<s>
iBoot	B-Operating_System
is	O
the	O
stage	O
2	O
bootloader	B-Application
for	O
all	O
Apple	O
products	O
.	O
</s>
<s>
It	O
replaces	O
the	O
old	O
bootloader	B-Application
,	O
BootX	B-Operating_System
.	O
</s>
<s>
Compared	O
with	O
its	O
predecessor	O
,	O
iBoot	B-Operating_System
improves	O
authentication	O
performed	O
in	O
the	O
boot	O
chain	O
.	O
</s>
<s>
For	O
x86	B-Operating_System
macOS	B-Application
,	O
the	O
boot	O
process	O
starts	O
by	O
running	O
code	O
stored	O
in	O
secured	O
UEFI	B-Architecture
Boot	O
ROM	O
(	O
first	O
stage	O
)	O
.	O
</s>
<s>
Boot	O
ROM	O
has	O
two	O
primary	O
responsibilities	O
:	O
to	O
initialize	O
system	O
hardware	O
and	O
to	O
select	O
an	O
operating	O
system	O
to	O
run	O
(	O
the	O
POST	B-Device
and	O
UEFI	B-Architecture
component	O
)	O
.	O
</s>
<s>
For	O
ARM	B-Architecture
macOS	B-Application
,	O
the	O
Boot	O
ROM	O
is	O
not	O
UEFI	B-Architecture
component	O
.	O
</s>
<s>
For	O
iOS	B-Application
,	O
the	O
boot	O
process	O
starts	O
by	O
running	O
the	O
device	O
's	O
Boot	O
ROM	O
code	O
.	O
</s>
<s>
In	O
systems	O
with	O
S1	O
processors	O
or	O
A9	O
or	O
earlier	O
A-series	O
processors	O
,	O
the	O
Boot	O
ROM	O
loads	O
the	O
Low-Level	O
Bootloader	B-Application
(	O
LLB	O
)	O
,	O
which	O
loads	O
iBoot	B-Operating_System
.	O
</s>
<s>
In	O
systems	O
with	O
newer	O
processors	O
,	O
the	O
Boot	O
ROM	O
loads	O
iBoot	B-Operating_System
itself	O
.	O
</s>
<s>
If	O
all	O
goes	O
well	O
,	O
iBoot	B-Operating_System
will	O
then	O
proceed	O
to	O
load	O
the	O
iOS	B-Application
kernel	B-Operating_System
as	O
well	O
as	O
the	O
rest	O
of	O
the	O
operating	O
system	O
.	O
</s>
<s>
If	O
the	O
LLB	O
or	O
iBoot	B-Operating_System
fails	O
to	O
load	O
iOS	B-Application
,	O
or	O
fails	O
to	O
verify	O
iOS	B-Application
,	O
the	O
bootloader	B-Application
jumps	O
to	O
DFU	O
(	O
Device	O
Firmware	O
Update	O
)	O
mode	O
;	O
otherwise	O
it	O
loads	O
the	O
remaining	O
kernel	B-Operating_System
modules	O
.	O
</s>
<s>
On	O
x86	B-Operating_System
macOS	B-Application
,	O
iBoot	B-Operating_System
is	O
located	O
in	O
/System/Library/CoreServices/boot.efi	O
.	O
</s>
<s>
Once	O
the	O
kernel	B-Operating_System
and	O
all	O
drivers	O
necessary	O
for	O
booting	O
are	O
loaded	O
,	O
the	O
boot	B-Application
loader	I-Application
starts	O
the	O
kernel	B-Operating_System
’s	O
initialization	O
procedure	O
.	O
</s>
<s>
At	O
this	O
point	O
,	O
enough	O
drivers	O
are	O
loaded	O
for	O
the	O
kernel	B-Operating_System
to	O
find	O
the	O
root	O
device	O
.	O
</s>
<s>
Apple	O
has	O
modified	O
the	O
C	B-Language
compiler	O
toolchain	B-General_Concept
that	O
is	O
used	O
to	O
build	O
iBoot	B-Operating_System
in	O
order	O
to	O
advance	O
memory	B-Application
safety	I-Application
since	O
iOS	B-Operating_System
14	I-Operating_System
.	O
</s>
<s>
This	O
advancement	O
is	O
designed	O
to	O
mitigate	O
entire	O
classes	O
of	O
common	O
memory	O
corruption	O
vulnerabilities	O
such	O
as	O
buffer	B-General_Concept
overflows	I-General_Concept
,	O
heap	B-General_Concept
exploitations	I-General_Concept
,	O
type	O
confusion	O
vulnerabilities	O
,	O
and	O
use-after-free	O
attacks	O
.	O
</s>
