• Choose a category
• All
• Classic-Fiction

Figure 7-4 A keyboard chip on a Pentium motherboard

Every time you press a key on your keyboard, a scanning chip in the keyboard notices which key you pressed. Then the scanner sends a coded pattern of ones and zeroes—called the scan code—to the keyboard controller. Every key on your keyboard has a unique scan code. The keyboard controller stores the scan code in its own register. Does it surprise you that the lowly keyboard controller has a register similar to a CPU? Lots of chips have registers—not just CPUs (Figure 7-6)!

Figure 7-5 Electronic view of the keyboard controller

Figure 7-6 Scan code stored in keyboard controller’s register

How does the CPU get the scan code out of the keyboard controller (see Figure 7-7)? While we’re at it, how does the CPU tell the keyboard to change the typematic buffer rate (when you hold down a key and the letter repeats) or to turn the number lock LED on and off, to mention just a few other jobs the keyboard needs to do for the system? The point is that the keyboard controller must be able to respond to multiple commands, not just one.

The keyboard controller accepts commands exactly as you saw the CPU accept commands in Chapter 5 “Microprocessors.” Remember when you added 2 to 3 with the 8088? You had to use specific commands from the 8088’s codebook to tell the CPU to do the addition and then place the answer on the external data bus. The keyboard controller has its own codebook—much simpler than any CPU’s codebook, but conceptually the same. If the CPU wants to know what key was last pressed on the keyboard, the CPU needs to know the command (or series of commands) that orders the keyboard controller to put the scan code of the letter on the external data bus so the CPU can read it.

Figure 7-7 The CPU ponders its dilemma…

Essentials

BIOS

The CPU can’t magically or otherwise automatically know how to talk with any device; it needs some sort of support programming loaded into memory that teaches it about a particular device. This programming is called basic input/output services (BIOS). The programs dedicated to enabling the CPU to communicate with devices are called services (or device drivers, as you’ll see later in the chapter). This goes well beyond the keyboard, by the way. In fact, every device on the computer needs BIOS! But let’s continue with the keyboard for now.

Bringing BIOS to the PC

A talented programmer could write BIOS for a keyboard if the programmer knew the keyboard’s codebook; keyboards are pretty simple devices. This begs the question: where would this support programming be stored? Well, programming could be incorporated into the operating system. Storing programming to talk to the hardware of your PC in the operating system is greatg—all operating systems have built-in code that knows how to talk to your keyboard, your mouse, and just about every piece of hardware you may put into your PC.

That’s fine once the operating system’s up and running, but what about a brand new stack of parts you’re about to assemble into a new PC? When a new system’s being built, it has no operating system. The CPU must have access to BIOS for the most important hardware on your PC: not only the keyboard, but also the monitor, hard drives, optical-media drives, USB ports, and RAM. This code can’t be stored on a hard drive or CD-ROM disc—these important devices need to be ready at any time the CPU calls them, even before installing a mass storage device or an operating system.

The perfect place to store the support programming is on the motherboard. That settles one issue, but another looms: What storage medium should the motherboard use? DRAM won’t work, because all of the data would be erased every time the computer was turned off. You need some type of permanent program storage device that does not depend on other peripherals to work. And you need that storage device to sit on the motherboard.

ROM

Motherboards store the keyboard controller support programming, among other programs,