Microprocessors- Evolution &

Introduction to 8085

Introduction Microprocessor is an electronic chip that functions as the central

processing unit (CPU) of a computer In other words, we can call microprocessor as the heart of any

computer system. Some may call the microprocessors as the brain of the computers. The microprocessor based systems with limited resources are

called as microcomputers. Now-a-days microprocessors are found in almost all electronic

machines and appliances in its different form. Some common devices using microprocessors are computer

printers, automobiles, washing machines, microwave ovens, mobile phones, fax machines, Xerox machines and advanced instruments like radar, satellites, flights etc.,

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Any middle class house-hold will have about a dozens microprocessor in different forms inside various appliances.

The recent developments in electronic industry and the large scale integration of devices has led to rapid cost reduction and more and more application of microprocessors and its derivatives.

Almost all microprocessors use the basic concept of “stored program execution”.

By this concept, programs or the instructions to be executed by the microprocessor are stored sequentially in memory locations.

The microprocessor or the processor in general will fetch the instructions one after the other and execute them it in its arithmetic and logic unit.

A microprocessor can be programmed to do any task that can be written and programmed by the user.

So, in order to work with the microprocessor, it is necessary for the user to know about the internal resources and features of the microprocessor.

The programmers must also understand the instructions that a microprocessor can support.

Every microprocessor will have its own associated set of instructions that it supports and this list is given by all the microprocessor manufacturers

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The instruction set for microprocessors is given in two formsa) One in mnemonic which is comparatively easy to understand b) The other in binary machine code which the microprocessor really

works upon and difficult to understand by us. Programs are written using mnemonics called the assembly level

language and then they are converted into binary machine level language.

This conversion can be done manually or using an application called assembler.

In general, the programs are written by the user for a microprocessor to work with real world data.

These data are available in many forms and are from many sources. To give this data to the microprocessor, the microprocessor based

systems need some input interfacing circuits and some electronic processing circuits.

These circuits include data converters and ports

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After processing the real world data, the output from the microprocessor must be taken out to give to final end effect instrument or circuits.

This again needs interfacing circuits and ports. So, a microprocessor based system will need a set of memory

units, set of interfacing circuits for inputs and a set of interfacing circuits for outputs.

All circuits put together along with microprocessor are called as microcomputer system.

The physical components of the microcomputer system are in general called as hardware.

The program which makes this hardware useful is called as software.

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Basic termsChip

a) A chip or an integrated circuit is a small, thin piece

of silicon with the required circuit and transistors

etched on it to perform a particular function.

b) Simpler processors might consist of a few thousand

transistors etched onto a silicon base just a few

millimeters square.

Bita) A bit means a single binary digit.

b) Also, the bit is the fundamental storage unit of computer memory. In binary, bit can have only two values, 0 or 1, whereas a decimal digit can have 10 values, represented by symbols 0 through 9.

Bit SizeThe bit size of a microprocessor refers to the number

of bits that can be processed simultaneously by the basic arithmetic circuits of the microprocessor.

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WordA number of bits grouped together for processing is called as word. In microprocessors, the word in general refers to the basic data size or bit size that can processed by the ALU of the processor. 16-bit binary number is called a word in a 16-bit processor.

Memory wordThe number of bits that can be stored in a register of memory element is called a memory word. Mostly all memory units use 8-bits as their memory word.

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ByteAn 8-bit word is referred to as a byte.

NibbleA 4-bit word is referred to as a nibble.

KilobyteA collection of 1024 bytes is called a kilobyte.

Megabyte A collection of 1024 Kbytes is called a

megabyte.

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RAM or R/W memory Random Access Memory or Read/ Write memory is

the type of semiconductor memory in which a particular memory location can be erased and written with a new data at any time.

These memory units are volatile, which means that the content of the memory is erased when the power to the chip is disrupted.

The access of the individual memory location can be done randomly. In microprocessors, the RAM is used to store data.

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DRAM

a) Dynamic Random Access Memory is a semiconductor

memory in which the stored contents need to be

refreshed repeatedly at about thousands of times per

second.

b) Without refreshing, the stored data will be lost.

c) These memory chips are preferred in a computer

system because it is economical although slower.

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SRAM

Static Random Access Memory chips keep the data

stored in it as long as power is available. There is no

need for refreshing. In terms of speed, SRAM is faster.

ROM

Read Only Memory are memory devices whose

contents are retained even after removing the power

supply.

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Arithmetic and Logic Unit (ALU)a) ALU is a digital circuit present in the microprocessor

to do performs arithmetic and logic operations on digital data.

b) The typical operations performed by the ALU are addition, subtraction, Logical AND, logical OR and comparison of binary data.

c) Generally, the functions of the ALU of a microprocessor will decide the functionality that can be performed by the processor.

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Microcontroller Microcontroller is a chip that includes

microprocessor, memory and I/O signal ports. Microcontrollers can be called as single chip microcomputers.

MicrocomputerThe system formed by interfacing

microprocessor with memory, and I/O devices to work with the required program is called microcomputer.

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BusThe bus in a microprocessor system refers to a group of wires or signals having a common functionality

System BusThe System bus is a group of signals used for communication between the microprocessor and peripherals.

FirmwareSoftware written for a microprocessor application without provision for changes. These are stored into permanent storage or ROM of the computer system.

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Input device The devices that are used for giving data to the microcomputer system are called as input devices. Usually keyboard and mouse are the input devices through which data and instructions are given to computer.

Output deviceThe devices that are used for getting data out from the microprocessor or microcomputer system are called as output devices. A display screen, printer and displays are the common output devices.

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Clock The circuit in the computer that generates the

sequence of evenly spaced pulses to synchronize the activities of the processor and its peripherals.

The clock speed determines the speed of the operation of the computer.

The computer with a high frequency clock works faster. Normally the clock frequency is in the range of Mega Hertz, MHz or Giga Hertz, GHz.

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Types of ProcessorsTwo types of processors are manufactured 

a) The microprocessor 

b) The microcontrollerThe general purpose microprocessors give the computers

all the necessary computing power. These microprocessors need additional circuitry elements

such as memory devices, I/O ports to connect the input and output devices.

All microprocessor based systems need two types of memories – RAM and ROM.

Microcontrollers The microcontrollers are the microprocessors designed especially for control

applications. Microcontrollers contain memory units and I/O ports inside a chip in addition

to the CPU part. Microcontrollers are otherwise called as embedded controllers are generally

used to control and operate smart machines. Some of the machines using microcontrollers are microwave ovens, washing

machines, sewing machines, automobile ignition systems, computer printers and fax machines etc.

Out of 100 processor chips manufactured, 99 are embedded processors and remaining 1 processor alone goes into general computers.

Lots of semiconductor companies are in the market of microcontrollers and any application development engineer is flooded with the variety of microcontrollers as the choice.

The system consists of CPU, memory and I/O ports

The interfacing of the processor with the other parts of the

microcomputer system needs three bus architecture.

The three buses are

a) Data bus,

b) Address bus

c) Control bus 

Microprocessor based system

Each memory location or I/O port is identified by a specific

address similar to the postal address.

Here, in microprocessor systems, the addresses are all in binary

and in general written in hexadecimal number format. 

The address is a unique pattern used to identify a location in

memory or I/O port.

The address bus consists of many lines that can have the digital

data sent by the processor.

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An address bus of 8 bits 28 different memory locations. Similarly, the 16-bit address bus can address 216 different

addresses. Its address ranges from 0000H to FFFFH. The higher the number of lines of address bus is, the more the

locations the processor is able to manage. A control bus is needed for proper data transfer between the

processor and other peripherals. The control bus basically consists of signals like selecting the

proper memory or I/O device from the address; signal to indicate the direction of data transfer; and the signal to synchronize data transfer between slow devices.

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Origin of Microprocessor

The breakthrough in transistor technology led to the

introduction of minicomputers of the 1960s and the

personal computer revolution of the 1970s.

Intel was the first MPU producer and has been holding a

large share in the world market of this product.

Microprocessors evolution is categorized into five

generations

first, second, third, fourth, and fifth generations.

First Generation (1971-73)

The microprocessors that were introduced in 1971 to

1972 were referred to as the first generation systems.

Intel Corporation introduced 4-bit 4004 at 108 kHz, the

first microprocessor in 1971, co-developed by Busicom, a

Japanese manufacturer of calculators.

In 1972, Intel made the 8-bit 8008 and 8080

microprocessors.

Second Generation (1974-78)

Very large-scale integration (VLSI) lead to chips which had speeds up to hundreds of millions of switching per second.

The second generation marked the beginning of very efficient 8 – bit microprocessors.

Some of the popular processors were Motorola’s 6800 and 6809 and Intel’s 8085, Zilog’s Z80.

The distinction between the first and second generation devices was primarily the use of newer semiconductor technology to fabricate the chips.

They were manufactured using NMOS technology.

Third Generation (1979-80)

Introduced in 1978, dominated by Intel’s 8086 and the Zilog Z8000, which were 16-bit processors with minicomputer-like performance, have 16-bit arithmetic and pipelined instruction processing.

Transistor counts about 250,000. Motorola’s MC68020, incorporated an on-chip cache for the first

time and the depth of the pipeline increased to five or more stages.

HMOS - speed-power-product of HMOS is four times better than that of NMOS.

HMOS can accommodate twice the circuit density compared to NMOS.

Fourth Generation (1981-95)

Designs with more than a million transistors in a package.

32 bits microprocessors introduced – Intel 80386 and

Motorola 68020/68030.

Fabricated using low-power version of the HMOS

technology called HCMOS.

Fifth Generation (1995 till date)

Employed decoupled super scalar processing.

Chips carry on-chip functionalities and improvements in

the speed of memory and I/O devices

Design surpassed 10 million transistors per chip.

Introduction of 64-bit processors

Intel leads the show with Pentium, Celeron and dual and

quad core processors working with up to 3.5GHz speed.

General Purpose Processors - Comparison

Microprocessors can be classified based on their purpose,

architecture, specifications and applications.

Based on the size of the data that the microprocessor can

handle, they are classified as 4-bit, 8-bit, 16-bit, 32-bit and

64-bit microprocessors.

Based on the application of the processors, they are

classified as i) General purpose processors, ii)

Microcontrollers and iii) Special purpose processors.

Classification of Microprocessors

General Purpose Processors

General purpose processors are those which are used in

general computer system integration and can be used by the

programmer for any application.

Common microprocessor such as Intel 8085 to Intel Pentium

processors are examples of general purpose processors.

Microcontrollers are the microprocessor chips with in built

hardware for the memory and ports.

These chips are can be programmed by the user for any generic

control applications.

Special Purpose Processors

Special purpose processors are designed specifically to

handle special functions required for an application.

The digital signal processors are examples for the special

purpose processors and these have special instructions to

handle signal processing.

The Application Specific Integrated Circuits (ASIC) chips

are also the examples of this category of microprocessors.

Classification of microprocessors Based on the architecture and hardware of the processors, they are classified as a)RISC processors,b)CISC processors, c)VLIW processors and d)Superscalar processors. Reduced Instruction Set Architecture’, is a processor architecture that supports limited or small number of machine language instructions.

RISC processors can execute the programs faster than CISC processors. CISC – Complex Instruction Set Computing architecture a) CISC processors have about 70 to few hundred instructions and are easier to program. b) CISC processors are slower and more expensive than RISC processors. c) Very Long Instruction Word (VLIW) processors have instruction composed of many machine operations which can be executed in parallel.

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This is achieved by many functional units operating in parallel. It has large number of registers and instruction level parallelism is achieved. Superscalar processors are using complex hardware to achieve parallelism. It is possible to have overlapping of execution of instructions in order to increase the speed of execution.

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Memory Unit – An Introduction Memory unit is the integral part of any microcomputer system and its primary purpose is to hold program and data. The major design goal of memory unit is to allow it to operate at a speed close to that of the processor. The cost factor inhibits the design of entire memory unit with single technology that guarantees high speed. In order to seek a trade-off between the cost and operating speed, a memory system is usually designed with different technologies such as solid state, magnetic and optical.

A microcomputer memory can be logically divided into four groupsa) Processor memory/ registerb) Cache memoryc) Primary or Main Memoryd) Secondary memory

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Processor Memory refers to a set of CPU registers. Processor registers are the first set of storage device available for the programmers to store any data. But they are generally few in number up to about few tens to hundreds. As these registers are available within the processor, they are the fastest memory registers. The main disadvantage is the cost involved which forces to restrict the number to very few registers of few bytes.

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Cache memory is the fastest external memory to the processor and they are placed close to the processor. The instructions to be executed are placed in cache memory for access by the processor. These are in few kilobytes in size. These are semiconductor RAMs and are volatile. The processor will fetch the next instruction from the cache memory and if the instruction is not in cache, it will refer to primary memory.

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Primary memory is the storage area from which all the programs are executed. All the programs and corresponding data must be within primary memory prior to execution. The size of primary memory is much larger compared to processor memory and cache memory but its operating speed is slower than them. The primary memory in a system varies from few Kilobytes to about 1 Megabyte. Secondary memory refers to the storage medium for huge files such as program source codes, compilers, operating systems etc.

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These are not accessed directly and very frequently by the microprocessor in a computer system. They comprises of slow devices such as magnetic tapes and optical disks. Sometimes they are referred to as auxiliary or backup store.Stored information in a magnetic tape or magnetic disk is not lost when the power is turned off. Therefore these storage devices are called nonvolatile memories.

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Primary Memory - Classification Primary memory normally includes ROM (Read Only Memory) and RAM (Random Access Memory). Microprocessor based systems have at least one RAM and ROM chips in it. Information stored in semiconductor random access memories RAM will be lost if the power is turned off. This property is known as volatility and hence, RAMs are usually called volatile memories. ROM is a nonvolatile memory.

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As the name implies, a ROM permits only a read access. There are many kinds of this category. Mask Programmable ROMS are custom made for the customer and their contents are programmed by the manufacturer. Since they are mass produced, they are inexpensive. The customer can not erase or program it afterwards.Programmable ROMs are the devices which can be programmed by the user at the user’s place or field. The main disadvantage of PROMs is that they cannot be erased and reprogrammed

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EPROM Erasable and Programmable ROMs (EPROMs) allow the erasure and reprogramming of the content by the user. In an EPROM, programs are entered using electrical impulses and the stored information is erased using UV rays. Electrically Erasable PROMs (EEPROMs) or Electrically Alterable ROMs (EAROMs) allow the users to erase its contents by electrical means and also reprogramming by electrical signals. EEPROMs are different from RAMs in the electrical signal required to erase and program. EEPROMs require a higher voltage for erasing and programming other than the normal 5V supply.

Static RAM In a semiconductor memory constructed using bipolar transistors, the information is stored in the form of voltage levels in flip-flops. Such memories are called static RAMs because stored information remains constant for some period of time.Static RAM maintains information in active circuits, power is required even when the chip is inactive or standby mode. Hence, static RAMs require large power supplies. Also each static RAM cell is about four times larger in area than an equivalent dynamic cell.

Dynamic RAMSemiconductor memories designed using MOS transistors, the information is held in the form of electrical charges in capacitors. The stored charge has the tendency of get leaked away. These memories are referred to as dynamic RAMs. In order to prevent any information loss, dynamic RAMS have to be refreshed at regular intervals. Refreshing means boosting the signal level and writing it back. This activity is performed using a hardware unit called “refresh logic”.

Differences

Input and Output Devices The input/output or I/O section allows the computer to take in data from the outside world or send data to the outside world. Peripherals such as keyboards, video display terminals, printers, and modems are connected to the I/O section. These allow the user and the computer to communicate with each other. The actual physical devices used to interface the computer buses to external systems are often called ports.

Function of I/O Ports An input port allows data from a keyboard, an A/D converter, or some other source to be read into the computer under control of the CPU. An output port is used to send data from the computer to peripheral devices, such as a video display terminal, a printer, or a D/A converter. Simplest type of input or output port is just a set of parallel D flip-flops. If they are being used as an input port, the D inputs are connected to the external device, and the Q outputs are connected to the data bus which can take the data to the CPU.

Data will then be transferred to the data bus when they are enabled by a control signal from the CPU. In a system, where they are being used as an output port, the D inputs of the latches are connected to the data bus, and the Q outputs are connected to some external device. Data sent out on the data bus by the CPU will be transferred to the external device when the latches are enabled by a control signal from the CPU.

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Input Devices Input devices include typewriter-like keyboards; hand-held devices such as the mouse, trackball, joystick, and special pen with pressure-sensitive pad; and microphones. The simplest input device is a switch. Input devices can also be sensors that provide information about their environment temperature, pressure, and so forth to a computer. Another direct-entry mechanism is the optical laser scanner (e.g., scanners used with point-of-sale terminals in retail stores) that can read bar-coded data or optical character fonts.

Output Devices An output is any device through which the user can receive the results from the computer. The output can be any display which can be changing fast or hard copies print out. Some other forms of output are sounds and alarms. The simplest output devices used in almost all microprocessor based systems or computer systems are the LEDs or the seven segment LED displays and LCD. displays.

The advanced video display terminals (either cathode-ray tubes or liquid crystal displays), ink-jet and laser printers, are the common output devices nowadays. Some output devices can be used to directly control machineries. Some devices, such as display terminals with touch screen, may provide both input and output. Modems and other network interface cards can also be called as output devices as they enable the transmission and reception of data between computers.

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Technological Improvements Technology improvements are taking place fast in the microprocessor, microcomputer and personal computer systems. The technical and research improvements in the field of microprocessor technology is listed below.

Increase in data bus / address bus width. The processing capability of the microprocessor can be drastically improved by increasing data size.

Increase in speed As the data to be processed by the microprocessors and computers increased in volume, it became a necessity to increase the speed of the processor. By high speed processors, the user can get the results faster even with the large data volumes.

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Reduction in size and increase in capability The trend in microprocessor technology is to include large amount of peripherals such as memory and I/O ports within a single chip. Microcontrollers are manufactured by all semiconductor manufacturers in this aspect. Development in the processing of large scale integration has lead to small chips of microprocessors with large built in peripherals. We have now processors with large amount of flash memory available in the market.

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Development of external peripherals The use of computers in all the fields have resulted in the development of many fast and advanced peripheral devices to be interfaced with the microprocessor. For example, applications of microprocessors in medical field has resulted in many handheld electronic devices with the specialized input sensors and output printers etc. The faster peripherals can increase the speed of processor execution and a good user interface.

Increase in memory size and speed The developments in the Integrated circuit technology have lead to developments in the reduction of the memory size and the increase in memory speed. This reduces the memory access time of the processor and so will result in the higher speed of execution. Also more amount of memory per unit area is possible

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The microprocessors are largely used in handheld devices operated from the battery source. This has resulted in the research towards reduction of power consumption in the microprocessor chips. As power consumption is reduced, the devices can work for more time with one full charge of batteries. There are many devices operating at 3.3V are even lower and have low power consumption.

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Summary Microprocessor is an electronic circuit that functions as the

central processing unit (CPU) of a computer, providing

computational control.

The microprocessor is the controlling element in a computer

system.

The microprocessor performs data transfers, does simple

arithmetic and logic operations, and makes simple decisions.

The basic operation of the microprocessor is to fetch instructions

stored in the memory and execute them one y one in sequence.

Microprocessors are used in almost all advanced electronic systems.

Microcontrollers are advanced forms of microprocessors with

memory and ports present within the chip.

A microcomputer system is made y interfacing memory and I/O

devices to a microprocessor

Microprocessor evolution is classified into five generations

The processors that are currently in use belong to the fifth

generation.

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