1.1 Architecture of 8085 | unit 1 architecture of

MICRO

Unit 1

8085

1.1 Architecture of 8085

8085 consists of the following functional units −

Accumulator

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

Arithmetic and logic unit

The arithmetic and logic unit performs arithmetic and logical operations like Addition, Subtraction, AND, OR, etc. on 8-bit data.

General purpose register

8085 consists of six general purpose registers in 8085 processor that is B, C, D, E, H & L. Each register holds 8-bit data. These registers work in pair to hold 16-bit data and their pairing combination is like B-C, D-E & H-L.

Program counter

It is a 16-bit register used to store the memory address location of the next instruction to be executed. Microprocessor increments the program whenever an instruction is being executed, so that the program counter points to the memory address of the next instruction that is going to be executed.

Stack pointer

It is 16-bit register which works like stack, which gets incremented/decremented by 2 during push & pop operations.

Temporary register

It is an 8-bit register, which holds the temporary data of arithmetic and logical operations.

Flag register

It is an 8-bit register which holds 0 or 1 depending upon the result stored in the accumulator.

These are : −

Sign (S)

Zero (Z)

Auxiliary Carry (AC)

Parity (P)

Carry (C)

Instruction register and decoder

It is an 8-bit register. When an instruction is fetched from memory then it is stored in the Instruction register. Instruction decoder decodes the information present in the Instruction register.

Timing and control unit

It provides timing and control signal to the microprocessor to perform operations. Following are the timing and control signals, which control external and internal circuits −

Control Signals: READY, RD’, WR’, ALE

Status Signals: S0, S1, IO/M’

DMA Signals: HOLD, HLDA

RESET Signals: RESET IN, RESET OUT

Interrupt control

It controls the interrupts during the process. When the microprocessor is executing the main program and whenever an interrupt occurs, the microprocessor shifts the control from the main program to process the incoming request. After the request is completed, the control returns to the main program.

There are 5 interrupt signals in 8085 microprocessors:

INTR

RST 7.5

RST 6.5

RST 5.5

TRAP.

Serial Input/output control

It controls the serial data communication by using the two instructions: SID (Serial input data) and SOD (Serial output data).

Address buffer and address-data buffer

The content stored in the stack pointer and program counter is loaded into the address buffer and address-data buffer to communicate with the CPU.

The memory and I/O chips are connected to these buses, the CPU can exchange the desired data with the memory and I/O chips.

Address bus and data bus

Data bus carries the data to be stored. It is bidirectional whereas address bus carries the location to where it should be stored, and it is unidirectional. It is used to transfer the data & Address I/O devices.

1.2 Classification of Instructions

The Classification of Instruction Set of 8085 is categorised into five different groups:

Data transfer Instructions

Arithmetic operations.

Logical operations

Branch operations and

Stack, Input/Output and Machine control operations

Data Transfer Operations:

The data transfer instructions load the data into register:

Copy data from register to register,

Copy data from register to memory location, and vice versa.

Therefore, we can say that data transfer instructions copy data from source to destination. The source can be data or contents of register or contents of memory location whereas destination can be register or memory location. These instructions do not affect the flag register of the processor.

Arithmetic Operations:

The arithmetic instructions perform addition, subtraction, increment and decrement operations.

Addition: For addition any 8-bit number or the contents of a register or the contents of a memory location can be added to the contents of the accumulator and the sum is stored in the accumulator. The resulted carry bit is stored in the carry flag.

Two registers cannot be added directly that is the contents of B and C registers cannot be added directly. To add two 16-bit numbers the 8085 provides DAD instruction. It adds the data within the register pair to the contents of the HL register pair and resulted sum is stored in the HL register pair.

Subtraction: 8- bit number or the contents of a register, or the contents of a memory location can be subtracted from the contents of the accumulator and the result is stored in the accumulator. The resulted borrow bit is stored in the carry flag.

Increment/Decrement: The 8085 has the increment and decrement instructions to increment and decrement the contents of any register, memory location or register pair by 1.

Logical Operations:

The logical instructions perform logical, rotate, compare and complement operations.

Logical : For logical instructions, any 8-bit number, or the contents of a register, or of a memory location can be logically ANDed, ORed, or Exclusive-ORed with the contents of the accumulator and the result is stored in the accumulator. The result also affects the flags.

Rotate: These instructions allow shifting of each bit in the accumulator either left or right by 1- bit position.

Compare: 8-bit number, or the contents of a register, or the contents of a memory location is compared for equality, greater than, or less than, with the contents of the accumulator.

Complement: The result of accumulator can be complemented with this. It replaces all 0s by 1s and all 1s by 0s.

Branching Operations:

These instructions allow the 8085 to change the sequence of the program, either unconditionally or under certain test conditions. These instructions include branch instructions, subroutine call and return instructions and restart instructions.

5.Stack, Input/Output and Machine Control Operations:

These instructions control the stack operations, input/output operations and machine operations. The stack instructions allow the transfer of data from register pair to stack memory and from stack memory to the register, pair.

The input/output instruction allows the transfer of 8-bit data to input/output port. Machine instructions control the Machine operations such as interrupt, halt, or do nothing.

1.3 Instruction set of 8085

Data Transfer Instruction

These instructions transfer data between registers or between memory and registers. The instruction is used to copy data from source to destination while copying the contents of source cannot be modified.

For example MOV B,C

MVI B,20H

LXI H, 4020H

LDA 6000H

Arithmetic Instructions

Addition

Subtraction

Increment

Decrement

For example:

ADD C

ADC M

ADI 40H

DAD HL

SUB B

SUI 20H

INR C

Logical Instructions

The logical operations are:

AND

XOR

Rotate

Compare

Complement

For example

CMP C

ANA B

ANI 40H

XRA M

ORA C

RLC

Branching Instructions

These instructions alter the normal sequential flow conditionally or unconditionally

OPCODE	OPERAND	DESCRIPTION
JMP	16-bit address	Jump unconditionally to the given location
JC	16-bit address	Jump if carry
JNC	16-bit address	Jump if no carry
JZ	16-bit address	Jump if zero
JNZ	16-bit address	Jump if no zero
CALL	16-bit address	Call unconditionaly
RET	16-bit address	Return unconditionally
RST	16-bit address	Restart (Software Interrupts)

For example

JMP 2000H

RET

RST 6

Control Instructions

It controls the operation of microprocessor

For example

NOP

SIM

1.4 Introduction to 8051 Microcontroller

8051 is the first most popular microcontroller known as MCS-51. It was introduced by Intel in the year 1981.

Initially it was N-type metal-oxide-semiconductor (NMOS) based microcontroller, but gradually versions were based on complementary metal-oxide-semiconductor (CMOS) technology. So, these microcontrollers were named as 80C51 where C it is based on CMOS technology.

It is an 8-bit microcontroller which means data bus is of 8-bits. Therefore, it can process 8-bits at a time. It is used in wide variety of embedded systems like robotics, remote controls, automotive industry, telecom applications, power tools etc.

8051 Microcontroller

System on a Chip :
It is also referred to as System on a Chip (SoC) microcontroller because it is a chip circuit/integrated circuit that holds many components of a computer together on a single chip which include CPU, memory, input output ports(I/O ports), timers and secondary storage.

Features –
Some of the features include:-

4 KB on-chip ROM (Program memory).

128 bytes on-chip RAM (Data memory).

8- bit data bus (bidirectional).

16- bit address bus (unidirectional).

Two 16-bit timers.

Instruction cycle of 1 microsecond with 12 MHz crystal.

Four 8-bit ports.

128 user defined flags.

Four register banks of 8 bit each.

16-byte bit-addressable RAM.

References

1. Microprocessor Architecture, Programming, and Applications with the 8085 Textbook by Ramesh S. Gaonkar

2. Microprocessor Book by A.P.Godse and D.A.Godse

3. Microprocessor And Microcontroller Book by A.P.Godse

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