prom

A "Programmable Read Only Memory" chip is a common component in computers, arcade games, and other assorted electronic devices.

These are blank when they are created, and can only be programmed a single time. They are actually written with binary ones when they are new, and the zeroes are written in afterwards, The PROM design is a lot like a bank of thousands of tiny fuses. The unblown fuses represent binary ones, while the blown fuses represent binary zeroes. Normally a PROM operates on 5 volt power, but they are programmed using 12 volt power. This in effect blows the "fuses" that need to be changed to zeroes. That is why programming a PROM is often referred to as "burning a ROM", and indeed the required hardware is called a "ROM burner".

Texas Instruments invented the PROM back in the seventies, and it went into widespread use almost immediately. They are most often used in applications where the data stored on them will never have to be changed, otherwise it is better to use the more expensive EPROMs, or Flash ROM chips instead.

Motherboards often use PROMs for permanent data storage, although EPROMs and Flash ROMs have largely replaced them, simply because the cost of ROM chips in general has fallen so low that it is affordable to use the reprogrammable chips instead.

Arcade games often use a PROM to store their color pallette (while keeping the rest of their data on EPROMs). The reason behind this is that the game's colors are usually fixed very early in development. So they are placed on a PROM. But the games code will undergo countless revisions before release, so the code is placed on EPROMs to avoid wasting thousands of PROMs during a games development. The final release will usually still use EPROMs, as switching to the cheaper PROMs would cause any future revisions to be very expensive (arcade games are occasionally "patched" even after they come out, Asteroids,Joust, Robotron 2084, and many others were originally shipped with fatal bugs that were only discovered after the machines went out on location). A few odd titles do indeed use nothing but PROMs, but these are few and far between.