<s>
An	O
application	B-Application
directory	I-Application
is	O
a	O
grouping	O
of	O
software	O
code	B-Application
,	O
help	O
files	O
and	O
resources	O
that	O
together	O
comprise	O
a	O
complete	O
software	B-Application
package	I-Application
but	O
are	O
presented	O
to	O
the	O
user	O
as	O
a	O
single	O
object	O
.	O
</s>
<s>
They	O
are	O
currently	O
used	O
in	O
RISC	B-Operating_System
OS	I-Operating_System
and	O
the	O
ROX	B-Language
Desktop	I-Language
,	O
and	O
also	O
form	O
the	O
basis	O
of	O
the	O
Zero	B-Application
Install	I-Application
application	O
distribution	O
system	O
.	O
</s>
<s>
Similar	O
technology	O
includes	O
VMware	B-Application
ThinApp	I-Application
,	O
and	O
the	O
NEXTSTEP/GNUstep/Mac	O
OS	O
X	O
concept	O
of	O
application	O
bundles	B-Application
.	O
</s>
<s>
Their	O
heritage	O
lies	O
in	O
the	O
system	O
for	O
automatically	O
launching	O
software	O
stored	O
on	O
floppy	B-Device
disk	I-Device
on	O
Acorn	O
's	O
earlier	O
8-bit	O
micros	O
such	O
as	O
the	O
BBC	B-Device
Micro	I-Device
(	O
the	O
!	O
BOOT	O
file	O
)	O
.	O
</s>
<s>
Bundling	O
various	O
files	O
in	O
this	O
manner	O
allows	O
tools	O
for	O
manipulating	O
applications	O
to	O
be	O
replaced	O
by	O
tools	O
for	O
manipulating	O
the	O
file	B-Application
system	I-Application
.	O
</s>
<s>
Applications	O
can	O
often	O
be	O
"	O
installed	O
"	O
simply	O
by	O
dragging	O
them	O
from	O
a	O
distribution	O
medium	O
to	O
a	O
hard	O
disk	O
,	O
and	O
"	O
uninstalled	O
"	O
by	O
deleting	O
the	O
application	B-Application
directory	I-Application
.	O
</s>
<s>
In	O
order	O
to	O
support	O
user	O
interaction	O
with	O
application	B-Application
directories	I-Application
,	O
several	O
files	O
have	O
special	O
status	O
.	O
</s>
<s>
Launching	O
an	O
application	B-Application
directory	I-Application
causes	O
the	O
included	O
file	O
AppRun	O
(	O
ROX	B-Language
Desktop	I-Language
)	O
or	O
!	O
Run	O
(	O
RISC	B-Operating_System
OS	I-Operating_System
)	O
to	O
be	O
launched	O
.	O
</s>
<s>
On	O
RISC	B-Operating_System
OS	I-Operating_System
this	O
is	O
generally	O
an	O
Obey	O
file	O
(	O
a	O
RISC	B-Operating_System
OS	I-Operating_System
command	O
script	O
)	O
which	O
allocates	O
memory	O
and	O
loads	O
OS	O
extension	O
modules	O
and	O
shared	O
libraries	O
before	O
executing	O
the	O
application	O
binary	O
,	O
usually	O
called	O
!	O
RunImage	O
.	O
</s>
<s>
Under	O
the	O
ROX	B-Language
Desktop	I-Language
,	O
it	O
is	O
not	O
uncommon	O
for	O
it	O
to	O
be	O
a	O
shell	B-Language
script	I-Language
that	O
will	O
launch	O
the	O
correct	O
system	O
binary	O
if	O
available	O
or	O
compile	O
a	O
suitable	O
binary	O
from	O
source	O
otherwise	O
.	O
</s>
<s>
Both	O
RISC	B-Operating_System
OS	I-Operating_System
and	O
the	O
ROX	B-Language
Desktop	I-Language
allow	O
the	O
user	O
to	O
view	O
help	O
files	O
associated	O
with	O
an	O
application	B-Application
directory	I-Application
without	O
launching	O
the	O
application	O
.	O
</s>
<s>
RISC	B-Operating_System
OS	I-Operating_System
relies	O
on	O
a	O
file	O
in	O
the	O
directory	O
named	O
!	O
Help	O
which	O
is	O
launched	O
as	O
if	O
the	O
user	O
double-clicked	O
on	O
it	O
when	O
help	O
is	O
requested	O
(	O
and	O
can	O
be	O
any	O
format	O
the	O
system	O
understands	O
,	O
but	O
plain	O
text	O
and	O
!	O
Draw	O
formats	O
are	O
common	O
)	O
,	O
while	O
the	O
ROX	B-Language
Desktop	I-Language
opens	O
the	O
application	O
's	O
Help	O
subdirectory	O
.	O
</s>
<s>
Similarly	O
,	O
custom	O
icons	O
for	O
the	O
Application	B-Application
Directory	I-Application
may	O
be	O
contained	O
within	O
it	O
—	O
either	O
.DirIcon	O
(	O
in	O
any	O
supported	O
image	O
format	O
)	O
or	O
AppIcon.xpm	O
under	O
the	O
ROX	B-Language
Desktop	I-Language
,	O
or	O
!	O
Sprites	O
and	O
optionally	O
!	O
Sprites22	O
under	O
RISC	B-Operating_System
OS	I-Operating_System
.	O
</s>
<s>
The	O
RISC	B-Operating_System
OS	I-Operating_System
Filer	O
automatically	O
loads	O
these	O
sprite	O
files	O
in	O
the	O
absence	O
of	O
a	O
!	O
Boot	O
file	O
,	O
otherwise	O
it	O
is	O
left	O
to	O
the	O
!	O
Boot	O
file	O
to	O
load	O
the	O
appropriate	O
icons	O
.	O
</s>
<s>
RISC	B-Operating_System
OS	I-Operating_System
application	B-Application
directories	I-Application
may	O
contain	O
a	O
file	O
named	O
!	O
Boot	O
which	O
is	O
executed	O
by	O
the	O
Filer	O
either	O
explicitly	O
via	O
the	O
FilerBoot	O
command	O
,	O
or	O
when	O
the	O
bundle	B-Application
is	O
first	O
displayed	O
(	O
unless	O
the	O
user	O
held	O
Control	B-Device
when	O
opening	O
that	O
Filer	O
window	O
,	O
which	O
defeats	O
application	O
booting	O
)	O
.	O
</s>
<s>
This	O
allows	O
an	O
application	O
to	O
do	O
tasks	O
such	O
as	O
register	O
supported	O
filetypes	O
(	O
by	O
defining	O
their	O
name	O
,	O
setting	O
command	O
aliases	O
for	O
running	O
or	O
printing	O
them	O
and	O
loading	O
appropriate	O
icons	O
)	O
,	O
or	O
choose	O
to	O
load	O
different	O
icons	O
such	O
as	O
ASprites	O
(	O
icons	O
with	O
an	O
alpha	O
channel	O
for	O
RISC	B-Operating_System
OS	I-Operating_System
Select	O
)	O
,	O
5Sprites	O
(	O
icons	O
with	O
a	O
RISC	B-Operating_System
OS	I-Operating_System
5	O
appearance	O
)	O
or	O
theme-specific	O
icons	O
.	O
</s>
<s>
!	O
Boot	O
files	O
are	O
the	O
most	O
popular	O
propagation	O
vector	O
for	O
such	O
RISC	B-Operating_System
OS	I-Operating_System
viruses	O
as	O
exist	O
(	O
such	O
as	O
the	O
well-known	O
Extend	O
virus	O
)	O
,	O
though	O
they	O
can	O
be	O
defeated	O
simply	O
by	O
holding	O
Control	B-Device
.	O
</s>
<s>
The	O
!	O
Boot	O
feature	O
is	O
not	O
duplicated	O
by	O
the	O
ROX	B-Language
Desktop	I-Language
.	O
</s>
<s>
Acorn	O
's	O
early	O
machines	O
allowed	O
a	O
medium	O
to	O
be	O
"	O
booted	O
"	O
(	O
automatically	O
run	O
)	O
by	O
executing	O
a	O
file	O
called	O
!	O
BOOT	O
if	O
present	O
(	O
this	O
action	O
,	O
"	O
booting	O
a	O
disc	O
"	O
,	O
was	O
toggled	O
with	O
the	O
Shift	B-Device
,	O
and	O
hence	O
became	O
known	O
as	O
shift-booting	O
)	O
.	O
</s>
<s>
This	O
was	O
exploited	O
in	O
the	O
RISC	B-Operating_System
OS	I-Operating_System
disc	O
structure	O
by	O
having	O
an	O
application	B-Application
directory	I-Application
in	O
the	O
root	O
of	O
the	O
default	O
disc	O
called	O
!	O
Boot	O
.	O
</s>
<s>
This	O
application	B-Application
directory	I-Application
is	O
executed	O
(	O
run	O
)	O
when	O
the	O
machine	O
boots	O
(	O
if	O
so	O
configured	O
)	O
,	O
therefore	O
causing	O
its	O
!	O
Run	O
file	O
to	O
be	O
executed	O
(	O
i.e.	O
</s>
<s>
!	O
Run	O
in	O
Acorn	O
pathname	B-Application
syntax	O
)	O
.	O
</s>
<s>
Thus	O
RISC	B-Operating_System
OS	I-Operating_System
's	O
entire	O
boot	O
sequence	O
can	O
be	O
avoided	O
merely	O
by	O
holding	O
Shift	B-Device
.	O
</s>
