Subroutine call convention (AAPCS) in ARM Architecture - Cheat Sheet & Quick Revision

Recall & Review

beginner

What does AAPCS stand for in ARM architecture?

AAPCS stands for ARM Architecture Procedure Call Standard. It defines rules for how functions receive parameters, return values, and manage registers.

beginner

Which registers are used to pass the first four arguments to a subroutine in AAPCS?

The first four arguments are passed using registers r0, r1, r2, and r3.

beginner

How does AAPCS specify the handling of the return value from a subroutine?

The return value is placed in register r0. For larger data, r1 may also be used.

intermediate

What is the role of the stack in the AAPCS calling convention?

The stack is used to store additional arguments beyond the first four, save registers that must be preserved, and manage local variables.

intermediate

Which registers must a subroutine preserve according to AAPCS?

Registers r4 to r11 and the stack pointer (sp) must be preserved by the called subroutine. These are called callee-saved registers.

In AAPCS, which register holds the return address when a subroutine is called?

A r14 (link register)

B r15 (program counter)

C r13 (stack pointer)

D r0

The link register (r14) holds the return address so the subroutine knows where to return after finishing.

How are more than four arguments passed to a subroutine in AAPCS?

A All arguments are passed in registers

B Extra arguments are passed on the stack

C Extra arguments are ignored

D Extra arguments are passed in special memory

Only the first four arguments use registers r0 to r3; any additional arguments are passed on the stack.

Which registers are caller-saved in AAPCS?

A r0 to r3 and r12

B r4 to r11

C r13 to r15

D All registers

Registers r0 to r3 and r12 are caller-saved, meaning the caller must save them if needed after the call.

What must a subroutine do with callee-saved registers before modifying them?

A Nothing, they can be freely modified

B Use different registers instead

C Pass them to the caller

D Save them on the stack and restore before returning

Callee-saved registers must be preserved by saving their values on the stack and restoring them before returning.

Where is the stack pointer stored in ARM architecture under AAPCS?

A r15

B r14

C r13

D r12

The stack pointer is stored in register r13.

Explain how arguments and return values are handled in the AAPCS calling convention.

Think about registers used for input and output of subroutines.

Describe the difference between caller-saved and callee-saved registers in AAPCS and why this matters.

Consider who is responsible for saving which registers.