CPU components
Describe the functions and interactions of the main CPU components
In this lesson
- The units inside the CPU: the arithmetic logic unit (ALU) and the control unit (CU)
- The registers IR, PC, MAR, MDR and AC, and what each one holds
- The address, data and control buses, and how they connect the CPU to memory
- Single-core processors, multi-core processors and co-processors
- A labelled diagram showing how these components relate to each other
LearnWhat is inside a CPU
The central processing unit (CPU) runs every program on a computer. It is made of a small number of components, and each one has one job. Reveal them one at a time.
LabThe CPU diagram
This is the kind of diagram the guide expects you to be able to draw. Explore it first: select any component to read what it does. Then switch to Test yourself and find each component from its description.
Select any part of the diagram to see what it does. You can also use Tab and Enter.
CheckMatch each register to its job
Select a term on the left, then its description on the right.
CheckQuick check
Which statement about the address bus is correct?
CodeHow wide is the address bus?
An address bus with n lines can carry 2ⁿ different addresses, so it can reach 2ⁿ memory locations. Run the code to see how quickly that grows.
Output appears here. The first run loads Python into the page (a few seconds).max_memory_gib(width) that returns how many GiB of memory an address bus of that width can reach. For example, max_memory_gib(32) should return 4. Test it by adding print(max_memory_gib(32)) and pressing Run.Show solution
def max_memory_gib(width):
return 2 ** width / 2 ** 30LearnSingle-core, multi-core and co-processors
A processor can contain one or more complete processing units, called cores.
CheckQuick check
A program processes its data in one long sequence of steps that cannot be split up. It is moved from a 2-core processor to an 8-core processor with the same core design and clock speed. What is most likely to happen?
ExamWorked exam answer
Describe the role of the control unit (CU).
[3]ReviewMistakes examiners see
The CU does not perform calculations. It decodes and directs; the ALU calculates.
The MAR holds an address. The MDR holds the data. Swapping them is the most common lost mark in this topic.
A multi-core processor does not automatically make every program faster. The task must be parallelisable.
Learn the bus directions: address bus one-way (CPU to memory), data bus two-way, control bus carries signals both ways.
When drawing the CPU, connect the MAR to the address bus, the MDR to the data bus and the CU to the control bus.
Use the full name once, then the abbreviation: “memory address register (MAR)”.
Exam practiceExam-style questions
5 questions · 14 marks. Answer each one first, then mark it yourself against the markscheme. Your answers are saved in this browser only.
A hospital's IT team is choosing new workstations for its radiology department. Staff will use the workstations to run the patient-records system and to process large medical images.
Identify the register that holds the address of the next instruction to be fetched.
Outline the function of the memory data register (MDR).
Distinguish between the address bus and the data bus.
Construct a labelled diagram showing the relationship between the CU, the ALU, the registers (PC, MAR, MDR, IR, AC), main memory and the three buses.
The IT team is considering workstations with a multi-core processor and a graphics co-processor. Explain why the image-processing software may run faster on these workstations, while the patient-records system may not.
0 of 5 questions marked. Mark every question to see your total.
Exam-style questions written for Praxis IB in the format of the 2027 specimen papers. They are not IB past-paper questions.