<s>
On	O
the	O
classic	B-Application
Mac	I-Application
OS	I-Application
(	O
the	O
original	O
Apple	B-Operating_System
Macintosh	I-Operating_System
operating	I-Operating_System
system	I-Operating_System
)	O
,	O
extensions	O
were	O
small	O
pieces	O
of	O
code	O
that	O
extended	O
the	O
system	O
's	O
functionality	O
.	O
</s>
<s>
Extensions	O
generally	O
filled	O
the	O
same	O
role	O
as	O
DOS	B-Device
's	O
terminate	O
and	O
stay	O
resident	O
programs	O
,	O
or	O
Unix	B-Application
's	O
daemons	B-Operating_System
,	O
although	O
by	O
patching	O
the	O
underlying	O
OS	O
code	O
,	O
they	O
had	O
the	O
capability	O
to	O
modify	O
existing	O
OS	O
behaviour	O
,	O
the	O
other	O
two	O
did	O
not	O
.	O
</s>
<s>
The	O
concept	O
of	O
extensions	O
was	O
not	O
present	O
in	O
the	O
original	O
Macintosh	B-Application
system	I-Application
software	I-Application
,	O
but	O
the	O
system	O
nevertheless	O
had	O
a	O
private	O
patching	O
mechanism	O
that	O
developers	O
soon	O
learned	O
to	O
take	O
advantage	O
of	O
-	O
the	O
INIT	B-Operating_System
loader	O
.	O
</s>
<s>
This	O
code	O
would	O
search	O
for	O
system	O
resources	O
of	O
type	O
'	O
INIT	B-Operating_System
 '	O
,	O
and	O
load	O
and	O
run	O
them	O
at	O
boot	O
time	O
.	O
</s>
<s>
The	O
code	O
resources	O
had	O
to	O
be	O
stored	O
directly	O
in	O
the	O
Mac	O
System	B-Application
suitcase	I-Application
's	O
resource	B-Operating_System
fork	I-Operating_System
,	O
meaning	O
it	O
was	O
only	O
really	O
available	O
to	O
"	O
power	O
users	O
"	O
who	O
would	O
be	O
comfortable	O
using	O
ResEdit	B-Operating_System
or	O
other	O
resource	O
editor	O
.	O
</s>
<s>
Since	O
taking	O
advantage	O
of	O
this	O
mechanism	O
was	O
an	O
unsupported	O
hack	O
,	O
and	O
only	O
32	O
INITs	B-Operating_System
could	O
be	O
loaded	O
in	O
this	O
manner	O
,	O
Apple	O
responded	O
by	O
providing	O
a	O
more	O
managed	O
solution	O
.	O
</s>
<s>
Initially	O
this	O
itself	O
was	O
in	O
the	O
form	O
of	O
an	O
'	O
INIT	B-Operating_System
 '	O
resource	O
with	O
ID	O
31	O
placed	O
in	O
the	O
System	B-Application
file	I-Application
that	O
would	O
search	O
for	O
further	O
files	O
of	O
type	O
'	O
INIT	B-Operating_System
 '	O
in	O
the	O
System	B-Operating_System
Folder	I-Operating_System
,	O
and	O
load	O
and	O
run	O
INIT	B-Operating_System
resources	O
inside	O
them	O
.	O
</s>
<s>
(	O
This	O
is	O
why	O
some	O
veteran	O
Mac	O
programmers	O
still	O
refer	O
to	O
the	O
extensions	O
loading	O
mechanism	O
as	O
the	O
"	O
INIT	B-Operating_System
31	O
trick	O
"	O
.	O
</s>
<s>
INITs	B-Operating_System
could	O
now	O
be	O
installed	O
simply	O
by	O
placing	O
a	O
file	O
in	O
the	O
System	B-Operating_System
Folder	I-Operating_System
,	O
well	O
within	O
the	O
abilities	O
of	O
the	O
average	O
user	O
.	O
</s>
<s>
Starting	O
with	O
System	B-Operating_System
7	I-Operating_System
,	O
extensions	O
were	O
relocated	O
to	O
the	O
Extensions	O
folder	O
within	O
the	O
System	B-Operating_System
Folder	I-Operating_System
for	O
convenience	O
,	O
and	O
an	O
auto-routing	O
mechanism	O
was	O
implemented	O
so	O
that	O
placing	O
an	O
extension	O
into	O
the	O
System	B-Operating_System
Folder	I-Operating_System
through	O
drag	O
and	O
drop	O
would	O
actually	O
place	O
the	O
file	O
in	O
the	O
appropriate	O
subfolder	O
.	O
</s>
<s>
Extensions	O
retained	O
the	O
resource	O
type	O
of	O
'	O
INIT	B-Operating_System
 '	O
throughout	O
their	O
lifetime	O
,	O
and	O
the	O
loader	O
was	O
gradually	O
enhanced	O
to	O
search	O
for	O
these	O
resources	O
in	O
numerous	O
places	O
,	O
including	O
in	O
the	O
resource	B-Operating_System
forks	I-Operating_System
of	O
control	B-Operating_System
panels	I-Operating_System
in	O
a	O
variety	O
of	O
formats	O
and	O
the	O
Chooser	B-Operating_System
.	O
</s>
<s>
INITs	B-Operating_System
evolved	O
into	O
system	O
extensions	O
,	O
gaining	O
additional	O
ad	O
hoc	O
protocols	O
along	O
the	O
way	O
,	O
such	O
as	O
supplying	O
an	O
icon	O
to	O
be	O
displayed	O
at	O
boot	O
time	O
(	O
the	O
origin	O
of	O
this	O
was	O
'	O
ShowINIT	O
 '	O
)	O
.	O
</s>
<s>
Apple	O
themselves	O
eventually	O
released	O
major	O
(	O
but	O
optional	O
)	O
pieces	O
of	O
the	O
operating	O
system	O
as	O
extensions	O
,	O
such	O
as	O
QuickTime	B-Application
,	O
QuickDraw	B-Operating_System
3D	I-Operating_System
and	O
many	O
others	O
.	O
</s>
<s>
A	O
substantial	O
number	O
of	O
services	O
and	O
drivers	O
in	O
Mac	B-Operating_System
OS	I-Operating_System
—	O
both	O
official	O
and	O
third	O
party	O
—	O
were	O
provided	O
as	O
extensions	O
,	O
allowing	O
for	O
the	O
OS	O
to	O
be	O
trimmed	O
down	O
by	O
disabling	O
them	O
.	O
</s>
<s>
In	O
addition	O
,	O
different	O
extensions	O
might	O
try	O
to	O
patch	O
the	O
same	O
part	O
of	O
the	O
system	O
,	O
which	O
could	O
lead	O
to	O
extension	B-Operating_System
conflicts	I-Operating_System
and	O
other	O
instability	O
.	O
</s>
<s>
Troubleshooting	O
Mac	B-Operating_System
OS	I-Operating_System
extensions	O
could	O
be	O
a	O
time-consuming	O
process	O
of	O
bisecting	O
and	O
trial	O
and	O
error	O
.	O
</s>
<s>
System	O
7.5	O
added	O
the	O
Extensions	B-Operating_System
Manager	I-Operating_System
,	O
which	O
allowed	O
the	O
user	O
to	O
quickly	O
enable	O
or	O
disable	O
particular	O
extensions	O
,	O
and	O
also	O
to	O
define	O
sets	O
of	O
them	O
that	O
would	O
work	O
correctly	O
together	O
.	O
</s>
<s>
Extensions	B-Operating_System
Manager	I-Operating_System
came	O
with	O
two	O
read-only	O
base	O
sets	O
provided	O
:	O
one	O
that	O
contained	O
the	O
subset	O
of	O
extensions	O
needed	O
for	O
basic	O
OS	O
operation	O
,	O
and	O
one	O
that	O
enabled	O
all	O
the	O
official	O
extensions	O
that	O
shipped	O
with	O
the	O
OS	O
but	O
disabled	O
all	O
third-party	O
extensions	O
.	O
</s>
<s>
People	O
figured	O
out	O
that	O
changing	O
the	O
first	O
character	O
in	O
the	O
file	O
name	O
could	O
change	O
the	O
extension	O
loading	O
order	O
,	O
which	O
caused	O
trouble	O
when	O
Mac	B-Operating_System
OS	I-Operating_System
8.1	O
moved	O
to	O
HFS+	O
.	O
</s>
<s>
With	O
System	B-Operating_System
7	I-Operating_System
,	O
control	B-Operating_System
panels	I-Operating_System
become	O
separate	O
Finder	O
plugins	O
on	O
disc	O
that	O
could	O
be	O
launched	O
by	O
the	O
user	O
.	O
</s>
<s>
By	O
inserting	O
INIT	B-Operating_System
code	O
into	O
a	O
control	B-Operating_System
panel	I-Operating_System
,	O
it	O
became	O
possible	O
to	O
build	O
extension/control	O
panel	O
hybrids	O
that	O
modified	O
the	O
operating	O
system	O
at	O
boot	O
time	O
and	O
contained	O
their	O
own	O
in-built	O
configuration	O
interface	O
in	O
the	O
same	O
form	O
as	O
any	O
other	O
operating	O
system	O
control	B-Operating_System
panel	I-Operating_System
.	O
</s>
<s>
MultiFinder	O
and	O
System	B-Operating_System
7	I-Operating_System
and	O
later	O
supported	O
faceless	O
background	O
applications	O
similar	O
to	O
UNIX	B-Operating_System
daemons	I-Operating_System
or	O
Windows	O
Services	O
,	O
though	O
using	O
cooperative	B-Operating_System
multitasking	I-Operating_System
.	O
</s>
<s>
The	O
Control	O
Strip	O
in	O
Mac	B-Operating_System
OS	I-Operating_System
8	O
and	O
9	O
was	O
an	O
example	O
of	O
a	O
faceless	O
background	O
application	O
that	O
displayed	O
a	O
global	O
floating	O
window	O
to	O
provide	O
user	O
interaction	O
.	O
</s>
<s>
Language	O
features	O
in	O
the	O
Open	O
Scripting	O
Architecture	O
(	O
and	O
thus	O
AppleScript	B-Application
)	O
were	O
initially	O
implemented	O
as	O
dynamically	O
loadable	O
plugins	O
known	O
as	O
"	O
scripting	O
additions	O
"	O
or	O
OSAXes	O
.	O
</s>
<s>
In	O
Mac	B-Operating_System
OS	I-Operating_System
8	O
and	O
9	O
,	O
these	O
were	O
augmented	O
by	O
faceless	O
background	O
applications	O
that	O
were	O
loaded	O
in	O
the	O
background	O
on	O
demand	O
.	O
</s>
<s>
INIT-type	O
extensions	O
were	O
loaded	O
at	O
boot	O
time	O
to	O
update	O
the	O
operating	O
system	O
.	O
</s>
<s>
INIT-type	O
files	O
were	O
not	O
the	O
only	O
type	O
of	O
system	O
extension	O
.	O
</s>
<s>
The	O
BootX	B-Application
Linux	O
bootloader	O
was	O
implemented	O
as	O
a	O
scri	O
simply	O
because	O
such	O
files	O
were	O
loaded	O
very	O
early	O
on	O
in	O
the	O
boot	O
process	O
,	O
before	O
all	O
other	O
extensions	O
.	O
</s>
<s>
BootX	B-Application
could	O
then	O
display	O
a	O
dialog	O
offering	O
to	O
let	O
the	O
user	O
finish	O
booting	O
Mac	B-Operating_System
OS	I-Operating_System
or	O
load	O
Linux	O
instead	O
.	O
</s>
