Introduction to microprocessor notes

Dr. C. SARITHADEPT. OF ELECTRONICS

UNIT – I

INTRODUCTION TO MICROCOMPUTER AND MICROPROCESSOR

Microprocessor :

A microprocessor is a CPU integrated into a small silicon chip that comprise of thousands of small

components such as - diodes, transistors and resistors that work together.

(OR)

The term microprocessor typically refers to the central processing unit (CPU) of a microcomputer,

containing the arithmetic logic unit (ALU) and the control units. It is typically

implemented on a single LSI chip.

(OR)

Microprocessor is a Central Processing Unit (CPU) etched on a single chip. A single Integrated

Circuit (IC) has all the functional components of a CPU namely Arithmetic Logic Unit (ALU),

Control Unit and registers.

Evolution of Microprocessors :

Microprocessor has turned into the brain of millions of gadgets, since year 1971. Now, we have a

look at the gadgets in which the microprocessors are playing an important role.

Business Calculator: A business calculator was invented in the year 1971. The Unicom 141P

business calculator was out of the foremost gadgets that feature a microprocessor.

Commodore PET: The PET was invented in the year 1971 and is broadly recognized as the

primary all-in-one home computer.

Washing Machine: The foremost microchip controlled washing machines were launched in the

year 1977 and gave a bang to the market, showcasing the varied usages of innovative technology.

Arcade Mania in the year 1980: Namco pioneered Pac-Man in the walkways of the United States

and ignited a new trend.

1 S.S.B.N. DEGREE & PG COLLEGE (AUTONOMOUS), ANANTAPURAMU


Osborne 1 Laptop: With five screen and 10.7kgs of weight, Osborne 1 Laptop was invented in the

year 1981. It actually was the great grand-father of most modern laptops.

Nintendo NES: Consoles revitalized the gaming industry in the year 1986 such as Nintendo

Entertainment System.

Computing Democratized: Personal & business computing blasted with a broad variety of

laptops, desktops & even early tabs. These inventions came up in the year 1991.

MP3 Player: The modern way to enjoy to music forever altered in the last 1990s with the foremost

MP3 player, which was invented in the year 1997.

BlackBerry: The Smartphone insurgence boosted with the launch of RIM’s Blackberry 850. The

1st BB was accessible in the year 1999.

Apple iPod: Apple launched its 1st iPod in the year 2001; its release gave the future of MP3 music

format a new selection of set tunes.

Microsoft Windows Tablet: Approximately a decade prior to the shopper’s fascination with tab,

Microsoft Windows Tablet was launched in the year 2002, business were employing these tabs for

more useful jobs.

Netbook: Netbooks were launched in the year 2008, as small and light-weighted gadget for

carrying out uncomplicated jobs and enjoying media & internet content on the move.

Apple iPod: Tabs strike the customers main-stream with the release of iPod in the year 2010.

Digital Signage in the year 2011: Digital Signage was 1st of the vast new usages for the

microprocessor. Intellectual, internet allied gadgets are more and more found in the daily life from

business and retail to farming and automobiles.

Ultrabook in the year 2011: The advancement of the Personal Computer takes an additional

gigantic step as trendy Ultrabook gadgets push ahead high performance computing experience.

Generations of microprocessors :

Microprocessors were categorized into five generations: first, second, third, fourth, and fifth



generations. Their characteristics are described below:

First-generation

The microprocessors that were introduced in 1971 to 1972 were referred to as the first generation

systems. First-generation microprocessors processed their instructions serially—they fetched the

instruction, decoded it, then executed it. When an instruction was completed, the microprocessor

updated the instruction pointer and fetched the next instruction, performing this sequential drill for

each instruction in turn.

Second generation

By the late 1970s, enough transistors were available on the IC to usher in the second generation

of microprocessor sophistication: 16-bit arithmetic and pipelined instruction processing.

Motorola’s MC68000 microprocessor, introduced in 1979, is an example. Another example is

Intel’s 8080. This generation is defined by overlapped fetch, decode, and execute steps (Computer

1996). As the first instruction is processed in the execution unit, the second instruction is decoded

and the third instruction is fetched.

The distinction between the first and second generation devices was primarily the use of

newer semiconductor technology to fabricate the chips. This new technology resulted in a five-fold

increase in instruction, execution, speed, and higher chip densities.

Third generation

The third generation, introduced in 1978, was represented by Intel’s 8086 and the Zilog Z8000,

which were 16-bit processors with minicomputer-like performance. The third generation came

about as IC transistor counts approached 250,000.

Motorola’s MC68020, for example, incorporated an on-chip cache for the first time and the

depth of the pipeline increased to five or more stages. This generation of microprocessors was

different from the previous ones in that all major workstation manufacturers began developing

their own RISC-based microprocessor architectures (Computer, 1996).

Fourth generation

As the workstation companies converted from commercial microprocessors to in-house designs,

microprocessors entered their fourth generation with designs surpassing a million transistors.

Leading-edge microprocessors such as Intel’s 80960CA and Motorola’s 88100 could issue and

retire more than one instruction per clock cycle.



Fifth generation

Microprocessors in their fifth generation, employed decoupled super scalar processing, and

their design soon surpassed 10 million transistors. In this generation, PCs are a low-margin,

high-volume-business dominated by a single microprocessor.

Salient features of 8085 microprocessor

It is a 40 pin LSI chip

Operates at Single + 5V Supply

Operates with 3MHz single phase clock

On-chip clock generator

It has 8 data lines and 16 address lines

It provides 74 instructions with 5 addressing modes

It provides 5 hardware interrupts and 8 software interrupts

It has one Serial In/Serial Out Port

It is an 8 bit parallel central processing unit (CPU).

It has Direct Addressing Capability to 64K bytes of memory

It uses a multiplexed data bus. The address is split between the 8bit address bus and the 8bit

data bus.

Architecture of 8085 Microprocessor :

The functional block diagram or architecture of 8085 Microprocessor, gives the complete details

about a Microprocessor. It includes the ALU (Arithmetic and logic unit), timing and control unit,

instruction registers and decoder, register array, interrupt control, and serial I/O control etc. To

connect all the blocks with each other we need some buses such as address bus, data bus and

control bus. Fig.1. shows the Block diagram of a Microprocessor.



Fig (1): Functional Block Diagram of 8085 Microprocessor

Arithmetic and Logic Unit

There is always a need to perform arithmetic operations like +, -, *, / and logical operations like

AND, OR, NOT etc. So there is a necessity for creating a separate unit which can perform such

type of operations. These operations are performed by the Arithmetic and Logic Unit (ALU). ALU

performs these operations on 8-bit data.

But these operations cannot be performed unless we have an input (or) data on which the desired

operation is to be performed. ALU gets its Input from accumulator and temporary register. After

processing the necessary operations, the result is stored back in accumulator.



Acumulator (A) :- It is an 8-bit register. It is used to store one of the operand in many instructions.

After the execution of most of the instructions the result is stored in the accumulator. That’s why

this is also called result register. It also works as a register for I/O access.

Temporary Register :- It is a 8-bit register. As the name suggests this register acts as a temporary

memory during the arithmetic and logical operations. Unlike other registers, this temporary register

can only be accessed by the microprocessor and it is completely inaccessible to programmers.

W and Z registers :- These are two 8- bit temporary registers used to hold temporary data

internally during the program execution. These are not accessible by the programmer.

Flags :-

Flags are nothing but a group of individual Flip-flops. The flags are mainly associated with

arithmetic and logic operations. The flags will show either a logic 0 or 1 (i.e.) a set or reset

depending on the data conditions in the accumulator or various other registers. A flag is actually a

latch which can hold some bits of information. It alerts the processor that some event has taken

place.

There are five flip-flops in the flag register. They are as follows:

1. Sign (S)

2. zero (Z)

3. Auxiliary carry (AC)

4. Parity (P)

5. Carry (C)

The bit position of the flip flops in flag register is:



1. Sign (S) – If MSB of the result of an operation has a value 1, this flag is set otherwise it is reset.

2. Zero (Z) - If the result of an operation has a value zero, this flag is set otherwise it is reset.

3. Auxiliary carry (AC) – During the arithmetic operation, if a carry is transferred from D3 to D4,

this flag is set otherwise it is reset.

4. Parity (P) - If the result of an operation contains even number of 1s, this flag is set otherwise it is

reset.

5. Carry(C) - If the instruction resulted in a carry (from addition) or borrow (from either

subtraction or comparision) out of higher order bit, this flag is set otherwise it is reset.

General Purpose Registers :-

Apart from the accumulator 8085 consists of six special type of registers called General Purpose

Registers. These general purpose registers are used to hold data like any other registers. The

general purpose registers in 8085 microprocessor are B, C, D, E, H and L. Each register can hold

8-bit data. These registers can also be used to work in pairs to hold 16-bit data.

They can work in pairs such as B-C, D-E and H-L to store 16-bit data. The H-L pair works as a

memory pointer. A memory pointer holds the address of a particular memory location. They can

store 16-bit address as they work in pair.

Program Counter :- It is a 16 bit special purpose register used to store the memory address of the

next instruction to be executed next. The execution of a program is initiated by loading the PC by

the address of the first instruction of the program. Once the first instruction is executed, the PC is



automatically incremented to point to the next instruction and this process is repeated till the end of

the program. Hence it is also called as ‘Memory Pointer’.

Stack Pointer :- It is a 16 bit special purpose register, which controls a portion of memory known

as stack and it holds the address of this stack top. This stack is used to save the content of a

register during the execution of a program.

Instruction registers (IR) :- It is an 8-bit register. It is used to hold the current instruction which

the microprocessor is about to execute. Note that this register is not accessible by the programmer.

Instruction Decoder :- It interprets the instruction stored in the instruction register. It generates

various machine cycles depending upon the instruction. The machine cycles are then given to the

Timing and Control Unit.

Incrementer/Decrementer Register :- It is a 16-bit register used to increment or decrement the

contents of PC and stack pointer. It is also not accessible by the programmer.

Timing and Control Unit :- The timing and control unit is a section of the CPU. It generates

timing and control signals which are necessary for the execution of instructions. It provides status,

control and timing signals which are required for the operation of memory and I/O devices. It

controls the entire operation of the microprocessor and peripherals connected to it. Thus it is seen

that control unit of the CPU acts as a brain of the computer.

There are two control signals:

1. - This is an active low control signal used for read operation.

2. -This is an active low control signal used for write operation.

There are three status signals used in microprocessor S0, S1 and IO/ . It changes its status

according to the provided input to these pins.



Serial Input/Output Control :- There are two pins in this unit SID and SOD . This unit is used

for serial data communication.

Interrupt control :- There are 6 interrupt pins in this unit. Generally an external hardware is

connected to these pins. These pins provide interrupt signal sent by the external hardware to the

microprocessor and microprocessor sends acknowledgement for receiving the interrupt signal.

Generally is used for acknowledgement.

Note : Registers are small memories within the CPU. They are used by the microprocessor for

temporary storage and manipulation of data and instructions. Data remain in the registers till they

are sent to the memory or I/O devices.

8085 Bus Structure :

There are three buses in 8085 Microprocessor:

1. Address Bus

2. Data Bus

3. Control Bus



Address Bus:- Genearlly, Microprocessor has 16 bit address bus. The bus over which the CPU

sends out the address of the memory location is known as Address bus. The address bus carries the

address of memory location to be written or to be read from.

The address bus is unidirectional. It means bit flow occurs only in one direction, only from

microprocessor to peripheral devices.

We can find that how much memory it can use by the formula 2N, where N is the number of bits

used for address lines.

Here, 216 = 65536 bytes or 64KB. So we can say that it can access upto 64 KB memory.

Data Bus:-

8085 Microprocessor has 8 bit data bus. So it can be used to carry the 8 bit data starting from

00000000H (00H) to 11111111H (FFH). Here 'H' tells the Hexadecimal Number. It is

bidirectional. These lines are used for data flowing in both direction means data can be transferred

or can be received through these lines. The data bus also connects the I/O ports and CPU. The

largest number that can appear on the data bus is 11111111.

It has 8 parallel lines of data bus. So it can access upto 28 = 256 data bus lines.

Control Bus:- The control bus is used for sending control signals to the memory and I/O devices.

The CPU sends control signal on the control bus to enable the outputs of addressed memory

devices or I/O port devices.



Some of the control bus signals are as follows:

1. Memory read

2. Memory write

3. I/O read

4. I/O write.

Pin configuration of 8085 Microprocessor and its description :

Intel 8085 is an 8-bit, N-channel Metal Oxide semiconductor (NMOS) microprocessor. It is a 40

pin IC package fabricated on a single Large Scale Integration (LSI) chip. The Intel 8085 uses a

single +5V DC supply for its operation. Its clock speed is about 3MHz. It has 80 basic instructions

and 246 opcodes. The 8085 is an enhanced version of its predecessor, the 8080A.

Pin diagram of 8085

The 8085 signals are grouped as follows:

1. Address bus

2. Address/data bus

3. Control and status signals



4. Interrupt signals

5. DMA signals

6. Timing and synchronization signals

7. Serial I/O signals

8. Power supply

(1) Address signals: A15 – A8

These signals form the higher order address lines

(2) Address/Data signals: AD7 – AD0

This is a time multiplexed address and data bus used for carrying both

lower order address signals

Data signal at different time intervals

Address bus is unidirectional and data bus is bidirectional

(3) Control and Status signals:

(a) Control Signals:

* RD - This is an active low signal. This signal indicates that selected I/O or memory device is to

be read and that the data is available on the data lines.

* WR- This is also an active low signal. This signal indicates that the data on the data bus is to be

written into the selected memory or I/O location.

(b) Status Signals:

* IO/M - used to differentiate between I/O and memory operation.

1 – I/O operation 0 – Memory operation

* S1, S0 – These signals along with IO/M are used to identify various operations of the

microprocessor.

* ALE – This signal is generated during the first clock period of every machine cycle. It is used to

demultiplex the multiplexed lower order address and data bus.

(4) Interrupt Signals:

An interrupt is a request to the microprocessor to suspend the execution of the main program

temporarily and execute another program called Interrupt Service Routine (ISR) corresponding to a



device which has requested microprocessor through any of the 5 interrupt lines. INTA is an

acknowledgement to a maskable interrupt.

(5) DMA Signals:

DMA (Direct Memory Access) is the process of transferring data from the I/O device to memory

without the interference of the microprocessor. We must keep in mind that for initiating the DMA

process microprocessor is needed.

HOLD – This signal indicates a peripheral such as DMA controller is requesting for the use of

address and data bus.

HLDA – This output signal acknowledges the HOLD request.

(6) Timing and synchronization signals:

* RESETIN - when the signal on this pin goes low, the program counter is set to 0, buses are

tristated and microprocessor is reset.

* RESET OUT – This signal indicates that the microprocessor is reset and can be used to reset

other devices.

* CLKOUT – This signal can be used as system clock for other devices.

* X1 and X2 – The crystal is connected across these pins. The frequency is internally divide by 2.

Thus, to operate a system at 3MHz, the crystal must have a frequency of 6MKz.

* READY – This input signal is used to delay the microprocessor read/write cycles until an I/O

device is ready to send/accept data.

(7) Serial I/O signals:

* SID – serial input data: The data on this line is loaded into accumulator bit – 7 whenever a RIM

instruction is executed.

* SOD – Serial output data: This line is set or reset as specified by the SIM instruction.

These two signals are used to establish serial communication between the microprocessor and

external serial I/O devices.

(8) Power supply signals:

VCC - +5V Power supply

VSS – ground reference



Timing Diagrams:

Timing Diagram is a graphical representation. It represents the execution time taken by each

instruction in a graphical format. The execution time is represented in T-states.

Instruction Cycle:

The time required to execute an instruction.

Machine Cycle:

The time required to access the memory or input/output devices.

T-State:

• The machine cycle and instruction cycle takes multiple clock periods.

• A portion of an operation carried out in one system clock period is called as T-state.

The 8085 microprocessor contains 6 basic machine cycles. They are

1. Op-code Fetch cycle (4T or 6T)

2. Memory read cycle (3T)

3. Memory write cycle (3T)

4. I/O read cycle (3T)

5. I/O write cycle (3T)

6. Interrupt Acknowledge cycle (6T or 12T)



Machine Cycle Status No. of Machine Cycles Control

S1 S0

Opcode fetch 0 1 1 4 =0

Memory Read 0 1 0 3 =0

Memory Write 0 0 1 3 =0

I/O Read 1 1 0 3 =0

I/O Write 1 0 1 3 =0

Interrupt Acknowledge

1 1 1 3 =0

Timing diagram for opcode fetch cycle (4T) :

The opcode fetch machine cycle is executed by the processor to fetch the opcode from the

memory. The time taken by the processor to execute the opcode fetch cycle is either 4T or 6T. In

this time the first 3 T-states are used for fetching the opcode from memory and the remaining T

states are used for internal operations by the processor.



Timing diagram for Memory Read cycle (3T) :

The memory read machine cycle is executed by the processor to read a data byte from the memory.

The processor takes 3 T-states to execute this machine cycle.



Timing diagram for Memory Write Cycle (3T):

The memory write machine cycle is executed by the processor to write a data byte in a memory

location. The processor takes 3 T-states to execute this machine cycle.



Timing diagram for I/O Read Cycle (3T) :

The I/O read cycle is executed by the processor to read a data byte from the I/O port or from the

peripheral which is I/O mapped in the system. The processor takes 3 T-states to execute this

machine cycle.



Timing diagram for I/O Write Cycle (3T) :

The I/O write cycle is executed by the processor to write a data byte in the I/O port or to a

peripheral which is I/O mapped in the system. The processor takes 3 T-states to execute this

machine cycle.



Instruction Cycle

The time taken for the execution of an instruction is called instruction cycle (IC). An instruction

cycle (IC) consists of a fetch cycle (FC) and an execute cycle (EC). A fetch cycle is the time

required for the fetch operation in which the machine code of an instruction (op-code) is fetched

from the memory. This time is a fixed slot of time. An execute cycle is of variable width which



depends on the instruction to be executed. The total time for the execution is given by IC = FC +

EC.

Fig (a): An instruction cycle showing FC, EC and IC

Machine Cycle

Machine cycle is defined as the time required for completing the operation of accessing either

memory or I/O device. In the 8085, the machine cycle may consist of three to six T states. The T-

state is defined as one sub division of the operation performed in one clock period. These sub

divisions are internal states synchronized with the system clock. In every machine cycle the first

operation is op-code fetch and the remaining will be read or write from memory or I/O devices.

Fetch Cycle

The first byte of an instruction is its op-code. An instruction may be more than one byte long. The

other bytes are data or operand address. The program counter (PC) keeps the memory address of

the next instruction to be executed. In the beginning of a fetch cycle the content of the program

counter, which is the address of the memory location where op-code is available, is sent to the

memory. The memory places the op-code on the data bus so as to transfer it to the microprocessor.

Execute Cycle

The op-code fetched from the memory goes to the instruction register, IR. From the instruction

register it goes to the decoder circuitry which decodes the instruction. After the instruction is



decoded, execution begins. If the operand is in the general purpose registers, execution is

immediately performed.

The time taken for decoding and execution is one clock cycle. If an instruction contains data or

operand and address which are still in the memory, the microprocessor has to perform some read

operations to get the desired data. After receiving the data it performs execute operation. A read

cycle is similar to a fetch cycle. In case of a read cycle the quantity received from the memory are

data or operand address instead of an op-code. In some instructions write operation is performed.

In write cycle data are sent from the microprocessor to the memory or an output device. Thus, in

some cases an execute cycle may involve one or more read or write cycles or both.

Applications of Microprocessor

Microprocessors are being used for numerous applications and the list of applications is becoming

longer and longer. Some of them are given below.

►Personal Computer ► Numerical Control

►Mobile Phones ►Automobiles

►Bending Machines ►Medical Diagnostic Equipment

►Automatic voice recognizing systems ►Prosthetics

►Traffic light Control ►Entertainment Games

►Digital Signal Processing ►Communication terminals

►Process Control ►Calculators

►Sophisticated Instruments ►Telecommunication Switching Systems

►Automatic Test Systems.

♣♣♣♣


Education

Introduction to microprocessor notes