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By Root 1384 0

CPU to communicate with a device starts with some kind of interconnectionc—a communication bus that enables the CPU to send commands to and from devices. To make this connection, let’s promote the MCC, giving it extra firepower to act not only as the interconnection between the CPU and RAM, but also the interconnection between the CPU and the other devices on the PC. The MCC isn’t just the memory controller anymore, so let’s now call it the Northbridge because it acts as the primary bridge between the CPU and the rest of the computer (Figure 7-1).

Figure 7-1 Meet the Northbridge

Your PC is full of devices, so the PC industry decided to delegate some of the interconnectivity work to a second chip called the Southbridge. The Northbridge only deals with high-speed interfaces such as the connection to your video card and RAM. The Southbridge works mainly with lower-speed devices, such as the USB controller and hard drive controllers. Chip makers design matched sets of particular models of Northbridge and Southbridge to work together. You don’t buy a Northbridge from one company and a Southbridge from another—they’re sold as a set. We call this set of Northbridge and Southbridge the chipset.

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NOTE Chipset makers rarely use the terms “Northbridge” and “Southbridge” anymore, but because most modern chipsets consist of only two or three chips with basically the same functions, techs continue to use the terms.

The chipset extends the data bus to every device on the PC. The CPU uses the data bus to move data to and from all of the devices of the PC. Data constantly flows on the external data bus among the CPU, chipset, RAM, and other devices on the PC (Figure 7-2).

The first use for the address bus, as you know, is for the CPU to tell the chipset to send or store data in memory and to tell the chipset which section of memory to access or use. Just as with the external data bus, the chipset extends the address bus to all of the devices (Figure 7-3). That way the CPU can use the address bus to send commands to devices, just as it sends commands to the chipset. You’ll see this in action a lot more in Chapter 8, “Expansion Bus,” but for now just go with the concept.

Figure 7-2 The chipset extending the data bus

It’s not too hard to swallow the concept that the CPU uses the address bus to talk to the devices, but how does it know what to say to them? How does it know all of the patterns of ones and zeroes to place on the address bus to tell the hard drive it needs to send a file? Let’s look at the interaction between the keyboard and CPU for insight into this process.

Figure 7-3 Every device in your computer connects to the address bus.

Talking to the Keyboard

The keyboard provides a great example of how the buses and support programming help the CPU get the job done. In early computers, the keyboard connected to the external data bus via a special chip known as the keyboard controller. Don’t bother looking for this chip on your motherboard—the Southbridge now handles keyboard controller functions. The way the keyboard controller—or technically, the keyboard controller circuitry—works with the CPU, however, has changed only a small amount in the past 20+ years, making it a perfect tool to illustrate how the CPU talks to a device.

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NOTE Techs commonly talk about various functions of the chipset as if those functions were still handled by discrete chips. So you’ll hear about memory controllers, keyboard controllers, mouse controllers, USB controllers, and so on, even though they’re all just circuits on the Northbridge or Southbridge chips.

The keyboard controller was one of the last single-function chips to be absorbed into the chipset. For many years—in fact, well into the Pentium III/Early Athlon era—most motherboards had separate keyboard controller chips. Figure 7-4 shows a typical keyboard controller from those days. Electronically, it looked like Figure 7-5.

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NOTE Even though the model numbers changed over the years, you’ll still hear techs refer to the keyboard controller