Preserving callee-saved registers in ARM Architecture - Practice Problems & Coding Challenges

Challenge - 5 Problems

🧠 Conceptual

intermediate

Why preserve callee-saved registers?

In ARM architecture, why must a function preserve callee-saved registers?

A Because the caller expects these registers to remain unchanged after the function call.

B Because callee-saved registers are used only for temporary calculations within the function.

C Because the operating system requires all registers to be saved by the callee.

D Because callee-saved registers are automatically saved and restored by hardware.

💡 Hint

Think about who is responsible for keeping the values intact across function calls.

Callee-saved registers must be preserved by the called function because the caller assumes their values remain unchanged after the call. This allows the caller to rely on these registers without saving them before the call.

📋 Factual

intermediate

Which ARM registers are typically callee-saved?

Identify the registers that are usually preserved by the callee in ARM calling conventions.

A r4 to r11 and the stack pointer (sp)

B r0 to r3 and the link register (lr)

C Program counter (pc) and r0 to r3

D Only r0 and r1

💡 Hint

Consider which registers are used for passing arguments and which are preserved.

In ARM, registers r4 to r11 and the stack pointer (sp) are callee-saved. Registers r0 to r3 are caller-saved and used for passing arguments.

🔍 Analysis

advanced

Effect of not preserving callee-saved registers

What is the likely outcome if a function modifies callee-saved registers but does not restore them before returning?

A The function will cause a hardware exception immediately.

B The caller may experience incorrect data or behavior due to unexpected register changes.

C The program will automatically fix the registers after the function returns.

D There will be no effect because callee-saved registers are not used by the caller.

💡 Hint

Think about the caller's expectations about register values after a function call.

If callee-saved registers are modified and not restored, the caller may read incorrect values, leading to bugs or crashes.

🚀 Application

advanced

Preserving callee-saved registers in assembly

Given this ARM assembly snippet, which option correctly preserves callee-saved registers?

ARM Architecture

push {r4, r5, lr}
// function body
pop {r4, r5, pc}

A No push or pop instructions are needed for callee-saved registers.

B Push r0, r1 at start; pop r0, r1 at end.

C Push only lr at start; pop only lr at end.

D Push r4, r5, and lr at start; pop r4, r5, and pc at end to restore registers and return.

💡 Hint

Remember which registers must be saved and how to return from a function.

r4 and r5 are callee-saved registers and must be saved and restored. lr (link register) is saved to return properly using pc in pop.

❓ Reasoning

expert

Stack frame and callee-saved registers interaction

In ARM, when a function uses callee-saved registers and creates a stack frame, what is the correct order of operations to preserve these registers and set up the frame?

A 1, 3, 2, 4

B 2, 1, 3, 4

C 1, 2, 3, 4

D 2, 3, 1, 4

💡 Hint

Think about saving registers before using stack space and restoring in reverse order.

The function first saves callee-saved registers, then adjusts the stack pointer for locals, performs operations, and finally restores the stack pointer and registers before returning.