2025-02-MicroprocessorApplication/notes/8.md

ARM Subroutines

Subroutine, Function, Procedure

It has a single-entry and single-exit.

• bl lbl
  • set lr to the next instruction location
  • set pc to the label
• bx lr
  • set pc to the saved lr

ARM register calling convention:

Passing Argument via registers:

    mov r0, #3
    mov r1, #4
    bl ssq
    mov r2, r0
    b endl
SSQ:
    mul r0, r0, r0
    mul r1, r1, r1
    add r0, r0, r1
    bx lr

pc is always incremented by 4. Each time, 4 bytes are fetched from the instruction memory. It is not depending on instruction size (2 or 4 bytes).

Using Stack

Stack is a LIFO data structure.

• push / pop

There is two stack growth convention:

• Ascending and Descending.
• Full and Empty

sp = r13 Coretex-M uses Full Descending stack.

commands

• push {register_list}
• stmdb sp!, {register_list}
• pop {register_list}
• ldmia sp!, {register_list}

push instruction

push {Rd}

pop instruction

multi-level subroutines

main:
    mov r0, #2
    bl quad
quad:
    push { lr }
    bl sq
    bl sq
    pop { lr }
    bx lr
sq:
    mul r0, r0
    bx lr

Initializing SP

Before using the stack, software has to define stack pointer

Recursive Function

No difference to multi-level subroutines.

It has multiple stack frames.