Classic Computer Magazine Archive COMPUTE! ISSUE 60 / MAY 1985 / PAGE 10

VIC Chip Replacement

I've noticed the VIC chip is removable in my VIC-20. The chip in my ExBASIC cartridge is also removable. If I replace the VIC chip with the ExBASIC chip, will ExBASIC be running when I turn on my VIC?

Also, what's that window-like thing on the ExBASIC chip?

Jeremy Faden

The VIC chip is responsible for generating and maintaining the computer screen display. It's an input/output chip. The ExBASIC chip is merely an EPROM (Erasable Programmable Read Only Memory), an erasable form of permanent memory used to store the ExBASIC program. Substituting this chip for your VIC chip simply wouldn't work, and might result in damage to your computer.

Your computer also contains ROM chips, similar to the EPROM used in the ExBASIC cartridge. Although it is electronically possible to plug the ExBASIC chip in place of one of your ROMs, the swap would never work. All programs depend upon the operating system built into these ROMs. If you replaced the ROMs with the ExBASIC, you'd be left with a useless operating system.

The window on your ExBASIC chip allows the chip to be erased so that it can be reprogrammed using a special device called an EPROM burner. Erasing is accomplished by exposing the the circuitry beneath the window on the chip to ultraviolet light, which resets all the memory locations in the EPROM. If you peel off the sticker and expose the window, you can actually look at the inside of the chip. Notice how the actual chip is a tiny square. What we usually refer to as a chip is just a plastic and wire carrier.

Don't leave the cover off the window, though. There's a chance that bright sunlight could alter some of the bits in the chip. Prolonged exposure will definitely erase the chip. Some experimenters erase EPROM chips simply by leaving the windows exposed to sunlight for a number of hours. Special high-energy ultraviolet bulbs are used in commercial EPROM erasers.

EPROMs are often used because they are easy and inexpensive to program, but when a manufacturer wants to turn them out by the thousands, it's cheaper to build the pattern into the chip when it's made, hence the nonerasable ROMs inside production models of computers.