以下ARM指令是什么意思？

  a7a4d8:   b530        push    {r4, r5, lr}
  a7a4da:   466c        mov r4, sp
  a7a4dc:   4605        mov r5, r0
  a7a4de:   682a        ldr r2, [r5, #0]
  a7a4e0:   ebad 0d02   sub.w   sp, sp, r2
  a7a4e4:   f104 0014   add.w   r0, r4, #20 ; 0x14
  a7a4e8:   4669        mov r1, sp
  a7a4ea:   b082        sub sp, #8
  a7a4ec:   466a        mov r2, sp
  a7a4ee:   462b        mov r3, r5
  a7a4f0:   f746 f1b8   bl  5c0864 <RoutineName>
  a7a4f4:   9800        ldr r0, [sp, #0]
  a7a4f6:   9901        ldr r1, [sp, #4]
  a7a4f8:   46a5        mov sp, r4
  a7a4fa:   bd30        pop {r4, r5, pc}

x_a7a4d8(dword *ptr_allocsize, void *arg1)
{
  alloca(*ptr_allocsize)
  dword p2;
  qword p1;
  x_5c0864(&arg1, &p1, &p2, ptr_allocsize)
  return p1;
}