Introduction to 8085 Microprocessor

1. EE-16102: MICROPROCESSOR & COMPUTER ORGANIZATION
Dr. Vishal Kumar Gaur
Lecture: Introduction

2. Text Books:
1. Ramesh S. Goankar, “Microprocessor Architecture, Programming and Applications with 8085”, 5th Edition,
Prentice Hall.
2. SSP Rao, “Microprocessors and Interfacing”, 3rd edition, McGraw Hill India Education Private Ltd.
2
Books
EE-16102: Microprocessor & Computer Organization

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Overview of Microprocessor
 The word “Microprocessor” comes from the combination of micro and processor.
 Processor means a device that processes whatever. In this context, processor means a
device that processes numbers, specifically binary numbers, 0’s and 1’s.
 Process means perform certain operations on numbers that depend on the microprocessor
application.

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Overview of Microprocessor
 Micro is a new addition
 In the late 1960’s, processors were built using discrete elements.
 These devices perform the required operation, but were too large and too slow.
 In the early 1970’s, the microchip (Integrated Circuits) was invented. All of the
components that made up the processor were now placed on a single piece of silicon.
 The size became several thousand times smaller and the speed became several hundred
times faster. The “Micro”Processor was born.

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Microprocessor and Basic Computer or Programmable Machine
 Definition: The microprocessor is a programmable, clock-driven, register based electronic
device that takes input as numbers, performs on them arithmetic or logical operations according
to the program stored in memory and then produces other numbers as a result.
 Programmable: Perform different set operation on the data depending on the sequence of instructions
supplied by the programmer
 Clock Driven: Whole task is divided into basic operations, are divided into precise system clock periods
 Register-based: Storage element
 Electronic device: Fabricated on a chip

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Microprocessor and Basic Computer or Programmable Machine
 Definition: The microprocessor is a programmable device that takes input as numbers, performs
on them arithmetic or logical operations according to the program stored in memory and then
produces other numbers as a result.
 Computer: A microprocessor combined with memory and
input/output devices forms a basic computer model.
 It reads binary instructions from a storage device called
memory, accepts binary data as input and processes data
according to those instruction, and provides results as output.
Microprocessor
Memory
Input
Output
Figure: Basic model of computer

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 Basic components of a microcomputer are;
1. Microprocessor: CPU and Clock Generator
2. Memory: Program Memory and Data Memory
3. Ports: Input and Output Ports
Input
Output
CPU
Microprocessor
Clock
Generator
Date Memory
Program Memory
Microprocessor and Basic Computer or Programmable Machine
Figure: Basic model of computer

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1A. CPU: Central Processing Unit
 CPU consists of ALU (Arithmetic and Logic Unit), register unit and control unit.
Microprocessor and Basic Computer or Programmable Machine

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Components of Microcomputer or Programmable Machine
1A. CPU: Central Processing Unit
 CPU consists of ALU (Arithmetic and Logic Unit), register unit and control unit.
 ALU: This unit performs computing functions on m-bit data (‘m’ is bit size of processor).
 These functions are arithmetic operations such as addition, subtraction and logical
operation such as AND, OR, XOR, rotate, compare etc.
 Results are stored either in registers or in memory or sent to output devices.

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Components of Computer or Programmable Machine
1A. CPU: Central Processing Unit
 CPU consists of ALU (Arithmetic and Logic Unit), register unit and control unit.
 Register Unit: It contains various 8-bit or 16-bit registers. These registers are used
primarily to store data temporarily during the execution of a program.
 Some of the registers are accessible to the user through instructions. It means their contents
can be read and/or changed through instructions.
 Some of the registers are not accessible to user but they are used by the processor for the
execution of an instruction.
 8085 A microprocessor contains 8 bit registers such as Accumulator (Reg A), B, C, D, E,
H, L, etc and 16 bit registers such as Program Counter (PC), Stack Pointer (SP).

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1A. CPU: Central Processing Unit
 CPU consists of ALU (Arithmetic and Logic Unit), register unit and control unit.
 Control Unit: It provides necessary timing & control signals required for the operation of
computer.
 It controls the flow of data between the microprocessor and peripherals (input, output &
memory).
 The control unit gets a clock signal which determines the speed of the microprocessor.
Components of Computer or Programmable Machine

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1A. CPU: In all, the CPU has the following basic functions:
 It fetches an instructions word stored in memory.
 It decodes the instruction to determine what the instruction is telling it to do.
 It executes the instruction. Executing the instruction may include some of the following major
tasks:
i. Transfer of data from one register to another register in the CPU itself.
ii. Transfer of data between a CPU register & specified memory location or input/output device.
iii. Performing arithmetic and logical operations on data from a specific memory location or a designated CPU register.
iv. Directing the CPU to change the sequence of fetching instructions, if processing the data created a specific condition.
 It looks for control signal such as interrupts and provides appropriate responses.
 After processing the data as per the instruction executed, stores the result in data memory or send
it to an output device.
Components of Computer or Programmable Machine

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1B. Clock Generator
 Operations inside the microprocessor as well as in other parts of the microcomputer are usually
synchronous by nature.
 This is done so that events in different parts of the system can proceed in a systematic fashion.
 The clock needed to perform this synchronous operation is provided by the clock generator.
 The clock generator generates the appropriate clock periods during which instruction executions
are carried out by the microprocessor.
Components of Computer or Programmable Machine

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2. Memory:
 Program Memory and Data Memory
 The basic task of a computer system is to ensure that its CPU executes the desired instructions
sequence i.e., the program properly.
 The instructions sequence is stored in the program memory.
 In typical processor based system, the program to be executed is fixed one which does not change.
 Therefore these programmes are stored in non-volatile memory such as ROM.
Components of Computer or Programmable Machine

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2. Memory:
 Program Memory and Data Memory
 A computer/CPU/micorprocessor works according to the algorithm given by the instruction in the
program in the program memory.
 These instructions may require intermediate results to be stored.
 The functional block used for this storage is the data memory.
 Microprocessors also have a small amount of memory in the form of internal registers which can also
be used if available for such storage. External data memory is needed if the storage requirement is
more.
Components of Computer or Programmable Machine

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3. Input/Output Ports:
 The input & output ports provide the capability to communicate with the outside world.
 The user can enter instruction (i.e. program) and data in memory through input devices such as
keyboard, or simple switches, etc.
 Computers are also used to measure and control physical quantities like temperature, pressure, speed
etc.
 For these purposes, transducers are used to convent physical quantitise into proportional electrical
signals. A/D converters are used to convert electrical signals into digital signals for input.
 The results are sent to the output devices e.g. LED, CRT, D/A converters, printers etc.
 These I/O devices are called peripherals.
Components of Computer or Programmable Machine

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Bussed Architecture of Microprocessor
 Bus: This is group of parallel lines that connect two or more devices.
 Computer contains three buses which carry all the address, data, and control information
involved in program execution.
 These buses connect the microprocessor to other elements- memory and I/O devices so that
transfer of information between the microprocessor and any of the elements can take place.

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Bussed Architecture of Microprocessor
1. Address Bus: (16 bit long in 8085A)
 The 8085 A microprocessor can generate 216 or 65,536 different addresses on this bus A memory location or an I/O
device can be represented by each one of these addresses.
 The processor uses the address bus to identify an I/O device or memory.
 The processor always selects the device for data transfer by putting the address of the device on the address bus.
 On the address bus, information (address) flow takes place only in one direction, i.e., from the microprocessor to the
memory or I/O devices.

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Bussed Architecture of Microprocessor
2. Data Bus: (8 bit long in 8085A)
 A set of data lines referred to as the data bus is shared by number of devices to transfer data between microprocessor and
peripherals. Basically, the data bus is a group of 8 lines used for data flow.
 The data can flow in both directions, i.e., to or from the microprocessor. Therefore, this is called bidirectional data bus
(BDB).
 In Intel 8085 A microprocessor lower 8 bits of the address (A7 - A0) are time multiplexed with the 8 bit data (D7 - D0)
and, therefore, this bus is called AD bus (AD7 - AD0).

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Bussed Architecture of Microprocessor
3. Control Bus:
 The control bus is used to send out or receive timing and control signals in order to coordinate and regulate its operation
and to communicate with other devices.
 Some of the signals of the control bus are issued by the processor and some of the signals are
 received by the processor Therefore, the control bus is called bidirectional control bus (BCB

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Computer Language
 Each machine has its own set of instructions based on the design of its microprocessor.
 To communicate with the computer one must give instruction in binary language or machine
language the form in which it is stored in memory, i.e, as patterns of 1s & 0s.
 Since it is difficult for most users to write programs in machine language, computers
manufactured have developed English-like words to represent the binary instructions of a
microprocessor. e.g. ADD, SUB or JMP etc.
 Users can write programs, called assembly language programs (ALP), using these words called
mnemonics.
 Special programs are available for each type of microprocessor that converts their assembly
language programs to the equivalent machine codes. These programs are called assemblers.

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Computer Language
 Because an assembly language is specific to a given machine, programs written in assembly
language are not transferable from one machine to another.
 To circumvent this limitation, such general purpose languages as BASIC, FORTRAN, PASCAL,
PL/M, C, have been devised, a program written in these languages are called high level
languages (HLL).
 The programmes written in HLL are converted to machine language by another program called
compiler or interpreter.

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Computer Language
Nibble
Or Small
Word
4 bit
Byte 8 bit
Medium
Word
16 bit
Long word 32 bit
Data Size or Word

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Computer Language
 Bit: a binary digit, 0 or 1
 Byte: a group of 8 bits
 Nibble: a group of 4 bits
 Word: a group of bits the computer recognizes and process at a time
 Instruction: a command in binary that is recognized and executed by the computer to accomplish a task
Some instructions are designed with one word, and some require multiple words.
 Mnemonic: a combination of letters to suggest the operation of an instruction
 Program: a set of instructions written in a specific sequence for the computer to accomplish a given
task
 Machine Language: the binary medium of communication with a computer through a designed
 set of instructions specific to each computer

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Computer Language
 Assembly Language a medium of communication with a computer in which programs are written in
mnemonics An assembly language is specific to a given computer.
 Assembler a computer program that translates an assembly language program from mnemonics to the
binary machine code of a computer.
 Manual Assembly a procedure of looking up the machine codes manually from the instruction set of a
computer and entering those into the computer through a key board.

26. Thank You
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