CS6 — Computer Architecture
- I can describe the role of the processor in a computer system
- I can explain the purpose of the control unit, ALU, registers and buses
- I can distinguish between address, data and control buses
- I can trace how data and instructions move around the processor and memory
- I can state that the processor executes instructions by controlling and processing data
- I can identify the ALU as the part that performs arithmetic and logical operations
- I can identify registers as small, fast storage locations inside the processor
- I can explain the direction and purpose of the address, data and control buses
Key vocabulary
Notes
The stored-program computer
A modern computer follows the stored-program concept: both program instructions and data are stored in main memory as binary. The processor fetches instructions from memory, decodes what each instruction means, and carries out the required operation. This might mean adding two numbers, comparing two values, copying data from one location to another, or sending a signal to an input/output device.
The important idea is that the processor does not understand a whole Python program, web page or image file at once. It works through very small machine-code instructions. Each instruction is fetched, decoded and executed using a collection of internal parts. The main parts named at Higher are the control unit, the ALU, registers and the buses that connect the processor to memory and other devices.
The CPU does the processing; main memory stores the current program and data; buses move addresses, values and control signals between them.
The control unit
The control unit directs the operation of the processor. It decodes instructions, works out which components must be used, and sends control signals to coordinate the movement of data. A useful way to think of the control unit is as the organiser: it does not perform the arithmetic itself, but it decides when the ALU should operate, which registers should be read or written, and whether memory should be read from or written to.
For example, if an instruction says to load a value from memory, the control unit places the required memory address onto the address bus and sends a read signal. If an instruction says to add two values, the control unit arranges for those values to be available to the ALU, then stores the result in a register.
The ALU
The arithmetic logic unit carries out the processor's arithmetic and logical operations. Arithmetic operations include addition and subtraction. Logical operations include comparisons such as equal to, greater than, less than, AND, OR and NOT. These operations matter because even complex programs are built from many small decisions and calculations.
When a program checks whether a password length is greater than 8, the ALU performs the comparison. When a program calculates a total price, the ALU performs the arithmetic. The ALU usually works with values that are already inside processor registers, because registers are much faster to access than main memory.
Registers: tiny, fast storage
Registers are small storage locations inside the processor. They are faster than main memory because they are physically part of the CPU. Registers temporarily hold instructions, addresses, data and results while the processor is executing instructions. Higher Computing Science does not require a large list of specific register names, but it does require you to understand the purpose: registers hold the immediate values the processor needs right now.
Buses: moving information around
A bus is a group of parallel wires used to transfer information between computer components. There are three buses named in the course: the address bus, the data bus and the control bus. They work together but carry different kinds of information.
The address bus identifies where to look; the data bus carries the value; the control bus carries signals such as read and write.
| Bus | What it carries | Typical direction | Purpose |
|---|---|---|---|
| Address bus | Memory addresses | Processor to memory | Selects the memory location to access. |
| Data bus | Actual data or instructions | Both directions | Transfers values between processor, memory and devices. |
| Control bus | Control signals | Both directions | Coordinates actions such as read, write, clock and interrupt signals. |
The address bus is usually described as one-way from processor to memory because the processor chooses which memory address it wants. The data bus is two-way because the processor may read data from memory or write data back to memory. The control bus is also two-way because the processor sends commands, while devices and memory can send status signals back.
A memory read in slow motion
To understand why the buses matter, imagine the processor needs to read a value stored in main memory. The processor first places the address of the required memory location on the address bus. The control unit sends a read signal on the control bus. Memory responds by putting the contents of that address onto the data bus. The processor then copies the value into a register so it can be used by the ALU or by the next instruction.
The processor selects the address first. Only then can memory return the value stored at that address.
Worked examples
score > 70 and stores the result.- Mixing up address and data: an address is the location; data is the value stored at that location.
- Saying the ALU controls the processor: the ALU performs arithmetic and logic. The control unit coordinates operations.
- Describing registers as main memory: registers are inside the processor and are much smaller and faster than RAM.
- Forgetting bus direction: the address bus usually goes from processor to memory, while the data bus transfers in both directions.
When asked about buses, always include both what the bus carries and why that matters. For example: "The address bus carries the address of the memory location being accessed." That is stronger than simply naming the bus.
For processor component questions, avoid vague phrases like "the CPU does everything". Split the answer: control unit controls, ALU calculates/compares, registers temporarily store immediate values.
Task Set A
Task Set B
Higher Computing Science → Computer Systems → CS6
Timing (120 min double):
5 min — warm up from CS5
8 min — vocabulary and architecture map
12 min — stored-program concept and processor components
15 min — control unit, ALU and registers with mini whiteboard checks
15 min — buses table and memory read flow-pipeline
15 min — worked examples and now-you-try bridge
5 min — common mistakes and exam tip
35 min — Task Set A, with A8/A10 as the main exam-style check
10 min — review or extension task depending on pace
Watch for: pupils treating the CPU as a single magic box. Keep asking "which part?" until they can separate control unit, ALU, registers and buses. The most common bus confusion is writing that the address bus carries data.
Practical demo: Draw a memory table with numbered addresses on the board. Have pupils physically point to where the address, value and read/write signal would travel. It makes the abstract bus vocabulary much less slippery.