RISC vs CISC design philosophy in ARM Architecture - Quick Revision & Key Differences

Recall & Review

beginner

What does RISC stand for in computer architecture?

RISC stands for Reduced Instruction Set Computer. It uses a small set of simple instructions for faster execution.

beginner

What is the main idea behind CISC design philosophy?

CISC stands for Complex Instruction Set Computer. It uses many complex instructions that can perform multiple tasks in one instruction.

intermediate

How does RISC simplify processor design?

RISC simplifies processor design by using fewer instructions, each taking about the same time to execute, which makes the processor faster and easier to build.

intermediate

Why might CISC processors use more power than RISC processors?

CISC processors have complex instructions that require more hardware and longer execution time, which can lead to higher power consumption compared to RISC processors.

beginner

Give a real-life example of a RISC and a CISC processor.

An example of a RISC processor is the ARM processor used in many smartphones. An example of a CISC processor is the Intel x86 processor used in many personal computers.

What is a key characteristic of RISC processors?

A They require more power than CISC

B They use a small set of simple instructions

C They execute instructions slowly

D They use many complex instructions

RISC processors use a small set of simple instructions to speed up execution.

Which design philosophy aims to reduce the number of instructions in a processor?

A RISC

B CISC

C Both CISC and RISC

D Neither

RISC aims to reduce the number of instructions to simplify and speed up processing.

Which processor type is commonly found in smartphones?

A RISC

B CISC

C Both equally

D Neither

Smartphones commonly use RISC processors like ARM for efficiency and power saving.

Why do CISC processors have complex instructions?

A To simplify hardware design

B To reduce power consumption

C To perform multiple tasks in one instruction

D To speed up instruction decoding

CISC processors use complex instructions to do more work per instruction.

Which design philosophy generally leads to simpler and faster instruction execution?

A CISC

B Both are equal

C Depends on the application

D RISC

RISC design leads to simpler and faster instruction execution by using fewer, simpler instructions.

Explain the main differences between RISC and CISC design philosophies.

Think about instruction complexity and processor speed.

Describe why ARM processors are considered RISC and Intel x86 processors are considered CISC.

Consider instruction set size and typical usage.

Practice

1. What is the main idea behind the RISC design philosophy?

easy

A. Use complex instructions that do more work

B. Focus on reducing power consumption only

C. Use simple and fast instructions

D. Use instructions that are all the same length

Solution

1. Step 1: Understand RISC basics

   RISC stands for Reduced Instruction Set Computer, which means it uses fewer, simpler instructions.

2. Step 2: Compare with CISC

   Unlike CISC, which uses complex instructions, RISC focuses on simple and fast instructions.

3. Final Answer:

   Use simple and fast instructions -> Option C

4. Quick Check:

   RISC = simple, fast instructions [OK]

Hint: RISC means simple instructions, CISC means complex [OK]

Common Mistakes:

• Confusing RISC with CISC
• Thinking RISC uses complex instructions
• Assuming RISC focuses only on power

2. Which of the following is a characteristic of CISC instruction sets?

easy

A. Instructions only use registers, no memory access

B. Instructions are all the same size

C. Instructions are very simple and fast

D. Instructions can perform multiple operations

Solution

1. Step 1: Recall CISC features

   CISC stands for Complex Instruction Set Computer, meaning instructions can do multiple tasks.

2. Step 2: Identify correct feature

   CISC instructions often combine several operations into one instruction.

3. Final Answer:

   Instructions can perform multiple operations -> Option D

4. Quick Check:

   CISC = complex, multi-operation instructions [OK]

Hint: CISC instructions do more work per instruction [OK]

Common Mistakes:

• Thinking CISC instructions are all the same size
• Confusing CISC with RISC simplicity
• Assuming CISC avoids memory access

3. Consider a processor that executes 4 simple instructions in 1 cycle each (RISC) versus 2 complex instructions in 2 cycles each (CISC). Which completes more instructions in 4 cycles?

medium

A. RISC completes 4 instructions

B. CISC completes 4 instructions

C. RISC completes 2 instructions

D. CISC completes 2 instructions

Solution

1. Step 1: Calculate RISC instructions done

   RISC does 1 instruction per cycle, so in 4 cycles it completes 4 instructions.

2. Step 2: Calculate CISC instructions done

   CISC does 1 instruction in 2 cycles, so in 4 cycles it completes 2 instructions.

3. Final Answer:

   RISC completes 4 instructions -> Option A

4. Quick Check:

   RISC faster with simple instructions [OK]

Hint: Count instructions per cycle to compare speed [OK]

Common Mistakes:

• Assuming complex instructions always finish faster
• Mixing cycles per instruction
• Ignoring instruction count per cycle

4. A programmer writes code assuming all instructions take the same time, but the processor uses CISC design. What problem might occur?

medium

A. Code timing is inaccurate due to variable instruction lengths

B. Code runs faster than expected

C. Code will not compile

D. Code uses less memory

Solution

1. Step 1: Understand CISC instruction timing

   CISC instructions vary in length and complexity, so execution time varies.

2. Step 2: Identify timing impact

   Assuming equal time causes inaccurate timing predictions and possible bugs.

3. Final Answer:

   Code timing is inaccurate due to variable instruction lengths -> Option A

4. Quick Check:

   CISC instructions vary in time, causing timing errors [OK]

Hint: CISC instructions vary in length and time [OK]

Common Mistakes:

• Thinking code won't compile
• Assuming code runs faster always
• Ignoring timing differences

5. A device needs to run simple tasks very fast and save power. Which design philosophy suits it best and why?

hard

A. CISC, because complex instructions reduce code size

B. RISC, because simple instructions use less power and run faster

C. CISC, because it uses fewer instructions overall

D. RISC, because it uses complex instructions to do more work

Solution

1. Step 1: Analyze device needs

   The device needs speed and low power for simple tasks.

2. Step 2: Match design philosophy

   RISC uses simple, fast instructions that consume less power, fitting the need.

3. Final Answer:

   RISC, because simple instructions use less power and run faster -> Option B

4. Quick Check:

   Simple fast tasks = RISC best fit [OK]

Hint: Simple fast tasks favor RISC for power and speed [OK]

Common Mistakes:

• Choosing CISC for simple fast tasks
• Confusing instruction complexity with speed
• Ignoring power consumption