<s>
In	O
computing	O
,	O
bus	B-Architecture
mastering	I-Architecture
is	O
a	O
feature	O
supported	O
by	O
many	O
bus	O
architectures	O
that	O
enables	O
a	O
device	O
connected	O
to	O
the	O
bus	O
to	O
initiate	O
direct	B-General_Concept
memory	I-General_Concept
access	I-General_Concept
(	O
DMA	O
)	O
transactions	O
.	O
</s>
<s>
It	O
is	O
also	O
referred	O
to	O
as	O
first-party	B-Architecture
DMA	I-Architecture
,	O
in	O
contrast	O
with	O
third-party	O
DMA	O
where	O
a	O
system	O
DMA	B-General_Concept
controller	I-General_Concept
actually	O
does	O
the	O
transfer	O
.	O
</s>
<s>
Some	O
types	O
of	O
buses	O
allow	O
only	O
one	O
device	O
(	O
typically	O
the	O
CPU	B-General_Concept
,	O
or	O
its	O
proxy	O
)	O
to	O
initiate	O
transactions	O
.	O
</s>
<s>
Most	O
modern	O
bus	O
architectures	O
,	O
such	O
as	O
PCI	B-Protocol
,	O
allow	O
multiple	O
devices	O
to	O
bus	B-Architecture
master	I-Architecture
because	O
it	O
significantly	O
improves	O
performance	O
for	O
general-purpose	B-General_Concept
operating	I-General_Concept
systems	I-General_Concept
.	O
</s>
<s>
Some	O
real-time	B-Operating_System
operating	I-Operating_System
systems	I-Operating_System
prohibit	O
peripherals	O
from	O
becoming	O
bus	B-Architecture
masters	I-Architecture
,	O
because	O
the	O
scheduler	O
can	O
no	O
longer	O
arbitrate	O
for	O
the	O
bus	O
and	O
hence	O
cannot	O
provide	O
deterministic	O
latency	O
.	O
</s>
<s>
While	O
bus	B-Architecture
mastering	I-Architecture
theoretically	O
allows	O
one	O
peripheral	O
device	O
to	O
directly	O
communicate	O
with	O
another	O
,	O
in	O
practice	O
almost	O
all	O
peripherals	O
master	O
the	O
bus	O
exclusively	O
to	O
perform	O
DMA	O
to	O
main	O
memory	O
.	O
</s>
<s>
If	O
multiple	O
devices	O
are	O
able	O
to	O
master	O
the	O
bus	O
,	O
there	O
needs	O
to	O
be	O
a	O
bus	B-Architecture
arbitration	I-Architecture
scheme	O
to	O
prevent	O
multiple	O
devices	O
attempting	O
to	O
drive	O
the	O
bus	O
simultaneously	O
.	O
</s>
<s>
A	O
number	O
of	O
different	O
schemes	O
are	O
used	O
for	O
this	O
;	O
for	O
example	O
SCSI	B-Architecture
has	O
a	O
fixed	O
priority	O
for	O
each	O
SCSI	B-Architecture
ID	O
.	O
</s>
<s>
PCI	B-Protocol
does	O
not	O
specify	O
the	O
algorithm	O
to	O
use	O
,	O
leaving	O
it	O
up	O
to	O
the	O
implementation	O
to	O
set	O
priorities	O
.	O
</s>
