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.
All activities in the computer system are composed of thousands of individual steps. These steps should follow in some order in fixed intervals of time. These intervals are generated by the clock unit. Every operation within the CPU takes place at the clock pulse. No operation, regardless of how simple, can be performed in less time than transpires between ticks of this clock. But some operations require more than one clock pulse. The faster the clock runs, the faster the computer performs. The clock rate is measured in megahertz (Mhz) or million ticks per second. At the moment of writing of these notes the highest clock rate is 200MHz but by the time you read them probably quicker microprocessors will be on the market. Larger systems are even faster. In "older" systems the clock unit is external to the microprocessor and resides on a separate chip. In most modern microprocessors the clock is usually incorporated within the CPU.
The two main features of the CPU are
its speed measured in millions of instructions per second;
the word size.
The word size is the number of bits the CPU can process at a time. An 8-bit processor manipulates with 8 bits simultaneously. Consider a simplified example: the CPU needs to add up two four-digit numbers. For an 8-bit CPU, it will take four operations, a separate operation for addition of each of the four digits. A 16-bit processor will be able to work on two digits simultaneously, so it will need only two operations. A 32-bit processor will complete addition in one operation.
Intel 80486 is a 32-bit processor so it can manipulate twice as much data at one time as, say, Intel 80286 which is a 16-bit processor. The maximum possible word size at the time of writing of these notes is 64 bits. Most supercomputers have such powerful processors. What is amazing that modern technology allows to produce 64-bit processors for personal computers and Pentium Pro would be one of the examples. The 32-bit processors are installed in many personal computers, minicomputers, and in most mainframes.
Today, there are two leading manufacturers of microprocessors : Intel and Motorola. Other microprocessors are compatible with them. For example, AMD-K5 is compatible with Pentium.
Modern CPUs are downwardly compatible with earlier CPUs of the same series which means that they can run programs
designed for earlier CPUs. For example, Pentium Pro can run programs written for all microprocessors from 80x86 series.
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