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Central processing unit

UNIT 4. CENTRAL PROCESSING UNIT

I. Practise reading the following words:

execute, fundamental, circuit, tolerances, miniaturization, standardization, manufacture, configure, digital, certain, configuration, contents, architecture, differentiate, retrieve, register, directly, cycle, entire, simultaneously, access, monolithic, separate, utilize, subsequent, execute, determine, processor, various, dramatically, exist, interpret.

II. Before reading the text try to answer these questions:

1. What is a central processing unit?

2. What is the main function of the CPU?

3. What are the advantages in making computers as small as possible?

 

III. Read and translate the following text.

Central processing unit

A central processing unit (CPU) is a logic machine that can execute computer programs. This definition can be applied to many computers that existed before the term «CPU» came into widespread usage. The form, design and implementation of CPUs have been changed dramatically since the earliest samples, but their fundamental operation has remained the same.

Early CPUs were custom-designed as a part of a larger computer. The integrated circuit (IC) has allowed increasingly complex CPUs to be designed and manufactured. Both the miniaturization and standardization of CPUs have increased the presence of these digital devices in modern life. Modern microprocessors appear in a wide range of things from automobiles to cell phones and children's toys.

On June 30, 1945, mathematician John von Neumann published the paper «First Draft of a Report on the EDVAC» in which he outlined the design of a stored-program computer that would eventually be completed in August 1949. EDVAC was designed to perform a certain number of instructions or operations of various types. With von Neumann's design, the programs that EDVAC ran could be changed by substituting the contents of the computer's memory.

The so-called Harvard architecture of the Harvard Mark I, which was completed before EDVAC, also utilized a stored-program design by using punched paper tape rather than electronic memory. The key difference between the von Neumann and Harvard architectures is that the latter separates the storage from treatment of instructions and data, while the former uses the same memory space for both. Most modern CPUs have von Neumann’s design, but elements of the Harvard architecture also exist.

The general task of most CPUs is to execute a sequence of instructions called a program. This program is represented by a series of numbers stored in the computer memory. There are four steps that von Neumann CPUs use: fetch, decode, execute and writeback.

The first step, fetch, involves retrieving an instruction from program memory. The location in program memory is determined by a program counter (PC), which keeps track of the next instruction to be performed in the main memory. The instruction that the CPU fetches from memory is used to determine what the CPU must perform.



In the decode step, the instruction is broken up into parts that are fed into the CPU. The CPU's instruction set architecture (ISA) defines the way numerical instruction value is interpreted. Often, one group of numbers in the instruction (called the opcode) indicates which operation to be performed. In earlier designs the portions of the CPU responsible for instruction decoding were unchangeable hardware devices. However, in more complicated CPUs and ISAs, a microprogram is often used to assist in translating instructions into various signals for the CPU. This microprogram is sometimes rewritable so that it can be modified to change the way the CPU decodes instructions even after it has been manufactured.

After the fetch and decode steps the execute step is performed. During this step various portions of the CPU are connected so they can perform the desired operation. If, for instance, an addition operation is required, an arithmetic logic unit (ALU) will be connected to a set of inputs and a set of outputs. The inputs provide the numbers to be added, and the outputs will contain the final sum. The ALU contains the circuitry to perform simple arithmetic and logical operations. If the addition operation produces the result too large for the CPU to handle, an arithmetic overflow flag in a flags register may also be set.

The final step, writeback, simply «writes back» the results of the execute step to some form of memory. The results are often written into some internal CPU register for a fast access by subsequent instructions. In other cases the results may be written into slower, but cheaper and larger main memory. Many instructions will also change the state of digits in a «flags» register. These flags can be used to influence how a program behaves, since they often indicate the outcome of various operations. The flag could then be used by a jump instruction to determine program flow.

A multi-core CPU combines two or more independent cores into a single package composed of a single integrated circuit, called a die and implements multiprocessing in a single physical package. A processor with all cores on a single die is called a monolithic processor. Cores in a multi-core device may share a single coherent cache at the highest on-device cache level (e.g., L2 for the Intel Core 2) or may have separate caches (e.g., current AMD dual-core processors). Each «core» implements optimizations such as superscalar execution, pipelining and multithreading independently. The most commercially significant multi-core processors are used in computers (primarily from Intel & AMD) and game consoles (e.g., the Cell processor in the PS3).

 

IV. Answer the following questions:

1. What is a definition of a central processing unit?

2. What is the impact of using integrated circuits in computer technology?

3. What is the John von Neumann famous for?

4. What architectures are there in modern computers?

5. What steps do von Neumann CPUs perform in their operation?

6. What operations are performed by the arithmetic logic unit?

7. What is a multi-core CPU?

8. What companies' multi-core CPUs are the most commercially significant?


Date: 2016-03-03; view: 892


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