Instruction cycle counting in ARM Architecture - Cheat Sheet & Quick Revision

Recall & Review

beginner

What is an instruction cycle in ARM architecture?

An instruction cycle is the basic time unit in which the processor fetches, decodes, and executes a single instruction.

beginner

Why is instruction cycle counting important?

It helps measure how long a program or instruction takes to run, which is useful for optimizing performance and power consumption.

intermediate

How does pipelining affect instruction cycle counting?

Pipelining allows overlapping of instruction steps, so multiple instructions are processed at once, reducing the total cycles needed per instruction.

intermediate

What factors can increase the number of instruction cycles needed?

Factors include cache misses, branch mispredictions, memory access delays, and complex instructions that take multiple cycles.

beginner

How can you estimate the total execution time of a program using instruction cycles?

Multiply the total number of instruction cycles by the clock cycle time (the duration of one cycle).

What does one instruction cycle typically include in ARM processors?

A Only fetch

B Fetch, decode, and execute

C Only execute

D Only decode

An instruction cycle includes fetching the instruction, decoding it, and executing it.

How does pipelining improve instruction cycle efficiency?

A By executing instructions one at a time

B By reducing instruction size

C By overlapping instruction steps

D By increasing clock speed

Pipelining overlaps the steps of multiple instructions, allowing faster overall processing.

Which of these can cause an increase in instruction cycles?

A Short clock cycles

B Cache hits

C Simple instructions

D Branch mispredictions

Branch mispredictions cause delays and increase the number of cycles needed.

If a program takes 1000 instruction cycles and the clock cycle time is 2 nanoseconds, what is the total execution time?

A 2000 nanoseconds

B 500 nanoseconds

C 1000 nanoseconds

D 200 nanoseconds

Execution time = instruction cycles × clock cycle time = 1000 × 2 ns = 2000 ns.

What is NOT a part of instruction cycle counting?

A Counting the number of instructions in a program

B Measuring program execution time

C Counting how many cycles an instruction takes

D Estimating power consumption

Instruction cycle counting focuses on cycles per instruction and execution time, not just counting instructions.

Explain what an instruction cycle is and why counting these cycles matters in ARM processors.

Think about the steps the processor takes to run one instruction and how this affects speed.

Describe how pipelining changes the way instruction cycles are counted and its effect on program speed.

Imagine a factory assembly line working on multiple tasks at once.

Practice

1. What does instruction cycle counting measure in ARM architecture?

easy

A. The size of the instruction in bytes

B. The number of steps an instruction takes to execute

C. The number of registers used by an instruction

D. The memory address of the instruction

Solution

1. Step 1: Understand instruction cycle counting

   Instruction cycle counting measures how many clock cycles an instruction takes to complete.

2. Step 2: Relate to ARM instructions

   In ARM, different instructions take different numbers of cycles, so counting cycles shows execution steps.

3. Final Answer:

   The number of steps an instruction takes to execute -> Option B

4. Quick Check:

   Instruction cycle counting = steps per instruction [OK]

Hint: Cycle count means steps to finish instruction [OK]

Common Mistakes:

• Confusing cycle count with instruction size
• Thinking it counts registers used
• Mixing cycle count with memory address

2. Which of the following is the correct syntax to represent an ARM instruction cycle count in documentation?

easy

A. ADD R0, R1, R2 ; cycles=1

B. ADD R0, R1, R2 cycles=1

C. ADD R0, R1, R2 (cycles=1)

D. ADD R0, R1, R2 cycles:1

Solution

1. Step 1: Identify comment syntax in ARM assembly

   ARM assembly uses semicolon (;) to start comments.

2. Step 2: Check which option uses semicolon before cycle count

   ADD R0, R1, R2 ; cycles=1 uses "; cycles=1" as a comment, which is correct.

3. Final Answer:

   ADD R0, R1, R2 ; cycles=1 -> Option A

4. Quick Check:

   Comments start with ; in ARM assembly [OK]

Hint: Use semicolon for comments in ARM assembly [OK]

Common Mistakes:

• Omitting semicolon before comments
• Using parentheses or colons incorrectly
• Writing cycle count as code, not comment

3. Given the following ARM instructions and their cycle counts:
MOV R0, #5 ; cycles=1
ADD R1, R0, #3 ; cycles=1
STR R1, [R2] ; cycles=2
What is the total number of cycles to execute all three instructions?

medium

A. 3

B. 6

C. 5

D. 4

Solution

1. Step 1: Add cycles for each instruction

   MOV takes 1 cycle, ADD takes 1 cycle, STR takes 2 cycles.

2. Step 2: Calculate total cycles

   Total = 1 + 1 + 2 = 4 cycles.

3. Final Answer:

   4 -> Option D

4. Quick Check:

   1+1+2 = 4 cycles [OK]

Hint: Sum individual cycles for total [OK]

Common Mistakes:

• Adding cycles incorrectly
• Ignoring cycle counts in comments
• Confusing instruction count with cycle count

4. Identify the error in this ARM instruction cycle count annotation:
LDR R3, [R4] ; cycles=1
ADD R5, R3, R6 ; cycles=1
Given that LDR takes 2 cycles and ADD takes 1 cycle, what is wrong?

medium

A. Both cycle counts are correct

B. Cycle count for ADD is incorrect

C. Cycle count for LDR is incorrect

D. Cycle counts should not be in comments

Solution

1. Step 1: Check cycle count for LDR

   LDR takes 2 cycles, but annotated as 1 cycle, so incorrect.

2. Step 2: Verify cycle count for ADD

   ADD takes 1 cycle, annotated as 1 cycle, correct.

3. Final Answer:

   Cycle count for LDR is incorrect -> Option C

4. Quick Check:

   LDR cycles = 2, not 1 [OK]

Hint: Check official cycle counts for each instruction [OK]

Common Mistakes:

• Assuming comments are always correct
• Mixing up cycle counts of different instructions
• Ignoring question focus on LDR

5. You have a loop running 100 times on an ARM processor. Each iteration executes:
SUBS R0, R0, #1 ; cycles=1
BNE loop ; cycles=3 if branch taken, 1 if not
How many total cycles does the loop take to complete?

hard

A. 398

B. 400

C. 299

D. 300

Solution

1. Step 1: Cycles for SUBS instructions

   100 iterations x 1 cycle = 100 cycles.

2. Step 2: Cycles for BNE instructions

   99 taken (3 cycles each) = 297 cycles; 1 not taken (1 cycle) = 1 cycle; total BNE = 298 cycles.

3. Step 3: Total cycles

   100 (SUBS) + 298 (BNE) = 398 cycles.

4. Final Answer:

   398 -> Option A

5. Quick Check:

   100x1 + 99x3 + 1x1 = 398 [OK]

Hint: Multiply cycles per iteration, add last iteration separately [OK]

Common Mistakes:

• Counting BNE cycles as always 1
• Ignoring last iteration branch not taken
• Adding cycles incorrectly