Introduction — what this topic is and why it matters
The CPU (Central Processing Unit) is the "brain" of a computer: the component that actually executes the instructions of a program. CPU organization is the study of the internal building blocks of that brain — the ALU, the control unit, and the registers — and how they are wired together by internal buses. The instruction cycle (also called the fetch–decode–execute cycle) is the never-ending loop the CPU repeats for every single instruction, from the moment you turn the machine on until you shut it down.
This is one of the highest-yield topics in BCS, Bank IT-officer and other government IT examinations. Questions appear in almost every paper — "What does the PC register hold?", "Which register holds the instruction currently being executed?", "What are the steps of the machine cycle?", "What is the function of the MAR/MBR?", "What is a fetch cycle?". If you master the small set of registers and the exact ordering of the micro-steps, you can answer any variation of these questions with confidence.
Learning objectives
After studying this lesson you will be able to:
- Name and describe the three main functional units of the CPU (ALU, CU, register set) and the buses that connect them.
- Define every important CPU register (PC, IR, MAR, MBR/MDR, AC, SP, PSW, general-purpose registers) and state exactly what each holds.
- List, in the correct order, the phases of the instruction cycle: fetch, decode, execute, and (when needed) operand-fetch and interrupt phases.
- Trace the micro-operations of a fetch cycle using register-transfer notation.
- Distinguish an instruction cycle from a machine cycle and a clock cycle.
- Calculate execution time, clock period, CPI, MIPS and effective CPU time from given numbers.
- Explain interrupts, the interrupt cycle, and how instruction pipelining overlaps cycles.
- Avoid the classic exam traps (PC vs IR, MAR vs MBR, instruction vs machine cycle).