PROM

Creating ROM chips totally from scratch is time-consuming and very expensive in s mall quantities. For this reason, mainly, developers created a type of ROM known as programmable read-only memory (PROM). Blank PROM chis 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. This 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 break the connection between the column and row by burning out the fuse. This process is known as burning the PROM.