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RIFT VALLEY INSTITUTE OF SCIENCE AND TECHNOLOGY- RVIST

DEPARTMENT OF ELECTRICAL AND ELECTRONIC ENGINEERING – EEE

CEEP/CEET MODULE II- 2015

COURSE TITLE: MICROELECTRONICS

Lecture notes

By

Dr. Cliff Orori Mosiori

(Ph.D- Electronics, M.Sc- Electronics, B.Ed –Physics)

©2015

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Table of Contents INTRODUCTION ....................................................................................................................... 3

Evolution of Computers ............................................................................................................... 4

Computer Generations .................................................................................................................. 5

COMPUTER STRUCTURES ...................................................................................................... 6

BASIC MICROPROCESSOR SYSTEMS.................................................................................... 8

The Microprocessor Architecture.............................................................................................. 8

The Microprocessor .................................................................................................................. 9

A microprocessor system ........................................................................................................ 10

The physical appearance of a microprocessor ......................................................................... 11

Terms used in microprocessors ............................................................................................... 12

Internal Structure of microprocessor ....................................................................................... 13

Microprocessor Families ........................................................................................................ 15

The Microprocessor Components ........................................................................................... 15

1. The Control Unit ......................................................................................................... 16

2. Arithmetic Logic Unit (ALU)...................................................................................... 17

3. Registers ..................................................................................................................... 18

4. BUSES ....................................................................................................................... 20

Interfacing Devices ................................................................................................................ 23

8085 Microprocessor .............................................................................................................. 25

8085 System Bus .................................................................................................................... 28

8085 Pin description ............................................................................................................... 30

8085 Functional Description ................................................................................................... 34

The 8085 Programming Model ............................................................................................... 37

The 8085 Addressing Modes .................................................................................................. 39

Arithmetic Operations ............................................................................................................ 41

Types of Memory ................................................................................................................... 45

Secondary Memory ................................................................................................................ 46

Programming ............................................................................................................................. 49

Check next attachment for programming notes on: ..................................................................... 49

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INTRODUCTION A computer is defined as a machine that is capable of taking in raw data (input) and

processing it into more meaningful output (information) under definite instructions

(programs) and stores the results for future use. Therefore a computer has four basic

operations or functions:

Input

Processing

Output

Storage

Definition of Terms in Computers

1. Data:

A collection of unorganized facts including words, symbols, numbers, images, sounds

etc with little or no meaning.

2. Data Processing:

The manipulation of data to give information by arithmetic (addition, subtraction,

multiplication, division etc) or logical operations (=, <, >, , , , etc)

3. Programs:

These are the step by step instructions used by the computer in data processing.

4. Information:

The finished product of data processing that has meaning and is useful for decision

making now and in future.

5. Hardware:

These are the tangible mechanical, electrical and electronic parts of a computer system.

6. Software:

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These are the series of instruction that tell hardware what to do and how to do it.

Programs are part of software but not synonymous.

Evolution of Computers Computers have evolved from simple to complex. The ABACUS was used by the Chinese

and Japanese about 500BC. The slide rule was developed in 1632 and assisted in arithmetic

operations of addition and subtraction. Wilhem Schickard made the first mechanical

machine in 1632 which could add and subtract automatically. In 1647, Blaise Pascal

developed the ‘Mechanical Calculator’ which could add, subtract and convert sums of

money between the different coinages of the time. His intention was to ease the burden on his

father who had the task of reorganizing finances and taxes. G. W. Leibnitz made the

‘Leibnitz Stepped Rocker’ which could add, subtract, multiply, divide and extract square

roots.

Start of the Computer Age

Charles Babbage in 1832 developed the ‘Analytic Engine’ which could deal with

arithmetic problems and make decisions based on the results. Babbage is also called the

‘Father of the Modern Computer’ because he:

i. Established the architecture of the universal computer i.e. Arithmetic Logic

Unit (ALU), control unit, Input and output units.

ii. Used perforated cards/punched cards to communicate with the machine of

which it is still important today.

iii. Made a digital machine thus showing the way to later computers.

NB: His insights were remarkable but unfortunately his plans were too ambitious for the

technology of his time thus his machine was never fully completed as per his ideas.

Herman Hollerinth made a card punching and reading machine to help in the USA’s

census of 1890. He also formed a company better known today as the International Business

Machines (IBM). 1906, Lee De Forest developed the ‘Thermionic/Vacuum tube’ which

helped in later computer electronic circuits. 1937 to 1944 the Mark I was developed by

Howard Aiken with IBM’s support. It was electromechanical and used relays and switches.

In 1946 the Electronic Numerical Integrator And Calculator (ENIAC) was built for use

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in World War two (WWII). It weighed 30 tones, had 18,000 vacuum tubes, occupied a 50 x

30 foot space and consumed 160 KW of power. 1947, W. Shockley, J. Bardeen and W.

Brattain invented the transfer of resistance device eventually called the transistor which

revolutionized computer development. In the late 1950’s, the integrated circuit (IC) was

developed paving way for the microprocessor developed in 1971 by Dr. Ted Hoff of Intel

Corporation who made the Intel 4004 microprocessor. The development of the computer

then became too fast and great.

Computer Generations The First Generation (1946 – 1956):

Mainly used the vacuum tubes which were short-lived consumed a lot of power and

generated a lot of heat. Speeds of up to 10K instructions per second and memories of 2KB

were reached. Modern computer architecture was developed by John Von Newman in 1945.

The Second Generation (1957 – 1963):

Relied on the transistor which is much more stable, lasts longer, consumes less power and

produces less heat compared to the vacuum tube. Magnetic core memories were developed

with up to 32KB of RAM. Speeds of 200K to 300K instructions per second were reached.

The Third Generation (1964 – 1979):

Relied on the integrated circuit (IC) which is made of combining many transistors, diodes

and other electronic semiconductor components in one ceramic or plastic package. The

magnetic disk memory was developed with up to 2MB of RAM. Speeds increased to five

million instructions per second (5MIPS). The first microcomputer was developed in 1974.

The Fourth Generation (1979 – 1989):

IC’s developed from the earlier Small Scale Integration (SSI) i.e. 1 – 10 transistors and

Medium Scale Integration (MSI) i.e. 10 – 100 transistors, to the Large Scale Integration

(LSI) i.e. 100 – 2000 transistors and Very Large Scale Integration (VLSI) i.e. 200,000 –

400,000 transistors per IC. Memories using magnetic disks, bubble memories and optical

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disks (CD-ROM’s) were developed thus several hundred MB’s. Speeds up to 50MIPS were

reached. The Personal Computer (PC) was developed by the IBM Company.

The Fifth Generation (1990 – Present):

Merging of the computer and telecommunications was done with large scale computer

networking (internet) turning the world into a global village. Supercomputers with speeds

between one gigahertz (1GHz) to one Terahertz (1THz) have been developed. Computer

technology is changing by the day having found usage in virtually all human activities.

COMPUTER STRUCTURES The computer consists of two main parts:

Hardware

Software

The relationship and parts of these structures in figure 1:

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The hardware parts consist of two main physical parts:

(i) System Unit:

This includes the mechanical, electrical and electronic parts that make up the main

computer parts i.e. the power supply unit, microprocessor system (motherboard and

CPU), the hard disk, the floppy drive(s), CD-ROM drive(s), zip drive and the casing

structure. The system unit can be an all in one, desktop or tower type.

NB: The CPU is just a part of the system unit on the motherboard and should not be

confused with the system unit.

(ii) Peripherals:

This consists of all input, output and other storage devices connected to the system

unit. Input devices include the keyboard, pointing devices(e.g. mouse, track ball,

light pen, touch screen, touch/track pad, pointing stick, joystick, pen input, pen

computing, graphics tablet, etc), scanning and reading devices (e.g. OCR, OMR,

MICR, POS terminals, scanner, etc), direct input from instruments (e.g. MIDI,

Computer Numerical Control (CNC) etc), video, audio and digital camera. The

output devices include printers, plotters, monitor/VDU, speakers, robots etc while

the other storage devices include USB flash disks, external drives, etc).

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Key:

Commands or

Signal flow

(Control bus)

Data or Instruction

Flow (data bus)

The components of computer hardware include: Central Processing Unit (CPU); Storage;

Input and Output.

All microcomputers are fully dependent on their CPU and the buses hence the term the Bus

Oriented System. The same block diagram above in this case is now drawn to show the

relationship between the parts as represented by the buses and the CPU.

BASIC MICROPROCESSOR SYSTEMS

The Microprocessor Architecture

The microprocessor was born

In 1971 two companies in the USA, introduced the world to its future by producing

microprocessors. They were a young company called Intel and their rival, Texas

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Instruments. The microprocessor, and its offspring, the microcontroller, were destined to

infiltrate every country, every means of production, and almost every home in the world.

There is now hardly a person on the planet that does not own or know of something that is

dependent on one of these devices. Yet curiously, so few people can give any sort of

answer to the simple question ‘What is a microprocessor?’ and ‘How does it work?’

The Microprocessor A microprocessor incorporates the functions of a computer's central processing unit

(CPU) on a single integrated circuit (IC) or at most a few integrated circuits. All modern

CPUs are microprocessors making the micro- prefix redundant. The microprocessor is a

multipurpose, programmable device that accepts digital data as input, processes it

according to instructions stored in its memory, and provides results as output. It is an

example of sequential digital logic, as it has internal memory. Microprocessors operate on

numbers and symbols represented in the binary numeral system.

The integration of a whole CPU onto a single chip or on a few chips greatly reduced the

cost of processing power. The integrated circuit processor was produced in large numbers

by highly automated processes, so unit cost was low. Single-chip processors increase

reliability as there are many fewer electrical connections to fail. As microprocessor

designs get faster, the cost of manufacturing a chip generally stays the same. Before

microprocessors, small computers had been implemented using racks of circuit boards

with many medium- and small-scale integrated circuits. Microprocessors integrated this

into one or a few large-scale ICs. Continued increases in microprocessor capacity have

since rendered other forms of computers almost completely obsolete with one or more

microprocessors used in everything from the smallest embedded systems and handheld

devices to the largest mainframes and supercomputers.

A system

The word ‘system’ is used to describe any organization or device that includes three

features. A system must have at least one input, one output and must do something, i.e. it

must contain a process. Often there are many inputs and outputs. Some of the outputs are

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required and some are waste products. A wide range of different devices meets these

simple requirements. For example, a motor car will usually require fuel, water for cooling

purposes and a battery to start the engine and provide for the lights and instruments. Its

process it to burn the fuel and extract the energy to provide transportation for people and

goods. The outputs are the wanted movement and the unwanted pollutants such as gases,

heat, water vapour and noise. The motor car contains other systems within it basic

microprocessor systems and the instruments but thereafter the battery is recharged by the

engine. There must, then, be an electrical system at work, so it is quite possible for

systems to have smaller systems inside or embedded within them. In a similar way, a

motor car is just a part of the transport system.

A microprocessor system A microprocessor has inputs, outputs and a process. The inputs and outputs of a

microprocessor are a series of voltages that can be used to control external devices. The

process involves analyzing the input voltages and using them to ‘decide’ on the required

output voltages. The decision is based on previously entered instructions that are followed

quite blindly, sensible or not.

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Garage door opener

Here is a little task that a simple microprocessor can solve for us. When the woman arrives

in her car, a light signal is flashed at the sensor and only her garage door opens. When the

man arrives home, his car flashes a light signal at the same sensor but this time his garage

door opens but hers remains closed. The cars are sending a different sequence of light

flashes to the light sensor. The light sensor converts the incoming light to electrical

voltage pulses that are recognized by the microprocessor. The output voltage now operates

the electrical motor attached to the appropriate door.

The physical appearance of a microprocessor A microprocessor is a very small electronic circuit typically 1⁄2 inch (12 mm) across. It is

easily damaged by moisture or abrasion so to offer it some protection it is encapsulated in

plastic or ceramic. To provide electrical connections directly to the circuit would be

impractical owing to the size and consequent fragility, so connecting pins are molded into

the case and the microprocessor then plugs into a socket. The size, shape and number of

pins on the microprocessor depend on the amount of data that it is designed to handle.

.

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Terms used in microprocessors

Integrated circuits

An electronic circuit fabricated out of a solid block of semiconductor material.

This design of circuit, often called a solid state circuit, allows for very complex

circuits to be constructed in a small volume. An integrated circuit is also called a

‘chip’.

Microprocessor (µp)

This is the device that you buy: just an integrated circuit on its own, without a

surrounding circuit and applied voltages it is quite useless. It will just lie on your

workbench staring back at you.

Microprocessor-based system

This is any system that contains a microprocessor, and does not necessarily have

anything to do with computing. In fact, despite all the hype, computers use only a

small proportion of all the microprocessors manufactured. Our garage door

opening system is a microprocessor-based system or is sometimes called a

microprocessor-controlled system.

Microcomputer

The particular microprocessor-based systems that happen to be used as a computer

are called microcomputers. The additional circuits required for a computer can be

built into the same integrated circuit giving rise to a single chip microcomputer.

Microcontroller

This is a complete microprocessor-based control system built onto a single chip. It

is small and convenient but doesn’t do anything that could not be done with a

microprocessor and a few additional components. We’ll have a detailed look at

these in a later chapter.

MPU and CPU

An MPU is a MicroProcessor Unit or microprocessor. A CPU is a Central

Processing Unit. This is the central ‘brain’ of a computer and can be (usually is)

made from one or more microprocessors. The IBM design for the ‘Blue Gene’

supercomputer includes a million processors!

Remember:

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*MPU is the thing

*CPU is the job.

Micro-

The word micro is used in electronics and in science generally, to mean ‘one-

millionth’ or 1/10–6. It has also entered general language to mean something very

small like a very small processor or microprocessor. It has also become an

abbreviation for microprocessor, microcomputer, microprocessor-based system or

a micro controller – indeed almost anything that has ‘micro’ in its name. In the

scientific sense, the word micro is represented by the Greek letter µ, (mu). It was

only a small step for microprocessor to become abbreviated to µP.

Internal Structure of microprocessor

The internal arrangement of a microprocessor varies depending on the age of the design

and the intended purposes of the microprocessor. The complexity of an integrated circuit

is bounded by physical limitations of the number of transistors that can be put onto one

chip, the number of package terminations that can connect the processor to other parts of

the system, the number of interconnections it is possible to make on the chip, and the heat

that the chip can dissipate. Advancing technology makes more complex and powerful

chips feasible to manufacture.

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A minimal hypothetical microprocessor might only include an arithmetic logic unit (ALU)

and a control logic section. The ALU performs operations such as addition, subtraction,

and operations such as AND or OR. Each operation of the ALU sets one or more flags in a

status register, which indicate the results of the last operation (zero value, negative

number, overflow, or others). The control logic section retrieves instruction operation

codes from memory, and initiates whatever sequence of operations of the ALU requires to

carry out the instruction. A single operation code might affect many individual data paths,

registers, and other elements of the processor.

As integrated circuit technology advanced, it was feasible to manufacture more and more

complex processors on a single chip. The size of data objects became larger; allowing

more transistors on a chip allowed word sizes to increase from 4- and 8-bit words up to

today's 64-bit words. Additional features were added to the processor architecture; more

on-chip registers sped up programs, and complex instructions could be used to make more

compact programs. Floating-point arithmetic, for example, was often not available on 8-

bit microprocessors, but had to be carried out in software. Integration of the floating point

unit first as a separate integrated circuit and then as part of the same microprocessor chip,

sped up floating point calculations. The microprocessor performs several functions

classified into three categories:

(i) Microprocessor initiated operations

(ii) Internal data operations

(iii) Peripheral (external) initiated operations

In order to achieve this, the microprocessor uses a group of logic circuits and sets of

control signals. Early processors had these circuits in different chips hence the term

Micro Processing Unit (MPU) or Central Processing Unit (CPU). However, later

microprocessors have most of the necessary circuitry in one chip called the

Microprocessor Chip. The microprocessor is defined broadly as a programmable logic

chip. Therefore, if a number of devices or chips are used the term MPU or CPU is used

but if only one chip contains the circuitry needed then microprocessor chip is used. But

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the terms MPU, CPU and microprocessor are used interchangeably/synonymously in

general day to day talk.

The microprocessor is usually found on the motherboard and the two are called the

microprocessor system. The complete set of devices required to make and operated a

computer i.e. microprocessor system (microprocessor and motherboard) and peripherals

(input, output and storage devices) form the microcomputer system.

Microprocessor Families

Microprocessor families depend on the manufacturers:

Intel

Intel has developed the 4004, 8008, 8080 and 8085 (8080A/8085A family), 8088,

80186, 80286, 80386, 80486, Pentium I, Pentium Pro, Pentium with MMXTM,

Pentium II, Pentium II Xeon, Celeron, Pentium III, Pentium III Xeon, Itanium and the

very latest Pentium Dual Core. Their processors are used to power PC’s.

Motorola

Motorola has developed the 6800, 68020, 68030, 68040 and PowerPC. Their processors

are used to power the Apple Macintosh computers.

American Device Manufacturers (AMD)

AMD and Cyrix make Intel-compatible generic microprocessor i.e. they have the same

internal design or architecture as Intel processors and perform the same functions but

often are less expensive.

Cyrix

Zilog Corporation

Zilog Corporation is known for the famous Z80 family of processors used in industrial

applications for process control.

Digital Equipment Corporation (DEC) etc.

The Microprocessor Components All microprocessors have the following:

- Internal buses and control lines

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- Program counter

- Instruction decoding circuitry

- Timing and control section

- Registers

- Bus buffers and latches

- Several controlled inputs and outputs

- Arithmetic Logic Unit (ALU)

- Several functional units e.g. RAM, ROM, Cache memory, serial I/O

lines, internal circuitry, programmable timers, serial to parallel interface

logic, Direct Memory Access (DMA) control logic, Interrupt logic

circuitry, etc.

This list can be reduced to four main functional elements:

(i) Control unit

(ii) ALU

(iii) Registers

(iv) Buses

1. The Control Unit This is the manager of the computer. It executes instructions stored in memory and

controls the other parts and peripherals in the data processing cycle. This is done as per

clock inputs for proper execution and sequencing. Therefore, the control unit is a fetch,

decode and execute part. The control unit does the following:

(i) Fetch instructions from the memory

(ii) Decodes the instructions to determine what the instruction

says/requires

(iii) Gets the data required to carry out the instruction

(iv) Activates the correct devices or electronic circuits for the instruction

to be executed.

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The control unit has the following parts:

(i) Program Manager (PM) or Sequence Control Register (SCR):

It is also referred to as the Timing and Control Unit. It controls the order of

execution of instructions as per the clock input signals. The timing and control

signals are sent to all the parts of the computer to coordinate the execution of

instructions.

(ii) Instruction Register:

This is used to store a copy of the instructions being processed. This ensures that

the instructions are readily available for decoding. For the 8085/8080A family it

is an 8-bit register.

(iii) Instruction Decoder:

It interprets the instructions to determine the type of action required and sends

signals to the control part so that the correct micro program is executed. In the

8080A/8085 family, it is together with the machine cycle encoding circuitry.

2. Arithmetic Logic Unit (ALU) It performs the arithmetic and logical operations for the whole computer system. The

accumulator (also called register A) holds one operand (part of the data being

processed) while the temporary register holds the other operand. The result is stored

in the accumulator which is an 8-bit register for the 8080A and 8085A microprocessors.

The ALU also includes five flip flops that are set or reset according to the data

conditions in the accumulator or other registers. This flip flops are called flag. The

8085A and 8080A has five flags:

(i) Carry (CY) flag

(ii) Zero (Z) flag

(iii) Sign (S) flag

(iv) Parity (P) flag

(v) Auxiliary Carry (AC) flag

S Z X AC X

P

X CY

X – Unused flip-flop

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The flag register is 8-bit but three of them are unused. If an operation results in data

which is more than 8-bits, the carry (CY) flag is set to one i.e. CY = 1. If the result is

zero, the zero (Z) flag is set to one i.e. Z = 1. Therefore, flags are used by the

microprocessor to test the data conditions especially in conditional jump instructions

e.g. JC – Jump on carry instruction is implemented if the carry (CY) flag is set i.e. CY

= 1. The data conditions of the accumulator (register A) and the flag register can be

examined by the instruction Program Status Word (PSW) in the instruction set.

3. Registers These are fast but volatile memories found within the microprocessor. They include:

(i) Accumulator or Register A

(ii) Flags

(iii) Internal Registers

(iv) Program Counter (PC)

(v) Stack Pointer (SP)

(vi) Index Register

Internal Registers or Data registers

The size of this registers gives the amount of data that the processor can operate

on at a time. The 8085A/8080A have six general purpose registers which are

programmable i.e. a programmer can use them to load or transfer data. The six

are B, C, D, E, H and L as shown below. They can also be used while combined

together as register pairs i.e. BC, DE and HL register pairs. The accumulator and

the flag register are included though used solely for ALU operations.

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Address Registers

These are storage parts for memory addresses and include the following parts:

(i) Program Counter (PC)

It contains the address of the next instruction to be executed hence used by the

microprocessor to sequence the execution of instructions. It is therefore referred

to as a memory pointer and is 16-bits because memory locations have 16-bit

addresses. Once an instruction is executed, the program counter is incremented

by one to point to the next instruction’s address.

(ii) Stack Pointer (SP)

It stores the addresses to be executed in order in a group of memory locations

in the data memory called the stack. The beginning of the stack is defined by

loading a 16-bit address in the stack pointer e.g. when an interrupt occurs, the

addresses of the program instructions running are stored in a stack temporarily

as the processor attends to the interrupt.

(iii) Index Register

It is used in accessing a number of memory locations using only one instruction

thus speeds up operations.

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4. BUSES These are sets of communication lines for the various parts of the computer to function.

The microprocessor performs four operations:

(i) Memory Read: reads/copies data from memory

(ii) Memory Write: writes/copies data into memory

(iii) Input/Output (I/O) Read: Accepts data from input devices

(iv) I/O Write: Sends data to output devices

To perform these operations, the microprocessor goes through the following steps:

Step 1 – Identify the peripheral or memory location using its address

Step 2 – Transfer data

Step 3 – Provide timing or synchronisation signals

There are three types of buses each used to perform one of the above steps:

(1) Address Bus

(2) Data Bus

(3) Control Bus

(1) ADDRESS BUS

This is a unidirectional 16- line communication medium usually labelled A0 to

A15 and it only one way i.e. from the microprocessor to the peripherals

(8080/8085). The number of address lines gives the capacity of the

microprocessor to identify different memory locations e.g. 216 = 65,536 memory

locations. (This when rounded off and referred to as 64KB (1KB = 1024 Bytes).

Most 8-bit microprocessors have 16-address lines. The address lines carry the

memory location of data to be acted on by the microprocessor or the location or

register in an input or output device.

(2) DATA BUS

This is a bidirectional (two way) medium for transmission of data to and from the

microprocessor. It consists of 8-lines hence 28 = 256 different numbers. The data bus

determines the word length and register size of a microprocessor hence they are

named as per their data bus.

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Microprocessor

No. of Data

Lines

Word Length Manufacturer

8080A/8085A 8 8-bits Intel

8088 8 8-bits Intel

80186 16 16-bits Intel

80286 16 16-bits Intel

6800 16 16-bits Motorola

Z80 16 16-bits Zilog

Corporation

80386,80486,

Pentium I

32 32-bits Intel

The data lines for an 8-bit microprocessor are usually labelled D0 to D7 i.e. 8-parallel

data lines carrying 8-bits of data or information simultaneously to or from different

parts of the computer. The data bus uses a bidirectional buffer i.e. allows data to flow

in both directions. It is also called the octal bus transceiver.

(3) CONTROL BUS

This are single lines that are unidirectional i.e. one way and do carry command

signals from the control unit to various parts of the computer system. The

microprocessor generates specific control signals for every operation it performs.

There are four main operations:

(i) I/O synchronisation

(ii) Memory Synchronisation

(iii) Microprocessor scheduling

(iv) Control various utilities

(i) Input/Output (I/O) Synchronisation:

Synchronisation is scheduling or sequencing of various activities to ensure that

there is no conflict or collusion and that all operations run smoothly and perfectly.

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Therefore the control unit sends to these devices READ or WRITE signals

together represented as R/W or separately as RD and WR. A read command is to

activate an I/O device to load or input data to the data bus. If it is a dedicated

command it will carry an identifying letter or abbreviation e.g. input/output read is

IOR or memory read is MEMR etc The write command is used to make the

peripheral devices to take or output data to the data bus. It can also be dedicated to

specific devices e.g. IOW, MEMW etc Once an I/O device receives the R/W

signal, it generates an “acknowledge” signal to confirm the reception of the

command or signal.

(ii) Memory Synchronisation:

These are signals used to regulate and sequence the transfer of data from memory

or to memory. The READ, WRITE and READY signals are used. The READY

signal acknowledges data availability and readiness to transfer.

(iii) Microprocessor Scheduling:

These are signals geared towards sequencing the working of various parts of the

microprocessor in the data processing cycle. Therefore, they are microprocessor

internal commands.

(iv) Control Utilities:

These are specific peripheral or externally initiated signals for which individual

pins of the microprocessor are assigned. These are:

(a) Reset

(b) Interrupt

(c) Ready

(d) Hold

(a) Reset

When activated, all internal operations are suspended and the program counter is

cleared. The program execution then begins at the zero memory address.

(b) Interrupt

– the microprocessor can be interrupted from its normal execution of instructions

so as to deal with other instructions called service routines e.g. emergency

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procedures, virus check/scanning etc. the microprocessor resumes its operations

after completing the service routine.

(c) Ready

– if the signal at the ready pin is low, the microprocessor enters a wait state. This is

used to synchronise slower peripheral devices with the microprocessor.

(d) Hold

– When the signal at the hold pin is high, the microprocessor gives up control of the

buses and allows an external peripheral device to use them e.g. during Direct Memory

Access (DMA).

5. POWER BUS

These are actual wires used to supply power to the various parts of the computer. They

can be two (+5V and 0V or +12V and 0V) or four (+12V, +5V, 0V and -5V) depending

on the type of microprocessor.

Interfacing Devices Interfacing devices are necessary to interconnect computer parts/components with

in bus oriented system. Interfacing components include:

(i) Buffer (ii) Decoder (iii) Encoder (iv) Latch

i) Buffers

- this is a logic circuit which amplifies the current or power of a signal. A simple buffer

gives the same output as the input but increases its driving capability hence known as a

driver. Buffers are commonly used to enable/disable the data and address bus and also

increase their signal driving capability.

Tri-state Devices:

These have three states: logic 1, logic 0 and high impedance.

- For (a) if the logic on the enable pin is 1, the device functions as an ordinary inverter. If it is

0, it will go into a high impedance state i.e. as if it were disconnected from the system.

- For (b) it will act as an inverter if the enable pin has logic 0 and go into high impedance if

the enable pin has 1.

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Decoder: -

This is a logic device that identifies each combination of signals present at its input

e.g. if a decoder has two input lines, it will have four output lines (22) since the two

input lines can have four different binary combinations of signals: 00, 01, 10 and 11.

A decoder is a commonly used device in interfacing I/O peripherals and memory.

Also internal decoders are found in memory devices to identify different memory

locations or addresses.

Encoders: -

An encoder is a logic device that provides an appropriate code (i.e. binary,

hexadecimal, decimal etc) as output for each input signal. Its process is the reverse

of decoding.

Latches: -

It is implemented by a D-flip flop. If the clock signal is high, then the logic on D

(input) is passed to Q (i.e. Q follows D if clock signal is high). If the clock signal

goes low, then data in Q is kept until the clock goes high again for any changes

to take place. A latch is mainly used to interface output devices. When the

microprocessor sends an output, data is available on the data bus for only a few

microseconds hence a latch is used to hold the data as the peripheral prepares or

saves it.

25

8085 Microprocessor Internal Architecture of 8085 Microprocessor

Control Unit

Generates signals within μP to carry out the instruction, which has been decoded.

In reality causes certain connections between blocks of the μP 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. it uses data from memory and from Accumulator to

perform arithmetic. Always stores result of operation in Accumulator.

Registers

The 8085-programming model includes six registers, one accumulator, and one

flag register, as shown in Figure above. In addition, it has two 16-bit registers: the

stack pointer and the program counter. They are described briefly as follows. The

8085 has six general-purpose registers to store 8-bit data; these are identified as B,

26

C, D, E, H, and L as shown in the figure above. 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 Zero (Z), Carry (CY), Sign (S), Parity (P), and 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 is

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.

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.

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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 "Stack and Subroutines."

Instruction Register/Decoder

It is the 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. It feeds the address

bus with addresses of location of the program under execution.

Control Generator

Generates signals within μP to carry out the instruction which has been decoded. In

reality causes certain connections between blocks of the μP 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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General Purpose Registers

μP requires extra registers for versatility. Can be used to store additional data

during a program. More complex processors may have a variety of differently

named registers.

Microprogramming

How does the μP knows what an instruction means, especially when it is only a

binary number? The microprogram in a μP/uC is written by the chip designer and

tells the μP/uC the meaning of each instruction μP/uC can then carry out operation.

8085 System Bus Typical system uses a number of busses, 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 busses: Address Bus, Data Bus

and Control Bus.

Address Bus

One wire for each bit, therefore 16 bits = 16 wires. Binary number carried alerts

memory to ‘open’ the designated box. Data (binary) can then be put in or taken

out. The Address Bus consists of 16 wires, therefore 16 bits. Its "width" is 16 bits.

A 16 bit binary number allows 216 different numbers, or 32000 different numbers,

i.e. 0000000000000000 μP to 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, eg 0000000000000011 (3 in decimal), to the

memory. The memory the selects box number 3 for reading or writing data.

Address bus is unidirectional, ie numbers only sent from microprocessor to

memory, not other way.

Question?: If you have a memory chip of size 256 kilobytes (256 x 1024 x 8 bits),

how many wires does the address bus need, in order to be able to specify an

address in this memory? Note: the memory is organized in groups of 8 bits per

location, therefore, how many locations must you be able to specify?

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Data Bus

Data Bus: carries ‘data’, in binary form, between μP and other external units, such

as memory. Typical size is 8 or 16 bits. Size determined by size of boxes in

memory and μP size helps determine performance of μP. The Data Bus typically

consists of 8 wires. Therefore, 28 combinations of binary digits are possible. Data

bus used to transmit "data", ie information, results of arithmetic, etc between

memory and the microprocessor.

Bus is bi-directional. Size of the data bus determines what arithmetic can be done.

If only 8 bits wide then largest number is 11111111 (255 in decimal). Therefore,

larger numbers 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.

Control Bus

Control Buses are various lines which have specific functions for coordinating and

controlling μP operations. Eg: Read/NotWrite line, single binary digit. Control

whether memory is being ‘written to’ (data stored in mem) or ‘read from’ (data

taken out of mem) 1 = Read, 0 = Write. May also include clock line(s) for

timing/synchronising, ‘interrupts’, ‘reset’ etc. Typically μP has 10 control lines.

The Control Bus carries control signals partly unidirectional, partly bi-directional.

Control signals are things like "read or write". This tells memory that we are either

reading from a location, specified on the address bus, or writing to a location

specified. Various other signals to the control and coordinate the operation of the

system.

Modern day microprocessors, like 80386, 80486 have much larger busses.

Typically, a 16 or 32 bit busses, which allow larger number of instructions, more

memory location, and faster arithmetic. Microcontrollers organized along same

lines, except: because microcontrollers have memory etc inside the chip, the busses

may all be internal. In the microprocessor the three busses are external to the chip

30

(except for the internal data bus). In case of external busses, the chip connects to

the busses via buffers, which are simply an electronic connection between external

bus and the internal data bus.

8085 Pin description Properties of 8085 microprocessor include the following;

Single + 5V SμPply

4 Vectored InterrμPts (One is Non Maskable)

Serial In/Serial Out Port

Decimal, Binary, and Double Precision Arithmetic

Direct Addressing Capability to 64K bytes of memory

The Intel 8085A is a new generation, complete 8 bit parallel central processing unit

(CPU). The 8085A uses a multiplexed data bus. The address is split between the 8-bit

address bus and the 8bit data bus. The following describes the function of each pin:

A6 - A1s (Output 3 State)

These are Address Buses. The most significant 8 bits of the memory address or the

8 bits of the I/0 address of which 3 stated during Hold and Halt modes.

AD0 - 7 (Input/Output 3 state)

Are multiplexed Address/Data Bus; Lower 8 bits of the memory address (or I/0

address) appear on the bus during the first clock cycle of a machine state. It then

becomes the data bus during the second and third clock cycles. 3 stated during

Hold and Halt modes.

ALE (Output)

Means the Address Latch Enable: It occurs during the first clock cycle of a

machine state and enables the address to get latched into the on chip latch of

peripherals. The falling edge of ALE is set to guarantee setup and hold times for

the address information. ALE can also be used to strobe the status information.

ALE is never 3 stated.

SO, S1 (Output)

These are Data Bus Status. Encoded status of the bus cycle:

S1 S0

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O O HALT

0 1 WRITE

1 0 READ

1 1 FETCH

This shows that S1 can be used as an advanced R/W status.

RD (Output 3state)

READ; indicates the selected memory or 1/0 device is to be read and that the Data

Bus is available for the data transfer.

WR (Output 3state)

WRITE; indicates the data on the Data Bus is to be written into the selected

memory or 1/0 location. Data is set μP at the trailing edge of WR 3stated during

Hold and Halt modes.

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READY (Input)

If Ready is high during a Read or Write cycle, it indicates that the memory or

peripheral is ready to send or receive data. If Ready is low, the CPU will wait for

Ready to go high before completing the read or write cycle.

HOLD (Input)

HOLD; indicates that another Master is requesting the use of the Address and Data

Buses. The CPU, upon receiving the Hold request will relinquish the use of buses

as soon as the completion of the current machine cycle. Internal processing can

continue. The processor can regain the buses only after the Hold is removed. When

the Hold is acknowledged, the Address, Data, RD, WR, and IO/M lines are

3stated.

HLDA (Output)

HOLD ACKNOWLEDGE; indicates that the CPU has received the Hold request

and that it will relinquish the buses in the next clock cycle. HLDA goes low after

the Hold request is removed. The CPU takes the buses one half clock cycles after

HLDA goes low.

INTR (Input)

INTERRΜPT REQUEST; is used as a general purpose interrupt. It is sampled

only during the next to the last clock cycle of the instruction. If it is active, the

Program Counter (PC) will be inhibited from incrementing and an INTA will be

issued. During this cycle a RESTART or CALL instruction can be inserted to jump

to the interrupt service routine. The INTR is enabled and disabled by software. It is

disabled by Reset and immediately after an interrupt is accepted.

INTA (Output)

INTERRΜPT ACKNOWLEDGE; is used instead of (and has the same timing as)

RD during the Instruction cycle after an INTR is accepted. It can be used to

activate the 8259 Interrupt chip or some other interrupt port.

RST 5.5

RST 6.5 - (Inputs)

RST 7.5

RESTART INTERRΜPTS

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These three inputs have the same timing as I NTR except they cause an internal

RESTART to be automatically inserted.

RST 7.5 ~~ Highest Priority

RST 6.5

RST 5.5 o Lowest Priority

The priority of these interrupts is ordered as shown above. These interrupts have a

higher priority than the INTR.

TRAP (Input)

Trap interrupt is a non-maskable restart interrupt. It is recognized at the same time

INTR

It is unaffected by any mask or Interrupt Enable. It has the highest priority of any

interrupt.

RESET IN (Input)

Reset sets the Program Counter to zero and resets the Interrupt Enable and HLDA

flip-flops. None of the other flags or registers (except the instruction register) are

affected The CPU is held in the reset condition as long as Reset is applied.

RESET OUT (Output)

Indicates CPU is being reset. It can be used as a system RESET. The signal is

synchronized to the processor clock.

X1, X2 (Input)

Crystal or R/C network connections to set the internal clock generator X1 can also

be an external clock input instead of a crystal. The input frequency is divided by 2

to give the internal operating frequency.

CLK (Output)

Clock Output for use as a system clock when a crystal or R/ C network is used as

an input to the CPU. The period of CLK is twice the X1, X2 input period.

IO/M (Output)

IO/M indicates whether the Read/Write is to memory or l/O Tri-stated during Hold

and Halt modes.

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SID (Input)

Serial input data line. The data on this line is loaded into accumulator bit 7

whenever a RIM instruction is executed.

SOD (output)

Serial output data line. The output SOD is set or reset as specified by the SIM

instruction.

Vcc - +5 volt suppply.

Vss - Ground Reference.

8085 Functional Description

The 8085A is a complete 8 bit parallel central processor. It requires a single +5 volt

sμPply. Its basic clock speed is 3 MHz thus improving on the present 8080's performance

with higher system speed. Also it is designed to fit into a minimum system of three IC's:

The CPU, a RAM/ IO, and a ROM or PROM/IO chip. The 8085A uses a multiplexed Data

Bus. The address is split between the higher 8-bit Address Bus and the lower 8bit

Address/Data Bus. During the first cycle the address is sent out. The lower 8bits are

latched into the peripherals by the Address Latch Enable (ALE). During the rest of the

machine cycle the Data Bus is used for memory or l/O data. The 8085A provides RD,

WR, and lO/Memory signals for bus control.

An Interrupt Acknowledge signal (INTA) is also provided. Hold, Ready, and all Interrupts

are synchronized. The 8085A also provides serial input data (SID) and serial output data

(SOD) lines for simple serial interface. In addition to these features, the 8085A has three

maskable, restart interrupts and one non-maskable trap interrupt. The 8085A provides RD,

WR and IO/M signals for Bus control.

Status Information

Status information is directly available from the 8085A. ALE serves as a status strobe.

The status is partially encoded, and provides the user with advanced timing of the type of

bus transfer being done. IO/M cycle status signal is provided directly also. Decoded So,

S1 Carries the following status information: HALT, WRITE, READ, FETCH

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S1 can be interpreted as R/W in all bus transfers. In the 8085A the 8 LSB of address are

multiplexed with the data instead of status. The ALE line is used as a strobe to enter the

lower half of the address into the memory or peripheral address latch. This also frees extra

pins for expanded interrupt capability.

Interrupt and Serial l/O

The8085A has 5 interrupt inputs: INTR, RST5.5, RST6.5, RST 7.5, and TRAP. INTR is

identical in function to the 8080 INT. Each of the three RESTART inputs (5.5, 6.5. 7.5)

has a programmable mask. TRAP is also a RESTART interrupt except it is nonmaskable.

The three RESTART interrupts cause the internal execution of RST (saving the program

36

counter in the stack and branching to the RESTART address) if the interrupts are enabled

and if the interrupt mask is not set. The non-maskable TRAP causes the internal execution

of a RST independent of the state of the interrupt enable or masks.

The interrupts are arranged in a fixed priority that determines which interrupt is to be

recognized if more than one is pending as follows: TRAP highest priority, RST 7.5, RST

6.5, RST 5.5, INTR lowest priority This priority scheme does not take into account the

priority of a routine that was started by a higher priority interrupt. RST 5.5 can interrupt a

RST 7.5 routine if the interrupts were re-enabled before the end of the RST 7.5 routine.

The TRAP interrupt is useful for catastrophic errors such as power failure or bus error.

The TRAP input is recognized just as any other interrupt but has the highest priority. It is

not affected by any flag or mask. The TRAP input is both edge and level sensitive.

Basic System Timing

The 8085A has a multiplexed Data Bus. ALE is used as a strobe to sample the lower 8bits

of address on the Data Bus. Figure 2 shows an instruction fetch, memory read and l/ O

write cycle (OUT). Note that during the l/O write and read cycle that the l/O port address

is copied on both the upper and lower half of the address. As in the 8080, the READY line

is used to extend the read and write pulse lengths so that the 8085A can be used with slow

memory. Hold causes the CPU to relinquish the bus when it is through with it by floating

the Address and Data Buses.

System Interface

8085A family includes memory components, which are directly compatible to the 8085A

CPU. For example, a system consisting of the three chips, 8085A, 8156, and 8355 will

have the following features:

· 2K Bytes ROM

· 256 Bytes RAM

· 1 Timer/Counter

· 4 8bit l/O Ports

· 1 6bit l/O Port

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· 4 Interrupt Levels

Serial In/Serial Out Ports

In addition to standard l/O, the memory mapped I/O offers an efficient l/O addressing

technique. With this technique, an area of memory address space is assigned for l/O

address, thereby, using the memory address for I/O manipulation. The 8085A CPU can

also interface with the standard memory that does not have the multiplexed address/data

bus.

The 8085 Programming Model

In the previous tutorial we described the 8085 microprocessor registers in reference to the

internal data operations. The same information is repeated here briefly to provide the

continuity and the context to the instruction set and to enable the readers who prefer to

focus initially on the programming aspect of the microprocessor. The 8085 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:

Registers

The 8085 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 above. 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.

38

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 Zero

(Z), Carry (CY), Sign (S), Parity (P), and Auxiliary Carry (AC) flags; 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 is 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. 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

39

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. This programming model will be used in

subsequent tutorials to examine how these registers are affected after the execution of an

instruction.

The 8085 Addressing Modes The instructions MOV B, A or MVI A, 82H are to copy data from a source into a

destination. In these instructions the source can be a register, an input port, or an 8-bit

number (00H to FFH). Similarly, a destination can be a register or an output port. The

sources and destination are operands. The various formats for specifying operands are

called the ADDRESSING MODES. For 8085, they are:

1. Immediate addressing.

2. Register addressing.

3. Direct addressing.

4. Indirect addressing.

(1) Immediate addressing

Data is present in the instruction. Load the immediate data to the destination provided.

40

Example: MVI R, data

(2) Register addressing

Data is provided through the registers.

Example: MOV Rd, Rs

(3) Direct addressing

Used to accept data from outside devices to store in the accumulator or send the data

stored in the accumulator to the outside device. Accept the data from the port 00H and

store them into the accumulator or Send the data from the accumulator to the port 01H.

Example: IN 00H or OUT 01H

(4) Indirect Addressing

This means that the Effective Address is calculated by the processor. And the contents of

the address (and the one following) is used to form a second address. The second address

is where the data is stored. Note that this requires several memory accesses; two accesses

to retrieve the 16-bit address and a further access (or accesses) to retrieve the data which is

to be loaded into the register.

Instruction Set Classification

An instruction is a binary pattern designed inside a microprocessor to perform a specific

function. The entire groμP of instructions, called the instruction set, determines what

functions the microprocessor can perform. These instructions can be classified into the

following five functional categories: data transfer (copy) operations, arithmetic operations,

logical operations, branching operations, and machine-control operations.

Data Transfer (Copy) Operations

This group of instructions copy data from a location called a source to another location

called a destination, without modifying the contents of the source. In technical manuals,

the term data transfer is used for this copying function. However, the term transfer is

misleading; it creates the impression that the contents of the source are destroyed when, in

41

fact, the contents are retained without any modification. The various types of data transfer

(copy) are listed below together with examples of each type:

Types Examples

1. Between Registers.

1. Copy the contents of the

register B into register D.

2. Specific data byte to a register

or a memory location.

2. Load register B with the

data byte 32H.

3. Between a memory location

and a register.

3. From a memory location

2000H to register B.

4. Between an I/O device and the

accumulator.

4. From an input keyboard to

the accumulator.

Arithmetic Operations These instructions perform arithmetic operations such as addition, subtraction, increment,

and decrement.

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. No two other 8-bit registers can be added directly (e.g., the

contents of register B cannot be added directly to the contents of the register C).

The instruction DAD is an exception; it adds 16-bit data directly in register pairs.

Subtraction

Any 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 results

stored in the accumulator. The subtraction is performed in 2's compliment, and the

results if negative, are expressed in 2's complement. No two other registers can be

subtracted directly.

Increment/Decrement

The 8-bit contents of a register or a memory location can be incremented or

decrement by 1. Similarly, the 16-bit contents of a register pair (such as BC) can

be incremented or decrement by 1. These increment and decrement operations

42

differ from addition and subtraction in an important way; i.e., they can be

performed in any one of the registers or in a memory location.

Logical Operations

These instructions perform various logical operations with the contents of the

accumulator.

AND, OR Exclusive-OR

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.

The results are stored in the accumulator.

Rotate

Each bit in the accumulator can be shifted either left or right to the next position.

Compare

Any 8-bit number or the contents of a register, or a memory location can be

compared for equality, greater than, or less than, with the contents of the

accumulator.

Complement

- The contents of the accumulator can be complemented. All 0s are replaced by 1s

and all 1s are replaced by 0s.

Branching Operations

This group of instructions alters the sequence of program execution either

conditionally or unconditionally.

Jump

- Conditional jumps are an important aspect of the decision-making process in the

programming. These instructions test for a certain conditions (e.g., Zero or Carry

flag) and alter the program sequence when the condition is met. In addition, the

instruction set includes an instruction called unconditional jump.

Call, Return, and Restart

- These instructions change the sequence of a program either by calling a

subroutine or returning from a subroutine. The conditional Call and Return

instructions also can test condition flags.

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Machine Control Operations

These instructions control machine functions such as Halt, Interrupt, or do nothing. The

microprocessor operations related to data manipulation can be summarized in four

functions:

1. copying data

2. performing arithmetic operations

3. performing logical operations

4. testing for a given condition and alerting the program sequence.

Some important aspects of the instruction set are noted below:

1. In data transfer, the contents of the source are not destroyed; only the

contents of the destination are changed. The data copy instructions do not

affect the flags.

2. Arithmetic and Logical operations are performed with the contents of the

accumulator, and the results are stored in the accumulator (with some

expectations). The flags are affected according to the results.

3. Any register including the memory can be used for increment and

decrement.

4. A program sequence can be changed either conditionally or by testing

for a given data condition.

Instruction Format

An instruction is a command to the microprocessor to perform a given task on a specified

data. Each instruction has two parts: one is task to be performed, called the operation

code (opcode), and the second is the data to be operated on, called the operand. The

operand (or data) can be specified in various ways. It may include 8-bit (or 16-bit) data, an

internal register, a memory location, or 8-bit (or 16-bit) address. In some instructions, the

operand is implicit.

Instruction word size

The 8085 instruction set is classified into the following three groups according to word

size:

1. One-word or 1-byte instructions

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2. Two-word or 2-byte instructions

3. Three-word or 3-byte instructions

In the 8085, "byte" and "word" are synonymous because it is an 8-bit microprocessor.

However, instructions are commonly referred to in terms of bytes rather than words.

(1) One-Byte Instructions

A 1-byte instruction includes the opcode and operand in the same byte. Operand(s) are

internal register and are coded into the instruction.

For example:

Task Op code

Operand Binary

Code

Hex

Code

Copy the contents of the

accumulator in the register C.

MOV

C,A 0100 1111 4FH

Add the contents of register B to

the contents of the accumulator.

ADD

B 1000 0000 80H

Invert (compliment) each bit in

the accumulator.

CMA

0010 1111 2FH

These instructions are 1-byte instructions performing three different tasks. In the first

instruction, both operand registers are specified. In the second instruction, the operand B

is specified and the accumulator is assumed. Similarly, in the third instruction, the

accumulator is assumed to be the implicit operand. These instructions are stored in 8- bit

binary format in memory; each requires one memory location.

Memory of the Computer

Memory or storage capacity is one of the important components of a computer. Any

storage unit of a computer system is classified on the basis of the following criteria:

1. Access time:

This is the time required to locate and retrieve stored data from the storage unit in

response to program instructions.

2. Storage capacity:

45

It is the amount of data that can be stored in the storage unit.

3. Cost per bit of storage.

4. Units of memory:

The computer stores a character in the storage cells with binary (0,1) mechanism.

Thus the basic unit of memory is a bit (binary digit – 0,1). To store a character, a computer

requires 8 bits or 1 byte. This is called the ― word lengthǁ of the storage unit. Hence the

storage capacity of the computer is measured in the number of words it can store and is

expressed in terms of bytes. The different units of measurement are;

8 Bits = 1 Byte

210 (or) 1024 Bytes = 1 Kilo Byte (KB)

210 (or)1024 KB = 1 Mega Byte (MB)

210 (or)1024 MB = 1 Gega Byte (GB)

American Standard Code for Information Interchange (ASCII)

This code has given alphabets like some numbers which can be converted to

Binary form.

A- 65 …….Z – 90 and a – 97 ………. z – 121

By using these codes the alphabets can be converted to digital & hence to Binary

form.

Types of Memory A computer memory is of two types;

1. Primary Memory (Internal storage)

2. Secondary Memory (External storage)

Primary Memory

Primary memory is also called internal memory and is an important part of a computer. It

is the main area in a computer where the data is stored. The stored data can be recalled

instantly and correctly whenever desired. This memory can be quickly accessed by the

CPU for reading or storing information. Primary memory is further classified into two

types:

Random Access Memory (RAM)

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Read- Only Memory (ROM)

RAM:

RAM is also known as read/write memory as information can be read from and written

onto it. RAM is a place in a computer that holds instructions for the computer, its

programs and the data. The CPU can directly access the data from RAM almost

immediately. However, the storage of data and instructions in RAM is temporary, till the

time the computer is running. It disappears from RAM as soon as the power to the

computer is switched off. i.e it is volatile memory.

ROM:

It is called Read-only memory as information can only be read from and not written or

changed onto ROM. ROM is the ‘built-in‘ memory of a computer. It stores some basic

input – output instructions put by the manufacturer to operate the computer. The storage of

data and instructions in ROM is permanent. It does not depend on the power supply. i.e it

is non-volatile memory.

Secondary Memory The primary memory which is faster (and hence expensive) is generally not sufficient for

large storage of data. As a result, additional memory, called the ―auxiliaryǁ or

―secondary memory is used. It is also referred as ―backup storageǁ as it is used to store

large volume of data on a permanent basis which can be transferred to the primary

memory whenever required for processing. Data are stored in secondary storage in the

same binary codes as in the main (primary memory) storage. Some of the devices of

secondary storages are Floppy Disk, Hard Disk, CD-ROM, DVD and Flash drive.

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1. Floppy Disk:

It is also referred as ―Diskette: and is made of flexible Vinyl material. It has a

small hole on one side called ―Right protect notch

which protects accidental writing/deleting the information from the disk. There is

a hole in the centre through which the spindle of drive unit rotates the disk. The

disks are available in two sizes of 5.25 and 3.5 inches and these could be either

low density or high-density floppies. Storage capacity of floppies are measured in

kilobytes (KB) and megabytes (MB). The details about the storage capacities of

the floppies are presented below:

Floppy Disk Storage Capacity Size (Diameter)

Low Density 360 KB 5.25 inches

High Density 1.2 MB 5.25 inches

High Density 1.44 MB 3.5 inches

Extended 2.8 MB 3.5 inches

2. Hard Disk:

The hard disk can hold more information than the floppy disk and the retrieval of

information from hard disk is faster when compared to floppies or tapes. A hard

disk is fixed inside the CPU and its capacity ranges from 20 MB onwards. The

hard disk is made up of a collection of discs (one below the other) known as

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platters on which the data is recorded. These platters are coated with magnetic

material. It is less sensitive to external environmental disorders and hence the

storage in hard disk is safe. A small hard disk might be as much as 25 times larger

than a floppy disk. Storage Capacity of hard disks varies from 20 MB to several

Gega bytes like 80GB, 160GB.

3. CD-ROM:

CD-ROM stands for Compact Disk–Read Only Memory. It is used to store a wide

variety of information. Its main advantage is that it is portable and can hold a large

amount of data. The storage capacity of most CD-ROMs is approximately 650 MB

or 700 MB.

CD-ROMs have the following variations

(i) CD-R(Compact disc Recordable): Data can be written onto it just once. The

stored data can be read. Data once written onto it cannot be erased.

(ii) CD-RW (Compact disc Rewritable): It is also called erasable CD. Data once

written onto it can be erased to write or record new information many times. To

use a CD-ROM, a device called CD drive is needed.

4. DVD:

DVD stands for Digital Versatile Disc. It is similar to a CD-ROM, except that it

can store larger amounts of data. The storage capacity of a DVD is at least 4.7MB.

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DVDs that can store up to 17GBs are also available. Because of their capacity,

DVDs are used to store a very large multimedia presentations and movies that

combine high quality sound and graphics.

5. Flash Drive

It is a small, portable device that can be used to store, access and transfer data. Due

to its small size, it is commonly called Pen drive. It is also called USB drive. We

can read, write, copy, delete, and move data from computer to pen drive or pen

drive to computer. It comes in various storage capacities of 2GB, 4GB, 8GB etc. It

is popular because it is easy to use and small enough to be carried in a pocket. This

device is plugged into the USB port of the computer and the computer

automatically detects this device.

6. Magnetic tapes

7. Magnetic cassettes

8. Magnetic disc

9. Bubble memories

10. Charge Coupled Devices (CCD)

Programming

Check next attachment for programming notes on: Assembly language program,

Data transfer

Data manipulation

Transfer of control

Input/output instruction

Machine control

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Memory organization

Memory mapping

Construct a memory map for 8085 microprocessor