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Architecture Of The 8085 Microprocessor

The Intel 8085/8085AH is a microprocessor, i.e., an 8-bit parallel central processing unit implemented in silicon gate NMOS/HMOS/C-MOS technology. It is available in a 40-pin IC package fabricated on a single LSI chip. It is designed with higher processing speed from 3 MHz to 5 MHz, comparably lower power consumption and power-down mode, thereby offering a high level of system integration. This processor uses a multiplexed address/data bus. The address bus is split between the 8-bit address bus and the 8-bit data bus.

The on-chip address latch allows a direct interface with the processor. The features of 8085 microprocessors are given below:



  • Power-down mode (HALT-HOLD)
  • Low Power Dissipation of about 50 mW
  • Single +3 to +6 V power supply
  • Operating temperature from –40 to +85°C
  • On-chip clock generator incorporating external crystal oscillators
  • On-chip system controller
  • Four vectored interrupt including one non-maskable
  • Serial input/Serial output port
  • Addressing capability to 64 kBytes of memory
  • TTL compatible
  • Available in 40-pin Plastic DIP package


Operation of the 8085 Microprocessor


Bus structure of the 8085 microprocessor


The step-by-step procedure of data flow is given below:

  • The 16-bit memory address is stored in the program counter. Therefore, the program counter sends the 16-bit address on the address bus. The memory address decoder is decoded and identifies the specified memory location.
  • The control unit sends the control signal  RD send signal in the next clock cycle and the memory IC is enabled. RD is active for two clock periods.
  • When the memory IC is enabled, the byte from the memory location is placed on the data bus, AD7–AD0. After that, data is transferred to the microprocessor.

Arithmetic Logic Unit (ALU)



Memory read operation


Data flow from memory to microprocessor


ALU functioning


The sequence of operations in ALU are given below:

  • One operand is in the A register.
  • The other operand may be in the general-purpose register or memory location, which will be transferred to the temporary register.
  • Then content of accumulator and temporary register are considered as inputs of ALU and the specified operation is carried out in the ALU.
  • The result of ALU operation is transferred in the A register through the internal data bus.
  • The content of the flag register will be changed depending on the result.

The arithmetic logic unit (ALU) performs the following operations:

  • Addition
  • Subtraction
  • Logical AND
  • Logical OR
  • Logical EXCLUSIVE OR
  • Complement
  • Increment by 1
  • Decrement by 1
  • Rotate Left, Rotate Right
  • Clear




The following registers of the Intel 8085 microprocessor are depicted in Fig

  • One 8-bit accumulator (ACC) known as register A
  • Six 8-bit general-purpose registers: B, C, D, E, H and L
  • One 16-bit stack pointer (SP)
  • One 16-bit program counter (PC)
  • Instruction register
  • Temporary register
  • Program status word register (PSW)


Accumulator The accumulator is an 8-bit register, which is part of the Arithmetic Logic Unit (ALU).


General-Purpose Registers The general-purpose registers of the 8085 microprocessor are B, C, D, E, H and L registers as shown in Fig These registers are used to store 8-bit operands.


Special-Purpose Registers In addition to the above general-purpose registers, the 8085 microprocessor has special-purpose registers, namely, Program Counter (PC), Stack Pointer (SP), Flags/Status Registers (SR), Instruction Register (IR), Memory Address Register (MAR), Temporary Register (TR), and Memory buffer register (MBR).

Registers of the 8085 microprocessor


  • Carry Flag (CY) The arithmetic operation generates a carry in case of addition or a borrow in case of subtraction after execution of an arithmetic instruction and the Carry Flag (CY) is set to 1.
  • Parity Flag (P) After an arithmetic or logical operation, if the number of 1s in the result is even (even parity), this parity status flag (P) is set, and if the number of 1s is odd (odd parity), this flag is reset.
  • Auxiliary Carry Flag (AC) In arithmetic operations of numbers, if a carry is generated by bit D3 and passed on to D4, the auxiliary carry flag (AC) is set.
  • Zero Flag (Z) When an 8-bit ALU operation results in zero, the Zero (Z) flag is set; otherwise it is reset. This flag is affected by the results of accumulator and general purpose registers.
  • Sign Flag (S) The sign flag has its importance only when signed arithmetic operation is performed. In arithmetic operations of signed numbers where bit D7 is used to indicate a sign, this flag is set to indicate the sign of a number.
  • PSW In flag register five bits (D7 D6 D4 D2 D0) indicate the five status flags and three bits D5 D3 and D1 are undefined. The combination of these 8-bits is known as Program Status Word (PSW). The PSW and the accumulator can be used as a 16-bit unit for stack operation.


System Bus: Address Bus, Data and Control Bus


Microprocessor and its buses


Address Bus Each memory location has a unique address. The address bus consists of 16 wires, therefore, the address bus has 16 bits. Its “width” is 16-bits.


Data Bus The data bus as 8-bit data is stored in each memory location. The data bus is used to move or transfer data in binary form.


Control Bus 

The control bus has various lines, which have specific functions for coordinating and controlling microprocessor operations.


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