Parameter passing in registers in ARM Architecture - Time & Space Complexity

Time Complexity: Parameter passing in registers

O(1)

Understanding Time Complexity

When a program calls a function, it often sends information called parameters. In ARM architecture, these parameters are passed using registers.

We want to understand how the time to pass parameters grows as the number of parameters increases.

Scenario Under Consideration

Analyze the time complexity of the following ARM code snippet that passes parameters in registers.

    MOV R0, param1
    MOV R1, param2
    MOV R2, param3
    BL  function_call
    

This code moves up to four parameters into registers R0 to R3 before calling a function.

Identify Repeating Operations

Look for repeated steps that take time.

• Primary operation: Moving each parameter into a register (MOV instructions).
• How many times: Once per parameter, up to four times.

How Execution Grows With Input

As the number of parameters increases, the number of MOV instructions grows linearly, but only up to four parameters.

Input Size (parameters)	Approx. Operations (MOV instructions)
1	1
3	3
4	4
5	4 (only first 4 in registers)

Pattern observation: The number of operations grows linearly with parameters but stops at four because only four registers are used.

Final Time Complexity

Time Complexity: O(1)

This means the time to pass parameters in registers does not grow beyond a fixed small number, even if more parameters exist.

Common Mistake

[X] Wrong: "Passing more parameters always takes more time because each parameter adds a new instruction."

[OK] Correct: Only the first four parameters use registers, so after four, extra parameters are passed differently and do not add register moves.

Interview Connect

Understanding how parameters are passed in registers helps you explain efficient function calls and low-level performance, a useful skill in systems programming and embedded development.

Self-Check

What if the architecture allowed passing eight parameters in registers instead of four? How would the time complexity change?

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