<s>
Key	B-Device
rollover	I-Device
is	O
the	O
ability	O
of	O
a	O
computer	B-Device
keyboard	I-Device
to	O
correctly	O
handle	O
several	O
simultaneous	O
keystrokes	O
.	O
</s>
<s>
A	O
keyboard	B-Device
with	O
n-key	O
rollover	O
(	O
NKRO	O
)	O
can	O
correctly	O
detect	O
input	O
from	O
each	O
key	O
on	O
the	O
keyboard	B-Device
at	O
the	O
same	O
time	O
,	O
regardless	O
of	O
how	O
many	O
other	O
keys	O
are	O
also	O
being	O
pressed	O
.	O
</s>
<s>
Keyboards	B-Device
that	O
lack	O
full	O
rollover	B-Device
will	O
register	O
an	O
incorrect	O
keystroke	O
when	O
certain	O
combinations	O
of	O
keys	O
are	O
pressed	O
simultaneously	O
.	O
</s>
<s>
Rollover	B-Device
has	O
applications	O
for	O
stenotype	O
,	O
electronic	O
music	O
keyboards	B-Device
,	O
gaming	O
,	O
and	O
touch-typing	B-Device
generally	O
.	O
</s>
<s>
During	O
normal	O
typing	O
on	O
a	O
conventional	O
computer	B-Device
keyboard	I-Device
,	O
only	O
one	O
key	O
is	O
usually	O
pressed	O
at	O
any	O
given	O
time	O
,	O
then	O
released	O
before	O
the	O
next	O
key	O
is	O
pressed	O
.	O
</s>
<s>
When	O
using	O
modifier	B-Device
keys	I-Device
such	O
as	O
Shift	O
or	O
Control	O
,	O
the	O
user	O
intentionally	O
holds	O
the	O
modifier	O
key(s )	O
while	O
pressing	O
and	O
releasing	O
another	O
key	O
.	O
</s>
<s>
Some	O
computer	O
games	O
require	O
multiple	O
keys	O
(	O
other	O
than	O
the	O
usual	O
modifier	B-Device
keys	I-Device
)	O
to	O
be	O
held	O
down	O
while	O
others	O
are	O
pressed	O
and	O
released	O
,	O
in	O
order	O
to	O
perform	O
actions	O
simultaneously	O
;	O
</s>
<s>
a	O
common	O
scenario	O
would	O
be	O
the	O
holding	O
down	O
of	O
two	O
Arrow	B-Device
keys	I-Device
or	O
WASD	O
keys	O
in	O
order	O
to	O
move	O
the	O
player	O
in	O
a	O
diagonal	O
direction	O
.	O
</s>
<s>
Other	O
scenarios	O
might	O
be	O
pressing	O
a	O
key	O
to	O
interact	O
with	O
an	O
object	O
or	O
item	O
in	O
the	O
game	O
while	O
holding	O
down	O
an	O
Arrow	B-Device
key	I-Device
to	O
signify	O
movement	O
.	O
</s>
<s>
Certain	O
high-end	O
keyboards	B-Device
have	O
-key	O
rollover	O
(	O
NKRO	O
)	O
.	O
</s>
<s>
This	O
means	O
that	O
each	O
key	O
is	O
scanned	O
completely	O
independently	O
by	O
the	O
keyboard	B-Device
hardware	O
,	O
so	O
that	O
each	O
keypress	O
is	O
correctly	O
detected	O
regardless	O
of	O
how	O
many	O
other	O
keys	O
are	O
being	O
pressed	O
or	O
held	O
down	O
at	O
the	O
time	O
.	O
</s>
<s>
Full	O
-key	O
rollover	O
is	O
required	O
for	O
stenotype	O
,	O
which	O
relies	O
on	O
chording	B-Device
to	O
input	O
text	O
far	O
faster	O
than	O
conventional	O
typing	O
methods	O
,	O
allowing	O
it	O
to	O
keep	O
pace	O
with	O
the	O
speed	O
of	O
human	O
speech	O
.	O
</s>
<s>
Most	O
music	O
keyboards	B-Device
use	O
isolation	O
diodes	O
in	O
their	O
keyboard	B-Device
matrix	O
to	O
implement	O
full	O
-key	O
rollover	O
,	O
making	O
them	O
immune	O
to	O
both	O
key	O
ghosting	O
and	O
key	O
jamming	O
.	O
</s>
<s>
When	O
the	O
data	O
is	O
sent	O
via	O
the	O
USB	O
protocol	O
,	O
there	O
are	O
two	O
operating	O
modes	O
:	O
Human	B-General_Concept
Interface	I-General_Concept
Device	I-General_Concept
(	O
HID	O
)	O
"	O
report	O
protocol	O
"	O
and	O
"	O
boot	O
protocol	O
"	O
.	O
</s>
<s>
The	O
(	O
optional	O
)	O
boot	O
protocol	O
,	O
which	O
is	O
solely	O
used	O
by	O
very	O
limited	O
USB	O
host	O
implementations	O
such	O
as	O
BIOS	O
,	O
is	O
limited	O
to	O
eight	O
modifier	B-Device
keys	I-Device
(	O
left	O
and	O
right	O
versions	O
of	O
Ctrl	O
,	O
Shift	O
,	O
Alt	O
,	O
and	O
Win	O
)	O
,	O
followed	O
by	O
maximum	O
six	O
key	O
codes	O
.	O
</s>
<s>
The	O
HID	O
specification	O
however	O
imposes	O
no	O
requirements	O
on	O
rollover	B-Device
and	O
low-end	O
keyboards	B-Device
may	O
impose	O
the	O
same	O
restrictions	O
regardless	O
of	O
whether	O
the	O
boot	O
protocol	O
or	O
the	O
HID	O
report	O
protocol	O
is	O
used	O
.	O
</s>
<s>
However	O
,	O
to	O
reduce	O
cost	O
and	O
design	O
complexity	O
,	O
most	O
computer	B-Device
keyboards	I-Device
do	O
not	O
isolate	O
all	O
keys	O
in	O
this	O
way	O
.	O
</s>
<s>
With	O
these	O
keyboards	B-Device
,	O
pressing	O
as	O
few	O
as	O
three	O
keys	O
can	O
cause	O
ghosting	O
effects	O
,	O
although	O
care	O
is	O
taken	O
when	O
laying	O
out	O
the	O
matrix	O
arrangement	O
that	O
this	O
does	O
not	O
happen	O
for	O
common	O
modifier	B-Device
key	I-Device
combinations	O
.	O
</s>
<s>
A	O
keyboard	B-Device
with	O
"	O
two-key	O
rollover	O
"	O
can	O
reliably	O
detect	O
only	O
any	O
two	O
keys	O
used	O
simultaneously	O
;	O
in	O
other	O
words	O
,	O
a	O
user	O
can	O
hold	O
down	O
any	O
key	O
on	O
the	O
keyboard	B-Device
and	O
press	O
a	O
second	O
key	O
,	O
and	O
be	O
sure	O
that	O
the	O
keypress	O
is	O
correctly	O
detected	O
by	O
the	O
computer	O
.	O
</s>
<s>
This	O
is	O
not	O
acceptable	O
for	O
quality	O
keyboards	B-Device
because	O
there	O
are	O
many	O
cases	O
when	O
more	O
than	O
two	O
keys	O
need	O
to	O
be	O
depressed	O
at	O
the	O
same	O
time	O
,	O
such	O
as	O
Ctrl-Alt-Delete	B-Device
,	O
or	O
when	O
more	O
than	O
two	O
keys	O
are	O
depressed	O
because	O
of	O
fast	O
typing	O
(	O
"	O
rolling	O
over	O
"	O
more	O
than	O
two	O
keys	O
)	O
.	O
</s>
<s>
"	O
Multi-key	O
rollover	O
"	O
is	O
considered	O
essential	O
for	O
quality	O
keyboards	B-Device
and	O
for	O
English-language	O
touch	B-Device
typing	I-Device
.	O
</s>
<s>
Key	O
ghosting	O
occurs	O
on	O
matrix	O
keyboards	B-Device
for	O
certain	O
combinations	O
of	O
three	O
keys	O
.	O
</s>
<s>
When	O
these	O
three	O
keys	O
are	O
pressed	O
simultaneously	O
,	O
a	O
fourth	O
keypress	O
is	O
erroneously	O
registered	O
by	O
the	O
keyboard	B-Device
controller	I-Device
.	O
</s>
<s>
Modern	O
keyboards	B-Device
detect	O
ghosting	O
,	O
and	O
instead	O
of	O
registering	O
a	O
fourth	O
key	O
,	O
they	O
will	O
ignore	O
the	O
third	O
key	O
,	O
which	O
is	O
known	O
as	O
jamming	O
.	O
</s>
<s>
Which	O
keys	O
jam	O
when	O
pressed	O
together	O
differs	O
between	O
brands	O
and	O
models	O
of	O
keyboards	B-Device
.	O
</s>
<s>
Due	O
to	O
the	O
keyboard	B-Device
matrix	O
most	O
consumer	O
keyboards	B-Device
use	O
,	O
jamming	O
and	O
ghosting	O
often	O
occur	O
when	O
three	O
out	O
of	O
four	O
keys	O
in	O
a	O
square	O
block	O
on	O
the	O
matrix	O
are	O
pressed	O
,	O
such	O
as	O
QASW	O
or	O
JKUI	O
.	O
</s>
<s>
Most	O
music	O
keyboards	B-Device
and	O
some	O
high-end	O
computer	B-Device
keyboards	I-Device
use	O
an	O
isolation	O
diode	O
with	O
each	O
keyswitch	B-Device
in	O
their	O
matrix	O
,	O
and	O
can	O
correctly	O
read	O
any	O
combination	O
(	O
chording	B-Device
)	O
of	O
keys	O
pressed	O
in	O
any	O
order	O
and	O
released	O
in	O
any	O
order	O
—	O
they	O
are	O
immune	O
to	O
both	O
key	O
ghosting	O
and	O
key	O
jamming	O
.	O
</s>
<s>
Some	O
"	O
gaming	B-Device
keyboards	I-Device
"	O
use	O
a	O
matrix	O
,	O
but	O
only	O
give	O
individual	O
diodes	O
to	O
the	O
most	O
frequently-used	O
keys	O
in	O
gaming	O
such	O
as	O
WASD	O
and	O
the	O
arrow	B-Device
keys	I-Device
.	O
</s>
<s>
Key	O
jamming	O
is	O
often	O
noticed	O
when	O
using	O
a	O
keyboard	B-Device
to	O
play	O
computer	O
games	O
where	O
many	O
keypresses	O
combine	O
to	O
movement	O
vectors	O
and	O
other	O
,	O
simultaneous	O
activities	O
rather	O
than	O
typing	O
text	O
.	O
</s>
<s>
The	O
original	O
Star	B-Application
Control	I-Application
game	O
included	O
a	O
utility	O
to	O
test	O
for	O
key	O
jamming	O
and	O
help	O
the	O
player	O
to	O
determine	O
the	O
best	O
key	O
mapping	O
for	O
their	O
keyboard	B-Device
,	O
since	O
during	O
gameplay	O
it	O
was	O
common	O
for	O
each	O
of	O
the	O
two	O
players	O
to	O
be	O
pressing	O
three	O
or	O
four	O
keys	O
at	O
the	O
same	O
time	O
.	O
</s>
<s>
Computer	B-Device
keyboards	I-Device
typically	O
are	O
designed	O
to	O
detect	O
these	O
keys	O
being	O
activated	O
in	O
addition	O
to	O
others	O
from	O
the	O
character	O
matrix	O
,	O
and	O
this	O
may	O
prevent	O
key	O
jamming	O
.	O
</s>
