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

87 Unit 7. Storage

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 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 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.