Билет № 1

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The Arithmetic Logic Unit

Because all computer data is stored as numbers, the processing involves comparing numbers or carrying out mathematical operations. The computer performs two types of operations: arithmetic operations and logical operations. Arithmetic operations include addition, subtraction, multiplication, and division. Logical operations include the following ones: “equal to”, “not equal to”, “greater than”, or “less than”. The ALU includes a group of registers – high-speed memory locations in the CPU that are used to hold the data currently being processed. For example, the control unit may load two numbers from memory into the registers in the ALU. Then it may tell the ALU to divide the two numbers (an arithmetic operation) or to see whether the numbers are equal (a logical operation). Each time the CPU executes an instruction, it takes a series of steps that is called a machine cycle. A machine cycle can be broken down into two smaller cycles: the instruction cycle and the execution cycle. During the instruction cycle the CPU takes two steps: fetching (the control unit retrieves, or “fetches”, a command or data from the computer's memory) and decoding (the control unit breaks down, or decodes, the command into instructions that correspond to those in the CPU's instruction set. At this point, the CPU is ready to begin the execution cycle. When the command is executed, the CPU carries out the instructions in order by converting them into microcode. The CPU may be required to store the results of an instruction in memory (but this condition is not always required).

Билет № 2

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Memory

The CPU contains the basic instructions to operate the computer, but it cannot store entire programs or large sets of data permanently. It contains registers which are small areas that can hold only a few bytes at a time. In addition to registers, the CPU has millions of bytes of space where it can quickly read or write programs and data in use. This area is called memory. Physically, memory consists of chips either on the motherboard or on a small circuit board attached to the motherboard. This electronic memory allows the CPU to store and retrieve data quickly. There are two types of memory: read-only memory, or ROM, and random access memory, or RAM. Read-only memory (ROM) is nonvolatile (or permanent); it holds instructions that run the computer when the power is first turned on. ROM contains a set of start-up instructions, which ensures that the rest of memory is functioning properly, checks for hardware devices, and checks for an operating system on the computer's disk drives. The data in ROM cannot be changed. Random-access memory (RAM) is volatile (read/write, or temporary); programs and data can be written to and erased from RAM as needed. The purpose of RAM is to hold programs and data while they are in use. The more RAM a computer has, the more it can do and the faster it can perform certain tasks. The CPU accesses each location in memory by using a unique number called the memory address. A memory address is a number that indicates a location on the memory chips. There are two types of RAM: dynamic and static. Dynamic RAM (DRAM) must be refreshed, or recharged with electricity frequently; otherwise, it will lose its contents. Static RAM is not refreshed often and can hold its contents longer than dynamic RAM. SRAM is also considerably faster than DRAM.