Difference between revisions of "BIOS"
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Latest revision as of 20:35, 6 February 2015
The Basic Input/Output System, or BIOS, also known as the System BIOS or ROM BIOS, is the program logic stored in read-only memory that enable a computer to start the operating system and to communicate with the various devices in the system, such as disk drives, keyboard, monitor, printer, and communications ports. The firmware is the program logic stored on the ROM chip memory.
Because RAM is faster than ROM, though, many computer manufacturers design systems so that the BIOS is copied from ROM to RAM each time the computer is booted. This is known as shadowing. Many modern PCs have a flash BIOS, which means that the BIOS has been recorded on a flash memory chip, which can be updated if necessary. The name "flash memory chip" comes from the early days when writing new firmware to BIOS required exposure to strong ultraviolet light in a window area under a protective sticker on the chip. This flashing of UV light to the chip became part of the terminology used today, "Flash the BIOS" even though the process is no longer used.
The window area on the old BIOS chip was transparent fused quartz on a chip known as an EPROM chip. This type of programmed EPROM retains its data for a minimum of ten to twenty years. The erasing window must be kept covered with an opaque label to prevent accidental erasure by the UV found in sunlight or camera flashes. Old PC BIOS chips were often EPROMs, and the erasing window was often covered with an adhesive label containing the BIOS publisher's name, the BIOS revision, and a copyright notice. Often this label was foil-backed to ensure its opacity to UV. Old EPROMs had a limited but large number of erase cycles; the silicon dioxide around the gates would accumulate damage from each cycle, making the chip unreliable after several thousand cycles.
Modern BIOS is stored on Electrically Erasable and Programmable ROM, or EEPROM (with two E's) which enables firmware updates to be performed electronically. This is a great technological improvement over the original EPROM which could only be erased with UV light. The term ROM or Read Only Memory now refers to the memory as being non-volatile in that the BIOS firmware is not lost if the computer does not have power and the CMOS battery is removed
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