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8085 Architecture

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8085 is pronounced as “eighty-eighty-five” microprocessor. It is an 8-bit microprocessor designed by Intel in 1977 using NMOS technology.

It has the following configuration −

  • 8-bit data bus
  • 16-bit address bus, which can address upto 64KB
  • A 16-bit program counter
  • A 16-bit stack pointer
  • Six 8-bit registers arranged in pairs: BC, DE, HL
  • Requires +5V supply to operate at 3.2 MHZ single phase clock

It is used in washing machines, microwave ovens, mobile phones, etc.

8085 Microprocessor – Functional Units

8085 consists of the following functional units −


It is an 8-bit register used to perform arithmetic, logical, I/O & LOAD/STORE operations. It is connected to internal data bus & ALU.

Arithmetic and logic unit

As the name suggests, it performs arithmetic and logical operations like Addition, Subtraction, AND, OR, etc. on 8-bit data.

General purpose register

There are 6 general purpose registers in 8085 processor, i.e. B, C, D, E, H & L. Each register can hold 8-bit data.

These registers can work in pair to hold 16-bit data and their pairing combination is like B-C, D-E & H-L.

Program counter

It is a 16-bit register used to store the memory address location of the next instruction to be executed. Microprocessor increments the program whenever an instruction is being executed, so that the program counter points to the memory address of the next instruction that is going to be executed.

Stack pointer

It is also a 16-bit register works like stack, which is always incremented/decremented by 2 during push & pop operations.

Temporary register

It is an 8-bit register, which holds the temporary data of arithmetic and logical operations.

Flag register

It is an 8-bit register having five 1-bit flip-flops, which holds either 0 or 1 depending upon the result stored in the accumulator.

These are the set of 5 flip-flops −

  • Sign (S)
  • Zero (Z)
  • Auxiliary Carry (AC)
  • Parity (P)
  • Carry (C)

Its bit position is shown in the following table −

D7 D6 D5 D4 D3 D2 D1 D0

Instruction register and decoder

It is an 8-bit register. When an instruction is fetched from memory then it is stored in the Instruction register. Instruction decoder decodes the information present in the Instruction register.

Timing and control unit

It provides timing and control signal to the microprocessor to perform operations. Following are the timing and control signals, which control external and internal circuits −

  • Control Signals: READY, RD’, WR’, ALE
  • Status Signals: S0, S1, IO/M’
  • DMA Signals: HOLD, HLDA

Interrupt control

As the name suggests it controls the interrupts during a process. When a microprocessor is executing a main program and whenever an interrupt occurs, the microprocessor shifts the control from the main program to process the incoming request. After the request is completed, the control goes back to the main program.

There are 5 interrupt signals in 8085 microprocessor: INTR, RST 7.5, RST 6.5, RST 5.5, TRAP.

Serial Input/output control

It controls the serial data communication by using these two instructions: SID (Serial input data) and SOD (Serial output data).

Address buffer and address-data buffer

The content stored in the stack pointer and program counter is loaded into the address buffer and address-data buffer to communicate with the CPU. The memory and I/O chips are connected to these buses; the CPU can exchange the desired data with the memory and I/O chips.

Address bus and data bus

Data bus carries the data to be stored. It is bidirectional, whereas address bus carries the location to where it should be stored and it is unidirectional. It is used to transfer the data & Address I/O devices.

8085 Architecture

The functional block diagram of architecture of 8085 Microprocessor is very important as it gives the complete details about a Microprocessor. Fig. shows the Block diagram of a Microprocessor.

8085 Bus Structure:
Address Bus:

The address bus is a group of 16 lines generally identified as A0 to A15.

The address bus is unidirectional: bits flow in one direction-from the MPU to peripheral devices.

The MPU uses the address bus to perform the first function: identifying a peripheral or a memory location.

Data Bus:

A data bus is a group of eight lines used for data flow.

These lines are bi-directional – data flow in both directions between the MPU and memory and peripheral devices.

The MPU uses the data bus to perform the second function: transferring binary information.

The eight data lines enable the MPU to manipulate 8-bit data ranging from 00 to FF (28 = 256 numbers).

The largest number that can appear on the data bus is 11111111.

Control Bus:

The control bus carries synchronization signals and providing timing signals.

The MPU generates specific control signals for every operation it performs. These signals are used to identify a device type with which the MPU wants to communicate.

Registers of 8085:

The 8085 have six general-purpose registers to store 8-bit data during program execution.

These registers are identified as B, C, D, E, H, and L.

They can be combined as register pairs-BC, DE, and HL-to perform some 16-bit operations.


Accumulator (A):

The accumulator is an 8-bit register that is part of the arithmetic/logic unit (ALU).

This register is used to store 8-bit data and to perform arithmetic and logical operations.

The result of an operation is stored in the accumulator.


The ALU includes five flip-flops that are set or reset according to the result of an operation.

The microprocessor uses the flags for testing the data conditions.

They are Zero (Z), Carry (CY), Sign (S), Parity (P), and Auxiliary Carry (AC) flags. The most commonly used flags are Sign, Zero, and Carry.

The bit position for the flags in flag register is,

1.Sign Flag (S):

       After execution of an arithmetic and logical operation, if D7 of the result is 1, the sign flag is set. Otherwise it is reset.

       D7 is reserved for indicating the sign; the remaining is the magnitude of the number.

       If D7 is 1, the number will be viewed as a negative number. If D7 is 0, the number will be viewed as a positive number.


2.Zero Flag (z):

       If the result of arithmetic and logical operation is zero, then zero flags is set otherwise it is reset.


3.Auxiliary Carry Flag (AC):

       If D3 generates any carry when doing any arithmetic and logical operation, this flag is set.

       Otherwise it is reset.


4.Parity Flag (P):

       If the result of arithmetic and logical operation contains even number of 1’s then this flag will be set and if it is an odd number of 1’s it will be reset.


5.Carry Flag (CY):

       If any arithmetic and logical operation result any carry then carry flag is set otherwise it is reset.

Temporary Register: It is used to hold the data during the arithmetic and logical operations.
Instruction Register:  When an instruction is fetched from the memory, it is loaded in the instruction register.
Instruction Decoder: It gets the instruction from the instruction register and decodes the instruction. It identifies the instruction to be performed.
Serial I/O Control:  It has two control signals named SID and SOD for serial data transmission.
Interrupt Control Unit:

It receives hardware interrupt signals and sends an acknowledgment for receiving the interrupt signal.

Arithmetic and Logic Unit (ALU):


It is used to perform the arithmetic operations like addition, subtraction, multiplication, division, increment and decrement and logical operations like AND, OR and EX-OR.


It receives the data from accumulator and registers.


According to the result, it set or reset the flags.


Program Counter (PC):


This 16-bit register sequencing the execution of instructions.


It is a memory pointer. Memory locations have 16-bit addresses, and that is why this is a 16-bit register.


The function of the program counter is to point to the memory address of the next instruction to be executed.


When an opcode is being fetched, the program counter is incremented by one to point to the next memory location.


Stack Pointer (Sp):


The stack pointer is also a 16-bit register used as a memory pointer.


It points to a memory location in R/W memory, called the stack.


The beginning of the stack is defined by loading a 16-bit address in the stack pointer (register).

Architecture/Functional block diagram of 8086 microprocessor