In simple terms
A friendly intro before the formal notes — no formulas yet.
The Computer's Conductor
The CPU is the brain of the computer, constantly fetching instructions from memory and executing them to perform tasks. This process, known as the machine instruction cycle, is the fundamental rhythm that powers all computation.
Imagine a master chef (the CPU) in a kitchen. The recipe book is the program stored in memory (RAM). To cook a dish, the chef first finds the right page and instruction (fetch), reads and understands what to do, like 'chop onions' (decode), and then actually chops the onions using their knives and hands (execute). The chopped onions might be placed in a special bowl on the counter (a register) before being used in the next step.
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The CPU's Program Counter (PC) holds the address of the next instruction to be executed.
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The CPU fetches this instruction from memory, placing it in the Instruction Register (IR) for decoding.
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The Control Unit (CU) interprets the instruction and sends signals to other components, like the ALU, to perform the required operation.
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The result of the operation is stored, either in a register like the Accumulator or back into primary memory, and the cycle repeats.
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Full topic notes
Formal explanation with the rigour you need for the exam.
The Central Processing Unit (CPU): The Computer's Brain
The CPU is the hardware component responsible for executing the instructions of a computer program. It performs the basic arithmetic, logic, controlling, and input/output (I/O) operations specified by the instructions. A CPU's performance is a critical factor in the overall speed of a computer system, often measured in Gigahertz (GHz), which relates to its clock speed.
Core Components of the CPU
Within the CPU, two main components work in tandem. The Control Unit (CU) acts as the manager, fetching instructions from memory, decoding them, and directing other parts of the computer to carry them out. It doesn't execute instructions itself but orchestrates the process. The Arithmetic Logic Unit (ALU) is the mathematician and logician, performing all calculations (like addition and multiplication) and logical comparisons (like AND, OR, and NOT) that the program requires.
Control Unit (CU): Fetches, decodes, and coordinates the execution of instructions by sending control signals.
Arithmetic Logic Unit (ALU): Executes all arithmetic and logical operations.
Clock: Synchronises all computer operations. The clock speed (e.g., 3.2 GHz) determines how many cycles the CPU can execute per second.
CPU Registers: High-Speed Local Storage
Registers are small amounts of very fast, temporary memory located directly within the CPU. They are used to store data, instructions, or memory addresses that are actively being used by the CPU. Because they are physically inside the CPU chip, access times are incredibly low compared to accessing primary memory (RAM).
Program Counter (PC): Holds the memory address of the next instruction to be fetched.
Memory Address Register (MAR): Holds the address in primary memory that is currently being read from or written to.
Memory Data Register (MDR): A two-way register that holds data fetched from memory (and ready for the CPU to process) or data waiting to be stored in memory.
Current Instruction Register (CIR): Holds the actual instruction that has just been fetched from memory and is being decoded and executed.
Accumulator (ACC): A general-purpose register used to hold data temporarily and to store the results of ALU operations.
IB questions frequently ask you to distinguish between the MAR and MDR. Remember: MAR holds the address (like a house number), while MDR holds the actual data or instruction (like the mail in the letterbox at that address).
The Machine Instruction Cycle
The fundamental operation of a CPU is to execute a sequence of instructions. This is done through a continuous cycle, often called the fetch-decode-execute cycle. Each cycle processes one machine-language instruction. This process is repeated billions of times per second in a modern processor.
Fetch: The CU fetches the instruction from the memory address held in the PC. The PC is then incremented to point to the next instruction. The instruction is loaded into the CIR via the MAR and MDR.
Decode: The CU interprets the instruction in the CIR. It determines what operation is to be performed and what data is needed.
Execute: The CU sends signals to the appropriate components (e.g., the ALU) to carry out the instruction. This might involve fetching data from memory or a register, performing a calculation, or changing the state of a register.
Store (optional): The result of the execution may be stored in a register (like the Accumulator) or written back to primary memory.
Primary Memory
Primary memory is the computer's main workspace. It is directly accessible by the CPU and is used to store the data and instructions for the currently running programs. It is much faster to access than secondary storage (like a hard drive or SSD) but is typically smaller in capacity and more expensive per byte. There are two main types: RAM and ROM.
Random Access Memory (RAM)
RAM is the workhorse of primary memory. The 'Random Access' means that any memory location can be accessed directly and in any order, taking roughly the same amount of time regardless of its physical location. RAM is volatile, which means it loses its contents when the computer is powered off. It is a read/write memory, used to store the operating system, application programs, and data currently in use so that they can be quickly reached by the computer's processor.
Read-Only Memory (ROM)
ROM is a type of non-volatile memory, meaning its contents are retained even when the computer is powered off. As the name suggests, the data stored in ROM is either unchangeable or requires a special operation to change. It is primarily used to store firmware or software that is rarely changed, such as the BIOS (Basic Input/Output System) or the bootstrap loader. This is the very first code that runs when you turn your computer on, responsible for testing the hardware and loading the operating system from the hard drive into RAM.
Worked examples
See the formulas applied — reveal one step at a time, like the exam.
A CPU is executing a program. The Program Counter (PC) currently holds the address 101. The instruction LOAD 105 is stored at address 101. The value 42 is stored at memory address 105. Trace the contents of the PC, MAR, MDR, and Accumulator (ACC) during the fetch stage and the execution of this instruction.
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The address from the PC is copied to the MAR. [1 mark]
The Accumulator holds the value 10. The instruction at address 204 is ADD 250, where memory location 250 contains the value 5. Trace the execution of this instruction, assuming it has already been fetched and decoded.
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The CU identifies the address part of the instruction (250) and copies it to the MAR. [1 mark]
How it all connects
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Glossary
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Quick check
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Revision flashcards
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What is the CPU?
The Central Processing Unit is the primary component of a computer that executes instructions. It is often called the 'brain' of the computer.
Key takeaways
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Control Unit (CU): Fetches, decodes, and coordinates the execution of instructions by sending control signals.
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Arithmetic Logic Unit (ALU): Executes all arithmetic and logical operations.
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Clock: Synchronises all computer operations. The clock speed (e.g., 3.2 GHz) determines how many cycles the CPU can execute per second.
Practice — then mark it
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