CPU Complete Flow

PC / IP

Program Counter

Address: 100

Instruction Register

Waiting...

CPU CORE

Control Unit + Decoder

Ready

Address Register

(MAR concept)

Data Register

(MDR concept - in CPU)

REGISTERS

RAX, RBX (in CPU)

RAX: - | RBX: -

ALU

Math Unit (in CPU)

RAM

No registers here!

Stores data

STEP 0

Ready to Start

Click "Next Step" to see how interrupts work!

CPU is running a user process in user mode.

Think of them as roles, not brand names.

Program Counter (PC / IP) — the pointer

What it is:

A pointer register

What it points to:

The next instruction in RAM

"Where do I go next?" — Pointer to instruction

Instruction Register (IR) — the instruction itself

What it is:

A register holding bits

What it holds:

The actual instruction

"What am I executing right now?" — Instruction lives here

Memory Address Register (MAR) — pointer to data

What it is:

A pointer register

What it points to:

A memory address in RAM

"Where is the data I want?" — Pointer to data

Memory Data Register (MDR) — the data itself

What it is:

A register holding bits

What it holds:

Data coming from or going to RAM

"Here is the data" — Data lives here

General-Purpose Registers (RAX, RBX, etc.) — working hands

What they are:

Data registers

What they hold:

Numbers, values, results

"What I'm working on right now" — CPU's hands

The Pattern (THIS IS THE KEY INSIGHT)

Every register fits one of two roles:

Pointer Registers

PC → points to instruction

MAR → points to data

Holder Registers

IR → holds instruction

MDR → holds data

GP registers → hold working data

Pointer vs Holder — That's the whole game.

Why this works so well

RAM is slow

CPU is fast

Registers are the bridge

OS + interrupts just change who owns the CPU

One-Sentence Truth (Tattoo this on your brain)

The CPU never works directly with RAM — it only works through registers.

KEY CONCEPTS - LOCK THESE IN YOUR BRAIN