Every program you run is carried out by the central processing unit, one tiny instruction at a time. Meet the parts inside it: two units, five registers and three buses, all working with one shared memory.
The central processing unit (CPU) processes the instructions and data that are input into the computer so that the result can be output. A microprocessor is a type of integrated circuit with the CPU on a single chip. In a Von Neumann architecture the instructions and data are both stored in the same memory (RAM) and travel along the same buses. The CPU contains the control unit (CU), the arithmetic logic unit (ALU) and registers: PC, MAR, MDR, CIR and ACC.
Fig. 1: the CPU's units and registers, linked to RAM by the address, data and control buses
Section 01
What the CPU does.
Explore each part in the lab to see its job. The CPU is the part of the computer that carries out program instructions: it takes in data, processes it and produces results that can be output or stored.
Role of the CPU: fetches instructions and data from memory, decodes the instructions and executes them, so that input is turned into output.
Microprocessor: an integrated circuit (a chip) that holds a whole CPU on one single chip. Phones, laptops and washing machines all contain microprocessors.
Von Neumann architecture: a design where a program's instructions and its data are stored together in the same main memory, and the CPU fetches them one after another.
Examiner trap
"The CPU stores all the files" is wrong. The CPU processes; files are kept in storage. Registers inside the CPU hold only a few values at a time.
Section 02
Units and registers.
A register is a tiny, very fast store inside the CPU that holds one value. Learn each register's full name and what it holds: exam questions often give a description and ask you to name the register.
Component
Purpose
Control unit (CU)
Decodes instructions and sends control signals to manage the other parts of the CPU and the flow of data; uses the clock to keep everything in step.
Arithmetic logic unit (ALU)
Carries out calculations (add, subtract) and logic operations (AND, OR, comparisons).
Program counter (PC)
Holds the address of the next instruction to be fetched.
Memory address register (MAR)
Holds the address of the memory location about to be read from or written to.
Memory data register (MDR)
Holds the data or instruction just read from memory, or about to be written to it.
Current instruction register (CIR)
Holds the instruction currently being decoded and executed.
Accumulator (ACC)
Holds the result of calculations done by the ALU.
Worked example Name the register
Which register holds the address of the next instruction, and which holds the instruction being executed?
"Address of the next instruction" describes a pointer to where the next instruction is: that is the program counter.
"Instruction being executed" is the current instruction: that is the current instruction register.
Answer: PC and CIR
Your turn
An instruction tells the CPU to add two numbers. Which unit does the addition, and which register will hold the answer?
The ALU does the addition; the result goes into the accumulator (ACC).
Examiner trap
Do not mix up the MAR and the MDR. The MAR holds an address (where); the MDR holds data or an instruction (what).
Stage 1 · Learn
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Which register holds the address of the next instruction to be fetched?
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In the lab, which part carries out comparisons and arithmetic?
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What does a Von Neumann architecture mean?
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Which bus is one-directional?
Section 03
Three buses.
A bus is a set of parallel wires that carries signals between the CPU, memory and input/output devices. Each bus has its own job and direction.
Bus
Carries
Direction
Address bus
Memory addresses, from the MAR to memory
One way (CPU to memory)
Data bus
Data and instructions, to and from the MDR
Two way
Control bus
Control signals from the CU, such as read, write and clock signals
Two way
Your turn
Why is the address bus one-directional?
Only the CPU sends addresses to memory; memory never needs to send an address back, so addresses travel one way, from the CPU to memory.
Examiner trap
The data bus carries instructions as well as data, because in a Von Neumann computer both are stored in the same memory.
Stage 2 · Exam
Exam-style questions
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Which register holds data that has just been read from memory?
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What is a microprocessor?
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What is the purpose of the control unit?
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Which statement about the buses is correct?
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