<s>
A	O
bootloader	B-Application
,	O
also	O
spelled	O
as	O
boot	B-Application
loader	I-Application
or	O
called	O
boot	B-Application
manager	I-Application
and	O
bootstrap	B-Operating_System
loader	I-Operating_System
,	O
is	O
a	O
computer	B-Application
program	I-Application
that	O
is	O
responsible	O
for	O
booting	B-Operating_System
a	O
computer	O
.	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
RAM	B-Architecture
(	O
especially	O
on	O
x86	O
systems	O
)	O
,	O
to	O
access	O
the	O
nonvolatile	O
device	O
(	O
usually	O
block	O
device	O
,	O
eg	O
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>
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	O
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	O
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
or	O
an	O
IBM	B-Device
PC	I-Device
compatible	O
)	O
at	O
a	O
predefined	O
address	O
(	O
some	O
CPUs	O
,	O
including	O
the	O
Intel	O
x86	B-General_Concept
series	I-General_Concept
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-Operating_System
loaders	I-Operating_System
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
earlier	O
IBM	B-Device
PC	I-Device
and	O
compatibles	O
,	O
a	O
boot	B-Device
sector	I-Device
should	O
typically	O
work	O
in	O
only	O
32KiB	O
(	O
later	O
relaxed	O
to	O
64	O
KiB	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>
The	O
first	O
stage	O
of	O
PC	O
boot	B-Operating_System
loaders	I-Operating_System
(	O
FSBL	O
,	O
first-stage	O
boot	B-Application
loader	I-Application
)	O
located	O
on	O
fixed	B-Device
disks	I-Device
and	O
removable	B-Device
drives	I-Device
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	O
Partition	O
(	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	O
and	O
FAT16	O
volumes	O
since	O
DOS	O
4.0	O
,	O
whereas	O
the	O
FAT32	O
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	O
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
32KiB	O
)	O
and	O
processor	O
support	O
(	O
8088/8086	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	O
,	O
FAT16	O
and	O
FAT32	O
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>
BIOS	B-Operating_System
and	O
UEFI	B-Architecture
not	O
only	O
load	O
the	O
operating	B-General_Concept
system	I-General_Concept
from	O
a	O
non-volatile	B-General_Concept
device	O
,	O
they	O
also	O
initialize	O
system	O
hardware	O
for	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
Examples	O
of	O
first-stage	O
bootloaders	B-Application
include	O
BIOS	B-Operating_System
,	O
coreboot	B-Device
,	O
Libreboot	B-Device
and	O
Das	B-Language
U-Boot	I-Language
.	O
</s>
<s>
Second-stage	O
boot	B-Operating_System
loaders	I-Operating_System
,	O
such	O
as	O
GNU	B-Application
GRUB	I-Application
,	O
rEFInd	B-Device
,	O
BOOTMGR	O
,	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	O
such	O
as	O
the	O
BIOS	B-Operating_System
or	O
Open	B-Architecture
Firmware	I-Architecture
,	O
though	O
typically	O
with	O
restricted	O
hardware	O
functionality	O
and	O
lower	O
performance	O
.	O
</s>
<s>
Many	O
boot	B-Operating_System
loaders	I-Operating_System
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-Operating_System
loaders	I-Operating_System
can	O
also	O
load	O
other	O
boot	B-Operating_System
loaders	I-Operating_System
;	O
for	O
example	O
,	O
GRUB	B-Application
loads	O
BOOTMGR	O
instead	O
of	O
loading	O
Windows	O
directly	O
.	O
</s>
<s>
Usually	O
,	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>
For	O
example	O
,	O
waiting	O
a	O
minute	O
for	O
a	O
digital	B-Application
television	I-Application
or	O
a	O
GPS	O
navigation	O
device	O
to	O
start	O
is	O
generally	O
unacceptable	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	O
booting	B-Operating_System
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	O
)	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-General_Concept
system	I-General_Concept
kernel	O
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	O
boot	B-Operating_System
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>
