Working of 8085 microprocessor Animation with English Subtitle

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Architecture Accumulator

Accumulator is an eight bit Register.It holds one of the data to be processed by Arithmetic

logic unit or ALU.It also stores the result of the operation.The Accumulator is also called

as A register.The Accumulator is connected to the eight bit internal data bus the bidirectional

arrow between the accumulator and the bus indicates that it allows the accumulator to

send or receive data.The two state output of the accumulator drives the ALU.

Temp.Register: The temporary register receives one of the

data to be processed by ALU from external memory or general purpose registers.The other

input for the ALU comes from the temporary register.This eight bit register stores the

operands of Arithematic logic operations.

General Purpose Registers: In Intel 8085 microprocessor there are six

8 bit general purpose registers.

They are B, C, D, E, H and L. They may be used individually or combined as register

pairs to perform some 16 bit operations.

The permitted combinations of register pairs are BC, DE and HL.

The HL register pair which is normally used to perform a 16 bit memory pointer.

Stack pointer or SP: Stack pointer is a 16 bit register used as

a memory pointer.

It maintains the address of the last byte enter into the stack.

Stack is the portion of RAM.

The Stack pointer is decremented each time when data is loaded into the stack and is

incremented when data is retrieved from the Stack.

Program Counter or PC: This 16 bit register deals with sequencing

the execution of instructions.

This register is also a memory pointer.

The microprocessor uses this register to sequence the execution of the instructions.

The function of the program counter is to point the address of the next instruction

to be executed.

At the end of the execution of an instruction the program counter is incremented by 1 pointing

to the next memory location where the next instruction is available.

Incrementer/Decrementer: It can add 1 or subtract 1 from the contents

of the stack pointer or program counter.

Arithmetic and Logic Unit (ALU): The ALU carries out the arithmetic and logic

operation on 8 bit words.

As shown the contents of the accumulator and the temporary register are the inputs to the

ALU.It can peform arithmetic operations such as addition, subtraction and logical operations

such as AND,OR and XOR.

The ALU result is then stored back in the Accumulator.

FLAGS: Flag register is a group of five individual


The content of the flag register will change 0 or 1 after the execution of arithmetic and

logic operations.

Instruction Register and Decoder: Instruction Register and Decoder is an 8 bit


When an instruction is fetched from memory it is loaded in the Instruction Register.

The instruction decoder decodes the contents of the Instruction register.

It also determines the operation to be followed in executing the entire instruction and directs

the timing and control unit accordingly.

Timing and Control unit: The Timing and control section of microprocessor

includes an oscillator and controller sequencer.

The oscillator generates the two phase clock signals, CLK and CLK' that synchronize all

registers .The controller sequencer also produces the control signals needed for internal and

external control.

The controller sequencer is microprogrammed.

It has a ROM that stores all the micro routines needed for executing the instructions.

Interrupt Control: Sometimes it is necessary to interrupt the

execution of the main program to answer a request from an IO device.

For instance an IO device may send an interrupt signal to the Interrupt control unit to indicate

the data is ready for input.

The computer temporarily stops the execution of main program inputs the data and then returns

to the main program.

SERIAL I/O CONTROL: Sometimes I/O devices works with serial data

rather than parallel.

The Serial input data enters 8085 through pin 5 SID Serial input Data and leaves through

pin 4 SOD serial output data.

Two new instructions known as SIM and RIM allows us to perform a serial parallel conversion

needed for seral IO device.

Address Buffer and Address Data Buffer: The contents of stack pointer or program counter

can be loaded into the address buffer an address data buffer.The output of these buffers then

drives the external address bus an address data Bus.

Memory and IO chips are connected to these buses.In this way the CPU can send a address

of desired data to the memory or IO chips.