8085 Microprocessor Contents

·         General definitions

·         Overview of 8085 microprocessor

·         Overview of 8086 microprocessor

·         Signals and pins of 8086 microprocessor

The salient features of 8085 µp are:

·         It is a 8 bit microprocessor.

·         It is manufactured with N-MOS technology.

·         It has 16-bit address bus and hence can address up to 216 =65536 bytes (64KB) memory locations through A0-A15.

·         The first 8 lines of address bus and 8 lines of data bus are multiplexed AD0 – AD7.

·         Data bus is a group of 8 lines D0 – D7.

·         It supports external interrupt request.

·         A 16 bit program counter (PC)

·         A 16 bit stack pointer (SP)

·         Six 8-bit general purpose register arranged in pairs: BC, DE, HL.

·         It requires a signal +5V power supply and operates at 3.2 MHZ single phase clock.

·         It is enclosed with 40 pins DIP (Dual in line package).

Overview of 8085 microprocessor

8085 Architecture

·         Pin Diagram

·         Functional Block Diagram

Instruction Set

8085 instruction set consists of the following instructions:

·         Data moving instructions.

·         Arithmetic – add, subtract, increment and decrement.

·         Logic – AND, OR, XOR and rotate.

·         Control transfer – conditional, unconditional, call subroutine, return from subroutine and restarts.

·         Input/Output instructions.

·        Other – setting/clearing flag bits, enabling/disabling interrupts, stack operations, etc.

Addressing mode

Register – references the data in a register or in a register pair.

Register indirect – instruction specifies register pair containing address, where the data is located.

Direct, Immediate – 8 or 16-bit data.

Advantages of 8086 over 8085 are:

1. pipelining is employed making the execution faster.

2. data bus width increased to 16 bits

3. higher memory of 1MB.

4. some instructions such as MUL or DIV are available for multiplication and division.

5. increased instruction set making the programming easier.

·         EU executes instructions from the instruction system byte queue.

·         Both units operate asynchronously to give the 8086 an overlapping instruction fetch and execution mechanism which is called as Pipelining. This results in efficient use of the system bus and system performance.

·         BIU contains Instruction queue, Segment registers, Instruction pointer, Address adder.

·         EU contains Control circuitry, Instruction decoder, ALU, Pointer and Index register, Flag register.