800XL, or Radio Shack Color Computer), you can pop ROM cartridges
into the computer. A ROM cartridge is a cartridge containing a PC
card full of ROM chips. Etched into those ROM chips is a program.
The typical ROM
cartridge contains a program that plays a video game, such as
Space Invaders or Pac Man or computer chess. You can also buy ROM
cartridges that contain programs for word processing, music, art,
or tutoring you. Each ROM cartridge costs about $30.
How ROM chips are made
The info in a ROM chip is said to be burned into the chip. To
burn in the info, the manufacturer can use two methods.
One method is to burn
the info into the ROM chip while the chip's being made. A ROM
chip produced by that method is called a custom ROM chip.
An alternate method is
to make a ROM chip that contains no info but can be fed info
later. Such a ROM chip is called a programmable ROM chip (PROM).
To feed it info later, you attach it to a device called a PROM
burner, which copies info from a RAM to the PROM. Info burned
into the PROM can't be erased, unless the PROM's a special kind:
an erasable PROM (EPROM).
To erase a typical
EPROM, shine an intense ultraviolet light at it for 20 minutes.
That's called an ultraviolet-erasable PROM (UV-EPROM).
A fancier kind of EPROM
can be erased quickly by sending it a 25-volt shock for a tenth
of a second. That's called an electrically erasable PROM (EEPROM)
or electrically alterable PROM (EAPROM).
After you erase an
EPROM, you can feed it new info.
If you're a manufacturer
designing a new computer, begin by using an erasable PROM
(EPROM), so you can make changes easily. When you decide not to
make any more changes, switch to a non-erasable PROM, which costs
less to manufacture. If your computer becomes so popular that you
need to manufacture over 10,000 copies of the ROM, switch to a
custom ROM, which costs more to design and ``tool up for'' but
costs less to make copies of.ROMIf a chip remembers information permanently, it's called a read-only memory chip (ROM chip), because you can read the information but can't change it. The ROM chip contains permanent, eternal truth