Instruction Cycle Definitions

Decoder

A circuit that decides which instruction the processor will execute. For example, It takes the instruction op-code from the IR as input and outputs a signal to the ALU to control the execution of the ADD instruction.

Program Counter (PC)

A register that holds the address of the next instruction to be fetched in the fetch execute cycle.

Instruction Register (IR)

A register that stores the instruction to be executed by the processor.

Memory Data Register (MDR)

A register used to store data that is being sent to or received from the MEM. The data that it stores can either be in the form of instructions or simple data such as an integer.

Memory Address Register (MAR)

A register used to store the address to a specific memory location in Main Storage so that data can be written to or read from that location.

Arithmetic Logic Unit (ALU)

Used to execute mathematical instructions such as ADD or SUB.

Accumulator (A)

Used to store data to be used as input to the ALU

Main Storage (MEM)

Used to store programs and data. Random Access Memory (RAM) is an implementation of MEM.