Program counter (R15/PC) in ARM Architecture - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to identify the register used as the program counter in ARM architecture.

ARM Architecture

The program counter in ARM is register [1].

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

AR12

BR15

CR7

DR0

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing R15 with other general-purpose registers like R0 or R7.

Thinking the program counter is a special separate register outside R0-R15.

In ARM architecture, the program counter is the register R15, often abbreviated as PC.

2fill in blank

medium

Complete the sentence to explain the role of the program counter (PC) in ARM.

ARM Architecture

The program counter [1] the address of the next instruction to execute.

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

Amodifies

Bdeletes

Cignores

Dstores

Attempts:

3 left

💡 Hint

Common Mistakes

Choosing 'modifies' or 'deletes' which do not describe the PC's role correctly.

Confusing the PC with a data register.

The program counter stores the address of the next instruction that the CPU will execute.

3fill in blank

hard

Fix the error in the statement about the program counter (PC).

ARM Architecture

In ARM, the program counter is register [1] which is also called R14.

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

AR15

BR13

CR14

DR12

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing the link register (R14) with the program counter (R15).

Assuming the PC is R14 because it is close in number.

The program counter is register R15, not R14. R14 is the link register.

4fill in blank

hard

Fill both blanks to complete the explanation of how the program counter (PC) works in ARM.

ARM Architecture

The program counter [1] the address of the current instruction plus [2] bytes to point to the next instruction.

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

Aholds

B4

C2

D8

Attempts:

3 left

💡 Hint

Common Mistakes

Using 2 or 8 bytes instead of 4 bytes for the instruction size.

Saying the PC modifies or deletes the address instead of holding it.

The PC holds the address of the current instruction plus 4 bytes (the size of one ARM instruction) to point to the next instruction.

5fill in blank

hard

Fill all three blanks to complete the dictionary comprehension that maps ARM registers to their roles, including the program counter.

ARM Architecture

register_roles = { [1]: [2] for [3] in ['R13', 'R14', 'R15'] }

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

A'PC'

B'reg'

Creg

D'LR'

Attempts:

3 left

💡 Hint

Common Mistakes

Using quotes around the loop variable name.

Mixing variable names and strings incorrectly.

This comprehension creates a dictionary where each register name (reg) maps to the string 'reg' as a placeholder role. To specifically map PC or LR, more logic would be needed.

Practice

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