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.Each different microprocessor type has its ownunique set of control signals, which it can generate or respond to.Many control bus signals are common to most microprocessors(or similar to those used by most processors).The following arethe more common control signals in use today:.System Clock (SYSCLK).Read/Write Line (R/W Line).Memory Read (MEMR).Memory Write (MEMW).I/O Read (IOR).I/O Write (IOW)One of the most important control signals in any microprocessor-based system is the system clock.This signal provides the timing infor-mation around which all the system s activities take place.Dependingupon the type of microprocessor being used, the clock signals maybe generated on the microprocessor chip, or by special IC signal A+ Certification Training Guide38generators.Microprocessors with internal clock generators usuallyrequire that an external crystal, or RC network, be connected totheir clock input pins.The control bus also carries the signals that enable selected mem-ory or I/O elements for Read and Write operations.These signalsmay range from a simple Read/Write line (R/W) to a collection ofsignals such as Memory Read (MEMR), Memory Write (MEMW), I/ORead (IOR), and I/O Write (IOW).These signals are used by themicroprocessor in conjunction with addresses on the address busto perform Read and Write operations at selected memory or I/Olocations.Microprocessor OperationTo better understand how a microprocessor-based system functions,consider a simplified computer system, which, for the purposes ofthis example, will be called the $1.98 Computer.This system is basedon a mythical 8-bit microprocessor, which has a 4-bit address busand is capable of performing 16 different operations.The 4-bitaddress bus means that this processor is capable of addressing only16 different memory locations, but, for these applications, thisshould be plenty.Consider your mythical microprocessor, its inter-nal block diagram, and a 16§8 (16 address locations, each storing8 bits) RAM memory block.The computer s input and output unitsdo not actually come into play during the discussions of the sys-tem s operation.Simply assume that the programs have beenentered into the RAM memory through the input unit and maybe displayed through the output unit.Internal RegistersThe microprocessor consists of a group of Internal Registers, anArithmetic Logic Unit (ALU), and a Control Unit.Different micro-processors have different numbers and types of internal registers.The ones depicted here are the same as, or similar to, the regis-ters found in nearly any microprocessor.They are described asfollows: Chapter 1 Microcomputer Fundamentals39Accumulator (ACCUM): This type of register is generally used bythe microprocessor to store the results of ALU operations.It isalso a source of one operand for most ALU operations.Manymicroprocessors contain more than one accumulator register.Program Counter (PC): This register/counter keeps track of In-struction Addresses and is always pointing at the address of thenext instruction to be fetched from memory.Each time an in-struction is fetched from memory, the PC is incremented by 1.The control unit may cause the PC to jump to an address out of itsnormal sequential order.When the control unit receives a branchinstruction, such as a JUMP (JMP) or a JUMP-ON-ZERO (JPZ)command, the control unit causes the PC to be loaded with anaddress portion of the instruction word.Upon execution of thejump instruction, the PC continues its normal sequential count,beginning at the new address.Note In the example, the PC is automatically reset to a value of 0.This corresponds to the beginning of the program.In realcomputers, the program counter is reset to some predeter-mined value, such as a memory location in ROM memorycontaining the monitor program.This location is referred to asthe microprocessor s vector address and is determined by themanufacturer of the microprocessor.Address Register (AR): This register is used to hold the addresscurrently being accessed by the microprocessor
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