Introduction to microprocessors notes

Dr. C. SARITHA
DEPT. 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

Dr. C. SARITHA
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

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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.

Dr. C. SARITHA
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.

Dr. C. SARITHA
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.

Dr. C. SARITHA
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:

Dr. C. SARITHA
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

Dr. C. SARITHA
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. RD - This is an active low control signal used for read operation.
2. WR -This is an active low control signal used for write operation.
There are three status signals used in microprocessor S0, S1 and IO/M . It changes its status
according to the provided input to these pins.

Dr. C. SARITHA
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 INTA 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

Dr. C. SARITHA
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.

Dr. C. SARITHA
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

Dr. C. SARITHA
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

Dr. C. SARITHA
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

Dr. C. SARITHA
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)

Dr. C. SARITHA
Machine Cycle Status No. of Machine Cycles Control
IO/ M S1 S0
Opcode fetch 0 1 1 4 RD=0
Memory Read 0 1 0 3 RD=0
Memory Write 0 0 1 3 WR =0
I/O Read 1 1 0 3 RD=0
I/O Write 1 0 1 3 WR =0
Interrupt
Acknowledge
1 1 1 3 INTA =
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.

Dr. C. SARITHA
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.

Dr. C. SARITHA
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.

Dr. C. SARITHA
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.

Dr. C. SARITHA
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.

Dr. C. SARITHA
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

Dr. C. SARITHA
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

Dr. C. SARITHA
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.
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Introduction to microprocessors notes

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