PROM

Creating ROM chips totally from scratch is time-consuming and
very expensive in small quantities. For this reason, mainly,
developers created a type of ROM known as programmable
read-only memory (PROM). Blank PROM chips can be bought
inexpensively and coded by anyone with a special tool called
a programmer.

PROM chips have a grid of columns and rows just as ordinary
ROMs do. The difference is that every intersection of
a column and row in a PROM chip has a fuse connecting them.
A charge sent through a column will pass through the fuse in
a cell to a grounded row indicating a value of 1. Since all
the cells have a fuse, the initial (blank) state of a PROM
chip is all 1s. To change the value of a cell to 0, you use
a programmer to send a specific amount of current to the cell.
The higher voltage breaks the connection between the column
and row by burning out the fuse. This process is known as
burning the PROM.

PROMs can only be programmed once. They are more fragile than
ROMs. A jolt of static electricity can easily cause fuses in
the PROM to burn out, changing essential bits from 1 to 0.
But blank PROMs are inexpensive and are great for prototyping
the data for a ROM before committing to the costly ROM
fabrication process.