Overview
of a Computer - Primary Storage
·
Primary storage, also known as main memory or RAM (random access
memory) is used to store information for immediate access by the Central
Processing Unit (CPU)
·
Memory can be viewed as a series
of memory cells with each cell
having its own individual address
·
E.g., think of a bank of
mailboxes at a post office
·
The information contained in a
memory cell is called the contents
of that cell
·
Memory cells can be used to store
data, such as characters or numbers
·
Internally, of course, they are
all numbers, but we can choose to interpret some numbers as characters
·
They can also be used to store program instructions
·
These are “special” numbers that
are meaningful to a CPU!
·
The smallest unit of computer
storage is a bit, as in binary
digit
·
Most computers group bits
together to form larger entities
·
E.g., 8 consecutive bits often
form a byte and 32 consecutive bits often form a word
·
A word on an Intel 286 computer
is 16 bits or 2 bytes
·
A word on an Intel 386, 486, and
Pentium computers is 32 bits or 4 bytes
·
The next generation of Intel
computers (e.g. the Merced) will use 64 bit words, i.e. 8 bytes
·
The DEC Alpha computer is
currently using 64 bit words
·
Many mainframe computers, such as
the Unisys A-Series, use 48 bit words
·
Memory cells in a computer are
typically one byte or one word in size
·
A word is a unit of information
that can be transferred to and from memory
·
A kilobyte of memory, as in 1K, is 1024 bytes of memory
·
A megabyte of memory, as in 1M, is 1024K of memory, or 2^20 bytes
·
A gigabyte of memory, as in 1G, is 1024M of memory, or 2^30 bytes
·
Note that a 32 bit word can
represent 2^32 different possible values
