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On the Motherboard

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Processor Basics

Processors come in a variety of speeds, measured in gigahertz (GHz). Hertz is a measurement of cycles per second. One hertz equals one cycle per second. One gigahertz is 1 billion cycles per second, or 1GHz. The original PC CPU, the 8088 microprocessor, ran at 4.77MHz. Today’s processors run at speeds near 5GHz.

The number of bits processed at one time is the processor’s register size (word size). Intel’s 8086 processor’s register size is 16 bits, or 2 bytes. Today’s CPUs have register sizes of 64 or 128 bits.


Processors operate on 1s and 0s. Processors operate on 1s and 0s. The 1s and 0s must travel from one place to another inside the processor, as well as outside to other chips. To move the 1s and 0s around, electronic lines called a bus are used. The electronic lines inside the CPU are known as the internal data bus or system bus. In the 8086, the internal data bus comprises 16 separate lines, with each line carrying one 1 or one 0. The word size and the number of lines for the internal data bus are equal. The 8086, for example, has a 16-bit word size, and 16 lines carry 16 bits on the internal data bus. In today’s processors, 64 or 128 internal data bus lines operate concurrently.

For a CPU to communicate with devices in the outside world, such as a printer, the 1s and 0s travel on the external data bus. The external data bus connects the processor to adapters, the keyboard, the mouse, the hard drive, and other devices. An external data bus is also known as an external data path. You can see the external data lines by looking between the expansion slots on the motherboard. Some solder lines between the expansion slots are used to send data out along the external data bus to the expansion slots. Today’s processors have 64- and 128-bit external data paths. Figure 3.3 shows the internal and external data buses.

Figure 3.3

Figure 3.3 Internal and external data buses


A processor has a special component called the arithmetic logic unit (ALU), which does all the calculations and comparison logic that the computer needs. Figure 3.3 shows the basic concept of how the ALU connects to the registers, control unit, and internal bus. The control unit coordinates activities inside the processor. The I/O unit manages data entering and leaving the processor. The registers in the CPU make up a high-speed storage area for 1s and 0s before the bits are processed.

To make sense of all of this, take a look at a letter typed on a computer that starts out DEAR MOM. To the computer, each letter of the alphabet is a different combination of eight 1s and 0s. For example, the letter D is 01000100, and the letter E is 01000101. Figure 3.4 demonstrates that the size of the bus greatly increases performance on a computer similar to the way that increasing the number of lanes of a highway decreases congestion.

Figure 3.4

Figure 3.4 Bus performance


Processors have multiple pipelines (separate internal buses) that operate simultaneously. To understand pipelining, take the example of a fast-food restaurant. In the restaurant, assume that there are five steps (and one employee per step) involved in making a burger and giving it to the customer. First, (1) take the order and input it into the computer system; (2) brown the buns and cook the burgers; (3) add the condiments to the buns and burgers; (4) wrap the burgers, add fries, and insert them into the bag; and then (5) take the customer’s money and give the bag to the customer. Keep in mind that the person taking the customer’s order and inputting the order can serve another customer once he or she has completed this task for the first customer. The same is true for each person along the line. To make this burger process go faster, you could (maybe) do one of the things shown in Figure 3.5. (1) Make your employees work faster; (2) break the tasks into smaller tasks (such as seven steps instead of five and have seven people); or, (3) have more lines of people doing exactly the same process tasks.

Figure 3.5

Figure 3.5 Ways to get faster processes

To relate this to processors, making the employees work faster is the same as increasing the CPU clock speed. Breaking the tasks into smaller tasks is the same as changing the structure of the CPU pipeline. Instead of performing the standard 5 tasks, the CPU might perform 6, 7, 14, 20, or even more steps. This allows each step to be acted upon more quickly, the task to be smaller, and production to be faster. Having more lines of people doing the same complete process is like having multiple pipelines.

A 32- or 64-bit CPU can have separate paths, each of which handles 32 or 64 bits. For example, if a processor has two pipelines, the Dear Mom letter can be in one pipeline, while a photo upload using a different application can be in the other pipeline.

A processor might have 12 pipelines for integers and 17 pipelines for floating-point numbers. (A floating-point number is a number that can include a decimal point.) Other processors contain anywhere from 20- to 31-stage pipelines. Debate continues about whether a longer pipeline improves performance.

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