CPU Structure and Function

Computer Organization & Embedded System — Control and Central Processing Units, NEC licence examination syllabus (Nepal Engineering Council).

CPU Structure and the Instruction Cycle

Zooming out from control memory to the whole processor.

Step back for a second — every single app, game, and website you've ever used eventually boils down to this one repeating loop running billions of times a second. Every CPU is built from four core pieces: an ALU to compute, registers to hold data, a control unit to manage everything, and a bus connecting them so data can move around.

👀 User-visible vs Control/Status Registers

User-visibleGeneral-purpose registers a programmer can use directly in assembly code.

Control/statusPC, IR, and flags — work only inside the CPU; the programmer never touches them directly.

Every CPU runs the same endless loop, called the instruction cycle: Fetch → Decode → Execute → check Interrupt → repeat.

Fetch gets the next instruction using PC. Decode figures out what it means. Execute performs it. Then the CPU checks for a pending interrupt before starting the next fetch.

💡 Draw this diagram with the indirect-addressing state added too (between Decode and Execute) — that fuller version is the one most commonly asked.

Syllabus points

CPU components: ALU, registers, control, internal bus
Register organization (user-visible vs control/status)
Instruction cycle: fetch → decode → execute → interrupt
Instruction cycle state diagram

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