The main unit inside the computer is the CPU. This unit is responsible for all events inside the computer. It controls all internal and external devices, performs arithmetic and logic operations. The operations a microprocessor performs are called the instruction set of this processor. The instruction set is "hard wired" in the CPU and determines the machine language for the CPU. The more complicated the instruction set is, the slower the CPU works. Processors differ from one another by the instruction set. If the same program can run on two different computer brands they are said to be compatible. Programs written for IBM compatible computers will not run on Apple computers because these two architectures are not compatible.
There is an exception to this rule. Apple Macintosh with a program SoftPC loaded can run programs written for IBM PC. Programs like SoftPC make one CPU "pretend" to be another. These programs are called software emulators. Although software emulators allow the CPU to run incompatible programs they severely slow down the performance.
The CPU is composed of several units...
Diagram 1. A simplified diagram of the CPU
The control unit directs and controls the activities of the internal and external devices. It interprets the instructions fetched into the computer, determines what data, if any, are needed, where it is stored, where to store the results of the operation, and sends the control signals to the devices involved in the execution of the instructions.
The arithmetic and logic unit (ALU) is the part where actual computations take place. It consists of circuits which perform arithmetic operations (e.g. addition, subtraction, multiplication, division) over data received from memory and capable to compare numbers.
While performing these operations the ALU takes data from the temporary storage area inside the CPU named registers. Registers are a group of cells used for memory addressing, data manipulation and processing. Some of the registers are general purpose and some are reserved for certain functions. It is a high-speed memory which holds only data for immediate processing and results of this processing. If these results are not needed for the next instruction, they are sent back to the main memory and registers are occupied by the new data used in the next instruction.
The microprocessor -- also known as a CPU or central processing unit -- is a complete computation engine that is fabricated on a single chip. The first microprocessor was the Intel 4004, introduced in 1971. The 4004 was not very powerful, all it could do was add and subtract, and it could only do that 4 bits at a time. But it was amazing that everything was on one chip. Prior to the 4004, engineers built computers either from collections of chips or from discrete components (transistors wired one at a time). The 4004 powered one of the first portable electronic calculators. The first microprocessor to make it into a home computer was the Intel 8080, a complete 8-bit computer on one chip, introduced in 1974. The first microprocessor to make a real splash in the market was the Intel 8088, introduced in 1979 and incorporated into the IBM PC (which first appeared around 1982). If you are familiar with the PC market and its history, you know that the PC market moved from the 8088 to the 80286 to the 80386 to the 80486 to the Pentium to the Pentium II to the Pentium III to the Pentium 4. All of these microprocessors are made by Intel and all of them are improvements on the basic design of the 8088. The Pentium 4 can execute any piece of code that ran on the original 8088, but it does it about 5,000 times faster!
The following table helps you to understand the differences between the different processors that Intel has introduced over the years.
Name |
Date | Transistors | Microns | Clock Speed | Data width | MIPS |
8080 |
1974 |
6,000 |
6 |
2 MHz |
8 bits |
0.64 |
8088 |
1979 |
29,000 |
3 |
5 MHz |
16 bits, 8-bit bus |
0.33 |
80286 |
1982 |
134,000 |
1.5 |
6 MHz |
16 bits |
1 |
80386 |
1985 |
275,000 |
1.5 |
16 MHz |
32 bits |
5 |
80486 |
1989 |
1,200,000 |
1 |
25 MHz |
32 bits |
20 |
Pentium |
1993 |
3,100,000 |
0.8 |
60 MHz |
32 bits, 64-bit bus |
100 |
Pentium II |
1997 |
7,500,000 |
0.35 |
233 MHz |
32 bits, 64-bit bus |
~300 |
Pentium III |
1999 |
9,500,000 |
0.25 |
450 MHz |
32 bits, 64-bit bus |
~510 |
Pentium IV |
2000 | 42,000,000 | 0.18 |
1.5 GHz | 32 bits, 64-bit bus | ~1,700 |
What's a Chip?
A chip is also called an integrated circuit. Generally it is a small, thin piece
of silicon onto which the transistors making up the microprocessor have been
etched. A chip might be as large as an inch on a side and can contain tens of
millions of transistors. Simpler processors might consist of a few thousand
transistors etched onto a chip just a few millimeters square.
The date is the year that the processor was first introduced.
Many processors are re-introduced at higher clock speeds for many years after
the original release date.
Transistors is the number of transistors on the chip. You
can see that the number of transistors on a single chip has risen steadily over
the years.
Microns is the width, in microns, of the smallest wire on
the chip. For comparison, a human hair is 100 microns thick. As the feature
size on the chip goes down, the number of transistors rises.
Clock speed is the maximum rate that the chip can be clocked
at. Clock speed will make more sense in the next section.
Data Width is the width of the ALU. An 8-bit ALU can add/subtract/multiply/etc.
two 8-bit numbers, while a 32-bit ALU can manipulate 32-bit numbers. An 8-bit
ALU would have to execute four instructions to add two 32-bit numbers, while
a 32-bit ALU can do it in one instruction. In many cases, the external data
bus is the same width as the ALU, but not always. The 8088 had a 16-bit ALU
and an 8-bit bus, while the modern Pentiums fetch data 64 bits at a time for
their 32-bit ALUs.
MIPS stands for "millions of instructions per second"
and is a rough measure of the performance of a CPU. Modern CPUs can do so many
different things that MIPS ratings lose a lot of their meaning, but you can
get a general sense of the relative power of the CPUs from this column.
From this table you can see that, in general, there is a relationship between clock speed and MIPS. The maximum clock speed is a function of the manufacturing process and delays within the chip. There is also a relationship between the number of transistors and MIPS. For example, the 8088 clocked at 5 MHz but only executed at 0.33 MIPS (about one instruction per 15 clock cycles). Modern processors can often execute at a rate of two instructions per clock cycle. That improvement is directly related to the number of transistors on the chip and will make more sense in the next section.
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