The circuits used in the CPU during the cycle are:
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Program counter (PC) - an incrementing counter that keeps track of the memory address of the instruction that is to be executed next.
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Memory address register (MAR) - holds the address of a memory block to be read from or written to.
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Memory data register (MDR) - a two-way register that holds data fetched from memory (and ready for the CPU to process) or data waiting to be stored in memory
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Instruction register (IR) - a temporary holding ground for the instruction that has just been fetched from memory
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Control unit (CU) - decodes the program instruction in the IR, selecting machine resources such as a data source register and a particular arithmetic operation, and coordinates activation of those resources
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Arithmetic logic unit (ALU) - performs mathematical and logical operations
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