Architechture of 8085

This is the functional block diagram of the 8085 Microprocessor.


It is a 8-bit register which is used to perform airthmetical and logical operation. It stores the output of any operation. It also works as registers for i/o accesses.

Temporary Register

It is a 8-bit register which is used to hold the data on which the acumulator is computing operation. It is also called as operand register because it provides operands to ALU.


These are general purposes registers. Microprocessor consists 6 general purpose registers of 8-bit each named as B,C,D,E,H and L. Generally theses registers are not used for storing the data permanently. It carries the 8-bits data. These are used only during the execution of the instructions.
These registers can also be used to carry the 16 bits data by making the pair of 2 registers. The valid register pairs available are BC,DE HL. We can not use other pairs except BC,DEand HL. These registers are programmed by user.


ALU performs the airthmetic operations and logical operation.

Flag Registers

It consists of 5 flip flop which changes its status according to the result stored in an accumulator. It is also known as status registers. It is connected to the ALU.
There are five flip-flops in the flag register are as follows:
  1. Sign(S)
  2. zero(z)
  3. Auxiliary carry(AC)
  4. Parity(P)
  5. Carry(C)
The bit position of the flip flop in flag register is:
D7 D6 D5 D4 D3 D2 D1 D0
All of the three flip flop set and reset according to the stored result in the accumulator.

If D7 of the result is 1 then sign flag is set otherwise reset. As we know that a number on the D7 always desides the sign of the number. if D7 is 1: the number is negative. if D7 is 0: the number is positive.

If the result stored in an accumulator is zero then this flip flop is set otherwise it is reset.

3.Auxiliary carry(AC)-
If any carry goes from D3 to D4 in the output then it is set otherwise it is reset.

If the no of 1's is even in the output stored in the accumulator then it is set otherwise it is reset for the odd.

If the result stored in an accumulator generates a carry in its final output then it is set otherwise it is reset.

Instruction registers(IR)

It is a 8-bit register. When an instruction is fetched from memory then it is stored in this register.

Instruction Decoder

Instruction decoder identifies the instructions. It takes the informations from instruction register and decodes the instruction to be performed.

Program Counter

It is a 16 bit register used as memory pointer. It stores the memory address of the next instruction to be executed. So we can say that this register is used to sequencing the program. Generally the memory have 16 bit addresses so that it has 16 bit memory. The program counter is set to 0000H.

Stack Pointer

It is also a 16 bit register used as memory pointer. It points to the memory location called stack. Generally stack is a reserved portion of memory where information can be stores or taken back together.

Timing and Control Unit

It provides timing and control signal to the microprocessor to perform the various operation.It has three control signal. It controls all external and internal circuits. It operates with reference to clock signal.It synchronizes all the data transfers.
There are three control signal:
  1. ALE-Airthmetic Latch Enable, It provides control signal to synchronize the components of microprocessor.
  2. RD- This is active low used for reading operation.
  3. WR-This is active low used for writing operation.
There are three status signal used in microprocessor S0, S1 and IO/M. It changes its status according the provided input to these pins.

Serial Input Output Control-

There are two pins in this unit. This unit is used for serial data communication.

Interrupt Unit

There are 6 interrupt pins in this unit. Generally an external hardware is connected to these pins. These pins provide interrupt signal sent by external hardware to microprocessor and microprocessor sends acknowledgement for receiving the interrupt signal. Generally INTA is used for acknowledgement.

Register Section

Many registers has been used in microprocessor. PIPO shift register It consists of PIPO(Parallel Input Parallel Output) register.