Microprocessor 8085 architecture

Microprocessor 8085

Microprocessor 8085 . It is a general purpose 8 bit microprocessor. This microprocessor was first designed in 1977 by intel. They made it using NMOS ( N-channel Metal Oxide Semiconductor ) technology.

The basic configuration of 8085 are –

Data bus of 8 bit
Address bus of 16 bit
Stack pointer of 16 bit
Program counter of 16 bit
Six registers of 8 bit. Arranged in three pair : BC, DE, HL.
It works at 5 V DC supply when operated at 3.2 MHz clock of single phase.

Functional units of microprocessor 8085

The microprocessor 8085 has the following functional units

Arithmetic and logic unit ( ALU )

This function is used to perform all kind of arithmetic and logical operations on 8 bit data. The arithmetic operations include addition, subtraction, division & multiplication. The logical operation includes AND, OR , NOT, etc.

General purpose register

Microprocessor 8085 has 6 general purpose registers. Names as B, C, D, E, H & L. each register has a capacity to store 8 bit data.

These registers can hold 16 bit data by working in pairs. The registers are pared like BC, DE, HL.

Accumulator

Accumulator is an 8 bit register. It is used to perform input output ( I/O ), logical and arithmetic operations. It is connected to Arithmetic Logic Unit ( ALU ) and internal data bus. The accumulator is also known as heart of microprocessor. It is because for most of the operations that are carried by different instructions are stored in accumulator after performance.

Program counter

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

Temporary register

Temporary register is a 8 bit register which holds the data of arithmetic and logical operations temporarily.

Flag register

Flag register is a 8 bit register having 5 flip flops of 1 bit. These flip flops hold digital value, either 0 or 1 that depends upon the result stored in accumulator.

The five 1 bit flip flops are known as

Sign (S) flag
Zero (Z) flag
Auxiliary Carry (AC) flag
Parity (P) flag
carry (CY) flag

The bit position is shown in the table below –

D₇	D₆	D₅	D₄	D₃	D₂	D₁	D₀
S	Z		AC		P		CY

Instruction register and decoder

This register is of 8 bit. An instruction register stores the instruction that is fetched from memory.

Stack pointer

Stack pointer is a 16 bit register that works like stack. This pointer is used for stack related operations like push and pop operations. The address of only one data is required, that is at the top.

Timing and control unit

The microprocessor gets timing and control signals from this unit.

The timing and control signals are –

Control Signals – RD’, WR’, ALE, READY

DMA Signals – HLDA, HOLD

Status Signals – IO/M’, S₀, S₁

Reset Signals – RESET OUT, RESET IN

Serial input/output control

It uses two instructions – SOD ( Serial Output Data ) & SID ( Serial Input Data ) to control serial data communication.

Interrupt control

Interrupt control signal controls the interrupts during processing. Whenever an interrupt occurs in a microprocessor during executing a main program, it shifts the control from main program to process the incoming interrupt request. The control again goes back to main program after the request is completed.

Address bus and Data bus

Address bus	Data bus

1. Address bus carries the location of data where it should be stored. It is used to transfer the data and address of I/O devices.	1. Data bus carries the data that is to be stored.
2. It is unidirectional.	2. it is bidirectional.