For the execution of an instruction, we need to perform an instruction cycle.  An instruction cycle consists of two phase,

• Fetch  cycle  and
• Execution   cycle.

Most of the operation of a CPU can be carried out by performing one or more of the following functions in some prespecified sequence:

1. Fetch the contents of a given memory location and load them into a CPU register.
2. Store a word of data from a CPU register into a given memory location.
3. Transfer a word of data from one CPU register to another or to the ALU.
4. Perform an arithmatic or logic operation, and store the result in a CPU register.

Now we will examine the way in which each of the above functions is implemented in a computer. Fetching a Word from Memory:

Information is stored in memory location indentified by their address. To fetch a word from memory, the CPU has to specify the address of the memory location where this information is stored and request a Read operation. The information may include both, the data for an operation or the instruction of a program which is available in main memory.