Inside the Machine

What actually runs your programs

Open the case of any computer and past the fans and cables you find the part that does the thinking: the CPU, the central processing unit. Everything a program does is broken down into tiny instructions, and the CPU works through them one at a time, just very fast. ⭐⭐ The CPU sits on the motherboard alongside memory, the power supply, and connections to input and output devices. The big idea behind almost every computer is the VON NEUMANN idea: the program instructions and the data they work on are stored together in the same memory, and the CPU fetches them from there. ⭐ Inside the CPU, three things share the work. The CONTROL UNIT directs everything and decodes each instruction. The ALU, the arithmetic and logic unit, does the sums and the comparisons. Small, very fast stores called REGISTERS and the CACHE hold the handful of things the CPU is working on right now. ⭐⭐ The CPU repeats one loop for every instruction: fetch it from memory, decode what it means, then execute it. Do that billions of times a second and you get a running program. This module is the processor and how it works. A separate module covers primary and secondary storage. ⚠️ Carry one question the whole way through: for each part, what job does it do in getting one instruction fetched, decoded and executed?

The parts that share the work

Three kinds of thing inside the processor, and what each is for.

Tap the two parts inside the CPU

Tap the TWO things that are found INSIDE the CPU itself, rather than elsewhere in the computer.

• The arithmetic and logic unit that does calculations.
• The registers that hold what is being worked on right now.
• The power supply unit that provides electricity.
• The monitor that displays the output to the user.

Why a change made it faster

Two computers are identical except that one has a much larger cache. That one runs the same program noticeably faster. Which explanation best fits?

• A larger cache holds more of the data and instructions close to the CPU, so it waits less often for slower memory
• A larger cache raises the clock speed
• A larger cache adds more cores
• A larger cache gives more secondary storage for files

The words the processor uses

Six terms this topic turns on. Learn them and the cycle reads clearly.

Pair each register with its job

Two ways to speed a processor up

Select the TWO changes that would typically make a processor do more work in the same time.

• A higher clock speed, so more cycles happen each second
• More cores, so more instructions can be handled at once
• A larger monitor
• More secondary storage for files

Explaining performance for top marks

The best answers do not just name a performance factor; they explain WHY it helps. ⭐⭐ SAY WHAT THE FACTOR DOES, THEN WHY IT MATTERS. Not just larger cache, but: a larger cache holds more of what the CPU needs nearby, so it waits less often for slower main memory. That because clause is where the top marks are. ⭐ KEEP THE FACTORS SEPARATE. Clock speed is cycles per second; the number of cores is how many things can run at once; cache size is how much fast memory sits near the CPU. Do not let one stand in for another. ⭐⭐ TIE IT TO THE CYCLE. Every factor helps because the CPU is repeating fetch, decode, execute; a faster clock runs the cycle more often, more cores run more cycles in parallel, more cache means fewer slow trips to memory during a cycle. DO NOT DRIFT INTO STORAGE OR THE OPERATING SYSTEM. Primary and secondary storage, and the operating system, have their own modules; here, keep the focus on the processor and the cycle. ⚠️ A note on accuracy. Describe speeds and sizes in words, as higher or larger, rather than inventing exact figures for a real chip. Keep the one question to hand: what job does this part do in getting one instruction fetched, decoded and executed?

Walk through one instruction

Put the steps the CPU takes to process a single instruction into the right order.

• The program counter gives the address of the next instruction
• The instruction is fetched from memory into the CPU
• The control unit decodes what the instruction means
• The instruction is executed, for example the ALU does a calculation
• The program counter updates ready for the next instruction

Build a sentence about the cycle

Assemble one sentence that describes how the processor handles an instruction.

The processor run

Five quick decisions about the processor. Three lives.

A chef and a recipe card

Leave the computer aside for a moment. Picture a chef working through a recipe written as a numbered list of steps, one line at a time. ⭐ For each line the chef does the same three things: read the next step, work out what it is asking for, and then carry it out. Read, understand, do. Then move a finger down to the next line and repeat, all the way to the end of the recipe. ⭐⭐ Now imagine three ways to get the meal out faster. Give the chef extra pairs of hands so several steps happen at once. Put a small shelf of the most-used ingredients within arm reach so there is no walk to the far larder each time. Or simply have the chef work through each line more quickly. Each change means more of the recipe done in the same time. Notice that a bigger freezer full of food does NOT make the cooking any faster. It stores more, but the speed of working through the recipe is unchanged. ⚠️ Hold that picture. A processor works exactly like the chef: read the next instruction, work out what it means, carry it out, and the same kinds of help make it faster.

Finish each processor line

The first step of the cycle, getting an instruction from memory into the CPU, is to _____ it. Working out what the instruction means is to _____ it. Carrying the instruction out is to _____ it. Fast memory close to the CPU that keeps recently used items to hand is the _____. Having more than one processing unit, so more work happens at once, means having more _____.

Three machines to reason about

Three situations are described. For each, choose the reading the architecture best supports.

• A program feels slow, and someone suggests fitting a bigger hard drive to speed it up.
• A student says the CPU does several completely different instructions all in one single step.
• A design team wants a processor to run many tasks at once for a busy server.

Explain how the processor works

Explain how a CPU processes a program: the Von Neumann idea, the roles of the control unit and the ALU, the fetch-decode-execute cycle, and what makes one processor faster than another. Explain WHY each performance factor helps, not just its name.

• Explain the Von Neumann idea that instructions and data share one memory
• Explain what the control unit and the ALU each do
• Explain the fetch-decode-execute cycle in order
• Name at least two CPU performance factors
• Explain WHY one of those factors makes the processor faster