<s>
Exec	B-Operating_System
is	O
the	O
kernel	B-Operating_System
of	O
AmigaOS	B-Application
.	O
</s>
<s>
It	O
is	O
a	O
13KB	O
multitasking	B-Operating_System
microkernel	B-Operating_System
which	O
enabled	O
pre-emptive	O
multitasking	B-Operating_System
in	O
as	O
little	O
as	O
256KB	O
of	O
memory	O
(	O
as	O
supplied	O
with	O
the	O
first	O
Amiga	B-Device
1000s	I-Device
)	O
.	O
</s>
<s>
Exec	B-Operating_System
provided	O
functions	O
for	O
multitasking	B-Operating_System
,	O
memory	B-General_Concept
management	I-General_Concept
,	O
and	O
handling	O
of	O
interrupts	B-Application
and	O
dynamic	B-Application
shared	O
libraries	B-Library
.	O
</s>
<s>
It	O
acts	O
as	O
a	O
scheduler	O
for	O
tasks	O
running	O
on	O
the	O
system	O
,	O
providing	O
pre-emptive	O
multitasking	B-Operating_System
with	O
prioritized	O
round-robin	B-Algorithm
scheduling	I-Algorithm
.	O
</s>
<s>
Exec	B-Operating_System
also	O
provides	O
access	O
to	O
other	O
libraries	B-Library
and	O
high-level	O
inter-process	B-Operating_System
communication	I-Operating_System
via	O
message	B-Architecture
passing	I-Architecture
.	O
</s>
<s>
Other	O
comparable	O
microkernels	B-Operating_System
have	O
had	O
performance	O
problems	O
because	O
of	O
the	O
need	O
to	O
copy	O
messages	O
between	O
address	B-General_Concept
spaces	I-General_Concept
.	O
</s>
<s>
Since	O
the	O
Amiga	B-Device
has	O
only	O
one	O
address	B-General_Concept
space	I-General_Concept
,	O
Exec	B-Operating_System
message	B-Architecture
passing	I-Architecture
is	O
quite	O
efficient	O
.	O
</s>
<s>
The	O
only	O
fixed	O
memory	O
address	O
in	O
the	O
Amiga	B-Device
software	O
(	O
address	O
4	O
)	O
is	O
a	O
pointer	O
to	O
exec.library	O
,	O
which	O
can	O
then	O
be	O
used	O
to	O
access	O
other	O
libraries	B-Library
.	O
</s>
<s>
Exec	B-Operating_System
was	O
designed	O
and	O
implemented	O
by	O
Carl	O
Sassenrath	O
.	O
</s>
<s>
Unlike	O
newer	O
modern	O
operating	B-General_Concept
systems	I-General_Concept
,	O
the	O
exec	B-Operating_System
kernel	B-Operating_System
does	O
not	O
run	O
"	O
privileged	O
"	O
.	O
</s>
<s>
Contemporary	O
operating	B-General_Concept
systems	I-General_Concept
for	O
the	O
Motorola	B-Device
68000	I-Device
,	O
such	O
as	O
Atari	B-Application
TOS	I-Application
and	O
SunOS	B-Operating_System
used	O
trap	B-General_Concept
instructions	O
to	O
invoke	O
kernel	B-Operating_System
functions	O
.	O
</s>
<s>
This	O
made	O
the	O
kernel	B-Operating_System
functions	O
run	O
in	O
the	O
68000	B-Device
's	O
supervisor	O
mode	O
,	O
while	O
user	O
software	O
ran	O
in	O
the	O
unprivileged	O
user	O
mode	O
.	O
</s>
<s>
By	O
contrast	O
,	O
exec	B-Operating_System
function	O
calls	O
are	O
made	O
with	O
the	O
library	O
jump	O
table	O
,	O
and	O
the	O
kernel	B-Operating_System
code	O
normally	O
executes	O
in	O
user	O
mode	O
.	O
</s>
<s>
Whenever	O
supervisor	O
mode	O
is	O
needed	O
,	O
either	O
by	O
the	O
kernel	B-Operating_System
or	O
user	O
programs	O
,	O
the	O
library	B-Library
functions	I-Library
Supervisor( )	O
or	O
SuperState( )	O
are	O
used	O
.	O
</s>
<s>
One	O
limit	O
of	O
the	O
Exec	B-Operating_System
kernel	B-Operating_System
was	O
that	O
an	O
uncooperative	O
program	O
could	O
disable	O
multitasking	B-Operating_System
for	O
a	O
long	O
time	O
,	O
or	O
indefinitely	O
,	O
by	O
invoking	O
Exec	B-Operating_System
's	O
calls	O
Forbid( )	O
or	O
Disable( )	O
,	O
with	O
no	O
later	O
invocation	O
of	O
corresponding	O
Permit( )	O
or	O
Enable( )	O
,	O
causing	O
the	O
environment	O
to	O
run	O
as	O
one	O
task	O
.	O
</s>
<s>
Multitasking	B-Operating_System
could	O
also	O
be	O
disabled	O
by	O
programs	O
which	O
,	O
by	O
software	O
bug	O
or	O
intent	O
,	O
modify	O
Exec	B-Operating_System
's	O
data	O
structures	O
or	O
the	O
code	O
stored	O
in	O
random-access	B-Architecture
memory	I-Architecture
(	O
RAM	B-Architecture
)	O
,	O
possibly	O
due	O
to	O
lack	O
of	O
memory	B-General_Concept
management	I-General_Concept
unit	I-General_Concept
(	O
MMU	O
)	O
support	O
.	O
</s>
<s>
Even	O
with	O
such	O
limits	O
,	O
Exec	B-Operating_System
satisfies	O
the	O
definition	O
of	O
preemptive	O
scheduling	O
algorithm	O
,	O
using	O
a	O
preemptive	O
scheduling	O
routine	O
and	O
basing	O
its	O
interrupt	B-Application
intervals	O
on	O
a	O
clock	O
.	O
</s>
<s>
Linux	B-Operating_System
kernel	I-Operating_System
developer	O
Linus	O
Torvalds	O
once	O
described	O
the	O
Amiga	B-Device
design	O
as	O
cooperative	O
,	O
even	O
though	O
it	O
uses	O
a	O
preemptive	O
scheduling	O
policy	O
.	O
</s>
<s>
The	O
reason	O
for	O
that	O
,	O
he	O
argued	O
,	O
was	O
because	O
the	O
lack	O
of	O
 [ memory ] 	B-General_Concept
protection	I-General_Concept
between	O
tasks	O
,	O
meant	O
a	O
task	O
could	O
hinder	O
the	O
system	O
from	O
operating	O
preemptively	O
.	O
</s>
<s>
ExecSG	O
(	O
Second	O
Generation	O
)	O
is	O
the	O
kernel	B-Operating_System
used	O
by	O
AmigaOS	B-Application
4	I-Application
.	O
</s>
<s>
This	O
kernel	B-Operating_System
was	O
licensed	O
by	O
Hyperion	B-Operating_System
Entertainment	I-Operating_System
from	O
Thomas	O
Frieden	O
and	O
Hans-Jörg	O
Frieden	O
.	O
</s>
<s>
It	O
exposes	O
the	O
same	O
functions	O
and	O
structures	O
as	O
the	O
Amiga	B-Operating_System
Exec	I-Operating_System
kernel	B-Operating_System
but	O
runs	O
on	O
PowerPC	B-Architecture
processors	I-Architecture
only	O
.	O
</s>
