Parameter passing in registers in ARM Architecture - Interactive Code Practice

Practice - 5 Tasks

1fill in blank

easy

Complete the code to identify the register used for the first parameter in ARM calling convention.

ARM Architecture

The first parameter is passed in register [1].

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Ar0

Br4

Cr7

Dr12

Attempts:

3 left

💡 Hint

Common Mistakes

Confusing r0 with r4 which is used for other purposes.

Assuming parameters are passed on the stack instead of registers.

In ARM architecture, the first parameter is passed using register r0.

2fill in blank

medium

Complete the code to specify how many parameters are passed in registers in ARM calling convention.

ARM Architecture

Up to [1] parameters are passed in registers before using the stack.

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A4

B2

C6

D8

Attempts:

3 left

💡 Hint

Common Mistakes

Assuming only 2 parameters are passed in registers.

Thinking all parameters are passed in registers regardless of count.

ARM calling convention passes up to 4 parameters in registers r0 to r3.

3fill in blank

hard

Fix the error in the statement about parameter passing registers in ARM.

ARM Architecture

The fifth parameter is passed in register [1].

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Ar4

Br0

Cr5

Dstack

Attempts:

3 left

💡 Hint

Common Mistakes

Assuming the fifth parameter uses r4 or r5 register.

Not knowing that parameters beyond four go to the stack.

Only the first four parameters are passed in registers (r0 to r3). The fifth parameter is passed on the stack.

4fill in blank

hard

Fill both blanks to complete the description of ARM parameter passing.

ARM Architecture

Parameters [1] to [2] are passed in registers r0 to r3.

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A1

B3

C4

D5

Attempts:

3 left

💡 Hint

Common Mistakes

Starting count from 0 instead of 1.

Including parameter 5 as passed in registers.

Parameters 1 to 4 are passed in registers r0 to r3 in ARM calling convention.

5fill in blank

hard

Fill all three blanks to complete the ARM parameter passing code snippet.

ARM Architecture

void func(int a, int b, int c, int d, int e) {
  int reg_params[] = { [1], [2], [3], d };
  // e is passed on the stack
}

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

Aa

Bb

Cc

De

Attempts:

3 left

💡 Hint

Common Mistakes

Including parameter e in the register array.

Mixing the order of parameters.

Parameters a, b, c, and d are passed in registers r0 to r3 respectively. Parameter e is passed on the stack.

Practice

1. In ARM architecture, which registers are primarily used to pass the first four function parameters?

easy

A. r8 to r11

B. r4 to r7

C. r0 to r3

D. r12 to r15

Solution

1. Step 1: Understand ARM calling convention basics

   ARM uses registers r0 to r3 to pass the first four parameters to functions for speed.

2. Step 2: Identify correct register range

   Registers r0, r1, r2, and r3 are used, not the higher registers.

3. Final Answer:

   r0 to r3 -> Option C

4. Quick Check:

   First four parameters = r0 to r3 [OK]

Hint: Remember first four params go in r0-r3 registers [OK]

Common Mistakes:

• Confusing r4-r7 as parameter registers
• Thinking all parameters fit in registers
• Assuming stack is always used for parameters

2. Which of the following is the correct syntax to pass five parameters to a function in ARM, considering register usage?

easy

A. First four in r0-r3, fifth on stack

B. All five in r0-r4

C. All five on stack

D. First three in r0-r2, last two in r3-r4

Solution

1. Step 1: Recall register parameter limits

   Only the first four parameters fit in registers r0 to r3.

2. Step 2: Understand handling of extra parameters

   Any parameter beyond the fourth is passed on the stack.

3. Final Answer:

   First four in r0-r3, fifth on stack -> Option A

4. Quick Check:

   5th parameter goes on stack [OK]

Hint: Only 4 params in registers; extras go to stack [OK]

Common Mistakes:

• Assuming registers can hold more than 4 parameters
• Passing all parameters on stack unnecessarily
• Misplacing parameters in wrong registers

3. Consider a function call in ARM with six integer parameters. Which registers and memory locations hold these parameters?

medium

A. r0, r1 hold first two; r2 to r5 hold next four

B. r0, r1, r2, r3 hold first four; stack holds 5th and 6th

C. Stack holds all six parameters

D. r0 to r5 hold all six parameters

Solution

1. Step 1: Identify register usage for parameters

   ARM passes first four parameters in r0 to r3 registers.

2. Step 2: Determine location for extra parameters

   Parameters beyond the fourth (5th and 6th) are passed on the stack.

3. Final Answer:

   r0, r1, r2, r3 hold first four; stack holds 5th and 6th -> Option B

4. Quick Check:

   Extra params after 4th go to stack [OK]

Hint: First 4 params in r0-r3; rest on stack [OK]

Common Mistakes:

• Assuming registers beyond r3 hold parameters
• Ignoring stack usage for extra parameters
• Mixing register order

4. A programmer passes seven parameters to a function in ARM but notices incorrect values for the last three parameters inside the function. What is the most likely cause?

medium

A. Registers r4 to r6 should be used for last three parameters

B. All parameters must be passed in registers; stack is never used

C. Parameters beyond four are ignored by ARM

D. Only first four parameters are passed in registers; last three must be on stack but were not

Solution

1. Step 1: Recall ARM parameter passing rules

   Only the first four parameters are passed in registers r0 to r3.

2. Step 2: Identify cause of incorrect values

   If last three parameters are not placed on the stack properly, their values will be incorrect inside the function.

3. Final Answer:

   Only first four parameters are passed in registers; last three must be on stack but were not -> Option D

4. Quick Check:

   Extra params must be on stack [OK]

Hint: Extra params need stack space; missing stack causes errors [OK]

Common Mistakes:

• Assuming all params fit in registers
• Using wrong registers for extra params
• Ignoring stack for extra parameters

5. You need to optimize a function call in ARM that takes eight parameters. How should you arrange parameter passing to maximize speed and follow ARM conventions?

hard

A. Pass first four parameters in r0-r3 registers; place remaining four on the stack

B. Pass all eight parameters on the stack to keep registers free

C. Pass first six parameters in r0-r5 registers; last two on stack

D. Pass first two parameters in r0-r1; remaining six on stack

Solution

1. Step 1: Understand ARM calling convention for parameters

   ARM passes up to four parameters in registers r0 to r3 for speed.

2. Step 2: Arrange remaining parameters

   Parameters beyond the fourth must be passed on the stack as per convention.

3. Step 3: Apply to eight parameters

   First four go in r0-r3; last four go on the stack to maximize speed and follow rules.

4. Final Answer:

   Pass first four parameters in r0-r3 registers; place remaining four on the stack -> Option A

5. Quick Check:

   4 in registers + rest on stack = fastest [OK]

Hint: Use r0-r3 for first 4 params; stack for others [OK]

Common Mistakes:

• Trying to pass more than 4 params in registers
• Passing all params on stack unnecessarily
• Ignoring ARM calling convention rules