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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 28, or 256
unique combinations of is ans os, and this is more than adequate to represent
all of the characters by which we communicate. data in the form of coded characters is 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.

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?