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Control unit figure 7 control unit specialized

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Control UnitFigure 7: Control UnitSpecialized electronic circuits (fetch and decode unit) in the control unit is designed tofetch and decode program instructions held in the main memory. Each instruction isread from the memory into the instruction register. The process of reading an instructionis often referred to as thefetch-execute process.1.3.2Arithmetic-Logic Unit (ALU)The arithmetic Logic unit (ALU) of a computer system is the place where the actualexecution takes place.All the Arithmetic calculations such as addition, subtraction, multiplication, anddivision are performed and all comparisons (decisions) are made in the ALU.Relational operator (=, <, >), i.e., equal to, less-than and greater-than are used todescribe the comparison operations by the ALU.ALU carries out arithmetic operations on integer (whole number) and real (witha decimal point) operands. It performs simple logical tests for integers operandsonly.The data and instructions, stored in the primary memory prior to the processing,are transferred as and when need to the ALU, when processing takes place.13Instruction RegisterProgram Counter+1Fetch UnitDecode UnitControl Logic Unit
Hardware and Software: AnIntroductionNo processing is done in the primary storage unit (or memory).Intermediate results generated in the ALU are temporarily transferred back tothe primary storage unit until needed at later time.Data may thus move from primary storage to ALU and back again to storagemany times before the processing is over.After the completion of processing, the final results, which are stored in thestorage unit, are released to an output device (monitor/screen or printer).Theaccumulatoris used to accumulate results. It is the place where theanswers of many operation are stored temporarily before being sent to thecomputer's memory (see Figure 6). Only the final result is transmitted to thememory unit for storage.To understand the operation of CU, ALU etc., let us execute the statement(or command) R=n1+n2. This statement adds two numbers n1and n2andplaces the result in a location X (refer the Figure 8).14Let R = n1+n2ADD INSTRUCTIONNUMBER 1NUMBER 2RESULTNumbers 1(n1)+CENTRAL PROCESSING UNITCONTROL UNITMEMORY UNITARITHMETIC AND LOGIC UNIT(ALU)MemoryLocationsNumbers 2(n2)Result R(ACCUMULATOR)Step 3The control unitsignals the ALUperform “additionoperation” on thetwo numbers andto store the resultin theaccumulatorStep 2The twonumbers arebrought frommemory andplaced in theALUStep 1Moveinstruction tothe control unitStep 4Send result inmemoryFigure 8: CPU Operation to find data and instructions (Adding two numbers)
Introduction to PC1.4TYPES OF MAIN MEMORYThe data and instructions which are entered into the computer system through inputdevices (keyboard, mouse etc.) have to be stored inside the computer before the actualprocessing starts. Similarly, after processing, the results produced by the computer mustbe kept somewhere inside the computer system before being passed onto the outputdevices (monitor/screen or printer). Thus, the storage unit (primary/main memory) of acomputer system is designed to take care of all these needs.

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Computer data storage

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