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DIGITAL FUNDAMENTALS We get into the heart of the microcomputer to discover what makes it tick. LESSON 8: INTRODUCTION TO MICROCOMPUTERS By Louis E. Frenzel AS YOU HAVE SEEN in previous lessons, digital circuits are a collection of gates and flipflops wired together to accept binary inputs from some source, process them, and generate one or more new outputs that will perform some useful function. There is nearly an infinite variety of ways that the various logic elements can be interconnected to process the inputs. The process itself may be nothing more than simple decoding or multiplexing performed by a combinational circuit. Or the circuit may be the more complex sequential type that performs various timing and sequencing operations. In either case, as shown in Fig.1, we can represent the circuit with a "black box." The inputs can come from a variety of sources: from a keyboard, switches of one type or another, transducers that sense various physical values, or binary information from some other piece of equipment that may be under test, evaluation or analysis. The outputs may operate displays such as LEDs, LCDs or CRTs; or drive actuators such as relays, solenoids or motors. Or they may simply be binary words that will be transferred to another piece of equipment for storage or further manipulation. So far, we have assumed that the digital circuits would be made up of individual integrated circuits: gates, flipflops, functional MSI (medium scale integration) circuits, PLAs (programmable logic arrays), or other LSI or VLSI circuits. The circuit may be combinational, sequential, or a mix of the two. However, there is an option to using conventional logic circuits, and that option is called a microcomputer. In this lesson, we are going to discuss microcomputers and show how they can be used to replace large collections of more conventional digital !Cs connected to form a custom circuit for some dedicated ap86 SILICON CHIP plication. In subsequent lessons, we will cover microcomputer input/output techniques and programming. What is a Microcomputer? A microcomputer is a miniature digital computer made with an LSI (large scale integration) circuit, which contains most of the circuitry ordinarily associated with a digital computer. This special LSI circuit is known as a microprocessor. We will speak more about microprocessors in just a minute, but first let's talk about digital computers in general. A digital computer is an electronic device that processes data. Data, of course, refers to binary words or numbers to be used in calculations, or information that must be stored or retrieved, such as ASCII text. Or the data may be simply random collections of binary signals that represent input information that must be processed in some way. Processing refers to the way data is manipulated. In its simplest form, processing may simply refer to the storage and retrieval of the data. Other kinds of processing might be mathematical operations, like addition or subtraction, or logical operations such as AND, OR, XOR and inversion. Processing may also mean Fig.1: when we are interested primarily in input and output signals, we can use a "black box" to represent some kind of digital circuit, combinational or sequential, that processes the inputs to generate new outputs. operations such as searching, sorting, editing or pattern matching. Digital circuits, as we have defined them in this series, meet this definition because they process data, and they may be designed to perform any of the abovementioned functions. Although a digital computer can perform the same functions, it does so in a different way. The key element in the definition of a digital computer is that the processing or manipulation takes place automatically. The digital computer is set up by programming, which specifies the way in which the data is going to be processed. We can accomplish our processing objective by replacing the black box in Fig.1 with a digital computer. The processing is automatic and preprogrammed. Classifying Digital Computers There are three basic types of digital computers: mainframes, minicomputers and microcomputers. Mainframes are the large computers used in business and government for large data processing tasks: they are used for financial and accounting systems, storage, retrieval and manipulation of customer credit files, airline reservations, and the like. Mainframes are super fast in their processing and store enormous amounts of data. Minicomputers are smaller than mainframes but are still very large and powerful. Because of their very high speed, they are generally associated with problem-solving in scientific and engineering applications. However, they are also used for business data processing and other functions. Microcomputers are the smallest classification of digital computers. They are low in cost and small in size. The most visible type is the personal computer which is used for a wide variety of data processing operations. The type of microcomputer we are most interested in, for the purpose of this series, is a special version usually ref erred to as a dedicated controller. This type of microcomputer is designed to perform a specific function. Unlike a personal computer, it is not general purpose in nature and cannot be used with a variety of software packages. Instead, this kind of microcomputer is built into the piece of equipment and is difficult to distinguish from the hardware itself. It is simply the control circuitry for the equipment that happens to be implemented by the microcomputer. In other words, the microcomputer replaces conventional logic circuitry that otherwise would have been implemented with individual gates, flipflops and MSI circuits. Dedicated microcomputer controllers are found in all kinds of equipment, such as TV sets, stereo hifi systems, auto dashboards and emission control systems, photocopiers, and so on. In any case, the key distinguishing characteristic of a microcomputer is that it is implemented with a special LSI device known as a microprocessor. Digital Computer Organisation As shown in Fig.2, a digital computer is made up of four basic sections: the memory, the control section, r CENTRAL PROCESSING UNIT (CPU~ I _ _ _ _ _..;.,__ MEMORY II CONTROL I.......,,._..,...... I I ________ ! _ .._ I I I I I INPUT/OUTPUT (l/0) I ARITHMETIC/LOGIC SECTION l... - - - - - - _.ll-----t'\. "-r------,1' I I _J DATA SIGNALS TO EXTERNAL DEVICES TO BE CONTROLLED OR MONITORED Fig.2: regardless of its type, a computer is made up of four sections - the memory, a control section, an arithmetic/logic unit, and an input/output unit. The control section can be a central processing unit (CPU), or a microprocessor. the arithmetic logic unit, and the input/output unit. Let's take a brief look at each of these sections. Memory The memory in a microcomputer is usually a combination of both RAM and ROM (see Lesson 7 in this series). This memory is used to store two types of information: data and instructions. Data represents those binary numbers or words that are to be processed. They may be numerical values, ASCII codes for the text of a written document, or simply random collections of binary signals that represent inputs or outputs that are collected and organised as binary words. In any case, it is the data words stored in the memory that will be manipulated by the computer. Instructions, which are stored in either RAM or ROM, are unique to computers. They are special binary codes that tell the computer how to manipulate the data. For example, an instruction may be an 8-bit binary number. With eight bits, 256 different instructions could be represented, which might specify arithmetic operations such as addition and subtraction, a logical operation such as AND or XOR, or data movement operations that cause binary words to be moved into or out of memory or cause transfers between registers. All computers have a special repertoire of these special codes known as an instruction set. They define the architecture of the computer and provide a wide range of ways in which the data can be manipulated. To process data, a number of instructions are written sequentially and stored in memory. Such a sequential list of instructions is called a program. A program defines a specific sequence of operations that process the data in some way. The purpose of the computer is to sequentially interpret and execute these instructions stored in memory and this, in turn, accomplishes the processing. JUNE 1988 87 This approach to digital processing is generally known as the stored program concept. It was invented by mathematician John Von Neumann and is the basis of operation for all digital computers. Control The control section of the digital computer fetches the instructions stored in memory one at time, interprets them, and executes them in sequence. The control unit gets one instruction from memory, decodes it and determines which function is to be performed next. It then issues control signals to the other sections of the computer so that the specified operations are carried out. Arithmetic-Logic The arithmetic/logic section is the section of the digital computer that generally carries out most processing operations. It is usually made up of a set of registers where the data to be manipulated is temporarily stored. In turn, these registers drive an arithmetic/logic unit (ALU), which is a collection of logical circuits which perform mathematical and logical operations. Serial shift and rotate operations can also be performed. It is usually the ALU that receives signals from the control unit to perform the operation specified by the instruction. As shown in Fig.2, the control section and ALU are closely related and interconnected. For the most part, they can be treated as a single block or section. The combination is usually called a central processing unit, or CPU. A microprocessor is simply a single-chip LSI CPU, which is sometimes referred to as a microprocessing unit, or CPU. How a Microprocessor Works Fig.3 shows a block diagram of a microprocessor. This has has been divided into two primary sections: the arithmetic/logic section and the control section. The control section of the CPU contains the instruction register and the program counter. The program counter holds the address of the memory location where an instruction is stored. To execute a program, the program counter is set to the address of the first instruction in the program. The content of the program counter (ie, the instruction address) is then transferred to an address register in memory. The address location is decoded (usually on the RAM or ROM chips) and that location in memory is enabled. The instruction stored there is transferred over the data bus into the instruction register. The instruction decoder looks at the instruction word and identifies the function to be performed. The timing and control circuits in the control section then generate the apDATA/INSTRUCTIONS TO/FROM MEMORY r----------- ---7 I I PROGRAM COUNTER I I I CONTROL SECTION A Typical Microcomputer The typical microcomputer consists of a single chip microprocessor, a set of RAM chips, and one or more ROMs that contain a dedicated control program. The input/output section is usually made with integrated circuits which have been specifically designed to interface the microcomputer to the external circuits or equipment involved in the application. For small, dedicated applications, single-chip microcomputers can be used. These devices contain the CPU, the ROM where the dedicated control program is stored, a small amount of RAM where data can be stored temporarily, and a variety of input/output circuits, which connect to the equipment being controlled. A good example of an application is the microcomputer used in most printers. SILICON CHIP I I TIMING AND CONTROL CIRCUITS The input/output (1/0) section of a digital computer is used to communicate with external circuits and equipment. Inputs to be processed are fed to the 1/0 section and either stored in memory or processed directly by the CPU. Binary words that are to be transferred to some external circuit or device are transferred from the memory or the CPU to the external equipment via the 1/0 section. As you might expect, special input/output instructions to the CPU are used to control the 1/0 section. I I 1/0 88 ADDRESS TO MEMORY I I I CONTROL SIGNALS TO ALL OTHER SECTIONS I r--------------1 I I - - -..✓ I ---II ...'"'ft"-~- I I ALU II I ____ I II II ---ARITHMETIC-LOGIC SECTION JI L....--------------- Fig.3: a CPU generally provides two independent functions - a control section and an arithmetic/logic unit which are interconnected through an external data bus. propriate control pulses that cause the desired action to be carried out. Once the instruction has been executed, the program counter is incremented so that the next instruction in sequence is fetched and executed. This process continues until the program is fully executed. The arithmetic/logic section of the computer consists of a main working register called the ac- 16-BIT REGISTERS 8-BIT REGISTERS 16-BIT REGISTERS 8-BIT REGISTERS PROGRAM COUNTER ACCUMULATOR A REGISTER PROGRAM COUNTER ACCUMULATOR REGISTER INDEX REGISTER ACCUMULATOR B REGISTER INDEX REGISTER X INSTRUCTION REGISTER STACK POINTER INSTRUCTION REGISTER INDEX REGISTER Y STATUS REGISTER CONDITION CODE REGISTER STACK POINTER REGISTER 6502 6800 (b) (a) Fig. 4: although there are differences in the internal architecture of the 6800 and 6502, both use the program counter to point to the next instruction in sequence. cumulator and the arithmetic/logic unit (ALU). The arithmetic/logic section carries out most of the operations designated by the computer's instruction set. All data transfers and arithmetic/logic operations take place in the accumulator. Most arithmetic and logic operations involve two operands. (Operand is just the name of a number to be involved in an arithmetic or logic computation). For example, an add operation involves the two numbers to be summed. One of the operands is stored in the accumulator while the other is stored in memory. The lwo operands are then used in the desired operation. The result of the operation, in this case the sum, is stored back in the accumulator. The operand previously stored in the accumulator is lost. The ALU in most microprocessors is capable of carrying out addition and subtraction as well as the basic logic operations AND, OR, XOR, and complement. Other instructions are used to manipulate data in the accumulator. For example, the accumulator can be cleared (set to zero), incremented, or decremented. Data can also be transferred from a desired memory location to the accumulator or taken from the accumulator and stored in a desired memory location. The arithmetic/logic section also permits data in the accumulator to be shifted or rotated to the right or to the left of a given number of bit positions. Typical Microprocessors Now let's look at a real microprocessor: in fact , we will examine two units which are similar in architecture and design. The first is the 6800 which, although introduced in the mid-1970s, is still in use today. The 16800 was used in some of the earliest personal computers, among them the MITS Altair 680, Southwest Technical Products SWTP 6800, and the Wavemate, but it has now virtually disappeared in this application. However, the 6800 is still widely used as a dedicated controller. The other microprocessor we will look at is the 6502. This CPU was designed by the same group that created the 6800. They left Motorola and developed the 6502 for MOS Technology. The 6502 is an improved or optimised version of the 6800. It too was widely used in personal computers and is still used in personal computers such as the Apple Ile/c and Commodore 64/128. Both the 6800 and 6502 have an architecture and operation that is simple and straightforward. In fact, they are essentially the same as the generic microprocessor described earlier. The 6800 and 6502 are 8-bit microprocessors (all microprocessors are rated or ranked by the basic number of bits they process simultaneously). That is, data transfers and arithmetic or logic operations are made on parallel 8-bit binary numbers or words. The 6800 and 6502 have 8-bit internal registers, an 8-bit ALU and an 8-bit data bus through which all data transfers between CPU, memory and I/O take place. A general block diagram of each of these microprocessors is shown in Fig.4. Note that only the main registers are shown. These will be explained next. CPU Registers The most predominant circuit in a micro (short for microprocessor) is the register, which is capable of storing one binary word. As the data and instruction words are moved from one place to another they are typically passed through or temporarily stored in the various registers. As data is processed, words are transferred into and out of these registers from external sources such as the memory and I/O devices. In addition, inter-register transfers in the micro also occur during processing. All micros have three basic registers: the program counter (PC); the instruction register (IR) and the accumulator (ACC). Fig.4 shows the register structure for the 6800 and 6502. Let's look at each. Program Counter The program counter (PC) contains the address of the next instruction to be fetched. As each instruction in a program is fetched and executed, the program counter is incremented so that it points to the next instruction in the sequence. The program counter also specifies how many bytes of RAM and ROM a CPU can address. The PC's output is sent to the memory where it selects a desired word. In the 6800 and 6502. the PC holds a 16-bit word, therefore, 216 or 65,536 (64K) words of RAM and/or ROM can be addressed. Instruction Register When the CPU fetches an instruction word from memory to be executed, that word is stored in the inJU NE 1988 89 struction register (IR). The word is then decoded to determine which operation is to be performed. In the 6800/6502, the instruction register holds an 8-bit instruction word or an op code as it is called. Accumulator The accumulator is the basic processing register of the micro. Words to be used in arithmetic or logic operations are stored here. Data transfers to or from memory and input/output devices are also passed through the accumulator. Not surprisingly, the accumulator size in the 6800 and 6502 is 8-bits. While all micros contain at least one accumulator register, some have more than one. By using more than one accumulator significant increases in speed can be achieved. Also, programs written for multiple accumulator machines typically involve fewer instructions and less programming effort, which significantly improves use of the available memory space. The 6800 has two accumulators, the 6502 has one. Some CPUs have sets of 8 or 16 accumulators usually called general-purpose registers. All micros feature these basic registers but they also feature additional registers, which further improve operations. Some of these are as follows. Index Register An index register stores a binary word that is used in address-modification operations. Typically, the content of the index register is added or subtracted from the address associated with an instruction. The index register content can be loaded from memory, stored in memory, incremented, or decremented with special index register instructions. This process of using an index register for address operations is called indexing. By using an index register, the number of instructions used in some programs can be significantly reduced. This is particularly true where sequential operations on a list or table of Mailbag continued from page 3 conductor. This arrangement has been prohibited by AS 3000 for some 12 years. The standard now requires that the main earthing conductor be run directly to an earth electrode. Where the water piping is metallically continuous from inside a building to the point of contact with the ground, such piping must be connected by an equipotential bonding conductor to the main earthing conductor. With the increasing use of non-metallic water piping, this bonding may not be necessary. • You refer to the fact that some current flowing to earth will do so via the earth electrode and water pipe as a "previously unconsidered fact". This is simply not true. The advantages and disadvantages of the MEN 90 SILICON CHIP data are to be performed. The 6800 has one 16-bit index register, the 6502 has two. Status Register The status or condition-code register is a group of flipflops that are either set or reset - depending upon the outcome of processing operations in the ALU. As arithmetic, logic or shift operations are performed, the various status flipflops are set or reset to indicate a specific machine state. The status word may be monitored and stored so that the condition of the computer at a given time can be determined. The various flipflops in the status word can also be tested under program control so that the program being executed can be modified. Jump or branch conditions that change the program sequence are usually determined by the information stored in the status register. Some of the conditions monitored by the conditioncode register are arithmetic operations, such as: accumulator equals zero, carry out of the most significant bit of the accumulator, accumulator overflow, and accumulator negative. In the 6800 and 6502, the status or condition code register contains eight bits. Stack Pointer The stack pointer is a 16-bit address register that is used to reference some particular part of the. micra's random-access memory. The stack itself is a specific portion of memory set aside to temporarily store data in a particular sequence. The stack pointer is used to address this data when it is being stored or retrieved. The stack is set aside especially for stack operations and is not used to store ordinary sequences of instructions or data. Nor does it have a fixed size. The number of memory locations used by the stack depends upon how it is used. The stack is a last-in, first-out (LIFO) memory. The data words to be stored in the stack are written and system are well understood by both SAA Committee EL/1, who are responsible for AS 3000, and the Electricity Supply Association of Australia. Every method of earthing has such advantages and disadvantages and is a point which would be conceded by supply authorities throughout the world. • Connections between an earth bonding clamp and an earth electrode may be subject to corrosion as you suggest. However, such connections, where exposed to weather, are required to be suitably protected. Also, the standards for household appliances that are likely to produce DC current, such as TV sets, hair dryers, and the like, have requirements which limit the amount of current that can be injected into the household wiring. Based on feedback from the elec- trical contracting industry and supply authorities, it is suggested that corrosion of an extent likely to degrade electrical safety is not a common occurrence. • While a warning is included in the article to point out that unlicenced persons should not tamper with any wiring, I would like further warning against the practice you describe and picture concerning the 3-core extension cord. Removal of the sheathing of a flexible cord to expose single insulated cable and then to further tamper with such wiring is not a practice which could be recommended. I hope these points have clarified some aspects of modern wiring arrangements. J. C. Tinslay Executive Officer Standards Association of Australia OP CODE OP CODE HALF OF ADDRESS OP CODE ADDRESS OR DATA HALF OF ADDRESS (a) SINGLE WORD (b) TWO WORO (c) THREE WORD Fig.5: the number of 8-bit (byte) instruction words depends on the particular computer. A one-byte word provides only the op code. A two-byte word provides the op code and an address or operand to be processed. A three-byte word provides both the op code and a 16-bit address. retrieved sequentially so that the last item stored is the first item to be retrieved; the first data item stored will be the last retrieved. The stack pointer register is used to determine the limits of the stack and to identify specific word locations in the stack. In the 6800 and 6502, the stack pointer register is 16-bits, and it can point to any one of 65,536 different memory locations. This means that the stack can be located anywhere within the maximum addressing range of the microprocessor. To set up the boundaries of the stack, the stack pointer register is usually loaded under program control with special instructions used for this purpose. Once the stack pointer has been initialised, it is then incremented or decremented to access sequential memory locations. Stack, store and retrieve operations are called push and pull (or pop) respectively. The Instruction Set The instruction set is the complete list of instructions that a micro is capable of executing. The programmer uses the instruction set to develop complete programs that perform a desired process, calculation, or control function. It is the instruction set that really defines the architecture of a micro. It specifies the number and types of registers and the logic circuits and how they are all interconnected. The instruction set for each micro is fixed and is unique to that device. Types of Instructions Two types of instructions are used in microprocessors: memory-reference and non-memory reference. A memory-reference instruction identifies some particular memory location where the operand to be used by that instruction is stored. The instruction usually contains an address that designates the location of the operand. For example, an ADD instruction contains an address that points to one of the operands to be added. The other operand is usually in the accumulator. A non-memory reference instruction does not have an address associated with it. This type of instruction simply defines a type of operation to be performed somewhere in the computer. The location of any data to be used, if any, is usually in a CPU register. Instructions are further classified by the types of operations they perform. Some specific types are listed below. Data Movement Instructions - specify data transfers from one location to another. The transfers can take place between internal registers or between the internal registers and the computer's memory. Arithmetic/Logic Instructions - identify unique arithmetic and logic operations such as add, subtract, logical AND, compare, or other operations. Data shift and rotate operations are usually included in this class of instructions. Decision-Making Instructions - test for certain conditions in the machine and cause the sequence of the program to be modified. If the test condition is met, the operation specified by the instruction is performed or otherwise the next instruction in sequence is executed. Usually, the operation is a jump or branch operation that causes the micro to begin executing a sequence of instructions different from the normal sequence specified by the program. Such instructions allow the micro to make decisions based on conditions that exist in the CPU or external circuits. Input/Output Instructions - cause data transfers to take place between the CPU and the I/O interface. Instructions for handling interrupts are usually included in this instruction class. Not all microprocessors have I/O instructions. For example, the 6800 and 6502 do not have I/O instructions as such; they simply use the data movement instructions to enable data transfers to external devices and circuits. Instruction-Word Formats - all microcomputers have a basic fixed word length. The 6800 and 6502 both feature an 8-bit word. The memory is organised as many sequential storage locations for 8-bit words. These words (bytes) may be data or instructions. Usually, data words are eight bits or less in length. However, this limits the number size to a maximum of 255; additional memory locations may be allocated if greater number sizes are required. For example, two sequential memory locations can be used to store a 16-bit word, thereby increasing the number size up to 32,767. (One-half of the word would be stored in each of the two adjacent memory locations). However, keep in mind that the micro will only process eight bits of data at a time. Instruction words are similar. Some instructions can be defined by a single 8-bit word. Others require two or three sequential 8-bit words. Fig.5 shows the three most commonly used instruction word formats. In Fig.5a, a single 8-bit word defines an instruction. This byte usually contains the op code - a specific bit pattern that causes some unique operation to take place. Such single word instructions are usually nonmemory reference instructions since they do not contain an address. Fig.5b shows a two-byte instruction. The first eight bits define the op code. The second 8-bit word in an adjacent memory location usually contains an address of the operand to be processed. When the instruction is executed, the operand stored in the second word location is used. If the second word is an address, it references some location in memory where the operand is stored. Fig.5c shows a three-byte instruction. As usual, the JUNE 1988 91 first 8-bit word contains the op code while the next two 8-bit words contain a 16-bit address which identifies the memory location where the operand is stored. MICROPROCESSOR (CPU) Addressing Modes Another important part of the architecture of a micro is the way that it addresses data or instruction words. The more different ways that memory words can be referenced, the more powerful and flexible the micro is. Many of the addressing modes greatly speed up and simplify the processing operations. The following addressing modes are used in most micros, including the 6800 and the 6502. Implied No specific address is used with implied addressing; instead, the location of the operand to be used in the processing is implied by the instruction itself. Implied addressing is used with non-memory reference locations. With these instructions, the operand is usually stored in a register that is the subject of the given instruction. For example, accumulator increment, decrement, or shift instructions imply that the operation is to take place on the operand stored in the accumulator. Immediate Immediate addressing assumes that the operand is contained within the instruction itself, usually as the second byte of a two-byte instruction word. With this arrangement, the operand is available immediately for processing, thereby eliminating the need to address memory and to perform a read operation prior to executing the instruction. Immediate instructions are used to speed up computation. Direct When the direct addressing mode is used, the address bits are a part of the instruction word. The adADDRESS 14 (DATA) ADD INSTRUCTION 63 (DATA) STA INSTRUCTION HALF OF ADDRESS HIIIII} = HALF OF ADDRESS H111111 16-BITS 1111111111111111 65535 ""' ~ 1 - - - - - ~ Fig.6: the microprocessor communicates with the memory and input/output functions through a data bus, an address bus and a control bus. The control bus provides the functions other than data transfers and memory addressing. 92 SILICON CHIP dress may be simply the second 8-bit byte of a 2-byte instruction, or it may be defined as the second and third bytes of a 3-byte instruction. Direct addressing is the simplest and most intuitive of all the address modes, and is the one most often used. Remember, the total number of address bits determines the maximum number of memory locations that can be referenced. In the 6800 and 6502, the address word size is 16 bits, thereby permitting a total of 65,536 words (64K bytes) to be addressed. Direct addressing is sometimes referred to as absolute addressing. Register In register addressing, the op code specifies a register where the operand is stored. Register Indirect In this address mode, the instruction op code specifies a register that contains the address of the operand. This register must be loaded with the proper memory address prior to executing the instruction. Relative RAM LOA INSTRUCTION 4 TO EXTERNAL Fig.7: program instructions for the computer are stored in memory in sequential order. An individual address might contain an op code, a data value, or another address where the program stores or retrieves data. In the relative addressing mode, the effective address of the operand to be used by the instruction is obtained by adding the direct address in the instruction word itself to the contents of the program counter. The address in the instruction word is used to specify a displacement from the address of the instruction currently being executed. (This address is contained in the program counter). This method of addressing permits the program and the associated data to be relocated anywhere in memory without changing the direct addresses in the program. The relocation of the program is simply a matter of adjusting the value of the program counter. Indexing Indexing or indexed addressing was discussed earlier in connection with the index register. As you may recall, the effective address designating the storage location of the operand is formed by adding the address in the instruction word to the contents of The microprocessor, or MPU, is just a CPU. To form a complete computer, memory, I/O and other circuits must be added. The MPU is usually indicated as a single box, as shown in Fig.6. The MPU communicates with the other circuits by way of many input and output lines. These lines are typically organised as buses. For example, all 8-bit data transfers into and out of the MPU take place over the data bus. [The data bus is eight lines over which data can travel in either direction). Another group of 16 lines on the MPU forms the address bus. An address from the program counter or other source in the MPU is put on the bus and sent to RAM, ROM, or an I/O device. The address identifies and enables a memory location or I/O circuit which will accept data from or send data to the MPU. The remaining lines of the MPU are collectively known as the control bus. These input and output signals are used to control MPU operation. value (14) to be transferred to the accumulator. This instruction uses immediate addressing. Locations 2 and 3 hold an add [ADD) instruction. Byte 2 is the op code, while byte 3 is the data word (63) to be added to the value in the accumulator. Locations 4, 5 and 6 hold a store [ST A) instruction. Byte 4 is the op code, while bytes 5 and 6 form a 16-bit address that tells where in RAM the contents of the accumulator will be stored. To start the program, the program counter - the PC - is loaded with O so that it will access the first instruction in the program. Byte O is loaded into the instruction register and decoded. The PC is incremented and the data in byte 1 is loaded into the accumulator. The PC is now incremented again and the ADD instruction is fetched from byte 2 and put into the instruction register. After decoding, the PC is incremented and the data in byte 3 is accessed, then added by the ALU to the data word in the accumulator. The sum 77 then appears in the accumulator. The PC is then incremented again to fetch the STA instruction in byte 4. The op code is loaded into the instruction register. Next, the PC is incremented twice to bring in bytes 5 and 6. Together they form a 16-bit address [65 ,535) that is sent to the RAM instead of the PC content. This enables the selected RAM location. The sum in the accumulator is sent via the data bus to this location. The program ends at this point. Executing a Program The Next Lesson the index register. Index registers can be loaded, stored, incremented, or decremented by using special index-register instructions. In addition, the contents of the index register may also be tested by decisionmaking instructions. Making a Microcomputer with a Microprocessor To complete our discussion of microprocessor operation, let's take a look at how a 6800 or 6502 would execute a simple program. Refer to Fig.7. This shows several bytes of RAM where the program is stored. In memory locations O and 1, a load accumulator [LDA) instruction is stored. The first byte contains the op code, the second contains the data In the next lesson we will cover I/O operations. Then, in the final lesson, we will examine other 6800 and 6502 instructions and show you the processes used to create programs that perform a variety of functions. ~ Reproduced from HANDS-ON ELECTRONICS by arrangement. Copyright (c) Gernsback Publications, USA. SHORT QUIZ ON DIGITAL FUNDAMENTALS 1. A microprocessor is an LSI _ _ _ _ _ _ __ 2. The four major parts of a digital computer are: a. b. c. d. LESSON 8 8. An area of RAM used to temporarily store data is called the _ _ _ . The register that contains the address of this area is called the _ _ _ _ __ 9. The address of the operand to be used in a computation is usually contained within the _ __ 3. A CPU is made up of the _ _ _ _ _ section and the _ _ _ _ _ _ section. 10. A microcomputer that replaces conventional logic circuits in performing a specific function is often called a ______________ 4. The two kinds of information stored in a computer memory are ___ and _ _ _ words. 11 . Communication with the world external to a microcomputer takes place via the _ _ _ section. 5. A program is a sequential list of ______ that performs a specific function. 6. The main processing register in a CPU is the 7. The register that points to the next instruction to be fetched and also deterrnJnes maximum memory size is called the __________ ANSWERS 0 /1 . ~ ~ Je110Jiuo:> pere:>1pap ·o ~ uo1pnJisu1 ·6 J8lU!0d >PBlS ')1:>BlS ·g Jaiuno:> weJ5oJd · L JOre1nwn:>:>e ·9 suo1pnJisu1 ·g SU0!PnJlSU! 'BlBP ·17 :>!50f/:>!I8Wl.fl!JB 'I0JlU0:> '8 0/1 ·p :>!50i/:>!l8Wl.fl!JB ·:> !0JlU0:> ·q A.Jowaw ·e l!Un 6u1sse:>0Jd IBJlU8:) · ~ ·z JUNE 1988 93