-The activity in computers is governed by Instruction. 

-To perform a given tosk, a set of instructions (program) is stored in memory.

→ Individual instructions are brought from the memory into the processor, which execute the specific operation.

– Data to be used of operands are also stored in memory

Example 

The instruction reined the several steps

1) The instruction is fetched from the memory into the processor.

2)Then operand at LOCA is fetched and added to the content of RO.

3)Resulting Sum is stored in RO.

→ The Add instruction comling a memory access operation with an ALU operation

Henceforth these two types of operation are performed by separate instruction for performance Reasons 

               Load LOCA, R₁   ; LOCA→R₁

               ADD R, Ro          ; R₁+Ro→Ro

1)The First instruction transfer the contents of memory location LOCA into processor Register R₁ 

2)The second instruction add  Ro & R₁ content and place the sum at Ro.

* Transfer between memory and the processor are sorted by sending the address of the memory location to be accessed to the memory unit and issuing appropriate control signals. Then data transfers from memory