Internal Architecture of 8085 Microprocessor ? Control unit , Arithmetic Logic Unit , Registers

Introduction to 8085

Architecture and Programming

Microprocessor The microprocessor is a programming integrated device that has computing and decision – making capability similar to that of the central processing unit (CPU) of a computer.

Internal Architecture of 8085 Microprocessor

Control unit

It generates  signals within up to carry out the instructions, which has been decoded, in reality causes certain connections between blocks of the up to be opened or closed, so that data goes where it is required, and so that ALU operations occur.

Arithmetic Logic Unit

The ALU performs the actual numerical and login operation such as add, subtract, AND, OR, etc. it uses data from memory and from accumulator to perform arithmetic operations. It always stores result of operations in Accumulator.


The 8085/8080A – programming model includes six registers, one accumulator, and one flag register. As shown in figure. In addition, it has two 16-bit registers : the stack pointer and the program counter.

The 8085/8080A has six general purpose registers to store 8-bit data, these are identified as B,C, D, E, H and L as shown in the figure. They can be combined as register pairs BC, DE and HL to perform some 16-bit operations. The programmer can use these registers to store or copy data into the registers by using data copy instruction.


The accumulator is an 8-bit register that is a part of 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 accumulator is also identified as register A.


The ALU includes five flip-flops, which are set or reset after an operation according to data conditions of the result in the accumulator and other registers. They are called Zero(Z), Carry (CY), sign (S) parity (P) and Auxiliary Carry (AC) flags. The most commonly used flags are Zero, Carry and Sign. The microprocessor uses these flags to test data conditions.

For example, after an addition of two numbers, if the sum in the accumulator is larger than 8 bit, the flip-flop uses to indicate a carry called the carry flag (CY) – is set to one. When an arithmetic operation results in zero, the flip-flop called the Zero (Z) flag is set to one. The first figure shows an 8-bit register, called the flag register, adjacent to the accumulator. However, it is not used as a register, 5 bit positions out of eight are used to store the outputs of the five flip-flops. The flags are stored in the 8-bit (data conditions) by accessing the register through an instruction. These flags have critical importance in the decision-making process of the microprocessor. The conditions (set or reset) of the flags are tested through the software instructions. For example, the instruction JC (jump on Carry) is implemented to change the sequence of a program when CY flag is set. The thorough understanding of flag is essential in writing assembly language programs.

Program Counter (PC)

The 16-bit register deals with sequencing the execution of instructions. This register is a memory pointer. Memory locations have 16 bit addresses and that is why this is a 16- bit register.

The microprocessor uses this register to sequence the execution of the instructions. The function of the program counter is to point to the memory address from which the next byte is to be fetched. When a byte (machine code) is being fetched, the program counter is incremented by one to point 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 16-bit address in the stack pointer. The stack concept is explained in the chapter stack and subroutines.

Instruction Register/Decoder

Temporary store for  the  current  instructions of a  program. Latest instruction sent here from memory prior to execution. Decoder then takes instruction and decodes or interprets the instruction. Decoded instruction then passed to next stage.

Memory Address Register

Holds address, received from PC of next program instruction. Feeds the address bus with address  of  location  of the  program  under  execution.

Control Generator

It generates signal within up to carry out the instructions which have been decoded. In reality causes certain connections between blocks of the up to be opened or closed, so that data goes where it is required, and so that ALU operations occur.

Register Selector

This block controls the use of the register stack in the example. Just a logic circuit which switches between different registers in the set will receive instructions from control unit.

General Purpose Registers

Up requires extra registers for versatility. It can be used to store additional data during a program. More complex processors may have a variety of differently named registers.


How does the up know what an instruction means, especially when it is only a binary number? The microprogram in a up/uC is written by the chip designer and tells the up/uc the meaning of each instruction, up/uc can then carry out operation.

  1. 8085 System Bus

Typical system uses a number of buses, collection of wires which transmit binary numbers, one bit per wire. A typical microprocessor communicates with memory and other devices (input and output) using three buses; Address bus, data bus and control bus.

Address Bus

One wire for each bit, therefore 16 bit = 16 wires. Binary number carried alerts memory to open the designed box. Data (binary) can then be put in or taken out. The address bus consists of 16 wires, therefore 16 bit. Its width is 16 bit. A 16 bit binary number allows 216 different numbers or 32000 different numbers, i.e., 0000000000000000 upto 1111111111111111. Because memory consists of boxes, each with a unique address, the size of the address bus determines the size of memory, which can be used. To communicate with memory the microprocessor sends an address on the address bus, e.g. 0000000000000011 (3 in decimal) to the memory. The memory selects box number 3 for reading or writing data. Address bus is unidirectional, i.e., numbers only sent from microprocessor to memory, not by other way.

The memory is organized in groups of 8 bit per location, therefore, how many locations must you be able to specify?

Data bus

Data bus carries data in binary form between up and other external units, such as memory. Typical size is 8 or 16 bit. Size determined by size of boxes in memory and up size helps determine performance of up. The data bus typically consists of 8 wires. Therefore, 28 combinations of binary digits. Data bus used to transmit data, i.e., information, results of arithmetic, etc between memory and the microprocessor. Bus is bidirectional. Size of the data bus determines what arithmetic can done. If only 8 bit wide then largest number is 11111111 (255 in decimal). Therefore, larger number have to be broken down into chunks of 255. This slows microprocessor. Data bus also carries instructions from memory to the microprocessor. Size of the bus therefore limits the number of possible instructions to 256, each specified by a separate number .