-conn b/w the processor and memory
individual instructions are brought form the memory into the processor for a specific operation
data to be used as operands are also sotred in the memory
MAR = memory addr register - holds the addr loc to be accessed
MDR = memory data register - stores the data to read/write an addr
PC = program counter
IR = insrtuction register
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execute a prog starts when pc is set to point the first instr of the prog
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contents of pc are transfered to MAR
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"read control signal" is sent to the memory
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addr word is read to memory and loaded to MDR
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contents of MDR are transferred to the IR and now decoded and executed
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if it is related to ALU also try to obtain the required operands
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when operand is read it is tranfered to ALU from MDR
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after a couple of repeated cycles the ALU can perform the desired operation
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result sent to MDR
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addr of the location of the result is stored to MAR & write cycle is initiated
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contents of the pc is incremented and points to next instruction/location
the decoded instr are stored to IR
instru eg:-> add LOCA,R0 = add the value of loca nad r0 and sotre it in r0
which registeers can interract with the secondary storage- MAR
During the executino of a prog PC is initilised
MAR is connected to Memory