The unit of data size
Bit : a binary digit that can have the value
0 or 1
Byte : 8 bits
Nibble : half of a bye, or 4 bits
Word : two bytes, or 16 bits
The terms used to describe amounts of
memory in IBM PCs and compatibles
Kilobyte (K): 210 bytes
Megabyte (M) : 220 bytes, over 1 million
Gigabyte (G) : 230 bytes, over 1 billion
Terabyte (T) : 240 bytes, over 1 trillion
Internal Organization of Computers
CPU (Central Processing Unit)
Execute information stored in memory
I/O (Input/output) devices
Provide a means of communicating with
CPU
Memory
RAM (Random Access Memory) –
temporary storage of programs that
computer is running
The data is lost when computer is off
ROM (Read Only Memory) – contains
programs and information essential to
operation of the computer
The information cannot be changed by use,
and is not lost when power is off
– It is called nonvolatile memory
The CPU is connected to memory and
I/O through strips of wire called a bus
Carries information from place to place
Address bus
Data bus
Control bus
Address bus
For a device (memory or I/O) to be
recognized by the CPU, it must be
assigned an address
The address assigned to a given device must
be unique
The CPU puts the address on the address bus,
and the decoding circuitry finds the device
Data bus
The CPU either gets data from the device
or sends data to it
Control bus
Provides read or write signals to the
device to indicate if the CPU is asking for
information or sending it information
The more data buses available, the
better the CPU
Think of data buses as highway lanes
More data buses mean a more
expensive CPU and computer
The average size of data buses in CPUs
varies between 8 and 64
Data buses are bidirectional
To receive or send data
The processing power of a computer is
related to the size of its buses
The more address buses available, the
larger the number of devices that can
be addressed
The number of locations with which a
CPU can communicate is always equal
to 2x, where x is the address lines,
regardless of the size of the data bus
ex. a CPU with 24 address lines and 16
data lines can provide a total of 224 or 16M
bytes of addressable memory
Each location can have a maximum of 1
byte of data, since all general-purpose
CPUs are byte addressable
The address bus is unidirectional
CPU’s Relation to RAM and ROM
For the CPU to process information,
the data must be stored in RAM or
ROM, which are referred to as primary
memory
ROM provides information that is fixed
and permanent
Tables or initialization program
RAM stores information that is not
permanent and can change with time
Various versions of OS and application
packages
CPU gets information to be processed
first form RAM (or ROM)
if it is not there, then seeks it from a mass
storage device, called secondary memory, and
transfers the information to RAM
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