RISC vs CISC design philosophy in ARM Architecture - Practice Questions

Challenge - 5 Problems

🧠 Conceptual

intermediate

Key difference between RISC and CISC

Which statement best describes the fundamental difference between RISC and CISC processor designs?

A RISC uses many simple instructions executed quickly, while CISC uses fewer complex instructions executed more slowly.

B RISC processors have more complex instructions than CISC processors.

C CISC processors use only fixed-length instructions, while RISC uses variable-length instructions.

D RISC processors are designed to minimize the number of instructions per program, unlike CISC.

💡 Hint

Think about how instruction complexity and execution speed differ between the two.

RISC (Reduced Instruction Set Computer) focuses on simple instructions that can be executed quickly, often in one clock cycle. CISC (Complex Instruction Set Computer) uses more complex instructions that may take multiple cycles to execute.

📋 Factual

intermediate

Instruction length in RISC vs CISC

Which of the following correctly describes instruction length characteristics in RISC and CISC architectures?

A Both RISC and CISC use fixed-length instructions.

B RISC instructions are typically fixed-length, while CISC instructions are variable-length.

C CISC instructions are fixed-length, while RISC instructions are variable-length.

D Both RISC and CISC use variable-length instructions.

💡 Hint

Consider how instruction decoding complexity relates to instruction length.

RISC architectures usually use fixed-length instructions to simplify decoding and speed up execution. CISC architectures often use variable-length instructions to allow more complex operations in a single instruction.

🔍 Analysis

advanced

Performance impact of RISC design

How does the RISC design philosophy typically impact processor performance compared to CISC?

A RISC processors achieve higher performance by executing simpler instructions faster and using pipelining effectively.

B RISC processors perform worse because they require more instructions to complete tasks.

C RISC processors have slower clock speeds due to complex instruction decoding.

D RISC processors rely on microcode to execute instructions, which slows down performance.

💡 Hint

Think about how instruction simplicity affects execution speed and pipeline design.

RISC processors use simple instructions that can be executed quickly and efficiently, often enabling effective pipelining. This generally leads to higher performance despite requiring more instructions for some tasks.

❓ Comparison

advanced

Memory usage differences between RISC and CISC

Which statement best explains how RISC and CISC architectures differ in memory usage for programs?

A CISC programs are larger because they use more instructions than RISC programs.

B RISC programs are smaller because they use fixed-length instructions.

C Both RISC and CISC programs use the same amount of memory on average.

D RISC programs tend to be larger in size because they use more simple instructions, while CISC programs are smaller due to complex instructions.

💡 Hint

Consider how instruction complexity affects the number of instructions needed.

RISC uses many simple instructions, so programs often have more instructions and thus larger size. CISC uses fewer but more complex instructions, resulting in smaller program size.

❓ Reasoning

expert

Why ARM architecture favors RISC design

Given ARM processors are based on RISC principles, which reason best explains why ARM uses RISC design over CISC?

A ARM uses RISC because it requires more transistors, which improves performance in mobile devices.

B CISC design would allow ARM to execute more complex instructions faster, which is better for mobile devices.

C RISC design allows ARM processors to have lower power consumption and simpler hardware, ideal for mobile devices.

D ARM uses RISC because it supports variable-length instructions, which reduces code size.

💡 Hint

Think about the priorities of mobile device processors like power and hardware simplicity.

ARM uses RISC design to keep hardware simple and power consumption low, which is critical for battery-powered mobile devices. CISC designs tend to be more complex and power-hungry.

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