<s>
Read-only	B-Device
memory	I-Device
(	O
ROM	B-Device
)	O
is	O
a	O
type	O
of	O
non-volatile	B-General_Concept
memory	I-General_Concept
used	O
in	O
computers	O
and	O
other	O
electronic	O
devices	O
.	O
</s>
<s>
Data	O
stored	O
in	O
ROM	B-Device
cannot	O
be	O
electronically	O
modified	O
after	O
the	O
manufacture	O
of	O
the	O
memory	B-General_Concept
device	I-General_Concept
.	O
</s>
<s>
Read-only	B-Device
memory	I-Device
is	O
useful	O
for	O
storing	O
software	O
that	O
is	O
rarely	O
changed	O
during	O
the	O
life	O
of	O
the	O
system	O
,	O
also	O
known	O
as	O
firmware	B-Application
.	O
</s>
<s>
Software	O
applications	O
(	O
like	O
video	O
games	O
)	O
for	O
programmable	O
devices	O
can	O
be	O
distributed	O
as	O
plug-in	B-Protocol
cartridges	I-Protocol
containing	I-Protocol
ROM	I-Protocol
.	O
</s>
<s>
Strictly	O
speaking	O
,	O
read-only	B-Device
memory	I-Device
refers	O
to	O
memory	O
that	O
is	O
hard-wired	O
,	O
such	O
as	O
diode	B-General_Concept
matrix	I-General_Concept
or	O
a	O
mask	B-Device
ROM	I-Device
integrated	O
circuit	O
(	O
IC	O
)	O
,	O
which	O
cannot	O
be	O
electronically	O
changed	O
after	O
manufacture	O
.	O
</s>
<s>
Floating-gate	B-Algorithm
ROM	B-Device
semiconductor	B-Architecture
memory	I-Architecture
in	O
the	O
form	O
of	O
erasable	B-General_Concept
programmable	I-General_Concept
read-only	I-General_Concept
memory	I-General_Concept
(	O
EPROM	B-General_Concept
)	O
,	O
electrically	B-General_Concept
erasable	I-General_Concept
programmable	I-General_Concept
read-only	I-General_Concept
memory	I-General_Concept
(	O
EEPROM	B-General_Concept
)	O
and	O
flash	B-Device
memory	I-Device
can	O
be	O
erased	O
and	O
re-programmed	O
.	O
</s>
<s>
The	O
term	O
"	O
ROM	B-Device
"	O
is	O
sometimes	O
used	O
to	O
refer	O
to	O
a	O
ROM	B-Device
device	O
containing	O
specific	O
software	O
or	O
a	O
file	O
with	O
software	O
to	O
be	O
stored	O
in	O
a	O
writable	O
ROM	B-Device
device	O
.	O
</s>
<s>
For	O
example	O
,	O
users	O
modifying	O
or	O
replacing	O
the	O
Android	B-Application
operating	I-Application
system	I-Application
describe	O
files	O
containing	O
a	O
modified	O
or	O
replacement	O
operating	B-General_Concept
system	I-General_Concept
as	O
"	O
custom	O
ROMs	O
"	O
after	O
the	O
type	O
of	O
storage	B-General_Concept
the	O
file	O
used	O
to	O
be	O
written	O
to	O
,	O
and	O
they	O
may	O
distinguish	O
between	O
ROM	B-Device
(	O
where	O
software	O
and	O
data	O
is	O
stored	O
,	O
usually	O
Flash	B-Device
memory	I-Device
)	O
and	O
RAM	B-Architecture
.	O
</s>
<s>
IBM	O
used	O
capacitor	O
read-only	B-Device
storage	I-Device
(	O
CROS	O
)	O
and	O
transformer	B-General_Concept
read-only	I-General_Concept
storage	I-General_Concept
(	O
TROS	O
)	O
to	O
store	O
microcode	B-Device
for	O
the	O
smaller	O
System/360	B-Application
models	O
,	O
the	O
360/85	B-Device
,	O
and	O
the	O
initial	O
two	O
System/370	B-Device
models	O
(	O
370/155	B-Device
and	O
370/165	B-Device
)	O
.	O
</s>
<s>
The	O
Apollo	B-Application
Guidance	I-Application
Computer	I-Application
used	O
core	B-General_Concept
rope	I-General_Concept
memory	I-General_Concept
,	O
programmed	O
by	O
threading	O
wires	O
through	O
magnetic	B-Architecture
cores	O
.	O
</s>
<s>
The	O
simplest	O
type	O
of	O
solid-state	O
ROM	B-Device
is	O
as	O
old	O
as	O
the	O
semiconductor	O
technology	O
itself	O
.	O
</s>
<s>
Combinational	O
logic	O
gates	O
can	O
be	O
joined	O
manually	O
to	O
map	O
-bit	O
address	O
input	O
onto	O
arbitrary	O
values	O
of	O
-bit	O
data	O
output	O
(	O
a	O
look-up	B-Data_Structure
table	I-Data_Structure
)	O
.	O
</s>
<s>
With	O
the	O
invention	O
of	O
the	O
integrated	O
circuit	O
came	O
mask	B-Device
ROM	I-Device
.	O
</s>
<s>
Mask	B-Device
ROM	I-Device
consists	O
of	O
a	O
grid	O
of	O
word	O
lines	O
(	O
the	O
address	O
input	O
)	O
and	O
bit	O
lines	O
(	O
the	O
data	O
output	O
)	O
,	O
selectively	O
joined	O
together	O
with	O
transistor	B-Application
switches	O
,	O
and	O
can	O
represent	O
an	O
arbitrary	O
look-up	B-Data_Structure
table	I-Data_Structure
with	O
a	O
regular	O
physical	O
layout	O
and	O
predictable	O
propagation	O
delay	O
.	O
</s>
<s>
Mask	B-Device
ROM	I-Device
is	O
programmed	O
with	O
photomasks	B-Algorithm
in	O
photolithography	B-Algorithm
during	O
semiconductor	B-Architecture
manufacturing	I-Architecture
.	O
</s>
<s>
The	O
mask	O
defines	O
physical	O
features	O
or	O
structures	O
that	O
will	O
be	O
removed	O
,	O
or	O
added	O
in	O
the	O
ROM	B-Device
chips	I-Device
,	O
and	O
the	O
presence	O
or	O
absence	O
of	O
these	O
features	O
will	O
represent	O
either	O
a	O
1	O
or	O
a	O
0	O
bit	O
,	O
depending	O
on	O
the	O
ROM	B-Device
design	O
.	O
</s>
<s>
In	O
mask	B-Device
ROM	I-Device
,	O
the	O
data	O
is	O
physically	O
encoded	O
in	O
the	O
circuit	O
,	O
so	O
it	O
can	O
only	O
be	O
programmed	O
during	O
fabrication	B-Architecture
.	O
</s>
<s>
It	O
is	O
only	O
economical	O
to	O
buy	O
mask	B-Device
ROM	I-Device
in	O
large	O
quantities	O
,	O
since	O
users	O
must	O
contract	O
with	O
a	O
foundry	B-Algorithm
to	O
produce	O
a	O
custom	O
design	O
for	O
every	O
piece	O
,	O
or	O
revision	O
of	O
software	O
.	O
</s>
<s>
The	O
turnaround	O
time	O
between	O
completing	O
the	O
design	O
for	O
a	O
mask	B-Device
ROM	I-Device
and	O
receiving	O
the	O
finished	O
product	O
is	O
long	O
,	O
for	O
the	O
same	O
reason	O
.	O
</s>
<s>
Mask	B-Device
ROM	I-Device
is	O
impractical	O
for	O
R&D	O
work	O
since	O
designers	O
frequently	O
need	O
to	O
quickly	O
modify	O
the	O
contents	O
of	O
memory	O
as	O
they	O
refine	O
a	O
design	O
.	O
</s>
<s>
If	O
a	O
product	O
is	O
shipped	O
with	O
faulty	O
mask	B-Device
ROM	I-Device
,	O
the	O
only	O
way	O
to	O
fix	O
it	O
is	O
to	O
recall	O
the	O
product	O
and	O
physically	O
replace	O
the	O
ROM	B-Device
in	O
every	O
unit	O
shipped	O
.	O
</s>
<s>
Programmable	B-General_Concept
read-only	I-General_Concept
memory	I-General_Concept
(	O
PROM	B-General_Concept
)	O
,	O
invented	O
by	O
Wen	O
Tsing	O
Chow	O
in	O
1956	O
,	O
allowed	O
users	O
to	O
program	O
its	O
contents	O
exactly	O
once	O
by	O
physically	O
altering	O
its	O
structure	O
with	O
the	O
application	O
of	O
high-voltage	O
pulses	O
.	O
</s>
<s>
This	O
addressed	O
problems	O
1	O
and	O
2	O
above	O
,	O
since	O
a	O
company	O
can	O
simply	O
order	O
a	O
large	O
batch	O
of	O
fresh	O
PROM	B-General_Concept
chips	O
and	O
program	O
them	O
with	O
the	O
desired	O
contents	O
at	O
its	O
designers	O
 '	O
convenience	O
.	O
</s>
<s>
The	O
advent	O
of	O
the	O
metal	B-Architecture
–	I-Architecture
oxide	I-Architecture
–	I-Architecture
semiconductor	I-Architecture
field-effect	I-Architecture
transistor	I-Architecture
(	O
MOSFET	B-Architecture
)	O
,	O
invented	O
at	O
Bell	O
Labs	O
in	O
1959	O
,	O
enabled	O
the	O
practical	O
use	O
of	O
metal	B-Architecture
–	I-Architecture
oxide	I-Architecture
–	I-Architecture
semiconductor	I-Architecture
(	O
MOS	B-Architecture
)	O
transistors	B-Application
as	O
memory	B-Algorithm
cell	I-Algorithm
storage	B-General_Concept
elements	O
in	O
semiconductor	B-Architecture
memory	I-Architecture
,	O
a	O
function	O
previously	O
served	O
by	O
magnetic	B-Architecture
cores	O
in	O
computer	B-General_Concept
memory	I-General_Concept
.	O
</s>
<s>
In	O
1967	O
,	O
Dawon	O
Kahng	O
and	O
Simon	O
Sze	O
of	O
Bell	O
Labs	O
proposed	O
that	O
the	O
floating	B-Algorithm
gate	I-Algorithm
of	O
a	O
MOS	B-Architecture
semiconductor	O
device	O
could	O
be	O
used	O
for	O
the	O
cell	O
of	O
a	O
reprogrammable	O
ROM	B-Device
,	O
which	O
led	O
to	O
Dov	O
Frohman	O
of	O
Intel	O
inventing	O
erasable	B-General_Concept
programmable	I-General_Concept
read-only	I-General_Concept
memory	I-General_Concept
(	O
EPROM	B-General_Concept
)	O
in	O
1971	O
.	O
</s>
<s>
The	O
1971	O
invention	O
of	O
EPROM	B-General_Concept
essentially	O
solved	O
problem	O
3	O
,	O
since	O
EPROM	B-General_Concept
(	O
unlike	O
PROM	B-General_Concept
)	O
can	O
be	O
repeatedly	O
reset	O
to	O
its	O
unprogrammed	O
state	B-Application
by	O
exposure	O
to	O
strong	O
ultraviolet	B-Application
light	O
.	O
</s>
<s>
Electrically	B-General_Concept
erasable	I-General_Concept
programmable	I-General_Concept
read-only	I-General_Concept
memory	I-General_Concept
(	O
EEPROM	B-General_Concept
)	O
,	O
developed	O
by	O
Yasuo	O
Tarui	O
,	O
Yutaka	O
Hayashi	O
and	O
Kiyoko	O
Naga	O
at	O
the	O
Electrotechnical	O
Laboratory	O
in	O
1972	O
,	O
went	O
a	O
long	O
way	O
to	O
solving	O
problem	O
4	O
,	O
since	O
an	O
EEPROM	B-General_Concept
can	O
be	O
programmed	O
in-place	O
if	O
the	O
containing	O
device	O
provides	O
a	O
means	O
to	O
receive	O
the	O
program	O
contents	O
from	O
an	O
external	O
source	O
(	O
for	O
example	O
,	O
a	O
personal	O
computer	O
via	O
a	O
serial	O
cable	O
)	O
.	O
</s>
<s>
Flash	B-Device
memory	I-Device
,	O
invented	O
by	O
Fujio	O
Masuoka	O
at	O
Toshiba	O
in	O
the	O
early	O
1980s	O
and	O
commercialized	O
in	O
the	O
late	O
1980s	O
,	O
is	O
a	O
form	O
of	O
EEPROM	B-General_Concept
that	O
makes	O
very	O
efficient	O
use	O
of	O
chip	O
area	O
and	O
can	O
be	O
erased	O
and	O
reprogrammed	O
thousands	O
of	O
times	O
without	O
damage	O
.	O
</s>
<s>
All	O
of	O
these	O
technologies	O
improved	O
the	O
flexibility	O
of	O
ROM	B-Device
,	O
but	O
at	O
a	O
significant	O
cost-per-chip	O
,	O
so	O
that	O
in	O
large	O
quantities	O
mask	B-Device
ROM	I-Device
would	O
remain	O
an	O
economical	O
choice	O
for	O
many	O
years	O
.	O
</s>
<s>
(	O
Decreasing	O
cost	O
of	O
reprogrammable	O
devices	O
had	O
almost	O
eliminated	O
the	O
market	O
for	O
mask	B-Device
ROM	I-Device
by	O
2000	O
.	O
)	O
</s>
<s>
Rewriteable	O
technologies	O
were	O
envisioned	O
as	O
replacements	O
for	O
mask	B-Device
ROM	I-Device
.	O
</s>
<s>
Its	O
designers	O
explicitly	O
broke	O
from	O
past	O
practice	O
,	O
stating	O
plainly	O
that	O
"	O
the	O
aim	O
of	O
NAND	O
flash	O
is	O
to	O
replace	O
hard	B-Device
disks	I-Device
,	O
"	O
rather	O
than	O
the	O
traditional	O
use	O
of	O
ROM	B-Device
as	O
a	O
form	O
of	O
non-volatile	B-General_Concept
primary	O
storage	B-General_Concept
.	O
</s>
<s>
,	O
NAND	O
has	O
nearly	O
completely	O
achieved	O
this	O
goal	O
by	O
offering	O
throughput	O
higher	O
than	O
hard	B-Device
disks	I-Device
,	O
lower	O
latency	O
,	O
higher	O
tolerance	O
of	O
physical	O
shock	O
,	O
extreme	O
miniaturization	O
(	O
in	O
the	O
form	O
of	O
USB	O
flash	O
drives	O
and	O
tiny	O
microSD	O
memory	B-Device
cards	I-Device
,	O
for	O
example	O
)	O
,	O
and	O
much	O
lower	O
power	O
consumption	O
.	O
</s>
<s>
Many	O
stored-program	O
computers	O
use	O
a	O
form	O
of	O
non-volatile	B-General_Concept
storage	I-General_Concept
(	O
that	O
is	O
,	O
storage	B-General_Concept
that	O
retains	O
its	O
data	O
when	O
power	O
is	O
removed	O
)	O
to	O
store	O
the	O
initial	O
program	O
that	O
runs	O
when	O
the	O
computer	O
is	O
powered	O
on	O
or	O
otherwise	O
begins	O
execution	O
(	O
a	O
process	B-Architecture
known	O
as	O
bootstrapping	O
,	O
often	O
abbreviated	O
to	O
"	O
booting	B-Operating_System
"	O
or	O
"	O
booting	B-Operating_System
up	I-Operating_System
"	O
)	O
.	O
</s>
<s>
Likewise	O
,	O
every	O
non-trivial	O
computer	O
needs	O
some	O
form	O
of	O
mutable	O
memory	O
to	O
record	O
changes	O
in	O
its	O
state	B-Application
as	O
it	O
executes	O
.	O
</s>
<s>
Forms	O
of	O
read-only	B-Device
memory	I-Device
were	O
employed	O
as	O
non-volatile	B-General_Concept
storage	I-General_Concept
for	O
programs	O
in	O
most	O
early	O
stored-program	O
computers	O
,	O
such	O
as	O
ENIAC	B-Device
after	O
1948	O
.	O
</s>
<s>
Read-only	B-Device
memory	I-Device
was	O
simpler	O
to	O
implement	O
since	O
it	O
needed	O
only	O
a	O
mechanism	O
to	O
read	O
stored	O
values	O
,	O
and	O
not	O
to	O
change	O
them	O
in-place	O
,	O
and	O
thus	O
could	O
be	O
implemented	O
with	O
very	O
crude	O
electromechanical	O
devices	O
(	O
see	O
historical	O
examples	O
below	O
)	O
.	O
</s>
<s>
With	O
the	O
advent	O
of	O
integrated	O
circuits	O
in	O
the	O
1960s	O
,	O
both	O
ROM	B-Device
and	O
its	O
mutable	O
counterpart	O
static	B-Architecture
RAM	I-Architecture
were	O
implemented	O
as	O
arrays	O
of	O
transistors	B-Application
in	O
silicon	O
chips	O
;	O
however	O
,	O
a	O
ROM	B-Device
memory	B-Algorithm
cell	I-Algorithm
could	O
be	O
implemented	O
using	O
fewer	O
transistors	B-Application
than	O
an	O
SRAM	O
memory	B-Algorithm
cell	I-Algorithm
,	O
since	O
the	O
latter	O
needs	O
a	O
latch	B-General_Concept
(	O
comprising	O
5-20	O
transistors	B-Application
)	O
to	O
retain	O
its	O
contents	O
,	O
while	O
a	O
ROM	B-Device
cell	O
might	O
consist	O
of	O
the	O
absence	O
(	O
logical	O
0	O
)	O
or	O
presence	O
(	O
logical	O
1	O
)	O
of	O
one	O
transistor	B-Application
connecting	O
a	O
bit	O
line	O
to	O
a	O
word	O
line	O
.	O
</s>
<s>
Consequently	O
,	O
ROM	B-Device
could	O
be	O
implemented	O
at	O
a	O
lower	O
cost-per-bit	O
than	O
RAM	B-Architecture
for	O
many	O
years	O
.	O
</s>
<s>
Most	O
home	O
computers	O
of	O
the	O
1980s	O
stored	O
a	O
BASIC	O
interpreter	O
or	O
operating	B-General_Concept
system	I-General_Concept
in	O
ROM	B-Device
as	O
other	O
forms	O
of	O
non-volatile	B-General_Concept
storage	I-General_Concept
such	O
as	O
magnetic	B-Architecture
disk	I-Architecture
drives	O
were	O
too	O
costly	O
.	O
</s>
<s>
For	O
example	O
,	O
the	O
Commodore	O
64	O
included	O
64	O
KB	O
of	O
RAM	B-Architecture
and	O
20	O
KB	O
of	O
ROM	B-Device
containing	O
a	O
BASIC	O
interpreter	O
and	O
the	O
KERNAL	B-Operating_System
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
Later	O
home	O
or	O
office	O
computers	O
such	O
as	O
the	O
IBM	B-Device
PC	I-Device
XT	I-Device
often	O
included	O
magnetic	B-Architecture
disk	I-Architecture
drives	O
,	O
and	O
larger	O
amounts	O
of	O
RAM	B-Architecture
,	O
allowing	O
them	O
to	O
load	O
their	O
operating	B-General_Concept
systems	I-General_Concept
from	O
disk	B-Device
into	O
RAM	B-Architecture
,	O
with	O
only	O
a	O
minimal	O
hardware	O
initialization	O
core	O
and	O
bootloader	B-Application
remaining	O
in	O
ROM	B-Device
(	O
known	O
as	O
the	O
BIOS	B-Operating_System
in	O
IBM-compatible	O
computers	O
)	O
.	O
</s>
<s>
This	O
arrangement	O
allowed	O
for	O
a	O
more	O
complex	O
and	O
easily	O
upgradeable	O
operating	B-General_Concept
system	I-General_Concept
.	O
</s>
<s>
In	O
modern	O
PCs	O
,	O
"	O
ROM	B-Device
"	O
is	O
used	O
to	O
store	O
the	O
basic	O
bootstrapping	O
firmware	B-Application
for	O
the	O
processor	O
,	O
as	O
well	O
as	O
the	O
various	O
firmware	B-Application
needed	O
to	O
internally	O
control	O
self-contained	O
devices	O
such	O
as	O
graphic	B-Device
cards	I-Device
,	O
hard	B-Device
disk	I-Device
drives	I-Device
,	O
solid	B-Device
state	I-Device
drives	I-Device
,	O
optical	O
disc	B-Device
drives	I-Device
,	O
TFT	B-Device
screens	I-Device
,	O
etc.	O
,	O
in	O
the	O
system	O
.	O
</s>
<s>
Today	O
,	O
many	O
of	O
these	O
"	O
read-only	O
"	O
memories	O
–	O
especially	O
the	O
BIOS/UEFI	O
–	O
are	O
often	O
replaced	O
with	O
EEPROM	B-General_Concept
or	O
Flash	B-Device
memory	I-Device
(	O
see	O
below	O
)	O
,	O
to	O
permit	O
in-place	O
reprogramming	O
should	O
the	O
need	O
for	O
a	O
firmware	B-Application
upgrade	O
arise	O
.	O
</s>
<s>
However	O
,	O
simple	O
and	O
mature	O
sub-systems	O
(	O
such	O
as	O
the	O
keyboard	O
or	O
some	O
communication	O
controllers	O
in	O
the	O
integrated	O
circuits	O
on	O
the	O
main	O
board	O
,	O
for	O
example	O
)	O
may	O
employ	O
mask	B-Device
ROM	I-Device
or	O
OTP	B-General_Concept
(	O
one-time	B-General_Concept
programmable	I-General_Concept
)	O
.	O
</s>
<s>
ROM	B-Device
and	O
successor	O
technologies	O
such	O
as	O
flash	O
are	O
prevalent	O
in	O
embedded	B-Architecture
systems	I-Architecture
.	O
</s>
<s>
all	O
of	O
which	O
are	O
designed	O
for	O
specific	O
functions	O
,	O
but	O
are	O
based	O
on	O
general-purpose	O
microprocessors	B-Architecture
.	O
</s>
<s>
With	O
software	O
usually	O
tightly	O
coupled	O
to	O
hardware	O
,	O
program	O
changes	O
are	O
rarely	O
needed	O
in	O
such	O
devices	O
(	O
which	O
typically	O
lack	O
hard	B-Device
disks	I-Device
for	O
reasons	O
of	O
cost	O
,	O
size	O
,	O
or	O
power	O
consumption	O
)	O
.	O
</s>
<s>
As	O
of	O
2008	O
,	O
most	O
products	O
use	O
Flash	O
rather	O
than	O
mask	B-Device
ROM	I-Device
,	O
and	O
many	O
provide	O
some	O
means	O
for	O
connecting	O
to	O
a	O
PC	O
for	O
firmware	B-Application
updates	O
;	O
for	O
example	O
,	O
a	O
digital	O
audio	O
player	O
might	O
be	O
updated	O
to	O
support	O
a	O
new	O
file	O
format	O
.	O
</s>
<s>
Some	O
hobbyists	O
have	O
taken	O
advantage	O
of	O
this	O
flexibility	O
to	O
reprogram	O
consumer	O
products	O
for	O
new	O
purposes	O
;	O
for	O
example	O
,	O
the	O
iPodLinux	B-General_Concept
and	O
OpenWrt	B-Application
projects	O
have	O
enabled	O
users	O
to	O
run	O
full-featured	O
Linux	B-Application
distributions	I-Application
on	O
their	O
MP3	O
players	O
and	O
wireless	O
routers	O
,	O
respectively	O
.	O
</s>
<s>
ROM	B-Device
is	O
also	O
useful	O
for	O
binary	O
storage	B-General_Concept
of	O
cryptographic	O
data	O
,	O
as	O
it	O
makes	O
them	O
difficult	O
to	O
replace	O
,	O
which	O
may	O
be	O
desirable	O
in	O
order	O
to	O
enhance	O
information	O
security	O
.	O
</s>
<s>
Since	O
ROM	B-Device
(	O
at	O
least	O
in	O
hard-wired	O
mask	O
form	O
)	O
cannot	O
be	O
modified	O
,	O
it	O
is	O
only	O
suitable	O
for	O
storing	O
data	O
which	O
is	O
not	O
expected	O
to	O
need	O
modification	O
for	O
the	O
life	O
of	O
the	O
device	O
.	O
</s>
<s>
To	O
that	O
end	O
,	O
ROM	B-Device
has	O
been	O
used	O
in	O
many	O
computers	O
to	O
store	O
look-up	B-Data_Structure
tables	I-Data_Structure
for	O
the	O
evaluation	O
of	O
mathematical	O
and	O
logical	O
functions	O
(	O
for	O
example	O
,	O
a	O
floating-point	B-General_Concept
unit	I-General_Concept
might	O
tabulate	O
the	O
sine	O
function	O
in	O
order	O
to	O
facilitate	O
faster	O
computation	O
)	O
.	O
</s>
<s>
This	O
was	O
especially	O
effective	O
when	O
CPUs	B-Device
were	O
slow	O
and	O
ROM	B-Device
was	O
cheap	O
compared	O
to	O
RAM	B-Architecture
.	O
</s>
<s>
Notably	O
,	O
the	O
display	B-Device
adapters	I-Device
of	O
early	O
personal	O
computers	O
stored	O
tables	O
of	O
bitmapped	O
font	O
characters	O
in	O
ROM	B-Device
.	O
</s>
<s>
This	O
was	O
the	O
case	O
for	O
both	O
the	O
CGA	B-Device
and	O
MDA	B-Device
adapters	O
available	O
with	O
the	O
IBM	B-Device
PC	I-Device
XT	I-Device
.	O
</s>
<s>
The	O
use	O
of	O
ROM	B-Device
to	O
store	O
such	O
small	O
amounts	O
of	O
data	O
has	O
disappeared	O
almost	O
completely	O
in	O
modern	O
general-purpose	O
computers	O
.	O
</s>
<s>
However	O
,	O
NAND	O
Flash	O
has	O
taken	O
over	O
a	O
new	O
role	O
as	O
a	O
medium	O
for	O
mass	B-Device
storage	I-Device
or	O
secondary	O
storage	B-General_Concept
of	O
files	O
.	O
</s>
<s>
Mask	B-Device
ROM	I-Device
is	O
a	O
read-only	B-Device
memory	I-Device
whose	O
contents	O
are	O
programmed	O
by	O
the	O
integrated	O
circuit	O
manufacturer	O
(	O
rather	O
than	O
by	O
the	O
user	O
)	O
.	O
</s>
<s>
The	O
desired	O
data	O
is	O
converted	O
into	O
a	O
custom	O
mask	B-Algorithm
layer	I-Algorithm
for	O
the	O
final	O
metallization	O
of	O
interconnections	O
on	O
the	O
memory	B-Architecture
chip	I-Architecture
(	O
hence	O
the	O
name	O
)	O
.	O
</s>
<s>
Mask	B-Device
ROM	I-Device
can	O
be	O
made	O
in	O
several	O
ways	O
,	O
all	O
of	O
which	O
aim	O
to	O
change	O
the	O
electrical	O
response	O
of	O
a	O
transistor	B-Application
when	O
it	O
is	O
addressed	O
on	O
a	O
grid	O
,	O
such	O
as	O
:	O
</s>
<s>
In	O
a	O
ROM	B-Device
with	O
transistors	B-Application
in	O
a	O
NOR	O
configuration	O
,	O
using	O
a	O
photomask	B-Algorithm
to	O
define	O
only	O
specific	O
areas	O
of	O
a	O
grid	O
with	O
transistors	B-Application
,	O
to	O
fill	O
with	O
metal	O
thus	O
connecting	O
to	O
the	O
grid	O
only	O
part	O
of	O
all	O
the	O
transistors	B-Application
in	O
the	O
ROM	B-Device
chip	I-Device
thus	O
making	O
a	O
grid	O
where	O
transistors	B-Application
that	O
are	O
connected	O
cause	O
a	O
different	O
electrical	O
response	O
when	O
addressed	O
,	O
than	O
spaces	O
in	O
the	O
grid	O
where	O
the	O
transistors	B-Application
are	O
not	O
connected	O
,	O
a	O
connected	O
transistor	B-Application
may	O
represent	O
a	O
1	O
and	O
an	O
unconnected	O
one	O
a	O
0	O
,	O
or	O
viceversa	O
.	O
</s>
<s>
This	O
is	O
the	O
least	O
expensive	O
,	O
and	O
fastest	O
way	O
of	O
making	O
mask	B-Device
ROM	I-Device
as	O
it	O
only	O
needs	O
one	O
mask	O
with	O
data	O
,	O
and	O
has	O
the	O
lowest	O
density	O
of	O
all	O
mask	B-Device
ROM	I-Device
types	O
as	O
it	O
is	O
done	O
at	O
the	O
metallization	O
layer	O
,	O
whose	O
features	O
can	O
be	O
relatively	O
large	O
in	O
respect	O
to	O
other	O
parts	O
of	O
the	O
ROM	B-Device
.	O
</s>
<s>
This	O
is	O
known	O
as	O
contact-programmed	O
ROM	B-Device
.	O
</s>
<s>
In	O
ROM	B-Device
with	O
a	O
NAND	O
configuration	O
,	O
this	O
is	O
known	O
as	O
metal-layer	O
programming	O
and	O
the	O
mask	O
defines	O
where	O
to	O
fill	O
the	O
areas	O
surrounding	O
transistors	B-Application
with	O
metal	O
which	O
short-circuits	O
the	O
transistors	B-Application
instead	O
,	O
a	O
transistor	B-Application
that	O
is	O
not	O
short	O
circuited	O
may	O
represent	O
a	O
0	O
,	O
and	O
one	O
that	O
is	O
may	O
represent	O
a	O
1	O
,	O
or	O
viceversa	O
.	O
</s>
<s>
Using	O
two	O
masks	O
to	O
define	O
two	O
types	O
of	O
ion	O
implantation	O
regions	O
for	O
transistors	B-Application
,	O
to	O
change	O
their	O
electrical	O
properties	O
when	O
addressed	O
in	O
a	O
grid	O
and	O
define	O
two	O
types	O
of	O
transistors	B-Application
.	O
</s>
<s>
The	O
type	O
of	O
transistor	B-Application
defines	O
if	O
it	O
represents	O
a	O
1	O
or	O
a	O
0	O
bit	O
.	O
</s>
<s>
One	O
mask	O
defines	O
where	O
to	O
deposit	O
one	O
type	O
of	O
ion	O
implantation	O
(	O
the	O
"	O
1	O
"	O
transistors	B-Application
)	O
,	O
and	O
another	O
defines	O
where	O
to	O
deposit	O
the	O
other	O
(	O
the	O
"	O
0	O
"	O
transistors	B-Application
)	O
.	O
</s>
<s>
This	O
is	O
known	O
as	O
voltage	O
threshold	O
ROM	B-Device
(	O
VTROM	O
)	O
as	O
the	O
different	O
ion	O
implantation	O
types	O
define	O
different	O
voltage	O
thresholds	O
in	O
the	O
transistors	B-Application
,	O
and	O
it	O
's	O
the	O
voltage	O
threshold	O
on	O
a	O
transistor	B-Application
that	O
defines	O
a	O
0	O
,	O
or	O
a	O
1	O
.	O
</s>
<s>
This	O
technique	O
offers	O
a	O
high	O
level	O
of	O
resistance	O
against	O
optical	O
reading	O
of	O
the	O
contents	O
as	O
ion-implantation	O
regions	O
are	O
difficult	O
to	O
distinguish	O
optically	O
,	O
which	O
may	O
be	O
attempted	O
with	O
decapping	O
of	O
the	O
ROM	B-Device
and	O
a	O
microscope	O
.	O
</s>
<s>
Using	O
two	O
levels	O
of	O
thickness	O
for	O
a	O
gate	O
oxide	O
in	O
transistors	B-Application
,	O
and	O
using	O
a	O
mask	O
to	O
define	O
where	O
to	O
deposit	O
one	O
thickness	O
of	O
oxide	O
,	O
and	O
another	O
mask	O
to	O
deposit	O
the	O
other	O
.	O
</s>
<s>
Depending	O
on	O
the	O
thickness	O
a	O
transistor	B-Application
can	O
have	O
different	O
electrical	O
properties	O
and	O
thus	O
represent	O
either	O
a	O
1	O
or	O
a	O
0	O
.	O
</s>
<s>
Using	O
several	O
masks	O
to	O
define	O
the	O
presence	O
or	O
absence	O
of	O
the	O
transistors	B-Application
themselves	O
,	O
on	O
a	O
grid	O
.	O
</s>
<s>
Addressing	O
a	O
non-existent	O
transistor	B-Application
may	O
be	O
interpreted	O
as	O
a	O
0	O
,	O
and	O
if	O
a	O
transistor	B-Application
is	O
present	O
it	O
may	O
be	O
interpreted	O
as	O
a	O
1	O
,	O
or	O
viceversa	O
.	O
</s>
<s>
Mask	B-Device
ROM	I-Device
transistors	B-Application
can	O
be	O
arranged	O
in	O
either	O
NOR	O
or	O
NAND	O
configurations	O
and	O
can	O
achieve	O
one	O
of	O
the	O
smallest	O
cell	O
sizes	O
possible	O
as	O
each	O
bit	O
is	O
represented	O
by	O
only	O
one	O
transistor	B-Application
.	O
</s>
<s>
NAND	O
offers	O
higher	O
storage	B-General_Concept
density	O
than	O
NOR	O
.	O
</s>
<s>
OR	O
configurations	O
are	O
also	O
possible	O
,	O
but	O
compared	O
to	O
NOR	O
it	O
only	O
connects	O
transistors	B-Application
to	O
Vcc	O
instead	O
of	O
Vss	O
.	O
</s>
<s>
Mask	O
ROMs	O
used	O
to	O
be	O
the	O
most	O
inexpensive	O
,	O
and	O
are	O
the	O
simplest	O
semiconductor	B-Architecture
memory	I-Architecture
devices	O
,	O
with	O
only	O
one	O
metal	O
layer	O
and	O
one	O
polysilicon	O
layer	O
,	O
making	O
it	O
the	O
type	O
of	O
semiconductor	B-Architecture
memory	I-Architecture
with	O
the	O
highest	O
manufacturing	O
yield	O
(	O
the	O
highest	O
number	O
of	O
working	O
devices	O
per	O
manufacturing	O
run	O
)	O
.	O
</s>
<s>
It	O
is	O
common	O
practice	O
to	O
use	O
rewritable	O
non-volatile	B-General_Concept
memory	I-General_Concept
–	O
such	O
as	O
UV-EPROM	O
or	O
EEPROM	B-General_Concept
–	O
for	O
the	O
development	O
phase	O
of	O
a	O
project	O
,	O
and	O
to	O
switch	O
to	O
mask	B-Device
ROM	I-Device
when	O
the	O
code	O
has	O
been	O
finalized	O
.	O
</s>
<s>
For	O
example	O
,	O
Atmel	O
microcontrollers	B-Architecture
come	O
in	O
both	O
EEPROM	B-General_Concept
and	O
mask	B-Device
ROM	I-Device
formats	O
.	O
</s>
<s>
The	O
main	O
advantage	O
of	O
mask	B-Device
ROM	I-Device
is	O
its	O
cost	O
.	O
</s>
<s>
Per	O
bit	O
,	O
mask	B-Device
ROM	I-Device
is	O
more	O
compact	O
than	O
any	O
other	O
kind	O
of	O
semiconductor	B-Architecture
memory	I-Architecture
.	O
</s>
<s>
Since	O
the	O
cost	O
of	O
an	O
integrated	O
circuit	O
strongly	O
depends	O
on	O
its	O
size	O
,	O
mask	B-Device
ROM	I-Device
is	O
significantly	O
cheaper	O
than	O
any	O
other	O
kind	O
of	O
semiconductor	B-Architecture
memory	I-Architecture
.	O
</s>
<s>
Design	O
errors	O
are	O
costly	O
:	O
if	O
an	O
error	O
in	O
the	O
data	O
or	O
code	O
is	O
found	O
,	O
the	O
mask	B-Device
ROM	I-Device
is	O
useless	O
and	O
must	O
be	O
replaced	O
in	O
order	O
to	O
change	O
the	O
code	O
or	O
data	O
.	O
</s>
<s>
,	O
four	O
companies	O
produce	O
most	O
such	O
mask	B-Device
ROM	I-Device
chips	O
:	O
Samsung	O
Electronics	O
,	O
NEC	O
Corporation	O
,	O
Oki	O
Electric	O
Industry	O
,	O
and	O
Macronix	O
.	O
</s>
<s>
Some	O
integrated	O
circuits	O
contain	O
only	O
mask	B-Device
ROM	I-Device
.	O
</s>
<s>
Other	O
integrated	O
circuits	O
contain	O
mask	B-Device
ROM	I-Device
as	O
well	O
as	O
a	O
variety	O
of	O
other	O
devices	O
.	O
</s>
<s>
In	O
particular	O
,	O
many	O
microprocessors	B-Architecture
have	O
mask	B-Device
ROM	I-Device
to	O
store	O
their	O
microcode	B-Device
.	O
</s>
<s>
Some	O
microcontrollers	B-Architecture
have	O
mask	B-Device
ROM	I-Device
to	O
store	O
the	O
bootloader	B-Application
or	O
all	O
of	O
their	O
firmware	B-Application
.	O
</s>
<s>
Classic	O
mask-programmed	O
ROM	B-Device
chips	I-Device
are	O
integrated	O
circuits	O
that	O
physically	O
encode	O
the	O
data	O
to	O
be	O
stored	O
,	O
and	O
thus	O
it	O
is	O
impossible	O
to	O
change	O
their	O
contents	O
after	O
fabrication	B-Architecture
.	O
</s>
<s>
It	O
is	O
also	O
possible	O
to	O
write	O
the	O
contents	O
of	O
a	O
Laser	O
ROM	B-Device
by	O
using	O
a	O
laser	O
to	O
alter	O
the	O
electrical	O
properties	O
of	O
only	O
some	O
diodes	O
on	O
the	O
ROM	B-Device
,	O
or	O
by	O
using	O
a	O
laser	O
to	O
cut	O
only	O
some	O
polysilicon	O
links	O
,	O
instead	O
of	O
using	O
a	O
mask	O
.	O
</s>
<s>
Programmable	B-General_Concept
read-only	I-General_Concept
memory	I-General_Concept
(	O
PROM	B-General_Concept
)	O
,	O
or	O
one-time	B-General_Concept
programmable	I-General_Concept
ROM	B-Device
(	O
OTP	B-General_Concept
)	O
,	O
can	O
be	O
written	O
to	O
or	O
programmed	O
via	O
a	O
special	O
device	O
called	O
a	O
PROM	B-General_Concept
programmer	O
.	O
</s>
<s>
Consequently	O
,	O
a	O
PROM	B-General_Concept
can	O
only	O
be	O
programmed	O
once	O
.	O
</s>
<s>
Erasable	B-General_Concept
programmable	I-General_Concept
read-only	I-General_Concept
memory	I-General_Concept
(	O
EPROM	B-General_Concept
)	O
can	O
be	O
erased	O
by	O
exposure	O
to	O
strong	O
ultraviolet	B-Application
light	O
(	O
typically	O
for	O
10	O
minutes	O
or	O
longer	O
)	O
,	O
then	O
rewritten	O
with	O
a	O
process	B-Architecture
that	O
again	O
needs	O
higher	O
than	O
usual	O
voltage	O
applied	O
.	O
</s>
<s>
Repeated	O
exposure	O
to	O
UV	O
light	O
will	O
eventually	O
wear	O
out	O
an	O
EPROM	B-General_Concept
,	O
but	O
the	O
endurance	O
of	O
most	O
EPROM	B-General_Concept
chips	O
exceeds	O
1000	O
cycles	O
of	O
erasing	O
and	O
reprogramming	O
.	O
</s>
<s>
EPROM	B-General_Concept
chip	O
packages	O
can	O
often	O
be	O
identified	O
by	O
the	O
prominent	O
quartz	B-General_Concept
"	O
window	O
"	O
which	O
allows	O
UV	O
light	O
to	O
enter	O
.	O
</s>
<s>
Some	O
EPROM	B-General_Concept
chips	O
are	O
factory-erased	O
before	O
they	O
are	O
packaged	O
,	O
and	O
include	O
no	O
window	O
;	O
these	O
are	O
effectively	O
PROM	B-General_Concept
.	O
</s>
<s>
Electrically	B-General_Concept
erasable	I-General_Concept
programmable	I-General_Concept
read-only	I-General_Concept
memory	I-General_Concept
(	O
EEPROM	B-General_Concept
)	O
is	O
based	O
on	O
a	O
similar	O
semiconductor	O
structure	O
to	O
EPROM	B-General_Concept
,	O
but	O
allows	O
its	O
entire	O
contents	O
(	O
or	O
selected	O
banks	O
)	O
to	O
be	O
electrically	O
erased	O
,	O
then	O
rewritten	O
electrically	O
,	O
so	O
that	O
they	O
need	O
not	O
be	O
removed	O
from	O
the	O
computer	O
(	O
whether	O
general-purpose	O
or	O
an	O
embedded	B-Architecture
computer	I-Architecture
in	O
a	O
camera	O
,	O
MP3	O
player	O
,	O
etc	O
.	O
)	O
.	O
</s>
<s>
Writing	O
or	O
flashing	O
an	O
EEPROM	B-General_Concept
is	O
much	O
slower	O
(	O
milliseconds	O
per	O
bit	O
)	O
than	O
reading	O
from	O
a	O
ROM	B-Device
or	O
writing	O
to	O
a	O
RAM	B-Architecture
(	O
nanoseconds	O
in	O
both	O
cases	O
)	O
.	O
</s>
<s>
Electrically	B-General_Concept
alterable	I-General_Concept
read-only	I-General_Concept
memory	I-General_Concept
(	O
EAROM	O
)	O
is	O
a	O
type	O
of	O
EEPROM	B-General_Concept
that	O
can	O
be	O
modified	O
one	O
bit	O
at	O
a	O
time	O
.	O
</s>
<s>
Writing	O
is	O
a	O
very	O
slow	O
process	B-Architecture
and	O
again	O
needs	O
higher	O
voltage	O
(	O
usually	O
around	O
12	O
V	O
)	O
than	O
is	O
used	O
for	O
read	O
access	O
.	O
</s>
<s>
EAROM	O
may	O
be	O
used	O
as	O
non-volatile	B-General_Concept
storage	I-General_Concept
for	O
critical	O
system	O
setup	O
information	O
;	O
in	O
many	O
applications	O
,	O
EAROM	O
has	O
been	O
supplanted	O
by	O
CMOS	B-Device
RAM	I-Device
supplied	O
by	O
mains	O
power	O
and	O
backed-up	O
with	O
a	O
lithium	O
battery	O
.	O
</s>
<s>
Flash	B-Device
memory	I-Device
(	O
or	O
simply	O
flash	O
)	O
is	O
a	O
modern	O
type	O
of	O
EEPROM	B-General_Concept
invented	O
in	O
1984	O
.	O
</s>
<s>
Flash	B-Device
memory	I-Device
can	O
be	O
erased	O
and	O
rewritten	O
faster	O
than	O
ordinary	O
EEPROM	B-General_Concept
,	O
and	O
newer	O
designs	O
feature	O
very	O
high	O
endurance	O
(	O
exceeding	O
1,000,000	O
cycles	O
)	O
.	O
</s>
<s>
Modern	O
NAND	O
flash	O
makes	O
efficient	O
use	O
of	O
silicon	O
chip	O
area	O
,	O
resulting	O
in	O
individual	O
ICs	O
with	O
a	O
capacity	O
as	O
high	O
as	O
32	O
GB	O
;	O
this	O
feature	O
,	O
along	O
with	O
its	O
endurance	O
and	O
physical	O
durability	O
,	O
has	O
allowed	O
NAND	O
flash	O
to	O
replace	O
magnetic	B-Architecture
in	O
some	O
applications	O
(	O
such	O
as	O
USB	O
flash	O
drives	O
)	O
.	O
</s>
<s>
NOR	O
flash	B-Device
memory	I-Device
is	O
sometimes	O
called	O
flash	B-Device
ROM	I-Device
or	O
flash	B-Device
EEPROM	I-Device
when	O
used	O
as	O
a	O
replacement	O
for	O
older	O
ROM	B-Device
types	O
,	O
but	O
not	O
in	O
applications	O
that	O
take	O
advantage	O
of	O
its	O
ability	O
to	O
be	O
modified	O
quickly	O
and	O
frequently	O
.	O
</s>
<s>
By	O
applying	O
write	B-Device
protection	I-Device
,	O
some	O
types	O
of	O
reprogrammable	O
ROMs	O
may	O
temporarily	O
become	O
read-only	B-Device
memory	I-Device
.	O
</s>
<s>
There	O
are	O
other	O
types	O
of	O
non-volatile	B-General_Concept
memory	I-General_Concept
which	O
are	O
not	O
based	O
on	O
solid-state	O
IC	O
technology	O
,	O
including	O
:	O
</s>
<s>
Optical	B-General_Concept
storage	I-General_Concept
media	O
,	O
such	O
CD-ROM	B-Device
which	O
is	O
read-only	O
(	O
analogous	O
to	O
masked	B-Device
ROM	I-Device
)	O
.	O
</s>
<s>
CD-R	O
is	O
Write	B-Device
Once	I-Device
Read	I-Device
Many	I-Device
(	O
analogous	O
to	O
PROM	B-General_Concept
)	O
,	O
while	O
CD-RW	O
supports	O
erase-rewrite	O
cycles	O
(	O
analogous	O
to	O
EEPROM	B-General_Concept
)	O
;	O
both	O
are	O
designed	O
for	O
backwards-compatibility	B-General_Concept
with	O
CD-ROM	B-Device
.	O
</s>
<s>
Diode	B-General_Concept
matrix	I-General_Concept
ROM	B-Device
,	O
used	O
in	O
small	O
amounts	O
in	O
many	O
computers	O
in	O
the	O
1960s	O
as	O
well	O
as	O
electronic	O
desk	O
calculators	B-Application
and	O
keyboard	O
encoders	O
for	O
terminals	B-General_Concept
.	O
</s>
<s>
This	O
ROM	B-Device
was	O
programmed	O
by	O
installing	O
discrete	O
semiconductor	O
diodes	O
at	O
selected	O
locations	O
between	O
a	O
matrix	O
of	O
word	O
line	O
traces	O
and	O
bit	O
line	O
traces	O
on	O
a	O
printed	O
circuit	O
board	O
.	O
</s>
<s>
Resistor	O
,	O
capacitor	O
,	O
or	O
transformer	B-Algorithm
matrix	O
ROM	B-Device
,	O
used	O
in	O
many	O
computers	O
until	O
the	O
1970s	O
.	O
</s>
<s>
Like	O
diode	B-General_Concept
matrix	I-General_Concept
ROM	B-Device
,	O
it	O
was	O
programmed	O
by	O
placing	O
components	O
at	O
selected	O
locations	O
between	O
a	O
matrix	O
of	O
word	O
lines	O
and	O
bit	O
lines	O
.	O
</s>
<s>
ENIAC	B-Device
's	O
Function	O
Tables	O
were	O
resistor	O
matrix	O
ROM	B-Device
,	O
programmed	O
by	O
manually	O
setting	O
rotary	O
switches	O
.	O
</s>
<s>
Various	O
models	O
of	O
the	O
IBM	B-Application
System/360	I-Application
and	O
complex	O
peripheral	O
devices	O
stored	O
their	O
microcode	B-Device
in	O
either	O
capacitor	O
(	O
called	O
BCROS	O
for	O
balanced	O
capacitor	O
read-only	B-Device
storage	I-Device
on	O
the	O
360/50	B-Device
and	O
360/65	O
,	O
or	O
CCROS	O
for	O
card	O
capacitor	O
read-only	B-Device
storage	I-Device
on	O
the	O
360/30	B-Device
)	O
or	O
transformer	B-Algorithm
(	O
called	O
TROS	O
for	O
transformer	B-General_Concept
read-only	I-General_Concept
storage	I-General_Concept
on	O
the	O
360/20	B-Device
,	O
360/40	B-Device
and	O
others	O
)	O
matrix	O
ROM	B-Device
.	O
</s>
<s>
Core	B-General_Concept
rope	I-General_Concept
,	O
a	O
form	O
of	O
transformer	B-Algorithm
matrix	O
ROM	B-Device
technology	O
used	O
where	O
size	O
and	O
weight	O
were	O
critical	O
.	O
</s>
<s>
This	O
was	O
used	O
in	O
NASA/MIT	O
'	O
s	O
Apollo	B-Application
Spacecraft	I-Application
Computers	I-Application
,	O
DEC	O
's	O
PDP-8	B-Device
computers	O
,	O
the	O
Hewlett-Packard	B-Device
9100A	I-Device
calculator	B-Application
,	O
and	O
other	O
places	O
.	O
</s>
<s>
This	O
type	O
of	O
ROM	B-Device
was	O
programmed	O
by	O
hand	O
by	O
weaving	O
"	O
word	O
line	O
wires	O
"	O
inside	O
or	O
outside	O
of	O
ferrite	O
transformer	B-Algorithm
cores	O
.	O
</s>
<s>
These	O
were	O
used	O
in	O
TXE	B-Device
telephone	O
exchanges	O
.	O
</s>
<s>
Although	O
the	O
relative	O
speed	O
of	O
RAM	B-Architecture
vs.	O
ROM	B-Device
has	O
varied	O
over	O
time	O
,	O
large	O
RAM	B-Architecture
chips	I-Architecture
can	O
be	O
read	O
faster	O
than	O
most	O
ROMs	O
.	O
</s>
<s>
For	O
this	O
reason	O
(	O
and	O
to	O
allow	O
uniform	O
access	O
)	O
,	O
ROM	B-Device
content	O
is	O
sometimes	O
copied	O
to	O
RAM	B-Architecture
or	O
shadowed	B-General_Concept
before	O
its	O
first	O
use	O
,	O
and	O
subsequently	O
read	O
from	O
RAM	B-Architecture
.	O
</s>
<s>
For	O
those	O
types	O
of	O
ROM	B-Device
that	O
can	O
be	O
electrically	O
modified	O
,	O
writing	O
speed	O
has	O
traditionally	O
been	O
much	O
slower	O
than	O
reading	O
speed	O
,	O
and	O
it	O
may	O
need	O
unusually	O
high	O
voltage	O
,	O
the	O
movement	O
of	O
jumper	O
plugs	O
to	O
apply	O
write-enable	O
signals	O
,	O
and	O
special	O
lock/unlock	O
command	O
codes	O
.	O
</s>
<s>
Modern	O
NAND	O
Flash	O
achieves	O
the	O
highest	O
write	O
speeds	O
of	O
any	O
rewritable	O
ROM	B-Device
technology	O
,	O
with	O
speeds	O
as	O
high	O
as	O
10	O
GB/s	O
.	O
</s>
<s>
This	O
has	O
been	O
enabled	O
by	O
the	O
increased	O
investment	O
in	O
both	O
consumer	O
and	O
enterprise	O
solid	B-Device
state	I-Device
drives	I-Device
and	O
flash	B-Device
memory	I-Device
products	O
for	O
higher	O
end	O
mobile	O
devices	O
.	O
</s>
<s>
On	O
a	O
technical	O
level	O
the	O
gains	O
have	O
been	O
achieved	O
by	O
increasing	O
parallelism	O
both	O
in	O
controller	O
design	O
and	O
of	O
storage	B-General_Concept
,	O
the	O
use	O
of	O
large	O
DRAM	O
read/write	O
caches	O
and	O
the	O
implementation	O
of	O
memory	B-Algorithm
cells	I-Algorithm
which	O
can	O
store	O
more	O
than	O
one	O
bit	O
(	O
DLC	O
,	O
TLC	O
and	O
MLC	O
)	O
.	O
</s>
<s>
The	O
latter	O
approach	O
is	O
more	O
failure	O
prone	O
but	O
this	O
has	O
been	O
largely	O
mitigated	O
by	O
overprovisioning	O
(	O
the	O
inclusion	O
of	O
spare	O
capacity	O
in	O
a	O
product	O
which	O
is	O
visible	O
only	O
to	O
the	O
drive	O
controller	O
)	O
and	O
by	O
increasingly	O
sophisticated	O
read/write	O
algorithms	O
in	O
drive	O
firmware	B-Application
.	O
</s>
<s>
Because	O
they	O
are	O
written	O
by	O
forcing	O
electrons	O
through	O
a	O
layer	O
of	O
electrical	O
insulation	O
onto	O
a	O
floating	B-Algorithm
transistor	I-Algorithm
gate	I-Algorithm
,	O
rewriteable	O
ROMs	O
can	O
withstand	O
only	O
a	O
limited	O
number	O
of	O
write	O
and	O
erase	O
cycles	O
before	O
the	O
insulation	O
is	O
permanently	O
damaged	O
.	O
</s>
<s>
In	O
the	O
earliest	O
EPROMs	B-General_Concept
,	O
this	O
might	O
occur	O
after	O
as	O
few	O
as	O
1,000	O
write	O
cycles	O
,	O
while	O
in	O
modern	O
Flash	B-Device
EEPROM	I-Device
the	O
endurance	O
may	O
exceed	O
1,000,000	O
.	O
</s>
<s>
The	O
limited	O
endurance	O
,	O
as	O
well	O
as	O
the	O
higher	O
cost	O
per	O
bit	O
,	O
means	O
that	O
Flash-based	O
storage	B-General_Concept
is	O
unlikely	O
to	O
completely	O
supplant	O
magnetic	B-Architecture
disk	I-Architecture
drives	O
in	O
the	O
near	O
future	O
.	O
</s>
<s>
The	O
timespan	O
over	O
which	O
a	O
ROM	B-Device
remains	O
accurately	O
readable	O
is	O
not	O
limited	O
by	O
write	O
cycling	O
.	O
</s>
<s>
The	O
data	O
retention	O
of	O
EPROM	B-General_Concept
,	O
EAROM	O
,	O
EEPROM	B-General_Concept
,	O
and	O
Flash	O
may	O
be	O
time-limited	O
by	O
charge	O
leaking	O
from	O
the	O
floating	B-Algorithm
gates	I-Algorithm
of	O
the	O
memory	B-Algorithm
cell	I-Algorithm
transistors	B-Application
.	O
</s>
<s>
Early	O
generation	O
EEPROM	B-General_Concept
's	O
,	O
in	O
the	O
mid	O
1980	O
's	O
generally	O
cited	O
5	O
or	O
6	O
year	O
data	O
retention	O
.	O
</s>
<s>
A	O
review	O
of	O
EEPROM	B-General_Concept
's	O
offered	O
in	O
the	O
year	O
2020	O
shows	O
manufacturers	O
citing	O
100	O
year	O
data	O
retention	O
.	O
</s>
<s>
Masked	B-Device
ROM	I-Device
and	O
fuse/antifuse	O
PROM	B-General_Concept
do	O
not	O
suffer	O
from	O
this	O
effect	O
,	O
as	O
their	O
data	O
retention	O
depends	O
on	O
physical	O
rather	O
than	O
electrical	O
permanence	O
of	O
the	O
integrated	O
circuit	O
,	O
although	O
fuse	O
re-growth	O
was	O
once	O
a	O
problem	O
in	O
some	O
systems	O
.	O
</s>
<s>
The	O
contents	O
of	O
ROM	B-Device
chips	I-Device
can	O
be	O
extracted	O
with	O
special	O
hardware	O
devices	O
and	O
relevant	O
controlling	O
software	O
.	O
</s>
<s>
This	O
practice	O
is	O
common	O
for	O
,	O
as	O
a	O
main	O
example	O
,	O
reading	O
the	O
contents	O
of	O
older	O
video	O
game	O
console	O
cartridges	B-Protocol
.	O
</s>
<s>
Another	O
example	O
is	O
making	O
backups	B-Protocol
of	O
firmware/OS	O
ROMs	O
from	O
older	O
computers	O
or	O
other	O
devices	O
-	O
for	O
archival	O
purposes	O
,	O
as	O
in	O
many	O
cases	O
,	O
the	O
original	O
chips	O
are	O
PROMs	O
and	O
thus	O
at	O
risk	O
of	O
exceeding	O
their	O
usable	O
data	O
lifetime	O
.	O
</s>
<s>
The	O
resultant	O
memory	O
dump	O
files	O
are	O
known	O
as	O
ROM	B-Device
images	O
or	O
abbreviated	O
ROMs	O
,	O
and	O
can	O
be	O
used	O
to	O
produce	O
duplicate	O
ROMs	O
-	O
for	O
example	O
to	O
produce	O
new	O
cartridges	B-Protocol
or	O
as	O
digital	O
files	O
for	O
playing	O
in	O
console	O
emulators	O
.	O
</s>
<s>
The	O
term	O
ROM	B-Device
image	O
originated	O
when	O
most	O
console	O
games	O
were	O
distributed	O
on	O
cartridges	B-Protocol
containing	O
ROM	B-Device
chips	I-Device
,	O
but	O
achieved	O
such	O
widespread	O
usage	O
that	O
it	O
is	O
still	O
applied	O
to	O
images	O
of	O
newer	O
games	O
distributed	O
on	O
CD-ROMs	B-Device
or	O
other	O
optical	O
media	O
.	O
</s>
<s>
ROM	B-Device
images	O
of	O
commercial	O
games	O
,	O
firmware	B-Application
,	O
etc	O
.	O
</s>
<s>
The	O
unauthorized	O
copying	O
and	O
distribution	O
of	O
copyrighted	O
software	O
is	O
a	O
violation	O
of	O
copyright	O
laws	O
in	O
many	O
jurisdictions	O
,	O
although	O
duplication	O
for	O
backup	B-Protocol
purposes	O
may	O
be	O
considered	O
fair	O
use	O
depending	O
on	O
location	O
.	O
</s>
