-Architecture
such	O
as	O
a	O
button	O
press	O
or	O
by	O
a	O
software	O
command	O
.	O
</s>
<s>
Booting	B-Operating_System
is	O
complete	O
when	O
the	O
operative	O
runtime	B-Device
system	I-Device
,	O
typically	O
the	O
operating	B-General_Concept
system	I-General_Concept
and	O
some	O
applications	O
,	O
is	O
attained	O
.	O
</s>
<s>
The	O
process	O
of	O
returning	O
a	O
computer	O
from	O
a	O
state	O
of	O
sleep	O
(	O
suspension	O
)	O
does	O
not	O
involve	O
booting	B-Operating_System
;	O
however	O
,	O
restoring	O
it	O
from	O
a	O
state	O
of	O
hibernation	O
does	O
.	O
</s>
<s>
Minimally	O
,	O
some	O
embedded	B-Architecture
systems	I-Architecture
do	O
not	O
require	O
a	O
noticeable	O
boot	B-Operating_System
sequence	I-Operating_System
to	O
begin	O
functioning	O
and	O
when	O
turned	O
on	O
may	O
simply	O
run	O
operational	O
programs	O
that	O
are	O
stored	O
in	O
ROM	B-Device
.	O
</s>
<s>
All	O
computing	O
systems	O
are	O
state	B-Architecture
machines	I-Architecture
,	O
and	O
a	O
reboot	B-Device
may	O
be	O
the	O
only	O
method	O
to	O
return	O
to	O
a	O
designated	O
zero-state	O
from	O
an	O
unintended	O
,	O
locked	O
state	O
.	O
</s>
<s>
In	O
addition	O
to	O
loading	O
an	O
operating	B-General_Concept
system	I-General_Concept
or	O
stand-alone	O
utility	O
,	O
the	O
boot	B-Operating_System
process	I-Operating_System
can	O
also	O
load	O
a	O
storage	O
dump	O
program	O
for	O
diagnosing	O
problems	O
in	O
an	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
Boot	B-Operating_System
is	O
short	O
for	O
bootstrap	O
or	O
bootstrap	O
load	O
and	O
derives	O
from	O
the	O
phrase	O
to	O
pull	O
oneself	O
up	O
by	O
one	O
's	O
bootstraps	O
.	O
</s>
<s>
The	O
invention	O
of	O
read-only	B-Device
memory	I-Device
(	O
ROM	B-Device
)	O
of	O
various	O
types	O
solved	O
this	O
paradox	O
by	O
allowing	O
computers	O
to	O
be	O
shipped	O
with	O
a	O
start	O
up	O
program	O
that	O
could	O
not	O
be	O
erased	O
.	O
</s>
<s>
Growth	O
in	O
the	O
capacity	O
of	O
ROM	B-Device
has	O
allowed	O
ever	O
more	O
elaborate	O
start	O
up	O
procedures	O
to	O
be	O
implemented	O
.	O
</s>
<s>
An	O
early	O
computer	O
,	O
ENIAC	B-Device
,	O
had	O
no	O
program	O
stored	O
in	O
memory	O
,	O
but	O
was	O
set	O
up	O
for	O
each	O
problem	O
by	O
a	O
configuration	O
of	O
interconnecting	O
cables	O
.	O
</s>
<s>
Bootstrapping	B-Application
did	O
not	O
apply	O
to	O
ENIAC	B-Device
,	O
whose	O
hardware	B-Architecture
configuration	O
was	O
ready	O
for	O
solving	O
problems	O
as	O
soon	O
as	O
power	O
was	O
applied	O
.	O
</s>
<s>
The	O
EDSAC	B-Device
system	O
,	O
the	O
second	O
stored-program	O
computer	O
to	O
be	O
built	O
,	O
used	O
stepping	O
switches	O
to	O
transfer	O
a	O
fixed	O
program	O
into	O
memory	O
when	O
its	O
start	O
button	O
was	O
pressed	O
.	O
</s>
<s>
The	O
first	O
programmable	O
computers	O
for	O
commercial	O
sale	O
,	O
such	O
as	O
the	B-Operating_System
UNIVAC	I-Operating_System
I	O
and	O
the	O
IBM	B-Device
701	I-Device
included	O
features	O
to	O
make	O
their	O
operation	O
simpler	O
.	O
</s>
<s>
The	O
same	O
hardware	B-Architecture
logic	O
could	O
be	O
used	O
to	O
load	O
the	O
contents	O
of	O
a	O
punch	B-Architecture
card	I-Architecture
(	O
the	O
most	O
typical	O
ones	O
)	O
or	O
other	O
input	O
media	O
,	O
such	O
as	O
a	O
magnetic	B-General_Concept
drum	I-General_Concept
or	O
magnetic	O
tape	O
,	O
that	O
contained	O
a	O
bootstrap	B-Operating_System
program	I-Operating_System
by	O
pressing	O
a	O
single	O
button	O
.	O
</s>
<s>
This	O
booting	B-Operating_System
concept	O
was	O
called	O
a	O
variety	O
of	O
names	O
for	O
IBM	O
computers	O
of	O
the	O
1950s	O
and	O
early	O
1960s	O
,	O
but	O
IBM	O
used	O
the	O
term	O
"	O
Initial	O
Program	O
Load	O
"	O
with	O
the	O
IBM	B-Device
7030	I-Device
Stretch	I-Device
and	O
later	O
used	O
it	O
for	O
their	O
mainframe	O
lines	O
,	O
starting	O
with	O
the	O
System/360	B-Application
in	O
1964	O
.	O
</s>
<s>
The	O
IBM	B-Device
701	I-Device
computer	O
(	O
1952	O
–	O
1956	O
)	O
had	O
a	O
"	O
Load	O
"	O
button	O
that	O
initiated	O
reading	O
of	O
the	O
first	O
36-bit	O
word	O
into	O
main	B-General_Concept
memory	I-General_Concept
from	O
a	O
punched	B-Architecture
card	I-Architecture
in	O
a	O
card	B-Device
reader	I-Device
,	O
a	O
magnetic	O
tape	O
in	O
a	O
tape	O
drive	O
,	O
or	O
a	O
magnetic	B-General_Concept
drum	I-General_Concept
unit	O
,	O
depending	O
on	O
the	O
position	O
of	O
the	O
Load	O
Selector	O
switch	O
.	O
</s>
<s>
The	O
loaded	O
boot	B-Operating_System
program	I-Operating_System
was	O
then	O
executed	O
,	O
which	O
,	O
in	O
turn	O
,	O
loaded	O
a	O
larger	O
program	O
from	O
that	O
medium	O
into	O
memory	O
without	O
further	O
help	O
from	O
the	O
human	O
operator	O
.	O
</s>
<s>
The	O
IBM	B-Device
704	I-Device
,	O
IBM	B-Device
7090	I-Device
,	O
and	O
IBM	B-Device
7094	I-Device
had	O
similar	O
mechanisms	O
,	O
but	O
with	O
different	O
load	O
buttons	O
for	O
different	O
devices	O
.	O
</s>
<s>
The	O
term	O
"	O
boot	B-Operating_System
"	O
has	O
been	O
used	O
in	O
this	O
sense	O
since	O
at	O
least	O
1958	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
IBM	B-Device
1401	I-Device
system	O
(	O
c	O
.	O
1958	O
)	O
used	O
a	O
card	B-Device
reader	I-Device
to	O
load	O
a	O
program	O
from	O
a	O
punched	B-Architecture
card	I-Architecture
.	O
</s>
<s>
The	O
80	O
characters	O
stored	O
in	O
the	O
punched	B-Architecture
card	I-Architecture
were	O
read	O
into	O
memory	O
locations	O
001	O
to	O
080	O
,	O
then	O
the	O
computer	O
would	O
branch	O
to	O
memory	O
location	O
001	O
to	O
read	O
its	O
first	O
stored	O
instruction	O
.	O
</s>
<s>
This	O
instruction	O
was	O
always	O
the	O
same	O
:	O
move	O
the	O
information	O
in	O
these	O
first	O
80	O
memory	O
locations	O
to	O
an	O
assembly	O
area	O
where	O
the	O
information	O
in	O
punched	B-Architecture
cards	I-Architecture
2	O
,	O
3	O
,	O
4	O
,	O
and	O
so	O
on	O
,	O
could	O
be	O
combined	O
to	O
form	O
the	O
stored	O
program	O
.	O
</s>
<s>
Once	O
this	O
information	O
was	O
moved	O
to	O
the	O
assembly	O
area	O
,	O
the	O
machine	O
would	O
branch	O
to	O
an	O
instruction	O
in	O
location	O
080	O
(	O
read	O
a	O
card	B-Architecture
)	O
and	O
the	O
next	O
card	B-Architecture
would	O
be	O
read	O
and	O
its	O
information	O
processed	O
.	O
</s>
<s>
Another	O
example	O
was	O
the	O
IBM	B-Device
650	I-Device
(	O
1953	O
)	O
,	O
a	O
decimal	O
machine	O
,	O
which	O
had	O
a	O
group	O
of	O
ten	O
10-position	O
switches	O
on	O
its	O
operator	O
panel	O
which	O
were	O
addressable	O
as	O
a	O
memory	O
word	O
(	O
address	O
8000	O
)	O
and	O
could	O
be	O
executed	O
as	O
an	O
instruction	O
.	O
</s>
<s>
Thus	O
setting	O
the	O
switches	O
to	O
7004000400	O
and	O
pressing	O
the	O
appropriate	O
button	O
would	O
read	O
the	O
first	O
card	B-Architecture
in	O
the	O
card	B-Device
reader	I-Device
into	O
memory	O
(	O
op	B-Language
code	I-Language
70	O
)	O
,	O
starting	O
at	O
address	O
400	O
and	O
then	O
jump	O
to	O
400	O
to	O
begin	O
executing	O
the	O
program	O
on	O
that	O
card	B-Architecture
.	O
</s>
<s>
The	O
IBM	B-Device
7040	I-Device
and	I-Device
7044	I-Device
have	O
a	O
similar	O
mechanism	O
,	O
in	O
which	O
the	O
Load	O
button	O
causes	O
the	O
instruction	O
set	O
up	O
in	O
the	O
entry	O
keys	O
on	O
the	O
front	B-Device
panel	I-Device
is	O
executed	O
,	O
and	O
the	O
channel	O
that	O
instruction	O
sets	O
up	O
is	O
given	O
a	O
command	O
to	O
transfer	O
data	O
to	O
memory	O
starting	O
at	O
address	O
00100	O
;	O
when	O
that	O
transfer	O
finishes	O
,	O
the	O
CPU	O
jumps	O
to	O
address	O
00101	O
.	O
</s>
<s>
The	O
CDC	B-Device
6600	I-Device
(	O
c	O
.	O
1964	O
)	O
had	O
a	O
dead	O
start	O
panel	O
with	O
144	O
toggle	O
switches	O
;	O
the	O
dead	O
start	O
switch	O
entered	O
12	O
12-bit	O
words	O
from	O
the	O
toggle	O
switches	O
to	O
the	O
memory	O
of	O
peripheral	O
processor	O
(	O
PP	O
)	O
0	O
and	O
initiated	O
the	O
load	O
sequence	O
by	O
causing	O
PP	O
0	O
to	O
execute	O
the	O
code	O
loaded	O
into	O
memory	O
.	O
</s>
<s>
The	O
GE	B-Device
645	I-Device
(	O
c	O
.	O
1965	O
)	O
had	O
a	O
"	O
SYSTEM	O
BOOTLOAD	O
"	O
button	O
that	O
,	O
when	O
pressed	O
,	O
caused	O
one	O
of	O
the	O
I/O	O
controllers	O
to	O
load	O
a	O
64-word	O
program	O
into	O
memory	O
from	O
a	O
diode	O
read-only	B-Device
memory	I-Device
and	O
deliver	O
an	O
interrupt	O
to	O
cause	O
that	O
program	O
to	O
start	O
running	O
.	O
</s>
<s>
The	O
first	O
model	O
of	O
the	O
PDP-10	B-Device
had	O
a	O
"	O
READ	O
IN	O
"	O
button	O
that	O
,	O
when	O
pressed	O
,	O
reset	O
the	O
processor	O
and	O
started	O
an	O
I/O	O
operation	O
on	O
a	O
device	O
specified	O
by	O
switches	O
on	O
the	O
control	O
panel	O
,	O
reading	O
in	O
a	O
36-bit	O
word	O
giving	O
a	O
target	O
address	O
and	O
count	O
for	O
subsequent	O
word	O
reads	O
;	O
when	O
the	O
read	O
completed	O
,	O
the	O
processor	O
started	O
executing	O
the	O
code	O
read	O
in	O
by	O
jumping	O
to	O
the	O
last	O
word	O
read	O
in	O
.	O
</s>
<s>
A	O
noteworthy	O
variation	O
of	O
this	O
is	O
found	O
on	O
the	O
Burroughs	B-Device
B1700	I-Device
where	O
there	O
is	O
neither	O
a	O
bootstrap	O
ROM	B-Device
nor	O
a	O
hardwired	O
IPL	O
operation	O
.	O
</s>
<s>
Instead	O
,	O
after	O
the	O
system	O
is	O
reset	O
it	O
reads	O
and	O
executes	O
microinstructions	O
sequentially	O
from	O
a	O
cassette	O
tape	O
drive	O
mounted	O
on	O
the	O
front	B-Device
panel	I-Device
;	O
this	O
sets	O
up	O
a	O
boot	B-Application
loader	I-Application
in	O
RAM	B-Architecture
which	O
is	O
then	O
executed	O
.	O
</s>
<s>
However	O
,	O
since	O
this	O
makes	O
few	O
assumptions	O
about	O
the	O
system	O
it	O
can	O
equally	O
well	O
be	O
used	O
to	O
load	O
diagnostic	O
(	O
Maintenance	O
Test	O
Routine	O
)	O
tapes	O
which	O
display	O
an	O
intelligible	O
code	O
on	O
the	O
front	B-Device
panel	I-Device
even	O
in	O
cases	O
of	O
gross	O
CPU	O
failure	O
.	O
</s>
<s>
In	O
the	O
IBM	B-Application
System/360	I-Application
and	O
its	O
successors	O
,	O
including	O
the	O
current	O
z/Architecture	B-Device
machines	O
,	O
the	O
boot	B-Operating_System
process	I-Operating_System
is	O
known	O
as	O
Initial	O
Program	O
Load	O
(	O
IPL	O
)	O
.	O
</s>
<s>
IBM	O
coined	O
this	O
term	O
for	O
the	O
7030	O
(	O
Stretch	O
)	O
,	O
revived	O
it	O
for	O
the	O
design	O
of	O
the	O
System/360	B-Application
,	O
and	O
continues	O
to	O
use	O
it	O
in	O
those	O
environments	O
today	O
.	O
</s>
<s>
In	O
the	O
System/360	B-Application
processors	O
,	O
an	O
IPL	O
is	O
initiated	O
by	O
the	O
computer	O
operator	O
by	O
selecting	O
the	O
three	O
hexadecimal	O
digit	O
device	O
address	O
(	O
CUU	O
;	O
C	O
=	O
I/O	O
Channel	O
address	O
,	O
UU	O
=	O
Control	O
unit	O
and	O
Device	O
address	O
)	O
followed	O
by	O
pressing	O
the	O
LOAD	O
button	O
.	O
</s>
<s>
On	O
the	O
high	O
end	O
System/360	B-Application
models	O
,	O
most	O
System/370	B-Device
and	O
some	O
later	O
systems	O
,	O
the	O
functions	O
of	O
the	O
switches	O
and	O
the	O
LOAD	O
button	O
are	O
simulated	O
using	O
selectable	O
areas	O
on	O
the	O
screen	O
of	O
a	O
graphics	O
console	O
,	O
often	O
an	O
IBM	O
2250-like	O
device	O
or	O
an	O
IBM	O
3270-like	O
device	O
.	O
</s>
<s>
For	O
example	O
,	O
on	O
the	O
System/370	B-Device
Model	O
158	O
,	O
the	O
keyboard	O
sequence	O
0-7-X	O
(	O
zero	O
,	O
seven	O
and	O
X	O
,	O
in	O
that	O
order	O
)	O
results	O
in	O
an	O
IPL	O
from	O
the	O
device	O
address	O
which	O
was	O
keyed	O
into	O
the	O
input	O
area	O
.	O
</s>
<s>
The	O
IPL	O
function	O
in	O
the	O
System/360	B-Application
and	O
its	O
successors	O
prior	O
to	O
IBM	B-Device
Z	I-Device
,	O
and	O
its	O
compatibles	O
such	O
as	O
Amdahl	O
's	O
,	O
reads	O
24	O
bytes	B-Application
from	O
an	O
operator-specified	O
device	O
into	O
main	O
storage	O
starting	O
at	O
real	O
address	O
zero	O
.	O
</s>
<s>
The	O
second	O
and	O
third	O
groups	O
of	O
eight	O
bytes	B-Application
are	O
treated	O
as	O
Channel	O
Command	O
Words	O
(	O
CCWs	O
)	O
to	O
continue	O
loading	O
the	O
startup	O
program	O
(	O
the	O
first	O
CCW	O
is	O
always	O
simulated	O
by	O
the	O
CPU	O
and	O
consists	O
of	O
a	O
Read	O
IPL	O
command	O
,	O
,	O
with	O
command	O
chaining	O
and	O
suppress	O
incorrect	O
length	O
indication	O
being	O
enforced	O
)	O
.	O
</s>
<s>
When	O
the	O
I/O	O
channel	O
commands	O
are	O
complete	O
,	O
the	O
first	O
group	O
of	O
eight	O
bytes	B-Application
is	O
then	O
loaded	O
into	O
the	O
processor	O
's	O
Program	B-Device
Status	I-Device
Word	I-Device
(	O
PSW	O
)	O
and	O
the	O
startup	O
program	O
begins	O
execution	O
at	O
the	O
location	O
designated	O
by	O
that	O
PSW	O
.	O
</s>
<s>
The	O
IPL	O
device	O
is	O
usually	O
a	O
disk	O
drive	O
,	O
hence	O
the	O
special	O
significance	O
of	O
the	O
read-type	O
command	O
,	O
but	O
exactly	O
the	O
same	O
procedure	O
is	O
also	O
used	O
to	O
IPL	O
from	O
other	O
input-type	O
devices	O
,	O
such	O
as	O
tape	O
drives	O
,	O
or	O
even	O
card	B-Device
readers	I-Device
,	O
in	O
a	O
device-independent	O
manner	O
,	O
allowing	O
,	O
for	O
example	O
,	O
the	O
installation	O
of	O
an	O
operating	B-General_Concept
system	I-General_Concept
on	O
a	O
brand-new	O
computer	O
from	O
an	O
OS	O
initial	O
distribution	O
magnetic	O
tape	O
.	O
</s>
<s>
For	O
disk	O
controllers	O
,	O
the	O
command	O
also	O
causes	O
the	O
selected	O
device	O
to	O
seek	O
to	O
cylinder	O
,	O
head	O
,	O
simulating	O
a	O
Seek	O
cylinder	O
and	O
head	O
command	O
,	O
,	O
and	O
to	O
search	O
for	O
record	O
,	O
simulating	O
a	O
Search	O
ID	O
Equal	O
command	O
,	O
;	O
seeks	O
and	O
searches	O
are	O
not	O
simulated	O
by	O
tape	O
and	O
card	B-Architecture
controllers	O
,	O
as	O
for	O
these	O
device	O
classes	O
a	O
Read	O
IPL	O
command	O
is	O
simply	O
a	O
sequential	O
read	O
command	O
.	O
</s>
<s>
The	O
disk	O
,	O
tape	O
or	O
card	B-Architecture
deck	I-Architecture
must	O
contain	O
a	O
special	O
program	O
to	O
load	O
the	O
actual	O
operating	B-General_Concept
system	I-General_Concept
or	O
standalone	O
utility	O
into	O
main	O
storage	O
,	O
and	O
for	O
this	O
specific	O
purpose	O
"	O
IPL	O
Text	O
"	O
is	O
placed	O
on	O
the	O
disk	O
by	O
the	O
stand-alone	O
DASDI	O
(	O
Direct	O
Access	O
Storage	O
Device	O
Initialization	O
)	O
program	O
or	O
an	O
equivalent	O
program	O
running	O
under	O
an	O
operating	B-General_Concept
system	I-General_Concept
,	O
e.g.	O
,	O
ICKDSF	O
,	O
but	O
IPL-able	O
tapes	O
and	O
card	B-Architecture
decks	I-Architecture
are	O
usually	O
distributed	O
with	O
this	O
"	O
IPL	O
Text	O
"	O
already	O
present	O
.	O
</s>
<s>
IBM	O
introduced	O
some	O
evolutionary	O
changes	O
in	O
the	O
IPL	O
process	O
,	O
changing	O
some	O
details	O
for	O
System/370	B-Device
Extended	O
Architecture	O
(	O
S/370	B-Device
-XA	I-Device
)	O
and	O
later	O
,	O
and	O
adding	O
a	O
new	O
type	O
of	O
IPL	O
for	O
z/Architecture	B-Device
.	O
</s>
<s>
Minicomputers	B-Architecture
,	O
starting	O
with	O
the	O
Digital	O
Equipment	O
Corporation	O
(	O
DEC	O
)	O
PDP-5	B-Device
and	O
PDP-8	B-Device
(	O
1965	O
)	O
simplified	O
design	O
by	O
using	O
the	O
CPU	O
to	O
assist	O
input	O
and	O
output	O
operations	O
.	O
</s>
<s>
This	O
saved	O
cost	O
but	O
made	O
booting	B-Operating_System
more	O
complicated	O
than	O
pressing	O
a	O
single	O
button	O
.	O
</s>
<s>
Minicomputers	B-Architecture
typically	O
had	O
some	O
way	O
to	O
toggle	O
in	O
short	O
programs	O
by	O
manipulating	O
an	O
array	O
of	O
switches	O
on	O
the	O
front	B-Device
panel	I-Device
.	O
</s>
<s>
Since	O
the	O
early	O
minicomputers	B-Architecture
used	O
magnetic-core	O
memory	O
,	O
which	O
did	O
not	O
lose	O
its	O
information	O
when	O
power	O
was	O
off	O
,	O
these	O
bootstrap	B-Operating_System
loaders	I-Operating_System
would	O
remain	O
in	O
place	O
unless	O
they	O
were	O
erased	O
.	O
</s>
<s>
Other	O
minicomputers	B-Architecture
with	O
such	O
simple	O
form	O
of	O
booting	B-Operating_System
include	O
Hewlett-Packard	O
'	O
s	O
HP	B-Device
2100	I-Device
series	O
(	O
mid-1960s	O
)	O
,	O
the	O
original	O
Data	B-Device
General	I-Device
Nova	I-Device
(	O
1969	O
)	O
,	O
and	O
DEC	O
's	O
PDP-4	B-Device
(	O
1962	O
)	O
and	O
PDP-11	B-Device
(	O
1970	O
)	O
.	O
</s>
<s>
As	O
the	O
I/O	O
operations	O
needed	O
to	O
cause	O
a	O
read	O
operation	O
on	O
a	O
minicomputer	B-Architecture
I/O	O
device	O
were	O
typically	O
different	O
for	O
different	O
device	O
controllers	O
,	O
different	O
bootstrap	B-Operating_System
programs	I-Operating_System
were	O
needed	O
for	O
different	O
devices	O
.	O
</s>
<s>
DEC	O
later	O
added	O
,	O
in	O
1971	O
,	O
an	O
optional	O
diode	B-General_Concept
matrix	I-General_Concept
read-only	B-Device
memory	I-Device
for	O
the	O
PDP-11	B-Device
that	O
stored	O
a	O
bootstrap	B-Operating_System
program	I-Operating_System
of	O
up	O
to	O
32	O
words	O
(	O
64	O
bytes	B-Application
)	O
.	O
</s>
<s>
It	O
consisted	O
of	O
a	O
printed	O
circuit	O
card	B-Architecture
,	O
the	O
M792	O
,	O
that	O
plugged	O
into	O
the	O
Unibus	B-Device
and	O
held	O
a	O
32	O
by	O
16	O
array	O
of	O
semiconductor	O
diodes	O
.	O
</s>
<s>
With	O
all	O
512	O
diodes	O
in	O
place	O
,	O
the	O
memory	O
contained	O
all	O
"	O
one	O
"	O
bits	O
;	O
the	O
card	B-Architecture
was	O
programmed	O
by	O
cutting	O
off	O
each	O
diode	O
whose	O
bit	O
was	O
to	O
be	O
"	O
zero	O
"	O
.	O
</s>
<s>
DEC	O
also	O
sold	O
versions	O
of	O
the	O
card	B-Architecture
,	O
the	O
BM792-Yx	O
series	O
,	O
pre-programmed	O
for	O
many	O
standard	O
input	O
devices	O
by	O
simply	O
omitting	O
the	O
unneeded	O
diodes	O
.	O
</s>
<s>
Following	O
the	O
older	O
approach	O
,	O
the	O
earlier	O
PDP-1	B-Device
has	O
a	O
hardware	B-Architecture
loader	O
,	O
such	O
that	O
an	O
operator	O
need	O
only	O
push	O
the	O
"	O
load	O
"	O
switch	O
to	O
instruct	O
the	O
paper	O
tape	O
reader	O
to	O
load	O
a	O
program	O
directly	O
into	O
core	O
memory	O
.	O
</s>
<s>
The	O
PDP-7	B-Device
,	O
PDP-9	B-Device
,	O
and	O
PDP-15	B-Device
successors	O
to	O
the	O
PDP-4	B-Device
have	O
an	O
added	O
Read-In	O
button	O
to	O
read	O
a	O
program	O
in	O
from	O
paper	O
tape	O
and	O
jump	O
to	O
it	O
.	O
</s>
<s>
The	O
Data	O
General	O
Supernova	O
used	O
front	B-Device
panel	I-Device
switches	O
to	O
cause	O
the	O
computer	O
to	O
automatically	O
load	O
instructions	O
into	O
memory	O
from	O
a	O
device	O
specified	O
by	O
the	O
front	B-Device
panel	I-Device
's	O
data	O
switches	O
,	O
and	O
then	O
jump	O
to	O
loaded	O
code	O
.	O
</s>
<s>
In	O
a	O
minicomputer	B-Architecture
with	O
a	O
paper	O
tape	O
reader	O
,	O
the	O
first	O
program	O
to	O
run	O
in	O
the	O
boot	B-Operating_System
process	I-Operating_System
,	O
the	O
boot	B-Application
loader	I-Application
,	O
would	O
read	O
into	O
core	O
memory	O
either	O
the	O
second-stage	O
boot	B-Application
loader	I-Application
(	O
often	O
called	O
a	O
Binary	O
Loader	O
)	O
that	O
could	O
read	O
paper	O
tape	O
with	O
checksum	B-Algorithm
or	O
the	O
operating	B-General_Concept
system	I-General_Concept
from	O
an	O
outside	O
storage	O
medium	O
.	O
</s>
<s>
Pseudocode	B-Language
for	O
the	O
boot	B-Application
loader	I-Application
might	O
be	O
as	O
simple	O
as	O
the	O
following	O
eight	O
instructions	O
:	O
</s>
<s>
A	O
related	O
example	O
is	O
based	O
on	O
a	O
loader	O
for	O
a	O
Nicolet	O
Instrument	O
Corporation	O
minicomputer	B-Architecture
of	O
the	O
1970s	O
,	O
using	O
the	O
paper	O
tape	O
reader-punch	O
unit	O
on	O
a	O
Teletype	B-Application
Model	I-Application
33	I-Application
ASR	O
teleprinter	O
.	O
</s>
<s>
The	O
bytes	B-Application
of	O
its	O
second-stage	O
loader	O
are	O
read	O
from	O
paper	O
tape	O
in	O
reverse	O
order	O
.	O
</s>
<s>
The	O
length	O
of	O
the	O
second	O
stage	O
loader	O
is	O
such	O
that	O
the	O
final	O
byte	B-Application
overwrites	O
location	O
7	O
.	O
</s>
<s>
The	O
second	O
stage	O
loader	O
then	O
waits	O
for	O
the	O
much	O
longer	O
tape	O
containing	O
the	O
operating	B-General_Concept
system	I-General_Concept
to	O
be	O
placed	O
in	O
the	O
tape	O
reader	O
.	O
</s>
<s>
The	O
difference	O
between	O
the	O
boot	B-Application
loader	I-Application
and	O
second	O
stage	O
loader	O
is	O
the	O
addition	O
of	O
checking	O
code	O
to	O
trap	O
paper	O
tape	O
read	O
errors	O
,	O
a	O
frequent	O
occurrence	O
with	O
relatively	O
low-cost	O
,	O
"	O
part-time-duty	O
"	O
hardware	B-Architecture
,	O
such	O
as	O
the	O
Teletype	B-Application
Model	I-Application
33	I-Application
ASR	O
.	O
</s>
<s>
The	O
earliest	O
microcomputers	B-Architecture
,	O
such	O
as	O
the	O
Altair	B-Architecture
8800	I-Architecture
(	O
released	O
first	O
in	O
1975	O
)	O
and	O
an	O
even	O
earlier	O
,	O
similar	O
machine	O
(	O
based	O
on	O
the	O
Intel	O
8008	O
CPU	O
)	O
had	O
no	O
bootstrapping	B-Application
hardware	B-Architecture
as	O
such	O
.	O
</s>
<s>
The	O
front	B-Device
panels	I-Device
of	O
these	O
machines	O
carried	O
toggle	O
switches	O
for	O
entering	O
addresses	O
and	O
data	O
,	O
one	O
switch	O
per	O
bit	O
of	O
the	O
computer	B-General_Concept
memory	I-General_Concept
word	O
and	O
address	O
bus	O
.	O
</s>
<s>
Simple	O
additions	O
to	O
the	O
hardware	B-Architecture
permitted	O
one	O
memory	O
location	O
at	O
a	O
time	O
to	O
be	O
loaded	O
from	O
those	O
switches	O
to	O
store	O
bootstrap	O
code	O
.	O
</s>
<s>
Once	O
correctly	O
loaded	O
,	O
the	O
CPU	O
was	O
enabled	O
to	O
execute	O
the	O
bootstrapping	B-Application
code	O
.	O
</s>
<s>
This	O
process	O
,	O
similar	O
to	O
that	O
used	O
for	O
several	O
earlier	O
minicomputers	B-Architecture
,	O
was	O
tedious	O
and	O
had	O
to	O
be	O
error-free	O
.	O
</s>
<s>
The	O
introduction	O
of	O
integrated	O
circuit	O
read-only	B-Device
memory	I-Device
(	O
ROM	B-Device
)	O
,	O
with	O
its	O
many	O
variants	O
,	O
including	O
mask-programmed	O
ROMs	O
,	O
programmable	O
ROMs	O
(	O
PROM	O
)	O
,	O
erasable	O
programmable	O
ROMs	O
(	O
EPROM	B-General_Concept
)	O
,	O
and	O
flash	B-Device
memory	I-Device
,	O
reduced	O
the	O
physical	O
size	O
and	O
cost	O
of	O
ROM	B-Device
.	O
</s>
<s>
This	O
allowed	O
firmware	B-Application
boot	B-Operating_System
programs	I-Operating_System
to	O
be	O
included	O
as	O
part	O
of	O
the	O
computer	O
.	O
</s>
<s>
The	O
Data	B-Device
General	I-Device
Nova	I-Device
1200	O
(	O
1970	O
)	O
and	O
Nova	O
800	O
(	O
1971	O
)	O
had	O
a	O
program	O
load	O
switch	O
that	O
,	O
in	O
combination	O
with	O
options	O
that	O
provided	O
two	O
ROM	B-Device
chips	I-Device
,	O
loaded	O
a	O
program	O
into	O
main	B-General_Concept
memory	I-General_Concept
from	O
those	O
ROM	B-Device
chips	I-Device
and	O
jumped	O
to	O
it	O
.	O
</s>
<s>
Digital	O
Equipment	O
Corporation	O
introduced	O
the	O
integrated-circuit-ROM-based	O
BM873	O
(	O
1974	O
)	O
,	O
M9301	O
(	O
1977	O
)	O
,	O
M9312	O
(	O
1978	O
)	O
,	O
REV11-A	O
and	O
REV11-C	O
,	O
MRV11-C	O
,	O
and	O
MRV11-D	O
ROM	B-Device
memories	I-Device
,	O
all	O
usable	O
as	O
bootstrap	O
ROMs	O
.	O
</s>
<s>
The	O
PDP-11/34	O
(	O
1976	O
)	O
,	O
PDP-11/60	O
(	O
1977	O
)	O
,	O
PDP-11/24	O
(	O
1979	O
)	O
,	O
and	O
most	O
later	O
models	O
include	O
boot	B-Operating_System
ROM	B-Device
modules	O
.	O
</s>
<s>
An	O
Italian	O
telephone	O
switching	O
computer	O
,	O
called	O
"	O
Gruppi	O
Speciali	O
"	O
,	O
patented	O
in	O
1975	O
by	O
Alberto	O
Ciaramella	O
,	O
a	O
researcher	O
at	O
CSELT	O
,	O
included	O
an	O
(	O
external	O
)	O
ROM	B-Device
.	O
</s>
<s>
Gruppi	O
Speciali	O
was	O
,	O
starting	O
from	O
1975	O
,	O
a	O
fully	O
single-button	O
machine	O
booting	B-Operating_System
into	O
the	O
operating	B-General_Concept
system	I-General_Concept
from	O
a	O
ROM	B-Device
memory	I-Device
composed	O
from	O
semiconductors	O
,	O
not	O
from	O
ferrite	O
cores	O
.	O
</s>
<s>
Although	O
the	O
ROM	B-Device
device	O
was	O
not	O
natively	O
embedded	O
in	O
the	O
computer	O
of	O
Gruppi	O
Speciali	O
,	O
due	O
to	O
the	O
design	O
of	O
the	O
machine	O
,	O
it	O
also	O
allowed	O
the	O
single-button	O
ROM	B-Device
booting	B-Operating_System
in	O
machines	O
not	O
designed	O
for	O
that	O
(	O
therefore	O
,	O
this	O
"	O
bootstrap	O
device	O
"	O
was	O
architecture-independent	O
)	O
,	O
e.g.	O
</s>
<s>
the	O
PDP-11	B-Device
.	O
</s>
<s>
Some	O
minicomputers	B-Architecture
and	O
superminicomputers	B-Device
include	O
a	O
separate	O
console	O
processor	O
that	O
bootstraps	O
the	O
main	O
processor	O
.	O
</s>
<s>
The	O
PDP-11/44	B-Device
had	O
an	O
Intel	B-General_Concept
8085	I-General_Concept
as	O
a	O
console	O
processor	O
;	O
the	O
VAX-11/780	O
,	O
the	O
first	O
member	O
of	O
Digital	O
's	O
VAX	B-Device
line	O
of	O
32-bit	O
superminicomputers	B-Device
,	O
had	O
an	O
LSI-11-based	O
console	O
processor	O
,	O
and	O
the	O
VAX-11/730	O
had	O
an	O
8085-based	O
console	O
processor	O
.	O
</s>
<s>
These	O
console	O
processors	O
could	O
boot	B-Operating_System
the	O
main	O
processor	O
from	O
various	O
storage	O
devices	O
.	O
</s>
<s>
Some	O
other	O
superminicomputers	B-Device
,	O
such	O
as	O
the	O
VAX-11/750	O
,	O
implement	O
console	O
functions	O
,	O
including	O
the	O
first	O
stage	O
of	O
booting	B-Operating_System
,	O
in	O
CPU	O
microcode	O
.	O
</s>
<s>
A	O
system	O
built	O
using	O
that	O
microprocessor	O
will	O
have	O
the	O
permanent	O
ROM	B-Device
occupying	O
these	O
special	O
locations	O
so	O
that	O
the	O
system	O
always	O
begins	O
operating	O
without	O
operator	O
assistance	O
.	O
</s>
<s>
For	O
example	O
,	O
Intel	B-Operating_System
x86	I-Operating_System
processors	O
always	O
start	O
by	O
running	O
the	O
instructions	O
beginning	O
at	O
F000:FFF0	O
,	O
while	O
for	O
the	O
MOS	B-General_Concept
6502	I-General_Concept
processor	O
,	O
initialization	O
begins	O
by	O
reading	O
a	O
two-byte	O
vector	O
address	O
at	O
$FFFD	O
(	O
MS	O
byte	B-Application
)	O
and	O
$FFFC	O
(	O
LS	O
byte	B-Application
)	O
and	O
jumping	O
to	O
that	O
location	O
to	O
run	O
the	O
bootstrap	O
code	O
.	O
</s>
<s>
Apple	O
Computer	O
's	O
first	O
computer	O
,	O
the	O
Apple	B-Device
1	I-Device
introduced	O
in	O
1976	O
,	O
featured	O
PROM	O
chips	O
that	O
eliminated	O
the	O
need	O
for	O
a	O
front	B-Device
panel	I-Device
for	O
the	O
boot	B-Operating_System
process	I-Operating_System
(	O
as	O
was	O
the	O
case	O
with	O
the	O
Altair	B-Architecture
8800	I-Architecture
)	O
in	O
a	O
commercial	O
computer	O
.	O
</s>
<s>
According	O
to	O
Apple	O
's	O
ad	O
announcing	O
it	O
"	O
No	O
More	O
Switches	O
,	O
No	O
More	O
Lights	O
...	O
the	O
firmware	B-Application
in	O
PROMS	O
enables	O
you	O
to	O
enter	O
,	O
display	O
and	O
debug	O
programs	O
(	O
all	O
in	O
hex	O
)	O
from	O
the	O
keyboard.	O
"	O
</s>
<s>
Due	O
to	O
the	O
expense	O
of	O
read-only	B-Device
memory	I-Device
at	O
the	O
time	O
,	O
the	O
Apple	B-Device
II	I-Device
series	I-Device
booted	O
its	O
disk	O
operating	B-General_Concept
systems	I-General_Concept
using	O
a	O
series	O
of	O
very	O
small	O
incremental	O
steps	O
,	O
each	O
passing	O
control	O
onward	O
to	O
the	O
next	O
phase	O
of	O
the	O
gradually	O
more	O
complex	O
boot	B-Operating_System
process	I-Operating_System
.	O
</s>
<s>
(	O
See	O
Apple	O
DOS	O
:	O
Boot	B-Application
loader	I-Application
)	O
.	O
</s>
<s>
Because	O
so	O
little	O
of	O
the	O
disk	O
operating	B-General_Concept
system	I-General_Concept
relied	O
on	O
ROM	B-Device
,	O
the	O
hardware	B-Architecture
was	O
also	O
extremely	O
flexible	O
and	O
supported	O
a	O
wide	O
range	O
of	O
customized	O
disk	O
copy	B-Application
protection	I-Application
mechanisms	O
.	O
</s>
<s>
Some	O
operating	B-General_Concept
systems	I-General_Concept
,	O
most	O
notably	O
pre-1995	O
Macintosh	B-Device
systems	O
from	O
Apple	O
,	O
are	O
so	O
closely	O
interwoven	O
with	O
their	O
hardware	B-Architecture
that	O
it	O
is	O
impossible	O
to	O
natively	O
boot	B-Operating_System
an	O
operating	B-General_Concept
system	I-General_Concept
other	O
than	O
the	O
standard	O
one	O
.	O
</s>
<s>
This	O
is	O
the	O
opposite	O
extreme	O
of	O
the	O
scenario	O
using	O
switches	O
mentioned	O
above	O
;	O
it	O
is	O
highly	O
inflexible	O
but	O
relatively	O
error-proof	O
and	O
foolproof	O
as	O
long	O
as	O
all	O
hardware	B-Architecture
is	O
working	O
normally	O
.	O
</s>
<s>
A	O
common	O
solution	O
in	O
such	O
situations	O
is	O
to	O
design	O
a	O
boot	B-Application
loader	I-Application
that	O
works	O
as	O
a	O
program	O
belonging	O
to	O
the	O
standard	O
OS	O
that	O
hijacks	O
the	O
system	O
and	O
loads	O
the	O
alternative	O
OS	O
.	O
</s>
<s>
This	O
technique	O
was	O
used	O
by	O
Apple	O
for	O
its	O
A/UX	B-Operating_System
Unix	O
implementation	O
and	O
copied	O
by	O
various	O
freeware	O
operating	B-General_Concept
systems	I-General_Concept
and	O
BeOS	B-Application
Personal	I-Application
Edition	I-Application
5	I-Application
.	O
</s>
<s>
Some	O
machines	O
,	O
like	O
the	O
Atari	B-Device
ST	I-Device
microcomputer	B-Architecture
,	O
were	O
"	O
instant-on	O
"	O
,	O
with	O
the	O
operating	B-General_Concept
system	I-General_Concept
executing	O
from	O
a	O
ROM	B-Device
.	O
</s>
<s>
Retrieval	O
of	O
the	O
OS	O
from	O
secondary	O
or	O
tertiary	O
store	O
was	O
thus	O
eliminated	O
as	O
one	O
of	O
the	O
characteristic	O
operations	O
for	O
bootstrapping	B-Application
.	O
</s>
<s>
To	O
allow	O
system	O
customizations	O
,	O
accessories	O
,	O
and	O
other	O
support	O
software	O
to	O
be	O
loaded	O
automatically	O
,	O
the	O
Atari	O
's	O
floppy	B-Device
drive	I-Device
was	O
read	O
for	O
additional	O
components	O
during	O
the	O
boot	B-Operating_System
process	I-Operating_System
.	O
</s>
<s>
There	O
was	O
a	O
timeout	O
delay	O
that	O
provided	O
time	O
to	O
manually	O
insert	O
a	O
floppy	B-Device
as	O
the	O
system	O
searched	O
for	O
the	O
extra	O
components	O
.	O
</s>
<s>
The	O
Atari	B-Device
ST	I-Device
hardware	B-Architecture
was	O
also	O
designed	O
so	O
the	O
cartridge	O
slot	O
could	O
provide	O
native	O
program	O
execution	O
for	O
gaming	O
purposes	O
as	O
a	O
holdover	O
from	O
Atari	O
's	O
legacy	O
making	O
electronic	O
games	O
;	O
by	O
inserting	O
the	O
Spectre	B-Device
GCR	I-Device
cartridge	O
with	O
the	O
Macintosh	B-Device
system	O
ROM	B-Device
in	O
the	O
game	O
slot	O
and	O
turning	O
the	O
Atari	O
on	O
,	O
it	O
could	O
"	O
natively	O
boot	B-Operating_System
"	O
the	O
Macintosh	B-Device
operating	B-General_Concept
system	I-General_Concept
rather	O
than	O
Atari	O
's	O
own	O
TOS	B-Application
.	O
</s>
<s>
The	O
IBM	B-Device
Personal	I-Device
Computer	I-Device
included	O
ROM-based	O
firmware	B-Application
called	O
the	O
BIOS	B-Operating_System
;	O
one	O
of	O
the	O
functions	O
of	O
that	O
firmware	B-Application
was	O
to	O
perform	O
a	O
power-on	B-Device
self	I-Device
test	I-Device
when	O
the	O
machine	O
was	O
powered	O
up	O
,	O
and	O
then	O
to	O
read	O
software	O
from	O
a	O
boot	B-Operating_System
device	O
and	O
execute	O
it	O
.	O
</s>
<s>
Firmware	B-Application
compatible	O
with	O
the	O
BIOS	B-Operating_System
on	O
the	O
IBM	B-Device
Personal	I-Device
Computer	I-Device
is	O
used	O
in	O
IBM	B-Device
PC	I-Device
compatible	O
computers	O
.	O
</s>
<s>
The	O
UEFI	B-Architecture
was	O
developed	O
by	O
Intel	O
,	O
originally	O
for	O
Itanium-based	O
machines	O
,	O
and	O
later	O
also	O
used	O
as	O
an	O
alternative	O
to	O
the	O
BIOS	B-Operating_System
in	O
x86-based	O
machines	O
,	O
including	O
Apple	B-Device
Macs	I-Device
using	I-Device
Intel	I-Device
processors	I-Device
.	O
</s>
<s>
Unix	B-Device
workstations	I-Device
originally	O
had	O
vendor-specific	O
ROM-based	O
firmware	B-Application
.	O
</s>
<s>
Sun	O
Microsystems	O
later	O
developed	O
OpenBoot	B-Architecture
,	O
later	O
known	O
as	O
Open	B-Architecture
Firmware	I-Architecture
,	O
which	O
incorporated	O
a	O
Forth	B-Application
interpreter	O
,	O
with	O
much	O
of	O
the	O
firmware	B-Application
being	O
written	O
in	O
Forth	B-Application
.	O
</s>
<s>
It	O
was	O
standardized	O
by	O
the	O
IEEE	O
as	O
IEEE	O
standard	O
;	O
firmware	B-Application
that	O
implements	O
that	O
standard	O
was	O
used	O
in	O
PowerPC-based	O
Macs	B-Device
and	O
some	O
other	O
PowerPC-based	O
machines	O
,	O
as	O
well	O
as	O
Sun	O
's	O
own	O
SPARC-based	O
computers	O
.	O
</s>
<s>
The	O
Advanced	B-Device
RISC	I-Device
Computing	I-Device
specification	O
defined	O
another	O
firmware	B-Application
standard	O
,	O
which	O
was	O
implemented	O
on	O
some	O
MIPS-based	O
and	O
Alpha-based	O
machines	O
and	O
the	O
SGI	B-Device
Visual	I-Device
Workstation	I-Device
x86-based	O
workstations	B-Device
.	O
</s>
<s>
When	O
a	O
computer	O
is	O
turned	O
off	O
,	O
its	O
softwareincluding	O
operating	B-General_Concept
systems	I-General_Concept
,	O
application	O
code	O
,	O
and	O
dataremains	O
stored	O
on	O
non-volatile	B-General_Concept
memory	I-General_Concept
.	O
</s>
<s>
When	O
the	O
computer	O
is	O
powered	O
on	O
,	O
it	O
typically	O
does	O
not	O
have	O
an	O
operating	B-General_Concept
system	I-General_Concept
or	O
its	O
loader	O
in	O
random-access	B-Architecture
memory	I-Architecture
(	O
RAM	B-Architecture
)	O
.	O
</s>
<s>
The	O
computer	O
first	O
executes	O
a	O
relatively	O
small	O
program	O
stored	O
in	O
read-only	B-Device
memory	I-Device
(	O
ROM	B-Device
,	O
and	O
later	O
EEPROM	B-General_Concept
,	O
NOR	O
flash	O
)	O
along	O
with	O
some	O
needed	O
data	O
,	O
to	O
initialize	O
CPU	O
and	O
motherboard	O
,	O
to	O
initialize	O
RAM	B-Architecture
(	O
especially	O
on	O
x86	B-Operating_System
systems	O
)	O
,	O
to	O
access	O
the	O
nonvolatile	O
device	O
(	O
usually	O
block	O
device	O
,	O
e.g.	O
</s>
<s>
NAND	O
flash	O
)	O
or	O
devices	O
from	O
which	O
the	O
operating	B-General_Concept
system	I-General_Concept
programs	O
and	O
data	O
can	O
be	O
loaded	O
into	O
RAM	B-Architecture
.	O
</s>
<s>
The	O
small	O
program	O
that	O
starts	O
this	O
sequence	O
is	O
known	O
as	O
a	O
bootstrap	B-Operating_System
loader	I-Operating_System
,	O
bootstrap	O
or	O
boot	B-Application
loader	I-Application
.	O
</s>
<s>
Often	O
,	O
multiple-stage	O
boot	B-Application
loaders	I-Application
are	O
used	O
,	O
during	O
which	O
several	O
programs	O
of	O
increasing	O
complexity	O
load	O
one	O
after	O
the	O
other	O
in	O
a	O
process	O
of	O
chain	B-Operating_System
loading	I-Operating_System
.	O
</s>
<s>
Some	O
earlier	O
computer	O
systems	O
,	O
upon	O
receiving	O
a	O
boot	B-Operating_System
signal	O
from	O
a	O
human	O
operator	O
or	O
a	O
peripheral	O
device	O
,	O
may	O
load	O
a	O
very	O
small	O
number	O
of	O
fixed	O
instructions	O
into	O
memory	O
at	O
a	O
specific	O
location	O
,	O
initialize	O
at	O
least	O
one	O
CPU	O
,	O
and	O
then	O
point	O
the	O
CPU	O
to	O
the	O
instructions	O
and	O
start	O
their	O
execution	O
.	O
</s>
<s>
Other	O
systems	O
may	O
send	O
hardware	B-Architecture
commands	O
directly	O
to	O
peripheral	O
devices	O
or	O
I/O	O
controllers	O
that	O
cause	O
an	O
extremely	O
simple	O
input	O
operation	O
(	O
such	O
as	O
"	O
read	O
sector	O
zero	O
of	O
the	O
system	O
device	O
into	O
memory	O
starting	O
at	O
location	O
1000	O
"	O
)	O
to	O
be	O
carried	O
out	O
,	O
effectively	O
loading	O
a	O
small	O
number	O
of	O
boot	B-Application
loader	I-Application
instructions	O
into	O
memory	O
;	O
a	O
completion	O
signal	O
from	O
the	O
I/O	O
device	O
may	O
then	O
be	O
used	O
to	O
start	O
execution	O
of	O
the	O
instructions	O
by	O
the	O
CPU	O
.	O
</s>
<s>
Smaller	O
computers	O
often	O
use	O
less	O
flexible	O
but	O
more	O
automatic	O
boot	B-Application
loader	I-Application
mechanisms	O
to	O
ensure	O
that	O
the	O
computer	O
starts	O
quickly	O
and	O
with	O
a	O
predetermined	O
software	O
configuration	O
.	O
</s>
<s>
In	O
many	O
desktop	O
computers	O
,	O
for	O
example	O
,	O
the	O
bootstrapping	B-Application
process	O
begins	O
with	O
the	O
CPU	O
executing	O
software	O
contained	O
in	O
ROM	B-Device
(	O
for	O
example	O
,	O
the	O
BIOS	B-Operating_System
of	O
an	O
IBM	B-Device
PC	I-Device
)	O
at	O
a	O
predefined	O
address	O
(	O
some	O
CPUs	O
,	O
including	O
the	O
Intel	B-Operating_System
x86	I-Operating_System
series	O
are	O
designed	O
to	O
execute	O
this	O
software	O
after	O
reset	O
without	O
outside	O
help	O
)	O
.	O
</s>
<s>
This	O
software	O
contains	O
rudimentary	O
functionality	O
to	O
search	O
for	O
devices	O
eligible	O
to	O
participate	O
in	O
booting	B-Operating_System
,	O
and	O
load	O
a	O
small	O
program	O
from	O
a	O
special	O
section	O
(	O
most	O
commonly	O
the	O
boot	B-Device
sector	I-Device
)	O
of	O
the	O
most	O
promising	O
device	O
,	O
typically	O
starting	O
at	O
a	O
fixed	O
entry	B-Language
point	I-Language
such	O
as	O
the	O
start	O
of	O
the	O
sector	O
.	O
</s>
<s>
Boot	B-Application
loaders	I-Application
may	O
face	O
peculiar	O
constraints	O
,	O
especially	O
in	O
size	O
;	O
for	O
instance	O
,	O
on	O
the	O
IBM	B-Device
PC	I-Device
and	O
compatibles	O
,	O
the	O
boot	B-Operating_System
code	O
must	O
fit	O
in	O
the	O
Master	B-Device
Boot	I-Device
Record	I-Device
(	O
MBR	B-Device
)	O
and	O
the	O
Partition	B-Device
Boot	I-Device
Record	I-Device
(	O
PBR	O
)	O
,	O
which	O
in	O
turn	O
are	O
limited	O
to	O
a	O
single	O
sector	O
;	O
on	O
the	O
IBM	B-Application
System/360	I-Application
,	O
the	O
size	O
is	O
limited	O
by	O
the	O
IPL	O
medium	O
,	O
e.g.	O
,	O
card	B-Architecture
size	O
,	O
track	O
size	O
.	O
</s>
<s>
On	O
systems	O
with	O
those	O
constraints	O
,	O
the	O
first	O
program	O
loaded	O
into	O
RAM	B-Architecture
may	O
not	O
be	O
sufficiently	O
large	O
to	O
load	O
the	O
operating	B-General_Concept
system	I-General_Concept
and	O
,	O
instead	O
,	O
must	O
load	O
another	O
,	O
larger	O
program	O
.	O
</s>
<s>
The	O
first	O
program	O
loaded	O
into	O
RAM	B-Architecture
is	O
called	O
a	O
first-stage	O
boot	B-Application
loader	I-Application
,	O
and	O
the	O
program	O
it	O
loads	O
is	O
called	O
a	O
second-stage	O
boot	B-Application
loader	I-Application
.	O
</s>
<s>
Examples	O
of	O
first-stage	O
(	O
Hardware	B-Architecture
initialization	O
stage	O
)	O
bootloaders	B-Application
include	O
BIOS	B-Operating_System
,	O
UEFI	B-Architecture
,	O
coreboot	B-Device
,	O
Libreboot	B-Device
and	O
Das	B-Language
U-Boot	I-Language
.	O
</s>
<s>
On	O
the	O
IBM	B-Device
PC	I-Device
,	O
the	O
boot	B-Application
loader	I-Application
in	O
the	O
Master	B-Device
Boot	I-Device
Record	I-Device
(	O
MBR	B-Device
)	O
and	O
the	O
Partition	B-Device
Boot	I-Device
Record	I-Device
(	O
PBR	O
)	O
was	O
coded	O
to	O
require	O
at	O
least	O
32KB	O
(	O
later	O
expanded	O
to	O
64KB	O
)	O
of	O
system	O
memory	O
and	O
only	O
use	O
instructions	O
supported	O
by	O
the	O
original	O
8088/8086	O
processors	O
.	O
</s>
<s>
Second-stage	O
(	O
OS	O
initialization	O
stage	O
)	O
boot	B-Application
loaders	I-Application
,	O
such	O
as	O
GNU	B-Application
GRUB	I-Application
,	O
rEFInd	B-Device
,	O
BOOTMGR	B-Device
,	O
Syslinux	B-Application
,	O
NTLDR	B-Application
or	O
iBoot	B-Operating_System
,	O
are	O
not	O
themselves	O
operating	B-General_Concept
systems	I-General_Concept
,	O
but	O
are	O
able	O
to	O
load	O
an	O
operating	B-General_Concept
system	I-General_Concept
properly	O
and	O
transfer	O
execution	O
to	O
it	O
;	O
the	O
operating	B-General_Concept
system	I-General_Concept
subsequently	O
initializes	O
itself	O
and	O
may	O
load	O
extra	O
device	B-Application
drivers	I-Application
.	O
</s>
<s>
The	O
second-stage	O
boot	B-Application
loader	I-Application
does	O
not	O
need	O
drivers	O
for	O
its	O
own	O
operation	O
,	O
but	O
may	O
instead	O
use	O
generic	O
storage	O
access	O
methods	O
provided	O
by	O
system	O
firmware	B-Application
such	O
as	O
the	O
BIOS	B-Operating_System
,	O
UEFI	B-Architecture
or	O
Open	B-Architecture
Firmware	I-Architecture
,	O
though	O
typically	O
with	O
restricted	O
hardware	B-Architecture
functionality	O
and	O
lower	O
performance	O
.	O
</s>
<s>
Many	O
boot	B-Application
loaders	I-Application
(	O
like	O
GNU	B-Application
GRUB	I-Application
,	O
rEFInd	B-Device
,	O
Windows	B-Application
's	O
BOOTMGR	B-Device
,	O
Syslinux	B-Application
,	O
and	O
Windows	B-Application
NT/2000/XP	O
'	O
s	O
NTLDR	B-Application
)	O
can	O
be	O
configured	O
to	O
give	O
the	O
user	O
multiple	B-Operating_System
booting	I-Operating_System
choices	O
.	O
</s>
<s>
These	O
choices	O
can	O
include	O
different	O
operating	B-General_Concept
systems	I-General_Concept
(	O
for	O
dual	B-Operating_System
or	I-Operating_System
multi-booting	I-Operating_System
from	O
different	O
partitions	O
or	O
drives	O
)	O
,	O
different	O
versions	O
of	O
the	O
same	O
operating	B-General_Concept
system	I-General_Concept
(	O
in	O
case	O
a	O
new	O
version	O
has	O
unexpected	O
problems	O
)	O
,	O
different	O
operating	B-General_Concept
system	I-General_Concept
loading	O
options	O
(	O
e.g.	O
,	O
booting	B-Operating_System
into	O
a	O
rescue	O
or	O
safe	B-Operating_System
mode	I-Operating_System
)	O
,	O
and	O
some	O
standalone	O
programs	O
that	O
can	O
function	O
without	O
an	O
operating	B-General_Concept
system	I-General_Concept
,	O
such	O
as	O
memory	O
testers	O
(	O
e.g.	O
,	O
memtest86+	B-Application
)	O
,	O
a	O
basic	O
shell	O
(	O
as	O
in	O
GNU	B-Application
GRUB	I-Application
)	O
,	O
or	O
even	O
games	O
(	O
see	O
List	B-Application
of	I-Application
PC	I-Application
Booter	I-Application
games	I-Application
)	O
.	O
</s>
<s>
Some	O
boot	B-Application
loaders	I-Application
can	O
also	O
load	O
other	O
boot	B-Application
loaders	I-Application
;	O
for	O
example	O
,	O
GRUB	B-Application
loads	O
BOOTMGR	B-Device
instead	O
of	O
loading	O
Windows	B-Application
directly	O
.	O
</s>
<s>
Usually	O
a	O
default	O
choice	O
is	O
preselected	O
with	O
a	O
time	O
delay	O
during	O
which	O
a	O
user	O
can	O
press	O
a	O
key	O
to	O
change	O
the	O
choice	O
;	O
after	O
this	O
delay	O
,	O
the	O
default	O
choice	O
is	O
automatically	O
run	O
so	O
normal	O
booting	B-Operating_System
can	O
occur	O
without	O
interaction	O
.	O
</s>
<s>
The	O
boot	B-Operating_System
process	I-Operating_System
can	O
be	O
considered	O
complete	O
when	O
the	O
computer	O
is	O
ready	O
to	O
interact	O
with	O
the	O
user	O
,	O
or	O
the	O
operating	B-General_Concept
system	I-General_Concept
is	O
capable	O
of	O
running	O
system	O
programs	O
or	O
application	O
programs	O
.	O
</s>
<s>
Many	O
embedded	B-Architecture
systems	I-Architecture
must	O
boot	B-Operating_System
immediately	O
.	O
</s>
<s>
Therefore	O
,	O
such	O
devices	O
have	O
software	O
systems	O
in	O
ROM	B-Device
or	O
flash	B-Device
memory	I-Device
so	O
the	O
device	O
can	O
begin	O
functioning	O
immediately	O
;	O
little	O
or	O
no	O
loading	O
is	O
necessary	O
,	O
because	O
the	O
loading	O
can	O
be	O
precomputed	O
and	O
stored	O
on	O
the	O
ROM	B-Device
when	O
the	O
device	O
is	O
made	O
.	O
</s>
<s>
Large	O
and	O
complex	O
systems	O
may	O
have	O
boot	B-Operating_System
procedures	O
that	O
proceed	O
in	O
multiple	O
phases	O
until	O
finally	O
the	O
operating	B-General_Concept
system	I-General_Concept
and	O
other	O
programs	O
are	O
loaded	O
and	O
ready	O
to	O
execute	O
.	O
</s>
<s>
Because	O
operating	B-General_Concept
systems	I-General_Concept
are	O
designed	O
as	O
if	O
they	O
never	O
start	O
or	O
stop	O
,	O
a	O
boot	B-Application
loader	I-Application
might	O
load	O
the	O
operating	B-General_Concept
system	I-General_Concept
,	O
configure	O
itself	O
as	O
a	O
mere	O
process	O
within	O
that	O
system	O
,	O
and	O
then	O
irrevocably	O
transfer	O
control	O
to	O
the	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
The	O
boot	B-Application
loader	I-Application
then	O
terminates	O
normally	O
as	O
any	O
other	O
process	O
would	O
.	O
</s>
<s>
Most	O
computers	O
are	O
also	O
capable	O
of	O
booting	B-Operating_System
over	O
a	O
computer	B-Architecture
network	I-Architecture
.	O
</s>
<s>
In	O
this	O
scenario	O
,	O
the	O
operating	B-General_Concept
system	I-General_Concept
is	O
stored	O
on	O
the	O
disk	O
of	O
a	O
server	B-Application
,	O
and	O
certain	O
parts	O
of	O
it	O
are	O
transferred	O
to	O
the	O
client	O
using	O
a	O
simple	O
protocol	O
such	O
as	O
the	O
Trivial	B-Device
File	I-Device
Transfer	I-Device
Protocol	I-Device
(	O
TFTP	B-Device
)	O
.	O
</s>
<s>
After	O
these	O
parts	O
have	O
been	O
transferred	O
,	O
the	O
operating	B-General_Concept
system	I-General_Concept
takes	O
over	O
the	O
control	O
of	O
the	O
booting	B-Operating_System
process	O
.	O
</s>
<s>
As	O
with	O
the	O
second-stage	O
boot	B-Application
loader	I-Application
,	O
network	B-Device
booting	I-Device
begins	O
by	O
using	O
generic	O
network	O
access	O
methods	O
provided	O
by	O
the	O
network	O
interface	O
's	O
boot	B-Operating_System
ROM	B-Device
,	O
which	O
typically	O
contains	O
a	O
Preboot	B-Device
Execution	I-Device
Environment	I-Device
(	O
PXE	B-Device
)	O
image	O
.	O
</s>
<s>
No	O
drivers	O
are	O
required	O
,	O
but	O
the	O
system	O
functionality	O
is	O
limited	O
until	O
the	O
operating	B-Operating_System
system	I-Operating_System
kernel	I-Operating_System
and	O
drivers	O
are	O
transferred	O
and	O
started	O
.	O
</s>
<s>
As	O
a	O
result	O
,	O
once	O
the	O
ROM-based	O
booting	B-Operating_System
has	O
completed	O
it	O
is	O
entirely	O
possible	O
to	O
network	B-Device
boot	I-Device
into	O
an	O
operating	B-General_Concept
system	I-General_Concept
that	O
itself	O
does	O
not	O
have	O
the	O
ability	O
to	O
use	O
the	O
network	O
interface	O
.	O
</s>
<s>
The	O
boot	B-Operating_System
device	O
is	O
the	O
storage	O
device	O
from	O
which	O
the	O
operating	B-General_Concept
system	I-General_Concept
is	O
loaded	O
.	O
</s>
<s>
A	O
modern	O
PC	O
's	O
UEFI	B-Architecture
or	O
BIOS	B-Operating_System
firmware	B-Application
supports	O
booting	B-Operating_System
from	O
various	O
devices	O
,	O
typically	O
a	O
local	O
solid	B-Device
state	I-Device
drive	I-Device
or	O
hard	B-Device
disk	I-Device
drive	I-Device
via	O
the	O
GPT	B-Application
or	O
Master	B-Device
Boot	I-Device
Record	I-Device
(	O
MBR	B-Device
)	O
on	O
such	O
a	O
drive	O
or	O
disk	O
,	O
an	O
optical	O
disc	O
drive	O
(	O
using	O
El	B-Device
Torito	I-Device
)	O
,	O
a	O
USB	B-Protocol
mass	B-Device
storage	I-Device
device	I-Device
(	O
USB	B-Protocol
flash	O
drive	O
,	O
memory	B-Device
card	I-Device
reader	I-Device
,	O
USB	B-Protocol
hard	B-Device
disk	I-Device
drive	I-Device
,	O
USB	B-Protocol
optical	O
disc	O
drive	O
,	O
USB	B-Protocol
solid	B-Device
state	I-Device
drive	I-Device
,	O
etc	O
.	O
</s>
<s>
)	O
,	O
or	O
a	O
network	O
interface	O
card	B-Architecture
(	O
using	O
PXE	B-Device
)	O
.	O
</s>
<s>
Older	O
,	O
less	O
common	O
BIOS-bootable	O
devices	O
include	O
floppy	B-Device
disk	I-Device
drives	I-Device
,	O
Zip	B-Operating_System
drives	I-Operating_System
,	O
and	O
LS-120	B-Device
drives	I-Device
.	O
</s>
<s>
Typically	O
,	O
the	O
system	O
firmware	B-Application
(	O
UEFI	B-Architecture
or	O
BIOS	B-Operating_System
)	O
will	O
allow	O
the	O
user	O
to	O
configure	O
a	O
boot	B-Operating_System
order	O
.	O
</s>
<s>
If	O
the	O
boot	B-Operating_System
order	O
is	O
set	O
to	O
"	O
first	O
,	O
the	O
DVD	O
drive	O
;	O
second	O
,	O
the	O
hard	B-Device
disk	I-Device
drive	I-Device
"	O
,	O
then	O
the	O
firmware	B-Application
will	O
try	O
to	O
boot	B-Operating_System
from	O
the	O
DVD	O
drive	O
,	O
and	O
if	O
this	O
fails	O
(	O
e.g.	O
</s>
<s>
because	O
there	O
is	O
no	O
DVD	O
in	O
the	O
drive	O
)	O
,	O
it	O
will	O
try	O
to	O
boot	B-Operating_System
from	O
the	O
local	O
hard	B-Device
disk	I-Device
drive	I-Device
.	O
</s>
<s>
For	O
example	O
,	O
on	O
a	O
PC	O
with	O
Windows	B-Application
installed	O
on	O
the	O
hard	B-Device
drive	I-Device
,	O
the	O
user	O
could	O
set	O
the	O
boot	B-Operating_System
order	O
to	O
the	O
one	O
given	O
above	O
,	O
and	O
then	O
insert	O
a	O
Linux	B-Application
Live	O
CD	O
in	O
order	O
to	O
try	O
out	O
Linux	B-Application
without	O
having	O
to	O
install	O
an	O
operating	B-General_Concept
system	I-General_Concept
onto	O
the	O
hard	B-Device
drive	I-Device
.	O
</s>
<s>
This	O
is	O
an	O
example	O
of	O
dual	B-Operating_System
booting	I-Operating_System
,	O
in	O
which	O
the	O
user	O
chooses	O
which	O
operating	B-General_Concept
system	I-General_Concept
to	O
start	O
after	O
the	O
computer	O
has	O
performed	O
its	O
Power-on	B-Device
self-test	I-Device
(	O
POST	O
)	O
.	O
</s>
<s>
In	O
this	O
example	O
of	O
dual	B-Operating_System
booting	I-Operating_System
,	O
the	O
user	O
chooses	O
by	O
inserting	O
or	O
removing	O
the	O
DVD	O
from	O
the	O
computer	O
,	O
but	O
it	O
is	O
more	O
common	O
to	O
choose	O
which	O
operating	B-General_Concept
system	I-General_Concept
to	O
boot	B-Operating_System
by	O
selecting	O
from	O
a	O
boot	B-Application
manager	I-Application
menu	O
on	O
the	O
selected	O
device	O
,	O
by	O
using	O
the	O
computer	O
keyboard	O
to	O
select	O
from	O
a	O
BIOS	B-Operating_System
or	O
UEFI	B-Architecture
Boot	B-Operating_System
Menu	I-Operating_System
,	O
or	O
both	O
;	O
the	O
Boot	B-Operating_System
Menu	I-Operating_System
is	O
typically	O
entered	O
by	O
pressing	O
or	O
keys	O
during	O
the	O
POST	O
;	O
the	O
BIOS	B-Operating_System
Setup	O
is	O
typically	O
entered	O
by	O
pressing	O
or	O
keys	O
during	O
the	O
POST	O
.	O
</s>
<s>
Several	O
devices	O
are	O
available	O
that	O
enable	O
the	O
user	O
to	O
quick-boot	O
into	O
what	O
is	O
usually	O
a	O
variant	O
of	O
Linux	B-Application
for	O
various	O
simple	O
tasks	O
such	O
as	O
Internet	O
access	O
;	O
examples	O
are	O
Splashtop	B-Device
and	O
Latitude	B-Application
ON	I-Application
.	O
</s>
<s>
Upon	O
starting	O
,	O
an	O
IBM-compatible	O
personal	B-Device
computer	I-Device
's	O
x86	B-Operating_System
CPU	O
,	O
executes	O
in	O
real	B-Application
mode	I-Application
,	O
the	O
instruction	O
located	O
at	O
reset	B-General_Concept
vector	I-General_Concept
(	O
the	O
physical	O
memory	O
address	O
on	O
16-bit	O
x86	B-Operating_System
processors	O
and	O
on	O
32-bit	O
and	O
64-bit	O
x86	B-Operating_System
processors	O
)	O
,	O
usually	O
pointing	O
to	O
the	O
firmware	B-Application
(	O
UEFI	B-Architecture
or	O
BIOS	B-Operating_System
)	O
entry	B-Language
point	I-Language
inside	O
the	O
ROM	B-Device
.	O
</s>
<s>
This	O
memory	O
location	O
typically	O
contains	O
a	O
jump	O
instruction	O
that	O
transfers	O
execution	O
to	O
the	O
location	O
of	O
the	O
firmware	B-Application
(	O
UEFI	B-Architecture
or	O
BIOS	B-Operating_System
)	O
start-up	O
program	O
.	O
</s>
<s>
This	O
program	O
runs	O
a	O
power-on	B-Device
self-test	I-Device
(	O
POST	O
)	O
to	O
check	O
and	O
initialize	O
required	O
devices	O
such	O
as	O
main	B-General_Concept
memory	I-General_Concept
(	O
DRAM	O
)	O
,	O
the	O
PCI	O
bus	O
and	O
the	O
PCI	O
devices	O
(	O
including	O
running	O
embedded	O
Option	O
ROMs	O
)	O
.	O
</s>
<s>
One	O
of	O
the	O
most	O
involved	O
steps	O
is	O
setting	O
up	O
DRAM	O
over	O
SPD	B-General_Concept
,	O
further	O
complicated	O
by	O
the	O
fact	O
that	O
at	O
this	O
point	O
memory	O
is	O
very	O
limited	O
.	O
</s>
<s>
After	O
initializing	O
required	O
hardware	B-Architecture
,	O
the	O
firmware	B-Application
(	O
UEFI	B-Architecture
or	O
BIOS	B-Operating_System
)	O
goes	O
through	O
a	O
pre-configured	O
list	O
of	O
non-volatile	B-General_Concept
storage	I-General_Concept
devices	I-General_Concept
(	O
"	O
boot	B-Operating_System
device	O
sequence	O
"	O
)	O
until	O
it	O
finds	O
one	O
that	O
is	O
bootable	B-Operating_System
.	O
</s>
<s>
A	O
bootable	B-Operating_System
MBR	B-Device
device	O
is	O
defined	O
as	O
one	O
that	O
can	O
be	O
read	O
from	O
,	O
and	O
where	O
the	O
last	O
two	O
bytes	B-Application
of	O
the	O
first	O
sector	O
contain	O
the	O
little-endian	O
word	O
,	O
found	O
as	O
byte	B-Application
sequence	O
,	O
on	O
disk	O
(	O
also	O
known	O
as	O
the	O
MBR	B-Device
boot	B-Operating_System
signature	O
)	O
,	O
or	O
where	O
it	O
is	O
otherwise	O
established	O
that	O
the	O
code	O
inside	O
the	O
sector	O
is	O
executable	O
on	O
x86	B-Operating_System
PCs	B-Device
.	O
</s>
<s>
Once	O
the	O
BIOS	B-Operating_System
has	O
found	O
a	O
bootable	B-Operating_System
device	O
it	O
loads	O
the	O
boot	B-Device
sector	I-Device
to	O
linear	O
address	O
(	O
usually	O
segment:offset	B-Device
:	O
,	O
but	O
some	O
BIOSes	O
erroneously	O
use	O
:	O
)	O
and	O
transfers	O
execution	O
to	O
the	O
boot	B-Operating_System
code	O
.	O
</s>
<s>
In	O
the	O
case	O
of	O
a	O
hard	B-Device
disk	I-Device
,	O
this	O
is	O
referred	O
to	O
as	O
the	O
Master	B-Device
Boot	I-Device
Record	I-Device
(	O
MBR	B-Device
)	O
.	O
</s>
<s>
The	O
conventional	O
MBR	B-Device
code	O
checks	O
the	O
MBR	B-Device
's	O
partition	O
table	O
for	O
a	O
partition	O
set	O
as	O
bootable	B-Operating_System
(	O
the	O
one	O
with	O
active	O
flag	O
set	O
)	O
.	O
</s>
<s>
If	O
an	O
active	B-Operating_System
partition	I-Operating_System
is	O
found	O
,	O
the	O
MBR	B-Device
code	O
loads	O
the	O
boot	B-Device
sector	I-Device
code	O
from	O
that	O
partition	O
,	O
known	O
as	O
Volume	B-Device
Boot	I-Device
Record	I-Device
(	O
VBR	B-Device
)	O
,	O
and	O
executes	O
it	O
.	O
</s>
<s>
The	O
MBR	B-Device
boot	B-Operating_System
code	O
is	O
often	O
operating-system	O
specific	O
.	O
</s>
<s>
The	O
boot	B-Device
sector	I-Device
code	O
is	O
the	O
first-stage	O
boot	B-Application
loader	I-Application
.	O
</s>
<s>
It	O
is	O
located	O
on	O
fixed	B-Device
disks	I-Device
and	O
removable	B-Device
drives	I-Device
,	O
and	O
must	O
fit	O
into	O
the	O
first	O
446	O
bytes	B-Application
of	O
the	O
Master	B-Device
Boot	I-Device
Record	I-Device
in	O
order	O
to	O
leave	O
room	O
for	O
the	O
default	O
64-byte	O
partition	O
table	O
with	O
four	O
partition	O
entries	O
and	O
the	O
two-byte	O
boot	B-Operating_System
signature	O
,	O
which	O
the	O
BIOS	B-Operating_System
requires	O
for	O
a	O
proper	O
boot	B-Application
loader	I-Application
or	O
even	O
less	O
,	O
when	O
additional	O
features	O
like	O
more	O
than	O
four	O
partition	O
entries	O
(	O
up	O
to	O
16	O
with	O
16	O
bytes	B-Application
each	O
)	O
,	O
a	O
disk	O
signature	O
(	O
6	O
bytes	B-Application
)	O
,	O
a	O
disk	O
timestamp	O
(	O
6	O
bytes	B-Application
)	O
,	O
an	O
Advanced	O
Active	B-Operating_System
Partition	I-Operating_System
(	O
18	O
bytes	B-Application
)	O
or	O
special	O
multi-boot	B-Operating_System
loaders	O
have	O
to	O
be	O
supported	O
as	O
well	O
in	O
some	O
environments	O
.	O
</s>
<s>
In	O
floppy	B-Device
and	O
superfloppy	O
Volume	B-Device
Boot	I-Device
Records	I-Device
,	O
up	O
to	O
59	O
bytes	B-Application
are	O
occupied	O
for	O
the	O
Extended	B-Device
BIOS	I-Device
Parameter	I-Device
Block	I-Device
on	O
FAT12	B-Application
and	O
FAT16	B-Application
volumes	O
since	O
DOS	O
4.0	O
,	O
whereas	O
the	O
FAT32	B-Application
EBPB	B-Device
introduced	O
with	O
DOS	O
7.1	O
requires	O
even	O
87	O
bytes	B-Application
,	O
leaving	O
only	O
423	O
bytes	B-Application
for	O
the	O
boot	B-Application
loader	I-Application
when	O
assuming	O
a	O
sector	O
size	O
of	O
512	O
bytes	B-Application
.	O
</s>
<s>
Microsoft	O
boot	B-Device
sectors	I-Device
therefore	O
traditionally	O
imposed	O
certain	O
restrictions	O
on	O
the	O
boot	B-Operating_System
process	I-Operating_System
,	O
for	O
example	O
,	O
the	O
boot	B-Operating_System
file	O
had	O
to	O
be	O
located	O
at	O
a	O
fixed	O
position	O
in	O
the	O
root	O
directory	O
of	O
the	O
file	O
system	O
and	O
stored	O
as	O
consecutive	O
sectors	O
,	O
conditions	O
taken	O
care	O
of	O
by	O
the	O
SYS	B-Language
command	I-Language
and	O
slightly	O
relaxed	O
in	O
later	O
versions	O
of	O
DOS	O
.	O
</s>
<s>
The	O
boot	B-Application
loader	I-Application
was	O
then	O
able	O
to	O
load	O
the	O
first	O
three	O
sectors	O
of	O
the	O
file	O
into	O
memory	O
,	O
which	O
happened	O
to	O
contain	O
another	O
embedded	O
boot	B-Application
loader	I-Application
able	O
to	O
load	O
the	O
remainder	O
of	O
the	O
file	O
into	O
memory	O
.	O
</s>
<s>
When	O
Microsoft	O
added	O
LBA	B-Device
and	O
FAT32	B-Application
support	O
,	O
they	O
even	O
switched	O
to	O
a	O
boot	B-Application
loader	I-Application
reaching	O
over	O
two	O
physical	O
sectors	O
and	O
using	O
386	O
instructions	O
for	O
size	O
reasons	O
.	O
</s>
<s>
At	O
the	O
same	O
time	O
other	O
vendors	O
managed	O
to	O
squeeze	O
much	O
more	O
functionality	O
into	O
a	O
single	O
boot	B-Device
sector	I-Device
without	O
relaxing	O
the	O
original	O
constraints	O
on	O
only	O
minimal	O
available	O
memory	O
(	O
32KB	O
)	O
and	O
processor	O
support	O
(	O
)	O
.	O
</s>
<s>
For	O
example	O
,	O
DR-DOS	B-Application
boot	B-Device
sectors	I-Device
are	O
able	O
to	O
locate	O
the	O
boot	B-Operating_System
file	O
in	O
the	O
FAT12	B-Application
,	O
FAT16	B-Application
and	O
FAT32	B-Application
file	O
system	O
,	O
and	O
load	O
it	O
into	O
memory	O
as	O
a	O
whole	O
via	O
CHS	B-Device
or	O
LBA	B-Device
,	O
even	O
if	O
the	O
file	O
is	O
not	O
stored	O
in	O
a	O
fixed	O
location	O
and	O
in	O
consecutive	O
sectors	O
.	O
</s>
<s>
The	O
VBR	B-Device
is	O
often	O
OS-specific	O
;	O
however	O
,	O
its	O
main	B-Language
function	I-Language
is	O
to	O
load	O
and	O
execute	O
the	O
operating	B-General_Concept
system	I-General_Concept
boot	B-Application
loader	I-Application
file	O
(	O
such	O
as	O
bootmgr	B-Device
or	O
ntldr	B-Application
)	O
,	O
which	O
is	O
the	O
second-stage	O
boot	B-Application
loader	I-Application
,	O
from	O
an	O
active	B-Operating_System
partition	I-Operating_System
.	O
</s>
<s>
Then	O
the	O
boot	B-Application
loader	I-Application
loads	O
the	O
OS	B-Operating_System
kernel	I-Operating_System
from	O
the	O
storage	O
device	O
.	O
</s>
<s>
If	O
there	O
is	O
no	O
active	B-Operating_System
partition	I-Operating_System
,	O
or	O
the	O
active	B-Operating_System
partition	I-Operating_System
's	O
boot	B-Device
sector	I-Device
is	O
invalid	O
,	O
the	O
MBR	B-Device
may	O
load	O
a	O
secondary	O
boot	B-Application
loader	I-Application
which	O
will	O
select	O
a	O
partition	O
(	O
often	O
via	O
user	O
input	O
)	O
and	O
load	O
its	O
boot	B-Device
sector	I-Device
,	O
which	O
usually	O
loads	O
the	O
corresponding	O
operating	B-Operating_System
system	I-Operating_System
kernel	I-Operating_System
.	O
</s>
<s>
In	O
some	O
cases	O
,	O
the	O
MBR	B-Device
may	O
also	O
attempt	O
to	O
load	O
secondary	O
boot	B-Application
loaders	I-Application
before	O
trying	O
to	O
boot	B-Operating_System
the	O
active	B-Operating_System
partition	I-Operating_System
.	O
</s>
<s>
If	O
all	O
else	O
fails	O
,	O
it	O
should	O
issue	O
an	O
INT	B-Device
18h	O
BIOS	B-Device
interrupt	I-Device
call	I-Device
(	O
followed	O
by	O
an	O
INT	B-Device
19h	O
just	O
in	O
case	O
INT	B-Device
18h	O
would	O
return	O
)	O
in	O
order	O
to	O
give	O
back	O
control	O
to	O
the	O
BIOS	B-Operating_System
,	O
which	O
would	O
then	O
attempt	O
to	O
boot	B-Operating_System
off	O
other	O
devices	O
,	O
attempt	O
a	O
remote	B-Device
boot	I-Device
via	O
network	O
.	O
</s>
<s>
Many	O
modern	O
systems	O
(	O
Intel	B-Device
Macs	I-Device
and	O
newer	O
PCs	B-Device
)	O
use	O
UEFI	B-Architecture
.	O
</s>
<s>
Unlike	O
BIOS	B-Operating_System
,	O
UEFI	B-Architecture
(	O
not	O
Legacy	O
boot	B-Operating_System
via	O
CSM	O
)	O
does	O
not	O
rely	O
on	O
boot	B-Device
sectors	I-Device
,	O
UEFI	B-Architecture
system	O
loads	O
the	O
boot	B-Application
loader	I-Application
(	O
EFI	O
application	O
file	O
in	O
USB	B-Protocol
disk	O
or	O
in	O
the	O
EFI	B-Device
System	I-Device
Partition	I-Device
)	O
directly	O
,	O
and	O
the	O
OS	B-Operating_System
kernel	I-Operating_System
is	O
loaded	O
by	O
the	O
boot	B-Application
loader	I-Application
.	O
</s>
<s>
Many	O
modern	O
CPUs	O
,	O
SoCs	O
and	O
microcontrollers	O
(	O
for	O
example	O
,	O
TI	B-Device
OMAP	I-Device
)	O
or	O
sometimes	O
even	O
digital	B-Architecture
signal	I-Architecture
processors	I-Architecture
(	O
DSPs	O
)	O
may	O
have	O
boot	B-Operating_System
ROM	B-Device
integrated	O
directly	O
into	O
their	O
silicon	O
,	O
so	O
such	O
a	O
processor	O
can	O
perform	O
a	O
simple	O
boot	B-Operating_System
sequence	I-Operating_System
on	O
its	O
own	O
and	O
load	O
boot	B-Operating_System
programs	I-Operating_System
(	O
firmware	B-Application
or	O
software	O
)	O
from	O
boot	B-Operating_System
sources	O
such	O
as	O
NAND	O
flash	O
or	O
eMMC	O
.	O
</s>
<s>
It	O
is	O
difficult	O
to	O
hardwire	O
all	O
the	O
required	O
logic	O
for	O
handling	O
such	O
devices	O
,	O
so	O
an	O
integrated	O
boot	B-Operating_System
ROM	B-Device
is	O
used	O
instead	O
in	O
such	O
scenarios	O
.	O
</s>
<s>
Also	O
,	O
a	O
boot	B-Operating_System
ROM	B-Device
may	O
be	O
able	O
to	O
load	O
a	O
boot	B-Application
loader	I-Application
or	O
diagnostic	O
program	O
via	O
serial	O
interfaces	O
like	O
UART	O
,	O
SPI	B-Architecture
,	O
USB	B-Protocol
and	O
so	O
on	O
.	O
</s>
<s>
This	O
feature	O
is	O
often	O
used	O
for	O
system	O
recovery	O
purposes	O
,	O
or	O
it	O
could	O
also	O
be	O
used	O
for	O
initial	O
non-volatile	B-General_Concept
memory	I-General_Concept
programming	O
when	O
there	O
is	O
no	O
software	O
available	O
in	O
the	O
non-volatile	B-General_Concept
memory	I-General_Concept
yet	O
.	O
</s>
<s>
flash	B-Device
memory	I-Device
controller	O
on	O
some	O
USB	B-Protocol
flash	O
drives	O
)	O
have	O
firmware	B-Application
ROM	B-Device
integrated	O
directly	O
into	O
their	O
silicon	O
.	O
</s>
<s>
Some	O
embedded	B-Architecture
system	I-Architecture
designs	O
may	O
also	O
include	O
an	O
intermediary	O
boot	B-Operating_System
sequence	I-Operating_System
step	O
.	O
</s>
<s>
For	O
example	O
,	O
Das	B-Language
U-Boot	I-Language
may	O
be	O
split	O
into	O
two	O
stages	O
:	O
the	O
platform	O
would	O
load	O
a	O
small	O
SPL	O
(	O
Secondary	O
Program	O
Loader	O
)	O
,	O
which	O
is	O
a	O
stripped-down	O
version	O
of	O
U-Boot	O
,	O
and	O
the	O
SPL	O
would	O
do	O
some	O
initial	O
hardware	B-Architecture
configuration	O
(	O
e.g.	O
</s>
<s>
DRAM	O
initialization	O
using	O
CPU	O
cache	O
as	O
RAM	B-Architecture
)	O
and	O
load	O
the	O
larger	O
,	O
fully	O
featured	O
version	O
of	O
U-Boot	O
.	O
</s>
<s>
It	O
is	O
also	O
possible	O
to	O
take	O
control	O
of	O
a	O
system	O
by	O
using	O
a	O
hardware	B-Architecture
debug	O
interface	O
such	O
as	O
JTAG	O
.	O
</s>
<s>
Such	O
an	O
interface	O
may	O
be	O
used	O
to	O
write	O
the	O
boot	B-Application
loader	I-Application
program	O
into	O
bootable	B-Operating_System
non-volatile	B-General_Concept
memory	I-General_Concept
(	O
e.g.	O
</s>
<s>
flash	O
)	O
by	O
instructing	O
the	O
processor	O
core	O
to	O
perform	O
the	O
necessary	O
actions	O
to	O
program	O
non-volatile	B-General_Concept
memory	I-General_Concept
.	O
</s>
<s>
Alternatively	O
,	O
the	O
debug	O
interface	O
may	O
be	O
used	O
to	O
upload	O
some	O
diagnostic	O
or	O
boot	B-Operating_System
code	O
into	O
RAM	B-Architecture
,	O
and	O
then	O
to	O
start	O
the	O
processor	O
core	O
and	O
instruct	O
it	O
to	O
execute	O
the	O
uploaded	O
code	O
.	O
</s>
<s>
This	O
allows	O
,	O
for	O
example	O
,	O
the	O
recovery	O
of	O
embedded	B-Architecture
systems	I-Architecture
where	O
no	O
software	O
remains	O
on	O
any	O
supported	O
boot	B-Operating_System
device	O
,	O
and	O
where	O
the	O
processor	O
does	O
not	O
have	O
any	O
integrated	O
boot	B-Operating_System
ROM	B-Device
.	O
</s>
<s>
Some	O
microcontrollers	O
provide	O
special	O
hardware	B-Architecture
interfaces	O
which	O
cannot	O
be	O
used	O
to	O
take	O
arbitrary	O
control	O
of	O
a	O
system	O
or	O
directly	O
run	O
code	O
,	O
but	O
instead	O
they	O
allow	O
the	O
insertion	O
of	O
boot	B-Operating_System
code	O
into	O
bootable	B-Operating_System
non-volatile	B-General_Concept
memory	I-General_Concept
(	O
like	O
flash	B-Device
memory	I-Device
)	O
via	O
simple	O
protocols	O
.	O
</s>
<s>
Then	O
at	O
the	O
manufacturing	O
phase	O
,	O
such	O
interfaces	O
are	O
used	O
to	O
inject	O
boot	B-Operating_System
code	O
(	O
and	O
possibly	O
other	O
code	O
)	O
into	O
non-volatile	B-General_Concept
memory	I-General_Concept
.	O
</s>
<s>
After	O
system	O
reset	O
,	O
the	O
microcontroller	O
begins	O
to	O
execute	O
code	O
programmed	O
into	O
its	O
non-volatile	B-General_Concept
memory	I-General_Concept
,	O
just	O
like	O
usual	O
processors	O
are	O
using	O
ROMs	O
for	O
booting	B-Operating_System
.	O
</s>
<s>
Most	O
notably	O
this	O
technique	O
is	O
used	O
by	O
Atmel	B-Architecture
AVR	I-Architecture
microcontrollers	O
,	O
and	O
by	O
others	O
as	O
well	O
.	O
</s>
<s>
In	O
other	O
cases	O
such	O
interfaces	O
could	O
be	O
created	O
by	O
software	O
running	O
in	O
integrated	O
on-chip	O
boot	B-Operating_System
ROM	B-Device
from	O
GPIO	B-Architecture
pins	O
.	O
</s>
<s>
Most	O
DSPs	O
have	O
a	O
serial	O
mode	O
boot	B-Operating_System
,	O
and	O
a	O
parallel	O
mode	O
boot	B-Operating_System
,	O
such	O
as	O
the	O
host	O
port	O
interface	O
(	O
HPI	O
boot	B-Operating_System
)	O
.	O
</s>
<s>
Such	O
a	O
processor	O
is	O
also	O
sometimes	O
referred	O
as	O
the	O
master	O
,	O
since	O
it	O
usually	O
boots	O
first	O
from	O
its	O
own	O
memories	O
and	O
then	O
controls	O
overall	O
system	O
behavior	O
,	O
including	O
booting	B-Operating_System
of	O
the	O
DSP	O
,	O
and	O
then	O
further	O
controlling	O
the	O
DSP	O
's	O
behavior	O
.	O
</s>
<s>
The	O
DSP	O
often	O
lacks	O
its	O
own	O
boot	B-Operating_System
memories	O
and	O
relies	O
on	O
the	O
host	O
processor	O
to	O
supply	O
the	O
required	O
code	O
instead	O
.	O
</s>
<s>
Many	O
FPGA	B-Architecture
chips	O
load	O
their	O
configuration	O
from	O
an	O
external	O
serial	O
EEPROM	B-General_Concept
(	O
"	O
configuration	O
ROM	B-Device
"	O
)	O
on	O
power-up	O
.	O
</s>
<s>
There	O
are	O
various	O
measures	O
have	O
been	O
implemented	O
which	O
enhance	O
the	O
security	O
of	O
the	O
booting	B-Operating_System
process	O
.	O
</s>
<s>
Traditionally	O
,	O
booting	B-Operating_System
did	O
not	O
involve	O
the	O
use	O
of	O
cryptography	O
.	O
</s>
<s>
The	O
security	O
can	O
be	O
bypassed	O
by	O
unlocking	B-Application
the	I-Application
bootloader	I-Application
,	O
which	O
might	O
or	O
might	O
not	O
be	O
approved	O
by	O
the	O
manufacturer	O
.	O
</s>
<s>
Matthew	O
Garrett	O
argued	O
that	O
booting	B-Operating_System
security	O
serve	O
a	O
legitimate	O
goal	O
but	O
in	O
doing	O
so	O
choose	O
defaults	B-General_Concept
that	O
are	O
hostile	O
to	O
users	O
.	O
</s>
<s>
Measured	O
boot	B-Operating_System
with	O
the	O
Trusted	O
Platform	O
Module	O
,	O
also	O
known	O
as	O
"	O
trusted	O
boot	B-Operating_System
"	O
.	O
</s>
