Making sense of ARM ISR prologue/epilogue

gistfile1.txt

00000004 <baz>:
   4:   4668            mov     r0, sp
   6:   f44f 4384       mov.w   r3, #16896      ; 0x4200
   a:   f020 0107       bic.w   r1, r0, #7
   e:   f2c4 0300       movt    r3, #16384      ; 0x4000
  12:   468d            mov     sp, r1
  14:   2280            movs    r2, #128        ; 0x80
  16:   b401            push    {r0}
  18:   601a            str     r2, [r3, #0]
  1a:   bc01            pop     {r0}
  1c:   4685            mov     sp, r0
  1e:   4770            bx      lr

00000038 <empty_isr>:
  38:   4668            mov     r0, sp
  3a:   f020 0107       bic.w   r1, r0, #7
  3e:   468d            mov     sp, r1
  40:   b401            push    {r0}
  42:   bc01            pop     {r0}
  44:   4685            mov     sp, r0
  46:   4770            bx      lr