CompTIA A+® Certification All-In-One For Dummies®, 2nd Edition

4.1. The BIOS and Its Purpose

I can't start a discussion on CMOS without first comparing it against the system BIOS; the two are closely related and are often confused by many IT professionals.

NOTE

BIOS — basic input-output system — is the low-level instructions used to communicate with the system devices. This is different than CMOS (complementary metal-oxide semiconductor), which is simply an inventory list for the system, containing information such as the hard drive space and amount of memory that exists on the system. CMOS is the inventory list; BIOS is the actual code that is run to communicate with those devices.

Originally, the BIOS was stored in a ROM chip on the motherboard. Because of this, you used to have to replace the entire chip with a new chip to update the BIOS. Today's systems, however, have the BIOS code stored in an EEPROM (electronically erasable programmable read-only memory) chip located on the motherboard. (For more information on ROM and EEPROM, check out the memory topic in Chapter 3 of this minibook.) The chip that contains the BIOS code is the BIOS chip. The BIOS chip should be easy to find — it's usually rectangular and clearly labeled, typically with the name of the manufacturer of the BIOS chip and the date when the chip was created. In Figure 4-1, the BIOS chip is the chip with the white label indicated with a number 1.

Figure 4.1. The BIOS chip (an EEPROM chip located on the motherboard) contains the BIOS code.

Along with the low-level code, ...