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on a special type of device called a read-only memory (ROM) chip. A ROM chip stores programs, called services, exactly like RAM: that is, like an 8-bit-wide spreadsheet. But ROM differs from RAM in two important ways. First, ROM chips are nonvolatile, meaning that the information stored on ROM isn’t erased when the computer is turned off. Second, traditional ROM chips are read-only, meaning that once you store a program on one, you can’t change it. Modern motherboards use a type of ROM called flash ROM that differs from traditional ROM in that you can update and change the contents through a very specific process called “flashing the ROM,” covered later in this chapter. Figure 7-8 shows a typical flash ROM chip on a motherboard. When the CPU wants to talk to the keyboard controller, it goes to the flash ROM chip to access the proper programming.

Every motherboard has a flash ROM, called the system ROM chip because it contains code that enables your CPU to talk to the basic hardware of your PC (Figure 7-9). As alluded to earlier, the system ROM holds BIOS for more than just the keyboard controller. It also stores programs for communicating with the floppy drives, hard drives, CD and DVD drives, video, USB ports, and other basic devices on your motherboard.

Figure 7-8 Typical flash ROM

Figure 7-9 Function of the flash ROM chip

To talk to all of that hardware requires hundreds of little services (2 to 30 lines of code each). These hundreds of little programs stored on the system ROM chip on the motherboard are called, collectively, the system BIOS (see Figure 7-10). Techs call programs stored on ROM chips of any sort firmware.

Figure 7-10 CPU running BIOS service

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EXAM TIP Programs stored on ROM chips—flash or any other kind of ROM chip—are known collectively as firmware, as opposed to programs stored on erasable media that are collectively called software.

The system ROM chips used on modern PCs store as much as 2 MB of programs, although only 65,536 bytes are used to store the system BIOS. This allows for backward compatibility with earlier systems. The rest of the ROM space is put to good use doing other jobs.

System BIOS Support

Every system BIOS has two types of hardware to support. First, the system BIOS supports all of the hardware that never changes, such as the keyboard. (You can change your keyboard, but you can’t change the keyboard controller built into the Southbridge.) Another example of hardware that never changes is the PC speaker (the tiny one that beeps at you, not the ones that play music). The system ROM chip stores the BIOS for these and other devices that never change.

Second, the system BIOS supports all of the hardware that might change from time to time. This includes RAM (you can add RAM), hard drives (you can replace your hard drive with a larger drive or add a second hard drive), and floppy drives (you can add another floppy drive, although that’s not common today). The system ROM chip stores the BIOS for these devices, but the system needs another place to store information about the specific details of a piece of hardware. This enables the system to differentiate between a Western Digital Caviar Black 1.5-TB hard drive and a Seagate Barracuda 60-GB drive, and yet still support both drives right out of the box.

CMOS

A separate memory chip, called the complementary metal-oxide semiconductor (CMOS) chip, stores the information that describes specific device parameters. CMOS does not store programs; it only stores data that is read by BIOS to complete the programs needed to talk to changeable hardware. CMOS also acts as a clock to keep the current date and time.

Years ago, CMOS was a separate chip on the motherboard, as shown in Figure 7-11. Today, the CMOS is almost always built into the Southbridge.

Figure 7-11 Old-style CMOS

Most CMOS chips store around 64 KB of data, but the PC usually needs only a very small amount—about 128 bytes—to store all of the necessary information on the changeable hardware. Don’t let the tiny size fool you. The information