In simple terms
A friendly intro before the formal notes — no formulas yet.
The Computer's Brain and Workspace
The CPU is the computer's brain, processing all instructions, while primary memory (RAM) is its short-term workspace, holding the information it needs right now. They work together constantly to run your programs.
Imagine a chef (the CPU) in a kitchen. The recipe book is the hard drive. To cook a dish, the chef first brings the specific recipe page and ingredients to the countertop (the RAM). The countertop is a limited but fast-access workspace. The chef reads the recipe (fetches/decodes), prepares the ingredients (executes), and places the finished part of the dish back on the counter (stores). The CPU, RAM, and storage work together in a similar way.
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Fetch: The CPU's Control Unit retrieves the next instruction from a specific memory address.
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Decode: The Control Unit interprets the instruction to understand what operation needs to be performed.
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Execute: The Arithmetic Logic Unit (ALU) performs the required calculation or logical operation.
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Store: The result of the execution is written back to a register or a location in primary memory.
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Key formulas
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Full topic notes
Formal explanation with the rigour you need for the exam.
The Central Processing Unit (CPU)
The CPU is the electronic circuitry that executes instructions comprising a computer program. It performs basic arithmetic, logic, controlling, and input/output (I/O) operations specified by the instructions in the program. Modern CPUs are microprocessors, meaning they are contained on a single integrated circuit (IC) chip.
Control Unit (CU): This component acts as the manager. It fetches instructions from memory, decodes them into commands, and controls the flow of data between the CPU and other devices.
Arithmetic Logic Unit (ALU): This is the calculator of the CPU. It performs all arithmetic calculations (addition, subtraction, etc.) and logical comparisons (AND, OR, NOT, greater than, etc.).
Registers: These are small, extremely fast storage locations within the CPU used to hold data temporarily. Key registers include the Program Counter (PC), which holds the address of the next instruction, and the accumulator, which stores the results of ALU operations.
The Role of MAR and MDR
To communicate with primary memory, the CPU uses two special-purpose registers: the Memory Address Register (MAR) and the Memory Data Register (MDR). These are crucial for the fetch-execute cycle. The MAR holds the address of the memory location to be accessed, while the MDR holds the data that is being read from or written to that location. Think of the MAR as the envelope with the address, and the MDR as the letter inside.
The Machine Instruction Cycle
The CPU executes a program by repeatedly performing the machine instruction cycle for each instruction. This cycle is the fundamental operation of the CPU and consists of four main stages.
1. Fetch: The CU fetches the next instruction from the memory address stored in the Program Counter (PC). The instruction is loaded into the Instruction Register (IR). The PC is then incremented to point to the next instruction.
2. Decode: The CU decodes the instruction in the IR. It interprets the opcode (operation code) to determine what action to perform and identifies any operands (data) required.
3. Execute: The CU sends signals to the appropriate components, usually the ALU, to carry out the instruction. For example, adding two numbers.
4. Store: The result of the execution is stored in a register or written back to a location in primary memory.
In exam questions about the machine instruction cycle, be precise about which register is used at each stage. For example, state that the address is copied to the MAR, and the instruction is copied to the MDR. Simply saying 'the CPU fetches from memory' is often not detailed enough for full marks.
Primary Memory
Primary memory is the only storage that the CPU can access directly. It holds the operating system, applications, and data currently in use. It is much faster to access than secondary storage (like a hard drive or SSD) but is generally smaller in capacity and volatile.
Random Access Memory (RAM): This is the main workspace of the computer. It is volatile, meaning its contents are erased when the power is switched off. You can both read from and write to RAM.
Read-Only Memory (ROM): This memory is non-volatile, meaning it retains its data without power. It contains the essential instructions to start up the computer, known as the bootstrap program or BIOS. As the name suggests, its contents cannot normally be changed.
Cache Memory: A small, very high-speed block of RAM located on or very close to the CPU. It stores frequently used instructions and data, reducing the need for the CPU to access the slower main RAM, thereby improving performance.
Memory Addressing
The amount of memory a CPU can access is determined by the width of its address bus. The address bus is a set of wires that carries the address of the memory location to be accessed. Each wire can carry one bit (a 0 or a 1).
Maximum Addressable Memory = locations, where is the number of lines in the address bus.
Worked examples
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A program needs to fetch an instruction from memory address 1101 (binary). Describe the roles of the MAR and MDR in this process.
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The address of the instruction, 1101, is copied from the Program Counter (PC) into the Memory Address Register (MAR). [1 mark]
A CPU has a 32-bit address bus. Calculate the maximum amount of primary memory it can address. Give your answer in Gibibytes (GiB). (Note: 1 KiB = bytes, 1 MiB = bytes, 1 GiB = bytes).
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The number of unique addresses is determined by the number of lines on the address bus, N. Here, N = 32. Total addresses = . [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 processes instructions. It is often called the 'brain' of the computer.
Key takeaways
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Control Unit (CU): This component acts as the manager. It fetches instructions from memory, decodes them into commands, and controls the flow of data between the CPU and other devices.
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Arithmetic Logic Unit (ALU): This is the calculator of the CPU. It performs all arithmetic calculations (addition, subtraction, etc.) and logical comparisons (AND, OR, NOT, greater than, etc.).
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Registers: These are small, extremely fast storage locations within the CPU used to hold data temporarily. Key registers include the Program Counter (PC), which holds the address of the next instruction, and the accumulator, which stores the results of ALU operations.
Practice — then mark it
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Test Your Knowledge on Computer Architecture
Test Your Knowledge on Computer Architecture
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