Although all components of the computer function together as a team, every team needs a leader—someone who gives out instructions and keeps everyone working toward the same goal. If any PC component were to be considered the team leader, it would probably be the CPU, or central processing unit. The key word here is “central,” which implies “center” or “focus.” The CPU can be considered the focus of the computer because it controls a large number of the computer system’s capabilities, such as the type of software that can run, the amount of total memory that the computer can physically see, and the speed at which the system will run.
This article will look at some of the features of the CPU that are responsible for setting the capabilities of the computer system. It will also discuss the importance of the CPU and its role as a PC component.
Processor Terminology
In this section, you will learn some basic terms that are used to describe characteristics of different processors—past and present. The exam may not ask for the specific definition of each term, but understanding the terms will help you answer the
related questions in this topic area.
Processor speed
Processor speed is the speed at which the processor executes its instructions or commands. This speed is measured in millions of cycles per second, or megahertz (MHz). Original CPUs had a speed of 4.77 MHz, while systems at the time of this writing are running around 1.5 GHz. Although processor speed is not the only factor affecting performance, in general, the larger the MHz the faster the system.
Data bus
A city bus is responsible for transferring people from one location to another. In the world of computers, a data bus is responsible for delivering data from one location on the PC to another (for example, from the processor to memory).
What would happen if 50 people needed to go from one location to another, but a city bus only had 25 available seats? The answer is simple: the bus would make two trips. But wouldn’t it be more efficient to upgrade the bus? If you upgraded the bus
to 50 seats, the bus would only have to make one trip to transfer the 50 people from one location to another, which increases the efficiency of our public transit system.
The data bus works the same way, only it transfers data in the form of bits (ones or zeros). All data processed by the computer is in the form of bits. The data bus has a “full capacity” point where it cannot handle any more bits of data, just as the bus system in your city has a “full capacity” point (50, measured in “seats”).
If a processor has a 16-bit data bus, it means that it can deliver 16 bits at any given time. If the same processor needs to deliver 32 bits of information, it will have to take two trips. Taking that same 32 bits of information and processing it on a 32-bit processor means that the information will get delivered in one trip as opposed to two, which increases the overall efficiency of the system. It is as if you had two pieces of paper to put in the garbage can, and instead of walking one piece of paper over to the garbage can, walking back, and then walking the second piece over, it would be more efficient to walk both pieces of paper over to the garbage can at the same time.
Data bus in terms of processors means the pathway to memory. The processor uses the data bus solely for delivering data back and forth from system memory and the processor. Because the processor accesses information from memory so often, an
entire bus—the data bus—is dedicated to this action.
Address bus
Figure shows that your system memory is organized like a spreadsheet, in rows and columns. These rows and columns make up blocks that can be written to or read from. If you want to store information in one of the blocks, you have to reference the location by address (by specifying “B2,” for example).
To store information into system memory, your processor would have to give an address that points to a particular storage location. Only the address does not look like “B2.” It looks something like “1-0,” or maybe “1-1,” which are two completely different memory locations.
Your processor accesses memory locations through the address bus. If, for example, the address bus is 2-bit, the processor has two address lines from the processor to system memory. The address lines are just tunnels to locations in memory, each with an on/off state (1 representing on and 0 representing off). The combination of the on/off states of both address lines at any given time is how a reference to an area in memory is made. Figure 2-2, Side A, illustrates a processor making a reference, or call, to Address 1-0, while Side B shows a reference to Address 1-1 being made. These two address calls are referencing completely different locations in memory.
If you add another address line to the address bus, there are even more possible addresses that the processor can access because the processor has more variations with three bits than with two. A two-bit address bus can make a reference to four possible memory addresses (2 × 2), while a three-bit address bus can make a reference to eight possible memory addresses (2 × 2 × 2).
Therefore, the address bus dictates how much physical memory the processor can access. For example, an 80286 processor has a 24-bit address bus, which means that it can access 16,777,216 memory addresses, or 16MB of system memory.
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