Lecture-04: 8085 Microprocessor (MPU)

 8085 Architecture

8085 is pronounced as “eighty-eighty-five” microprocessor. It is an 8 bit microprocessor designed by Intel in 1917  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 paris: BC, DE, HL

·       Requires +5v supply to operate at 3.2 MHZ single phase clock

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

8085 Microprocessor-Functional Unit

8085 consists of the following functional unit-

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

2.    Arithmetic and logic unit: As the name suggests, it performs arithmetic and logical operations like Addition, Substraction, AND, OR, etc. on 8-bit data.

3.    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 combinations is like B-C, D-E & H-L.

4.    Program counter: It is a 16-bit register used to store the memory address location of the next instruction to be executed.

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

6.    Temporary Register: It is an 8-bit register, which holds either 0 or 1 depending upon the result stored in the accumulator.

 System Bus in microprocessor 8085

System Bus is a group of communication lines to carry information bits. Different components of a microprocessor system are organized around bus. 

The 8085 microprocessor has three buses: address bus, data bus and control bus.

Address Bus: A collection of wires used to identify location in main memory is called Address Bus. It is a group of 16 lines generally marked as A0 to A15. It is unidirectional which flow from microprocessor to Input Output devices. The address bus carries address bits. It is used to identify IO peripheral or a memory location.

Data Bus: A collection of wires through which data is transmitted from one part of a computer to another is called Data Bus. It is a group of 8 lines used for data flow generally mark as D0 to D7. These lines are bidirectional. This bus connects all the computer components to the CPU and main memory. Data flow in both directions between microprocessor and memory. The data bus is used to transfer binary information.

   2⁸ = 256

Control Bus: The connections that carry control information between the CPU and other devices within the computer is called Control Bus.

      i.          Memory read

    ii.          Memory writes

  iii.          I/O read

  iv.          I/O write

  v.     Opcode Fetch (MOV AH, AL)