How ROM Works

Read-only memory (ROM), also known as firmware, is
an integrated circuit programmed with specific data when it
is manufactured. ROM chips are used not only in computers,
but in most other electronic items as well.

ROM Types
There are five basic ROM types:

* ROM
* PROM
* EPROM
* EEPROM
* Flash memory

Each type has unique characteristics, which you'll learn
about in this article, but they are all types of memory with
two things in common:

* Data stored in these chips is nonvolatile -- it is not
lost when power is removed.
* Data stored in these chips is either unchangeable or
requires a special operation to change (unlike RAM, which can
be changed as easily as it is read).

This means that removing the power source from the chip will
not cause it to lose any data.

ROM at Work

Similar to RAM, ROM chips (Figure 1) contain a grid of
columns and rows. But where the columns and rows intersect,
ROM chips are fundamentally different from RAM chips. While
RAM uses transistors to turn on or off access to a capacitor
at each intersection, ROM uses a diode to connect the lines
if the value is 1. If the value is 0, then the lines are not
connected at all.

A diode normally allows current to flow in only one direction
and has a certain threshold, known as the forward breakover,
that determines how much current is required before the diode
will pass it on. In silicon-based items such as processors
and memory chips, the forward breakover voltage is
approximately 0.6 volts. By taking advantage of the unique
properties of a diode, a ROM chip can send a charge that is
above the forward breakover down the appropriate column with
the selected row grounded to connect at a specific cell. If
a diode is present at that cell, the charge will be conducted
through to the ground, and, under the binary system, the cell
will be read as being "on" (a value of 1). The neat part of
ROM is that if the cell's value is 0, there is no diode at
that intersection to connect the column and row. So the
charge on the column does not get transferred to the row.

As you can see, the way a ROM chip works necessitates the
programming of perfect and complete data when the chip is
created. You cannot reprogram or rewrite a standard ROM chip.
If it is incorrect, or the data needs to be updated, you have
to throw it away and start over. Creating the original
template for a ROM chip is often a laborious process full of
trial and error. But the benefits of ROM chips outweigh the
drawbacks. Once the template is completed, the actual chips
can cost as little as a few cents each. They use very little
power, are extremely reliable and, in the case of most small
electronic devices, contain all the necessary programming to
control the device. A great example is the small chip in the
singing fish toy. This chip, about the size of your
fingernail, contains the 30-second song clips in ROM and the
control codes to synchronize the motors to the music.