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1. what are the functional units of a digital computer? 2. what units make up the central processing unit? 3. how is computer system organized? 4. what are the two main types of storage units? 5. what do they contain? 6. what is the function of a primary storage? 7. why is primary storage often called memory? 8. in what respect does computer memory differ from human memory? 9. what are codes based on? 10. what is secondary storage and what is it used for?

storage units

computer system architecture is organized around the primary storage unit because all data and instructions used by the computer system must pass through primary storage. our discussion of computer system units will begin with the functions of the primary and secondary storage units. this leads to the examination of the central processing unit and from there to the consideration of the input and output units. therefore, the sequence in which we'll describe the functional units of a digital computer is: 1) storage units, primary and secondary; 2) central processing unit; 3) input and output units.

as you know, there are primary and secondary storage units. both contain data and the instructions for processing the data. data as well as instructions must flow into and out of primary storage.

primary storage is also called main storage or internal storage. the specific functions of internal storage are to hold (store): 1) all data to be processed; 2) intermediate results of processing; 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 important 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 usually communicate, 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 1, 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 1's and 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 represent 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 available to main storage as needed.

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