Instruction cycle counting in ARM Architecture - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to calculate the total cycles for a single instruction execution.

ARM Architecture

total_cycles = base_cycles + [1]

Drag options to blanks, or click blank then click option'

Astall_cycles

Binstruction_count

Ccache_miss

Dbranch_penalty

Attempts:

3 left

💡 Hint

Common Mistakes

Using instruction_count instead of stall_cycles adds wrong values.

Confusing branch_penalty with general stall cycles.

The total cycles include the base cycles plus any stall cycles caused by hazards or delays.

2fill in blank

medium

Complete the code to compute cycles for multiple instructions with stalls.

ARM Architecture

total_cycles = ([1] * base_cycles) + stall_cycles

Drag options to blanks, or click blank then click option'

Ainstruction_count

Bstall_cycles

Cbranch_penalty

Dcache_miss

Attempts:

3 left

💡 Hint

Common Mistakes

Using stall_cycles instead of instruction_count in multiplication.

Adding branch_penalty instead of multiplying instruction count.

Multiplying the number of instructions by base cycles gives the cycles without stalls.

3fill in blank

hard

Fix the error in the cycle calculation formula.

ARM Architecture

total_cycles = instruction_count * base_cycles [1] stall_cycles

Drag options to blanks, or click blank then click option'

A*

B-

C+

D/

Attempts:

3 left

💡 Hint

Common Mistakes

Using '*' causes incorrect multiplication of stall cycles.

Using '-' subtracts stall cycles incorrectly.

Stall cycles add to the total cycles, so the correct operator is '+'.

4fill in blank

hard

Fill both blanks to calculate average cycles per instruction.

ARM Architecture

average_cycles = total_cycles [1] instruction_count [2] 1.0

Drag options to blanks, or click blank then click option'

A/

B*

C+

D-

Attempts:

3 left

💡 Hint

Common Mistakes

Using '+' or '-' instead of division.

Not multiplying by 1.0 causes integer division in some languages.

Dividing total cycles by instruction count gives average cycles per instruction. Multiplying by 1.0 ensures floating-point division.

5fill in blank

hard

Fill all three blanks to create a dictionary of instruction cycles with conditions.

ARM Architecture

cycles = {instr: base_cycles [1] stall_cycles for instr in instructions if instr [2] 'load' [3] 'store'}

Drag options to blanks, or click blank then click option'

A+

B==

Cor

Dand

Attempts:

3 left

💡 Hint

Common Mistakes

Using '*' instead of '+' for adding cycles.

Using 'and' instead of 'or' in the condition.

Using '=' instead of '==' for comparison.

Add stall cycles to base cycles for each instruction. Filter instructions that are either 'load' or 'store' using '==' and 'or'.

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