Function call in ARM instructions

SP stack pointer, each exception mode has its own independent r13, which usually points to the stack dedicated to the exception mode, that is to say, the five exception modes and non-exception modes (user mode and system mode) have their own independent stacks, indexed by different stack pointers. In this way, when ARM enters the exception mode, the program can push general registers into the stack and pop them when returning, ensuring the integrity of the program status in various modes

LR Link register, generally used to store the return address; when executing instructions such as BL or BLX, it automatically stores the address of the next instruction, that is, the return address

PC Program Counter, stores the instruction to be executed

2. Program jump

B ;--> PC, jump to label

BL ; Store the address of the next instruction of the current instruction in LR, and then jump to label --> PC

BX Rm; switch instruction set, jump to the label of another instruction set;

BLX Rm; Store the address of the next instruction of the current instruction in LR, and then jump to the address of another instruction set label --> PC

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Seeing this, we need to note that not all function calls follow the AAPCS calling method. On the contrary, some ARM C compilers may force the return address to be stored on the stack, and then use forced jump instructions such as B or BX to jump.

Note that, in practice, not all function calls follow the AAPCS calling conventions: Instead of transferring the return address to lr register, the ARM C compiler may enforce the return address to be push onto the stack and afterwards performs a direct branch to the function through a B or BX instruction.