`
`OVERVIEW OF MICROPROCESSORS
`
`1.1 GENERAL
`A microprocessor is one of the most exciting technological innovations in electronics since the
`appearance of the transistor in 1948. This wonder device has not only set in the process of
`revolutionizing the field of digital electronics, but it is also getting entry into almost every
`sphere of human life. Applications of microprocessors range from the very sophisticated process
`controllers and supervisory control equipment to simple game machines and even toys.
`It is, therefore, imperative for every engineer, specially electronics engineer, to know
`about microprocessors. Every designer of electronic products needs to learn how to use
`microprocessors. Even if he has no immediate plans to use a microprocessor, he should have
`knowledge of the subject so that he can intelligently plan his future projects and can make
`sound engineering judgements when the time comes.
`The subject of microprocessors is overviewed here with the objective that a beginner gets
`to know what a microprocessor is, what it can do, how it fits in a system and gets an overall
`idea of the various components of such a system. Once he has understood signam of each
`component and its place in the system, he can go deeper into the working details and design of
`individual components without difficulty.
`
`1.2 WHAT IS A MICROCOMPUTER?
`To an engineer who is familiar with mainframe and mini computers, a microcomputer is simply
`a less powerful mini computer. Microcomputers have smaller instruction sets and are slower
`than mini computers, but then they are far less expensive and smaller too.
`To an engineer with a hardware background and no computer experience, a microcomputer
`will look like a sequential state machine that can functionally replace thousands of random
`logic chips, but occupies a much lesser space, costs much lesser and the number of device
`interconnections being fewer in it, is much more reliable.
`A microcomputer is primarily suited, because of its very low cost and very small size, to
`dedicated applications. On the same grounds, the mainframe computer is as a rule suitable as
`a general purpose computer. Mini computer finds applications in both areas.
`
`1.3 WHAT IS A MICROPROCESSOR?
`A computer, large or small, can be represented functionally (in a simplified form) by the block
`diagram in Figure. 1.1. As shown, it comprises of three basic parts or sub-systems:
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`2 Advanced Microprocessors
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`MICRO-
`PROCESSOR
`
`MEMORY
`
`I / O
`
`Figure 1.1 Block Digram of Microcomputer
`(a) Central Processing Unit (CPU)
`It performs the necessary arithmetic and logic operations and controls the timing and general
`operation of the complete system.
`(b) Input/Output (I/O) Devices
`Input devices are used for feeding data into the CPU, examples of these devices are toggle
`switches, analog-to-digital converters, paper tape readers, card readers, keyboards, disk etc.
`The output devices are used for delivering the results of computations to the outside world;
`examples are light emitting diodes, cathode ray tube (CRT) displays, digital-to-analog converters,
`card and paper-tape punches, character printers, plotters, communication lines etc. The input-
`output subsystem thus allows the computer to usefully communicate with the outside world.
`Input-output devices are also called as peripherals.
`(c) Memory
`It stores both the instructions to be executed (i.e., the program) and the data involved. It
`usually consists of both RAMs (random-access memories) and ROMS (read-only memories).
`A microprocessor is an integrated circuit designed to function as the CPU of a
`microcomputer.
`
`1.4 WHAT IS INSIDE A MICROPROCESSOR ?
`The microprocessor or CPU reads each instruction from the memory, decodes it and executes
`it. It processes the data as required in the instructions. The processing is in the form of arithmetic
`and logical operations. The data is retrieved from memory or taken from an input device and
`the result of processing is stored in the memory or delivered to an appropriate output device,
`all as per the instructions.
`To perform all these functions, the µP (microprocessor) incorporates various functional
`units in an appropriate manner. Such an internal structure or organizational structure of µP,
`which determines how it operates, is known as its architecture.
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`
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`A typical microprocessor architecture is shown in Figure 1.2. The various functional units
`are as follows:
`
`µP
`
`Overview of Microprocessors 3
`
`INPUT
`
`OUTPUT
`
`ALU
`
`REGISTER
`ARRAY
`
`INTERNAL BUS
`(ADDRESS, DATA
`
`)
`
`CONTROL
`
`ROM
`
`RAM
`
`MEMORY
`
`Figure 1.2 Architecture of Microprocessor
`
`1.4.1 Busses
`µC (microcomputer), like all computers, manipulates binary information. The binary information
`is represented by binary digits, called bits. µC operates on a group of bits which are referred to
`as a word. The number of bits making-µP a word varies with the µP. Common word sizes are 4,
`8, 12 and 16 bits (µPs with 32 bit-word have also of late entered the market). Another binary
`terms that will be of interest in subsequent discussions are the byte and the nibble, which
`represent a set of 8 bits and 4 bits, respectively.
`Figure 1.2 shows busses interconnecting various blocks. These busses allow exchange of
`words between the blocks. A bus has a wire or line for each bit and thus allows exchange of all bits
`of a word in parallel. The processing of bits in the µP is also in parallel. The busses can thus be
`viewed as data highways. The width of a bus is the number of signal lines that constitute the bus.
`The figure shows for simplicity three busses for distinct functions. Over the address bus,
`the µP transmits the address of that I/O device or memory locations which it desires to access.
`This address is received by all the devices connected to the processor, but only the device which
`has been addressed responds. The data bus is used by the µP to send and receive data to and
`from different devices (I/O and memory) including instructions stored in memory. Obviously
`the address bus is unidirectional and the data bus is bi-directional. The control bus is used for
`transmitting and receiving control signals between the µP and various devices in the system.
`
`1.4.2 Arithmetic-Logic Unit (ALU)
`The arithmetic-logic unit is a combinational network that performs arithmetic and logical
`operations on the data.
`
`1.4.3 Internal Registers
`A number of registers are normally included in the microprocessor. These are used for temporary
`storage of data, instructions and addresses during execution of a program. Those in the Intel
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`8085 microprocessor are typical and are described below:
`(i) Accumulator (Acc) or Result Register
`This is an 8-bit register used in various arithmetic and logical operations. Out of the two
`operands to be operated upon, one comes from accumulator (Acc), whilst the other one may be
`in another internal register or may be brought in by the data bus from the main memory. Upon
`completion of the arithmetic/logical operation, the result is placed in the accumulator (replacing
`the earlier operand). Because of the later function, this register is also called as result register.
`(ii) General Purpose Registers or Scratch Pad Memory
`There are six general purpose 8-bit registers that can be used by the programmer for a variety
`of purposes. These registers, labelled as B, C, D, E, H and L, can be used individually (e.g.,
`when operation on 8-bit data is desired) or in pairs (e.g., when a 16-bit address is to be stored).
`Only B-C, D-E and H-L pairs are allowed.
`(iii) Instruction Register (IR)
`This 8-bit register stores the next instruction to be executed. At the proper time this stored
`word (instruction) is fed to an instruction decoder which decodes it and supplied appropriate
`signals to the control unit. When the execution has been accomplished the new word in the
`instruction register is processed.
`(iv) Program Counter (PC)
`This is a 16-bit register which holds the address of the next instruction that has to be fetched
`from the main memory and loaded into the instruction register. The program controlling the
`operation is stored in the main memory and instructions are retrieved from this memory
`normally in order. Therefore, normally the address contained in the PC is incremented after
`each instruction is fetched. However, certain classes of instruction can modify the PC so that
`the programmer can provide for branching away from the normal program flow. Examples are
`instructions in the “jump” and ‘call subroutine’ groups.
`(v) Stack Pointer (SP)
`This is also a 16-bit register and is used by the programmer to maintain a stack in the memory
`while using subroutines.
`(vi) Status Register or Condition Flags
`A status register consisting of a few flip-flops, called as condition flags (in 8085 the number of
`flags is five) is used to provide indication of certain conditions that arise during arithmetic and
`logical operations. These are:
`Flag is set if result of instruction is 0.
`‘zero’
`Set if MSB of result is 1.
`‘sign’
`Set if result has even parity.
`‘parity’
`Set if carry or borrow resulted.
`‘carry’
`‘auxiliary carry’ Set if instruction caused a carry out of bit 3 and
`into bit 4 of the resulting value.
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`Overview of Microprocessors 5
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`(vii) Dedicated Registers
`Several other registers are incorporated in the µP for its internal operation. They cannot be
`accessed by the programmer and hence do not concern much a µP user.
`
`1.4.4 Instruction Decoder and Control Unit
`It decodes each instruction and under the supervision of a clock controls the external and
`internal units ensuring correct logical operation of the system.
`
`1.5 SEMICONDUCTOR MEMORIES
`As mentioned earlier, semiconductor memories are required in a microcomputer for storing
`information which may comprise of (a) the data to be used for computation, (b) instructions and
`(c) computational results. A program starts as a set of instructions on a paper, then this is
`transferred to a set of cards with the instructions punched in code on them. These instructions
`also can be transferred to magnetic tape, paper tape or directly into semiconductor memory
`which is the eventual storage space for a program. The semiconductor memory chips are
`connected to the µP through the address bus, data bus and control bus. (This is also the way
`that I/O devices are connected to the µP). See Figure 1.3.
`
`8
`
`A
`ROM
`D
`
`A
`ROM
`D
`
`C
`
`C
`
`Microprocessor
`
`8
`
`8
`
`8
`
`C
`
`C
`
`8
`
`A
`Input device
`D
`
`A
`Output device
`D
`
`Address
`bus
`
`Data
`bus
`
`Control
`bus
`
`Figure 1.3 Connection of I/O Devices and Memory
`
`1.5.1 Memory Classes
`Memories may be broadly divided into two classes:
`(a) Random Access Memory (RAM) or Read/Write Memory (RWM)
`There is provision in RAMs (RWMs) for writing information into the memory and reading it
`when the microcomputer is in operation. It is, therefore, used to store information which
`changes or may change during the operation of the system, viz. data for calculations and results
`of calculations. It is also used to store the programs which are to be changed frequently.
`Semiconductor RAM is a volatile memory.
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`6 Advanced Microprocessors
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`A RAM can be of static or dynamic type. Dynamic RAMs have higher packing densities,
`are faster and consume less power in the quiescent state. However, because of external
`refreshing circuitry requirement, the dynamic RAMs are profitable only in large sizes.
`(b) Read-Only Memory (ROM)
`The ROM functions as a memory array whose contents once programmed, are permanently
`fixed and can not be altered by the µP while the system is operating. It is non-volatile. ROMs
`exist in many forms.
`(i) Mask ROM : It is custom programmed or mask programmed when manufactured and
`can not be altered thereafter. The cost of a custom built mask for programming is so
`high that thousands of ROMs storing the same information must be produced to pay
`for the mask.
`(ii) Programmable ROM (PROM) : This type is programmable by the user (typically by
`electrically overheating fusible links in selected manner). Once programmed, the
`contents can not be altered. The memory may be programmed one at a time by the
`user and is thus suitable for the cases where small quantities of a ROM are needed.
`(iii) Electrically Alterable ROM (EAROM) : In this type of memory, the contents can be
`electrically erased (by applying a large negative voltage to control gates of memory
`cells) and the memory can be then reprogrammed (by applying a large positive voltage
`to control gates). This type is convenient when the user is not sure of the program
`and may wish to modify it. This is a typical requirement in prototype development.
`(iv) Erasable Programmable ROM (EPROM) : Like EAROM, this type of memory can also
`be erased and reprogrammed. However, erasing is by exposing the memory chips to
`high intensity ultravoilet light of a wavelength close to 2537 Å. It has the same
`application filed as the EAROM.
`1.5.2 Bipolar v/s MOS Memories
`Basically there are two semiconductor technologies, namely, bipolar and MOS unipolar. Mask
`ROMs and PROMs are available in both types whereas EAROMs and EPROMs are made with
`MOS technology only.
`In general, bipolar devices (including memories) are faster and have higher drive
`capabilities. On the other hand, MOS devices consume less space and power and are cheaper.
`Therefore, MOS memories are preferred where speed is not a critical factor.
`
`1.6 PERIPHERAL INTERFACING
`1.6.1 Functions
`When one or more I/O devices (peripherals) are to be connected to a µP, an interface network
`for each device, called peripheral interface, is required.
`The interface incorporate commonly the following four functions:
`(a) Buffering : Which is necessary to take care of incompatibility between the µP and the
`peripheral.
`(b) Address Decoding : Which is required to select one of the several peripherals connected
`in the system.
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`Overview of Microprocessors 7
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`(c) Command Decoding : Which is required for peripherals that perform actions other
`than data transfers.
`(d) Timing and Control : All the above functions require timing and control.
`
`1.6.2 Data Transfer
`Data exchange or transfers which occur between a peripheral device and the µC fall into one of
`the following two broad categories:
`(i) Programmed Data Transfer
`A software routine residing in memory requests the peripheral device for data transfer to or
`from the µP. Generally, the data is transferred to or from the accumulator though in some µPs,
`others internal registers may also participate in the transfer.
`Programmed data transfers are generally used when a small amount of data is transferred
`with relatively slow I/O devices, e.g., A/D and D/A converters, peripheral multiplier, peripheral
`floating point arithmetic unit etc. In these cases, usually one word of data is transferred at a
`time.
`(ii) Direct Memory Access (DMA) Transfer or Cycle Stealing Transfer
`In this mode, the data transfer is controlled by the peripheral device. The µP is forced to hold
`on by an I/O device until the data transfer between the device and the memory is complete.
`Since the data control transfer is controlled entirely by hardware, the interface is more complex
`than that required for a programmed data transfer. DMA transfer is used when a large block of
`data is to be transferred, for example, for transferring data from peripheral mass storage
`devices like the floppy disk and high-speed card reader.
`
`1.6.3 Interfacing Devices
`Extensive hardware (IC chips) is now available for designing custom interfaces. Included in this
`are multiplexer, and demultiplexers, line drivers and receivers, level translators and buffers a
`stable and monostable multivibrators, latches, gates, shift registers, and so on.
`Then there are more sophisticated interfaces-the programmable inter-faces, whose functions
`may be altered by an instruction from the mP. These interfaces can be of general purpose type
`or special purpose (dedicated function) type.
`1.7 µµµµµP SOFTWARE AND PROGRAM LANGUAGES
`The foregoing was a brief account of the µC hardware and now follows a concise picture of the
`µC software. A sequence of instructions designed to perform a particular task in a computer is
`known as the program and a set of programs written for a computer-based system is called
`software for that system.
`1.7.1 Programming Languages
`(i) Machine Language (ML)
`Since a machine (computer) can handle (i.e., store and process) information in binary form
`only, the instruction (program) must be finally encoded in binary for feeding to the machine. A
`program in this form is thus in machine language.
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`8 Advanced Microprocessors
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`800 MIPS ?
`Eventually 450 MHz,
`Eventually 200 MHz.
`Eventually 200 MHz
`MIPS
`Eventually 50 MHz, 41
`MIPS
`Eventually 33 MHz, 11.4
`MIPS at 12MHz
`IBP ATs. Up to 2.66
`First IBM PC
`First home computers
`
`32 bits, 64 bit bus1,000 MIPS ?
`32 bits, 64 bit bus400 MIPS ?
`
`450 MHz
`233 MHz
`
`0.25
`0.35
`
`9,500,000
`7,500,000
`
`1999
`1997
`
`Pentium III
`Pentium II
`
`32 bits, 64 bit bus100 MIPS
`
`60 MHz
`
`0.8
`
`3,100,000
`
`1993
`
`Pentium
`
`20 MIPS
`
`32 bits
`
`25 MHz
`
`5 MIPS
`
`1 MIPS
`
`32 bits
`
`16 MHz
`
`16 bits
`
`6 MHz
`
`0.33 MIPS
`0.64 MIPS
`
`16 bits, 8 bit bus
`
`8
`
`MIPS
`
`Data width
`
`5 MHz
`2 MHz
`
`speed
`Clock
`
`1
`
`1.5
`
`1.5
`
`6
`
`3
`
`1,200,000
`
`1989
`
`275,000
`
`1985
`
`134,000
`
`1982
`
`29,000
`6,000
`
`1979
`1974
`
`80486
`
`80386
`
`80286
`
`8088
`8080
`
`Microns
`
`Transistors
`
`Date
`
`Name
`
`Comparison between Various Microprocessors
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`Overview of Microprocessors 9
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`(ii) Assembly Language (AL)
`In an assembly language programme, the instructions including the storage locations are
`represented by alphanumeric symbols, called mnemonics. It is relatively easier to write a
`programme in assembly language than in machine language. However, if written in AL, it
`must be translated to ML before it can be stored and executed in µC. Mostly one statement in
`AL translates to one instruction in ML.
`(iii) High-Level Language (HLL)
`Programming in AL is very tedious and time consuming. High Level languages like Fortran,
`Cobol, Algol, Pascal and PL/M can be used for programming and then the program translated
`into ML program. One statement in HL generally corresponds to several ML instructions.
`
`1.7.2 Software Tools
`(i) Assembler
`It is a computer program that translates an AL program to ML program (also called object
`code). A cross assembler is an assembler that is executed on a machine other than the one for
`which it is producing the ML program. A self assembler or resident assembler, on the other
`hand, is meant to be run on the machine for which ML program is required to be produced.
`(ii) Compiler
`A computer program that translates a HLL program to ML program (object code). Like the
`assembler, the compiler can be a cross compiler or a self (resident) compiler.
`(iii) Editor
`During the process of program entry into the memory or debugging, it may be necessary to
`make changes in the program text in order to correct any errors or modify the logic. An editor
`helps the programmer to do this.
`(iv) System Monitor
`All single boards µCs, design kits and µP development systems have system monitor as an
`integrated component. The monitor (a ML program) resides in ROM. It helps a programmer in
`performing such functions as entering a program into memory, getting a program executed,
`modifying contents of one or more memory locations or µP registers, displaying the contents of
`any memory location of a register in the µP, entering ML program in convenient hexadecimal
`format, etc.
`
`1.8 MICROCOMPUTER INSTRUCTION SET
`The instruction set of a microprocessor will typically comprise of five groups of instructions:
`(i) Data Transfer Group
`These instructions help to move data between registers within the microprocessor, between a
`register and a memory location or between memory location.
`(ii) Arithmetic Group
`Instructions in this group add, subtract, increment or decrement data in registers or in memory
`(e.g., an instruction to add the contents of two registers within the microprocessor).
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`10 Advanced Microprocessors
`(iii) Logical Group
`These are used to AND, OR, EXCLUSIVE-OR, Compare, rotate or complement data in registers
`or in memory (e.g., OR the contents of two registers within the microprocessor).
`(iv) Branch Group
`This group includes conditional and unconditional jump instructions, subroutine-call instructions
`and return-from-subroutine instructions. A conditional instruction specifies that a certain
`operation be performed only if a certain condition has been met (e.g., jump to a particular
`instruction if the result of the last operation was zero). Conditional instructions provide decision
`making capability in programs.
`(v) Stack, I/O and Machine Control Group
`This group of instructions performs data transfer between the microprocessor and I/O devices,
`manipulates the stack and alters internal control flags. These instructions make it possible for
`the programmer to halt the microprocessor, put it in a “no operation” state, enable/disable its
`interrupt system and so on.
`Instructions, which are stored alongwith data in the memory, may be one or more bytes
`in length. Multiple byte instructions are stored in successive memory locations: the address of
`the first byte is always used as the address of the instruction. Also, the first byte is always the
`operation code (abbreviated as OPCODE).
`
`1.9 MICROPROCESSOR DEVELOPMENT CHRONOLOGY
`The first microprocessor was announced in 1971 by Intel Corporation, U.S.A. This was the
`Intel 4004. It was on a single chip and was a 4-bit microprocessor (i.e., operated on 4 bits of data
`at a time).
`Encouraged by the success of 4004, Intel Corp. introduced its enhanced version, the Intel
`4040. Many other companies also announced 4-bit microprocessors, examples are Rockwell
`International’s PPS4, NEC’s µCOM 4 and Toshiba’s T3472.
`The first 8-bit microprocessor was announced in 1973, again by Intel Corp. This was the
`Intel 8008. An improved version, Intel 8030, followed. Several other companies followed the
`suit. Today the batter known 8-bit mPs are Intel’s 8085, Motorola’s M6800, NEC’s µCOM85AF,
`National * SC/MP, Zilog Corporation’s Z80 and Fairchild’s F8.
`Then followed 12-bit and 16-bit µPs. Examples of 12-bit µPs are Intersil’s IM 6100 and
`Toshiba’s T3190 and those of 16-bit µPs Intel’s 8086, Fairchild’s 9440, Texas Instrument’s TMS
`9940 and TMS 9980, Zilog’s Z8000, Motorola’s M68000.
`The developments in µP since 1971 have been in the direction of (a) improving architecture,
`(b) improving instruction set, (c) increasing speeds, (d) simplifying power requirements and (e)
`incorporating more and more memory space and I/O facilities in the same chip (thus giving use
`to single chip computers).
`• The date is the year that the processor was first introduced. Many processors are re-
`introduced at higher clock speeds for many years after the original release date.
`• Transistors is the number of transistors on the chip. You can see that the number of
`transistors on a single chip has risen steadily over the years.
`• Microns is the width, in microns, of the smallest wire on the chip. For comparison, a
`human hair is 100 microns thick. As the feature size on the chip goes down, the
`number of transistors rises.
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`Overview of Microprocessors 11
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`• Clock speed is the maximum rate that the chip can be clocked. Clock speed will make
`more sense in the next section.
`• Data Width is the width of the ALU. An 8-bit ALU can add/subtract/multiply/etc. two
`8-bit numbers, while a 32-bit ALU can manipulate 32-bit numbers. An 8-bit ALU
`would have to execute 4 instructions to add two 32- bit numbers, while a 32-bit ALU
`can do it in one instruction. In many cases the external data bus is the same width as
`the ALU, but not always. The 8088 had a 16-bit ALU and an 8-bit bus, while the
`modern Pentiums fetch data 64 bits at a time for their 32-bit ALUs.
`• MIPS stands for Millions of Instructions Per Second, and is a rough measure of the
`performance of a CPU. Modern CPUs can do so many different things that MIPS
`ratings lose a lot of their meaning, but you can get a general sense of the relative
`power of the CPUs from this column.
`From this table you can see that, in general, there is a relationship between clock speed
`and MIPS. The maximum clock speed is a function of the manufacturing process and delays
`within the chip. There is also a relationship between the number of transistors and MIPS. For
`example, the 8088 clocked at 5 MHz but only executed at 0.33 MIPS (about 1 instruction per 15
`clock cycles). Modern processors can often execute at a rate of 2 instructions per clock cycle.
`That improvement is directly related to the number of transistors on the chip.
`
`1.10 MANUFACTURING TECHNOLOGIES
`Broadly two technologies have been used in the manufacturer of µPs: MOS and Bipolar. The
`majority of µPs available in the market use MOS technology because of its two distinct merits,
`namely, a higher component density and a lower manufacturing cost. The bipolar-technology-
`based µPs are limited to special applications that call for high speeds in which respect MOS
`devices are inferior. Because of the size problem the bipolar µPs are usually made in bit-slice
`configuration; examples being Intel’s 3002 (2-bit slice, TTL), Transitron’s 1601 (4-bit slice, TTL)
`and Texas Instrument’s SBP 0400 (4-bit slice, TIL).
`The first few types of µPs to be announced (e.g., 4004, 4040, 8008) were based on PMOS
`technology, which is now obsolete for µPs because of its speed limitation. The NMOS is the
`main technology today in use for low cost µPs (e.g., 8080, 8085, Z-80, 6800 808, 6800, 8086,
`Z-8000, 68000). The CMOS technology based µPs (e.g., RCA’s COSMAC) have limited application
`because of lower packing density and higher cost. The exceptions are the less cost-sensitive
`military and aerospace applications, where low power dissipation (typical of the CMOS devices)
`is of prime importance.
`
`ASSIGNMENTS
`1. What is a general purpose microcomputer?
`2. Name three classes of computers.
`3. What is the heart of microcomputer system called?
`4. How much ROM is provided in the PC?
`5. What are the standard data word lengths for which microprocessors have been developed?
`6. What is the difference between microprocessor and microcomputer?
`7. Define bit, byte, word and instruction.
`8. What is an assembler?
`9. Explain the difference between compiler and interpreter.
`10. What is an assembler?
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