UNIT I – INTEL 8085
1.1 INTRODUCTION TO MICROPROCESSOR BASED SYSTEM
The microprocessor is a semiconductor device (Integrated Circuit) manufactured by the 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 by using microprocessor
as a CPU and interfacing memory, input and output devices to it. A system designed using a
microprocessor as its CPU is called a microcomputer.
The Microprocessor based system (single board microcomputer) consists of microprocessor 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 and floppy disk and the output devices are printer,
LED/LCD displays, CRT monitor, etc.
The above block diagram shows the organization of a microprocessor based system. In this system,
the microprocessor 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 processor are reading/writing the memory and
reading/writing the J/O devices or the peripherals. If the peripheral requires the attention of the
master then it can interrupt the master and initiate an operation.
The microprocessor is the master, which controls all the activities of the system. To perform a
specific job or task, the microprocessor 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 appropriate action.
BUSES:
The buses are group of lines that carries data, address or control signals.
· 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.
· 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 tristate
logic and hence normally remain in high impedance state. Only when the slave is
selected it comes to the normal logic.
PERIPHERAL DEVICES:
· The EPROM memory is used to store permanent programs and data.
· The RAM memory is used to store temporary programs and data.
· The input device is used to enter the program, data and to operate the system.
· The output device is used for examining the results.
Since the speed of I/O devices does not match with the speed of microprocessor, an interface
device is provided between system bus and I/O devices. Generally I/O devices are slow
devices.

Advantages of Microprocessor based system
1. Computational/processing speed is high.
2. Intelligence has been brought to systems.
3. Automation of industrial processes and office administration.
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 and cost less. Also it is more reliable.
6. Operation and maintenance are 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 microprocessors does not support floating point operations.