Introduction to 8085 Architecture and Programming

Introduction to 8085 Architecture and Programming

Contents

1. Internal architecture of 8085 microprocessor

2. 8085 system bus

3. 8085 pin description.

4. 8085 functional description.

5. Programming model of 8085 microprocessor

6. Addressing modes.

7. Instruction set classification.

8. Instruction format.

9. Sample programs.

Control Unit

        Generates signals within uP to carry out the instruction, 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 logic operation such as ‘add’, ‘subtract’,

‘AND’, ‘OR’, etc. Uses data from memory and from Accumulator to perform

arithmetic. Always stores result of operation in Accumulator.

Registers

       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. They are described briefly as follows.

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 instructions.

Accumulator

          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.

Flags

     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

1. Zero(Z)
2. Carry (CY)
3.  Sign (S)
4.  Parity (P)
5.  Auxiliary Carry (AC) flags

they are listed in the Table and their bit positions in the flag register are shown in the Figure

below. 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 id larger

than eight bits, 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; five bit

positions out of eight are used to store the outputs of the five flip-flops. The flags are

stored in the 8-bit register so that the programmer can examine these flags (data

conditions) by accessing the register through an instruction 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)

         This 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 to 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 instruction 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 addresses of location of the program under execution.

Control Generator

        Generates signals within uP to carry out the instruction 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.

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.

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