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Текст 1. ЕДИНИЦЫ ХРАНЕНИЯАрхитектура компьютерной системы организована вокруг единицы первичного хранения потому, что все данные и инструкции, используемые системой компьютера должны пройти через основное хранилище. Наше обсуждение единиц компьютерной системы начнется с функциями единиц первичного и вторичного хранения. Это приводит к рассмотрению центрального процессора и оттуда к рассмотрению входных и выходных единиц. Таким образом, последовательность, в которой мы рассмотрим функциональные единицы цифрового компьютера является: 1) единиц хранения, первичный и вторичный; 2) центральный процессор; 3) входных и выходных единиц.Как вы знаете, есть единицы первичного и вторичного хранения. Оба содержат данные и инструкции для обработки данных. Данные, а также инструкции должны поступать в и из основного хранилища.Primary storage is also called main storage or internal stor¬age. The specific functions of internal storage are to hold (store): 1) all data to be processed; 2) intermediate results of process¬ing; 3) final results of processing; 4) all the instructions required for ongoing process. Another name for primary storage is memory, because of its similarity to a function of the human brain. However, computer storage differs from human memory in im¬portant respects. Computer memory must be able to retain very large numbers of symbol combinations, without forgetting or changing any details. It must be able to locate all its contents quickly upon demand. The combinations of characters, that is, the letters, numbers, and special symbols by which we usuallycommunicate, are coded. The codes used by computer designers are based upon a number system that has only two possible values, 0 and 1 .'A number system with only two digits, 0 and I, is called a binary number system. Each binary digit is called a bit, from Binary digiT. As the information capacity of a single bit is limited to 2 alternatives, codes used by computer designers are based upon combinations of bits. These combinations are called binary codes. The most common binary codes are 8-bit codes because an 8-bit code provides for 2/8, or 256 unique combinations of l's ans O's, and this is more than adequate to represent all of the characters by which we communicate.Data in the form of coded characters are stored in adjacent storage locations in main memory in two principal ways : 1) as "strings" of characters — in bytes; and 2) within fixed-size "boxes" — in words. A fixed number of consecutive bits that repre¬sent a character is called a byte. The most common byte size is 8-bit byte. Words are usually 1 or more bytes in length.Secondary storage. Primary storage is expensive because each bit is represented by a high-speed device, such as a semiconductor. A million bytes (that is, 8 million bits) is a large amount of primary storage. Often it is necessary to store many millions, sometimes billions, of bytes of data. Therefore slower, less expensive storage units are available for computer systems. These units are called secondary storage. Data are stored in them in the same binary codes as in main storage and are made avail-able to main storage as needed.
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