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ARM Architectureknowledge~20 mins

Program counter (R15/PC) in ARM Architecture - Practice Problems & Coding Challenges

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Challenge - 5 Problems
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ARM Program Counter Mastery
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🧠 Conceptual
intermediate
2:00remaining
Role of the Program Counter in ARM Architecture

What is the primary function of the Program Counter (R15/PC) in ARM processors?

AIt stores the result of arithmetic operations.
BIt holds the address of the next instruction to be executed.
CIt manages the stack pointer for function calls.
DIt controls the interrupt enable flags.
Attempts:
2 left
💡 Hint

Think about what the processor needs to know to continue running instructions in order.

📋 Factual
intermediate
2:00remaining
Program Counter Behavior During Branch Instructions

What happens to the Program Counter (R15/PC) when a branch instruction is executed in ARM architecture?

AIt increments by 4 to point to the next sequential instruction.
BIt resets to zero to restart the program.
CIt is set to the target address specified by the branch instruction.
DIt stores the current instruction's opcode.
Attempts:
2 left
💡 Hint

Consider how the processor jumps to a different part of the program.

🔍 Analysis
advanced
2:00remaining
Effect of Pipeline on Program Counter Value

In ARM processors with pipelining, the Program Counter (R15/PC) does not always point to the current instruction being executed. Which statement best explains this behavior?

AThe Program Counter points to the instruction currently being decoded, which is ahead of the executing instruction due to pipeline stages.
BThe Program Counter always points to the last executed instruction to allow rollback.
CThe Program Counter is frozen during pipelining to prevent errors.
DThe Program Counter points to a random instruction to optimize speed.
Attempts:
2 left
💡 Hint

Think about how pipelining fetches instructions ahead of execution.

❓ Comparison
advanced
2:00remaining
Difference Between Program Counter and Link Register

Which of the following best describes the difference between the Program Counter (R15/PC) and the Link Register (R14) in ARM architecture?

AThe Program Counter manages interrupts; the Link Register manages arithmetic operations.
BThe Program Counter stores data values; the Link Register holds the next instruction address.
CBoth registers hold the same value but are used in different modes.
DThe Program Counter holds the next instruction address; the Link Register stores the return address during function calls.
Attempts:
2 left
💡 Hint

Consider what happens when a function is called and then returns.

❓ Reasoning
expert
2:00remaining
Impact of Directly Modifying the Program Counter

What is the likely effect if a program directly writes a new value into the Program Counter (R15/PC) register during execution?

AThe processor will jump to the new address immediately, changing the flow of execution.
BThe Program Counter will reset to the start of the program automatically.
CThe program will crash due to illegal register modification.
DThe processor will ignore the change and continue sequential execution.
Attempts:
2 left
💡 Hint

Think about what happens when you change the instruction pointer in a running program.

Practice

(1/5)
1. What is the primary role of the Program Counter (PC) in ARM architecture?
easy
A. It holds the address of the next instruction to execute.
B. It stores the result of arithmetic operations.
C. It manages input/output operations.
D. It controls the power management of the processor.

Solution

  1. Step 1: Understand the function of PC

    The Program Counter (PC) is a special register that keeps track of the address of the next instruction to be executed in the program.

  2. Step 2: Compare with other options

    Other options describe functions of different registers or components, not the PC.

  3. Final Answer:

    It holds the address of the next instruction to execute. -> Option A

  4. Quick Check:

    PC = Next instruction address [OK]
Hint: PC always points to the next instruction address [OK]
Common Mistakes:
  • Confusing PC with general-purpose registers
  • Thinking PC stores data instead of addresses
  • Mixing PC role with I/O or power control
2. Which register number corresponds to the Program Counter (PC) in ARM processors?
easy
A. R14
B. R13
C. R15
D. R12

Solution

  1. Step 1: Recall ARM register naming

    In ARM architecture, the Program Counter is known as register R15.

  2. Step 2: Verify other register roles

    R14 is the Link Register, R13 is the Stack Pointer, and R12 is a general-purpose register.

  3. Final Answer:

    R15 -> Option C

  4. Quick Check:

    PC = R15 [OK]
Hint: PC is always R15 in ARM [OK]
Common Mistakes:
  • Confusing PC with Link Register (R14)
  • Mixing up Stack Pointer (R13) with PC
  • Assuming PC is a general-purpose register
3. Consider this ARM assembly snippet:
ADD R0, R0, #4
MOV PC, R0

What happens when the MOV PC, R0 instruction executes?
medium
A. The program jumps to the address stored in R0.
B. The value in R0 is added to the PC without changing flow.
C. The PC is set to zero, causing a reset.
D. The instruction causes a syntax error.

Solution

  1. Step 1: Understand MOV PC, R0

    Moving a value into PC changes the program flow to the address in R0, causing a jump.

  2. Step 2: Analyze the ADD instruction

    ADD R0, R0, #4 increments R0 by 4, so PC will jump to this new address.

  3. Final Answer:

    The program jumps to the address stored in R0. -> Option A

  4. Quick Check:

    Setting PC = R0 causes jump [OK]
Hint: MOV to PC changes program flow to that address [OK]
Common Mistakes:
  • Thinking MOV PC,R0 just copies value without jump
  • Assuming PC increments automatically ignoring MOV
  • Believing MOV PC,R0 causes error
4. Identify the error in this ARM assembly code snippet:
MOV PC, #1000

What is wrong with this instruction?
medium
A. The immediate value #1000 is too large for MOV instruction.
B. You cannot move an immediate value directly into PC.
C. PC cannot be used as a destination register.
D. MOV instruction requires two registers, not immediate values.

Solution

  1. Step 1: Understand MOV limitations with PC

    In ARM, you cannot move an immediate value directly into PC; it must come from a register.

  2. Step 2: Check other options

    PC can be destination, MOV can use immediate but not directly to PC, and #1000 size is not the main issue here.

  3. Final Answer:

    You cannot move an immediate value directly into PC. -> Option B

  4. Quick Check:

    Immediate to PC invalid [OK]
Hint: MOV immediate to PC is invalid; use register first [OK]
Common Mistakes:
  • Trying to load PC directly with immediate values
  • Confusing PC with general-purpose registers
  • Assuming MOV always accepts immediate to any register
5. In ARM assembly, how can you implement a conditional jump to address stored in R4 only if R1 equals zero, using the Program Counter (PC)?
hard
A. Use JMP R4 if R1 equals zero.
B. Use BEQ R4 to jump if R1 is zero.
C. Use MOV PC, R4 unconditionally after checking R1.
D. Use CMP R1, #0 followed by MOVEQ PC, R4.

Solution

  1. Step 1: Compare R1 with zero

    The CMP R1, #0 instruction sets condition flags based on R1's value.

  2. Step 2: Use conditional move to PC

    MOVEQ PC, R4 moves R4 into PC only if the zero flag is set (R1 == 0), causing a conditional jump.

  3. Final Answer:

    Use CMP R1, #0 followed by MOVEQ PC, R4. -> Option D

  4. Quick Check:

    Conditional MOV to PC enables jump [OK]
Hint: Use CMP then conditional MOV to PC for jumps [OK]
Common Mistakes:
  • Using BEQ with a register operand (invalid syntax)
  • Unconditionally moving PC ignoring condition
  • Using JMP which is not an ARM instruction
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