Execution of a Complete Instruction

3.1 Introduction
An instruction cycle consists of an instruction fetch, followed by zero or more operand fetches, followed by zero or more operand stores, followed by an interrupt check (if interrupts are enabled) The major computer system components (processor, main memory, I/O modules)
need to be interconnected in order to exchange data and control signals. The most popular means on interconnection is the use of a shared system bus consisting on multiple lines.

3.2 Control unit
A computer can be programmed by using a small set of basic logic components that store binary data and perform arithmetic and logical operations on data. If a particular computation is to be performed, a configuration of logic components designed specifically for that computation could be constructed. Programming in hardware is termed a hardwired program. This customized hardware system accepts data and produces results. This customized hardware system is not very
flexible because for each new program, this customized hardware must be rewired.
As an alternative, construct a general-purpose configuration of arithmetic and logic functions. This set of hardware will perform various functions on data depending on control signals applied to the hardware. This general-purpose hardware system accepts data and control signals and produces results. For each new program, the programmer merely needs to supply a new set of control signals. How shall control signals be supplied? The entire program is actually a sequence of steps. At each step, some arithmetic or logic operation is performed on some
data. For each step, a new set of control signals is needed. Simply provide a unique code for each possible set of control signals, and add to the general-purpose hardware a segment that can accept a code and generate control signals.
Programming is now much easier. Instead of rewiring the hardware for each new program, simply provide a new sequence of codes. Each code is, in effect, an instruction, and part of the hardware interprets each instruction and generates
control signals. To distinguish this new method of programming, a sequence of codes or instructions is called software(microprogram).