<s>
In	O
information	B-General_Concept
technology	I-General_Concept
,	O
a	O
write-only	B-General_Concept
memory	I-General_Concept
(	O
WOM	O
)	O
is	O
a	O
memory	O
location	O
or	O
register	O
that	O
can	O
be	O
written	O
to	O
but	O
not	O
read	O
.	O
</s>
<s>
The	O
most	O
common	O
occurrence	O
of	O
the	O
latter	O
situation	O
is	O
when	O
a	O
processor	O
writes	O
data	O
to	O
a	O
write-only	B-General_Concept
register	I-General_Concept
of	O
hardware	O
the	O
processor	O
is	O
controlling	O
.	O
</s>
<s>
This	O
can	O
lead	O
to	O
problems	O
in	O
producing	O
device	B-Application
drivers	I-Application
for	O
the	O
hardware	O
.	O
</s>
<s>
Write-only	B-General_Concept
memories	I-General_Concept
also	O
find	O
applications	O
in	O
security	O
and	O
cryptography	O
as	O
a	O
means	O
of	O
preventing	O
data	O
being	O
intercepted	O
as	O
it	O
is	O
being	O
decrypted	O
.	O
</s>
<s>
In	O
1972	O
,	O
WOM	O
,	O
an	O
antithesis	O
of	O
read-only	B-Device
memory	I-Device
(	O
ROM	B-Device
)	O
,	O
was	O
introduced	O
as	O
an	O
inside	O
practical	O
joke	O
perpetrated	O
by	O
Signetics	O
.	O
</s>
<s>
However	O
,	O
it	O
was	O
soon	O
recognized	O
that	O
this	O
concept	O
actually	O
describes	O
certain	O
functionalities	O
in	O
microprocessor	B-Architecture
systems	O
.	O
</s>
<s>
The	O
most	O
frequent	O
occurrences	O
of	O
write-only	B-General_Concept
memories	I-General_Concept
are	O
where	O
the	O
memory	O
locations	O
are	O
registers	O
or	O
an	O
integrated	O
circuit	O
being	O
used	O
to	O
control	O
,	O
or	O
pass	O
information	O
to	O
,	O
hardware	O
outside	O
the	O
processor	O
.	O
</s>
<s>
A	O
central	B-General_Concept
processing	I-General_Concept
unit	I-General_Concept
(	O
CPU	O
)	O
can	O
write	O
to	O
these	O
locations	O
,	O
and	O
thus	O
control	O
the	O
hardware	O
,	O
but	O
cannot	O
read	O
back	O
the	O
information	O
and	O
discover	O
the	O
current	O
state	O
of	O
the	O
hardware	O
.	O
</s>
<s>
Such	O
arrangements	O
are	O
common	O
on	O
small	O
embedded	B-Architecture
microcontroller	I-Architecture
systems	I-Architecture
to	O
save	O
costs	O
and	O
can	O
cause	O
difficulties	O
for	O
authors	O
of	O
device	B-Application
driver	I-Application
software	O
.	O
</s>
<s>
However	O
,	O
this	O
strategy	O
is	O
only	O
reliable	O
if	O
the	O
register	O
is	O
read-only	B-Device
from	O
the	O
perspective	O
of	O
the	O
hardware	O
.	O
</s>
<s>
A	O
large	O
part	O
of	O
this	O
was	O
occupied	O
by	O
the	O
BIOS	B-Operating_System
and	O
the	O
video	O
card	O
,	O
resulting	O
in	O
only	O
of	O
contiguous	O
addressable	O
RAM	B-Architecture
being	O
available	O
.	O
</s>
<s>
One	O
method	O
of	O
overcoming	O
the	O
limitation	O
was	O
by	O
the	O
use	O
of	O
bank-switched	B-General_Concept
RAM	I-General_Concept
.	O
</s>
<s>
A	O
number	O
of	O
banks	O
of	O
RAM	B-Architecture
are	O
provided	O
,	O
but	O
are	O
not	O
permanently	O
assigned	O
to	O
a	O
memory	O
location	O
.	O
</s>
<s>
Another	O
example	O
concerns	O
some	O
functions	O
carried	O
out	O
by	O
graphics	B-Architecture
processing	I-Architecture
units	I-Architecture
(	O
GPUs	B-Architecture
)	O
.	O
</s>
<s>
For	O
instance	O
,	O
a	O
GPU	B-Architecture
might	O
be	O
carrying	O
out	O
shader	O
processing	O
on	O
the	O
contents	O
of	O
graphics	O
memory	O
.	O
</s>
<s>
It	O
can	O
be	O
faster	O
and	O
more	O
efficient	O
for	O
the	O
GPU	B-Architecture
to	O
take	O
input	O
for	O
the	O
shader	O
process	O
from	O
read-only	B-Device
locations	O
and	O
write	O
the	O
shader	O
output	O
to	O
different	O
write-only	O
locations	O
without	O
having	O
to	O
copy	O
data	O
between	O
the	O
read	O
and	O
write	O
buffers	B-General_Concept
after	O
each	O
iteration	O
.	O
</s>
<s>
An	O
example	O
that	O
is	O
still	O
of	O
contemporary	O
relevance	O
can	O
be	O
found	O
in	O
the	O
16550	B-Device
UART	I-Device
,	O
whose	O
derivatives	O
are	O
still	O
in	O
widespread	O
use	O
.	O
</s>
<s>
To	O
add	O
a	O
data	O
FIFO	B-Operating_System
without	O
breaking	O
compatibility	O
with	O
the	O
8250	B-Device
UART	I-Device
's	O
8	O
configuration	O
registers	O
,	O
the	O
write-only	O
"	O
FIFO	B-Operating_System
control	O
register	O
"	O
was	O
assigned	O
the	O
same	O
port	O
address	O
as	O
the	O
read-only	B-Device
"	O
interrupt	O
identification	O
register	O
"	O
.	O
</s>
<s>
Writes	O
to	O
that	O
address	O
program	O
the	O
FIFO	B-Operating_System
control	O
register	O
,	O
but	O
there	O
is	O
no	O
way	O
to	O
read	O
it	O
back	O
.	O
</s>
<s>
Locking	O
the	O
frame	B-Algorithm
buffer	I-Algorithm
of	O
a	O
GPU	B-Architecture
to	O
effectively	O
turn	O
it	O
into	O
a	O
write-only	B-General_Concept
memory	I-General_Concept
can	O
be	O
useful	O
in	O
protecting	O
encrypted	O
data	O
.	O
</s>
<s>
This	O
weakness	O
may	O
be	O
alleviated	O
by	O
carrying	O
out	O
the	O
decryption	O
within	O
the	O
GPU	B-Architecture
and	O
writing	O
the	O
unencrypted	O
data	O
directly	O
to	O
display	B-Algorithm
memory	I-Algorithm
.	O
</s>
<s>
Nothing	O
else	O
can	O
read	O
the	O
data	O
while	O
the	O
frame	B-Algorithm
buffer	I-Algorithm
is	O
locked	O
,	O
and	O
when	O
it	O
is	O
unlocked	O
it	O
will	O
be	O
displaying	O
something	O
else	O
.	O
</s>
