请帮忙看一下这个程序，为什么LED常亮不闪呢

.text
.global _start
_start:
        ldr r0,=0x56000050

        mov r1,#0x00000015

        str r1,[r0]

        ldr r0,=0x56000054

        mov r1,#0x00000000
 16
 17         str r1,[r0]
 18
 19         mov r4,#134217728
 20
 21 @set cricle program
 22
 23                                 @close led
 24 loop:
 25         bl dely
 26
 27         ldr r0,=0x56000054
 28
 29         mov r1,#0x000000ff
 30
 31         str r1,[r0]
 32
 33                                 @open led
 34         bl dely
 35
 36         ldr r0,=0x56000054
 37
 38         mov r1,#0x00000000
 39
 40         bl dely
 41
 42         b loop
 43
 44
 45 dely:
 46
 47         sub r2,r2,#1
 48
 49         cmp r2,#0
 50
 51         bne dely