Figure1.1 shows block diagram of a simple microcomputer. The major parts are
the central processing unit or CPU, memory and the input and output circuitry or
Input/output. Connecting these parts are three sets of parallel line is called buses and
control bus. In a microcomputer the CPU is a microprocessor and is often referred to as
the microprocessor unit (MPU). Its purpose is to decode the instruction and use them to
control the activity with in the system. It performs all arithmetic and logical
Memory: Memory section usually consists of a mixture of RAM and ROM. It may also
magnetic floppy disks, magnetic hard disks or optical disks, to store the data.
Input/output: The input/output section allows the computer to take in data from the
outside world or send data to the outside world. Peripherals such as keyboards, video
display terminals. Printers and modem are connected to the input/output section. These
allow the user and computer to communicate with each other. The actual physical devices
used to interface the computer buses to external systems are often called ports. An
input/output port allows data from keyboard, an analog to digital converter (ADC) or
some other source to be read into the computer under the control of the CPU. An output
port is used to send data from the computer to some peripheral, such as a video display
terminal, a printer or a digital to analog converter (DAC).
Central processing Unit (CPU): CPU controls the operation of the computer .In a
microcomputer the CPU is a microprocessor. The CPU fetches the binary coded
instructions from memory, decodes the instructions into a series of simple action and
carries out these actions in sequence of steps.
CPU contains an a address counter or instruction pointer register which holds the
address of the next instruction or data item to be fetched from memory, general purpose
register, which are used for temporary storage or binary data and circuitry, which
generates the control bus signals.
Address bus: The address bus consists of 16, 20, 24 or 32 parallel lines. On these lines
the CPU sends out the address of the memory locations that are to be written to or read
from. The number of memory locations that the CPU can addresses is determined by the
number of address lines, then it can directly address 2n memory location. When the CPU
reads data from or writes data to a port, it sends the port address on the address bus.
CPU has 16 address lines can address 216 or 65536 memory locations.
Data bus: It consists of 8, 16, 32 parallel signal lines. The data bus lines are
bidirectional. This means that the CPU can read, data from memory or from a port on
these lines, or it can send data out to memory or to port on these lines.
Control bus: The control bus consists of 4 to 10 parallel signals lines. The CPU sends
out signals on the control bus enable the outputs of addressed memory devices or port