Introduction to Microprocessor
The MP is a semiconductor device (Integrated Circuit) manufactured by VLSI (Very Large Scale
Integration) technique. It includes the ALU, Register Arrays, and Control Circuit on a single chip. To
perform a function or useful task we have to form a system using MP as a CPU and interfacing memory, input,
and output devices to it. A system designed using a MP as its CPU is called a Microcomputer.
The MP based system consists of MP as CPU, semiconductor memories like EPROM and RAM, input
device, output device, and interfacing devices. The memories, input device, output device, and interfacing
devices are called peripherals. The popular input devices are keyboard, mouse and the output devices are
printer, LED/LCD displays, CRT monitors, etc.
Fig. Microprocessor Based System (Organization of Microcomputer)
The above block diagram shows the organization of a microcomputer based system. In this system,
the MP is the master, and all other peripherals are slaves. The master controls all the peripherals, and initiates
all operations.
The work done by the processor can be classified into the following three groups –
1. Work done internal to the processor
2. Work done external to the processor
3. Operations initiated by the slaves or peripherals
The work done internal to the processors are addition, subtraction, logical operations, data transfer
operations, etc. The work done external to the processors are reading/writing the memory and
reading/writing the I/O devices or the peripherals. If the peripherals requires the attention of the master
(CPU) then it can interrupt the master and initiates an operation.
The MP is the master, which controls all the activities of the system. To perform a specific job or task,
the MP has to execute a program stored in memory. The program consists of a set of instructions. It issues
address and control signals and fetches the instruction and data from memory. The instruction is executed one
by one internal to the processor and based on the result it takes an appropriate action.
Buses
The buses are group of lines that carries data, address or control signals.
1. The CPU Bus has multiplexed lines i.e. same line is used to carry different signals.
The CPU interface is provided to demultiplex the multiplexed lines, to generate chip select signals and
additional control signals.
2. The System Bus has separate lines for each signal.
All the slaves in the system are connected to the same system bus. At any time instant communication
takes place between the master and one of the slaves. All the slaves have tri-state logic and hence normally
remain in high impedance state. Only when the slave is selected it comes to the normal logic.
Peripheral Devices
1. The EPROM memory is used to store permanent programs and data.
2. The RAM memory is used to store temporary programs and data.
3. The input device is used to enter the program, data and to operate the system.
4. The output device is used for examining the results.
Since the speed of the I/O devices does not match with the speed of the MP, an interfacing device
is provided between the system bus and the I/O devices.
Advantages of Microprocessor Based System
1. Processing speed is high.
2. Intelligence has been brought to systems.
3. Automation of industrial processes and office automation.
4. Since the devices are programmable, there is flexibility to alter the system by changing the software
alone.
5. Less number of components, compact in size, cost less and more reliable.
6. Operation and maintenance is easier.
Disadvantages of Microprocessor Based System
1. It has limitations on the size of data.
2. The applications are limited by the physical address space.
3. The analog signals cannot be processed directly and digitizing the analog signals introduces errors.
4. The speed of execution is slow and so real time applications are not possible.
5. Most of the microprocessor does not support floating point operations.
The Intel 8085 is an 8-bit microprocessor introduced by Intel in 1977. The '8085' in the 8085
microprocessor is the designation given to the microprocessor by Intel after 8080. The "5" in the model number
came from the fact that the 8085 requires only a single +5V power supply as opposed to the 8080 which
needed +5V, -5V, and +12V.
In 8085, ‘5’ means the microprocessor operates at +5 volt, third 8 means 8 bits, and its word length is 1
byte. 80 indicates it was developed in eighties (1977).
Some terms:
ALU – the group of circuits that performs arithmetic and logic operations and it’s a part of CPU.
Control Unit – The group of circuits that provides timing and signals to all operations in the computer and
controls data flow.
LSI – the process of designing more than a thousand circuit on a single chip
ROM – a memory that stores binary information permanently but cannot be altered.
Register – stores 8 bit - data. (B, C, D, E, H & L are General Purpose Registers)
Interrupt – a signal send by an external device to the MP to do a particular job/task.
Tri-state – 0, 1 and high impedance (as if it were disconnected from the system) .i.e. no current is drawn from
the system.