Most PCs come with factory programmed chips containing software called the Basic Input/Output System, used by the computer to run the keyboard, mouse, hard drive and other devices. When the manufacturer releases new BIOS software you can in many cases update the existing chips with the latest programs. For very old computers, however, you must replace the BIOS chips -- they cannot be reprogrammed.
Read-Only Memory chips are etched permanently with data during manufacture; although this makes them less expensive than other types of BIOS memory, it also means you can't update them. Some of the earliest personal computers, as well as cartridge-based games and other mass-market electronics, used ROM-based technology. Some PCs had the ROMs soldered into the motherboard, making updates to the BIOS all but impossible. More expensive designs used sockets for the ROMs, allowing you to pry the old chips out and install new ones for software updates.
An Erasable, Programmable Read-Only Memory chip is a step up from ROM; you can change its programming, although the process takes special equipment. Unlike a ROM, which comes in a sealed package, the EPROM features a quartz window that exposes the integrated circuit to light. A specialized ultraviolet light box erases the EPROM's contents and readies it for new programming. To update the chip, you place it in an EPROM programmer consisting of a powered socket connected to a computer. The programmer writes new data onto the chip and the information remains until it's erased with UV light.
An Electrically Erasable Programmable Read Only Memory chip is easier to update than an EPROM, because it uses an electric current instead of ultraviolet light to erase data. You can update an EEPROM without removing the chip from the computer, simplifying the task and making it considerably quicker. Although EEPROMs require no external equipment, the computer needs internal circuitry to support the reprogramming.
The BIOS chips that are easiest to update use flash memory technology. Flash takes new data in microseconds, and it can be reprogrammed millions of times. Flash is commonly and widely used as semi-permanent data storage in PCs, smartphones and tablets as well as network routers and other devices. As with EEPROM, you can update flash BIOS without the need to remove the chips.