Execution of an instruction

The execution of an instruction by the computer means completion of the work by the computer as a specified by the instruction, the fetch cycle brings the instruction to the Instruction Register, which holds this instruction temporarily as long as it is being interpreted (decoded) and executed, for the computer having word size of say 16 bit the instruction held in IR has two parts namely, 1. a 4 bit operation code 2. 12 bit address of operand.
The operation code is transferred to the control unit, this control unit sends op-code to decoder where it is decoded, after decoding the op-code the control unit issues commands in the form of control signals to various other units such as arithmetic logic unit to perform the necessary operations, the address part of instruction in IR is transferred to a decoder which decodes it as address of a particular memory location, this address id transmitted to memory  address register (MAR) which transmits it to the main memory, under the directions of the control unit the operand stored in that particular memory location is transmitted to the memory Buffer Register from where it is transmitted to the ALU

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