How cores, cache and clock affect CPU performance

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Quick Questions

Shade in the A or B circle for each question.

1. A CPU has a larger cache. What is the main benefit?

A The clock ticks more times per second

B Fewer slow trips to RAM to fetch data and instructions

2. A program can only run one task at a time and cannot be split up. What happens when you move it from a dual-core CPU to a quad-core CPU?

A Little or no speed-up, because the extra cores sit unused

B It runs about twice as fast because there are twice as many cores

3. Which change means more fetch-decode-execute cycles per second?

A Raising the clock speed from 2.5 GHz to 3.5 GHz

B Adding more RAM to the computer

4. What is a downside of making the clock speed much higher?

A The CPU produces more heat and may need better cooling

B The CPU has fewer cores

5. CPU A: 4 cores, 3.0 GHz, 8 MB cache. CPU B: 2 cores, 3.0 GHz, 4 MB cache. Which is likely faster on software that uses all cores?

A They are the same, because the clock speed is the same

B CPU A, because it has more cores and a larger cache

Match the Pairs

Draw a line connecting each item on the left to its match on the right.

Left column		Right column
1Clock speed: 3.5 GHz		AThe CPU completes 3.5 billion cycles per second, so it can run more instructions each second, but it also produces more heat.
2Cache: 8 MB		BCPU A: 2 cores, 3.8 GHz, 4 MB cache \| CPU B: 4 cores, 3.0 GHz, 8 MB cache
3Extra cores only help when software can split its work into tasks that run at the same time.		CCores: 4 \| Program: single task \| Speed-up: little or none
4To compare two CPUs, look at their cores, clock speed and cache together, not just one figure.		DA small, very fast memory inside the CPU that holds frequently used data and instructions, so the CPU makes fewer slow trips to RAM.
5A CPU with 8 cores always runs every program 8 times faster than a CPU with 1 core.		EExtra cores do not speed up every program. A program only gets faster if it can split its work into tasks that run on different cores.
6Doubling the clock speed forever is the best way to improve performance because it has no downsides.		FA faster clock speed makes the CPU produce more heat, which limits how high it can go without overheating or needing better cooling.

Definitions

CPU performance

Fill in the Blanks

Complete each line of code by filling in the missing parts.

Clock speed = 3.5 ___

More _____ = more tasks at the same time

Bigger cache = fewer trips to ___

Higher clock speed = more ____ produced

IF coresA _ coresB THEN OUTPUT "A can run more tasks" ENDIF

Spot the Error

Shade in the letter of the line that contains an error.

Question 1

A IF data is in cache THEN

B use it quickly

C ELSE fetch it from the ALU

D END IF

Question 2

A cacheSize ← cacheSize + 4

B ramTrips ← ramTrips + 5

C OUTPUT ramTrips

Question 3

A clock ← 3.0 // GHz

B cycles ← clock / 1000000000

C OUTPUT cycles

Question 4

A tasks ← 4

B cores ← 4

C IF software splits tasks THEN

D tasks run one by one

E END IF

Question 5

A IF clock speed decreases THEN

B heat produced increases

C END IF

Answer Key

How cores, cache and clock affect CPU performance

Section A: Quick Questions

B — Fewer slow trips to RAM to fetch data and instructions

A — Little or no speed-up, because the extra cores sit unused

A — Raising the clock speed from 2.5 GHz to 3.5 GHz

A — The CPU produces more heat and may need better cooling

B — CPU A, because it has more cores and a larger cache

Section B: Match the Pairs

Clock speed: 3.5 GHz → The CPU completes 3.5 billion cycles per second, so it can run more instructions each second, but it also produces more heat.

Cache: 8 MB → A small, very fast memory inside the CPU that holds frequently used data and instructions, so the CPU makes fewer slow trips to RAM.

Extra cores only help when software can split its work into tasks that run at the same time. → Cores: 4 | Program: single task | Speed-up: little or none

To compare two CPUs, look at their cores, clock speed and cache together, not just one figure. → CPU A: 2 cores, 3.8 GHz, 4 MB cache | CPU B: 4 cores, 3.0 GHz, 8 MB cache

A CPU with 8 cores always runs every program 8 times faster than a CPU with 1 core. → Extra cores do not speed up every program. A program only gets faster if it can split its work into tasks that run on different cores.

Doubling the clock speed forever is the best way to improve performance because it has no downsides. → A faster clock speed makes the CPU produce more heat, which limits how high it can go without overheating or needing better cooling.

Section C: Definitions

CPU performance — How fast a CPU runs programs. More cores, a bigger cache or a faster clock speed can raise it, but each has limits: software must split tasks, cache is costly, and a faster clock makes more heat.

Section D: Fill in the Blanks

Clock speed = 3.5 GHz

More cores = more tasks at the same time

Bigger cache = fewer trips to RAM

Higher clock speed = more heat produced

IF coresA > coresB THEN
  OUTPUT "A can run more tasks"
ENDIF

Section E: Spot the Error

C — On a cache miss the CPU makes a slower trip to RAM, not to the ALU. The ALU only does calculations and does not store data. It should be: ELSE fetch it from RAM.

B — A bigger cache holds more data, so the CPU makes fewer slow trips to RAM. The trips should go down: ramTrips ← ramTrips - 5.

B — 1 GHz means one billion cycles per second, so the clock speed in GHz must be multiplied by 1 000 000 000, not divided. It should be: cycles ← clock * 1000000000.

D — When software can split its work, each core takes a task and they run at the same time (in parallel). One by one is what happens when the work cannot be split. It should be: tasks run at the same time.

A — A higher clock speed means more cycles per second, which makes more heat. The condition should say increases, to match the heat produced increasing.