I cannot write the sp register in the monitor mode

I use a Cortex-A7 board and write start up code.

I try to use Security Extension.

I use `smc` instruction and make cpu mode monitor mode.

In the monitor handler, I tried to changed stack pointer value for calling other functions.

But after execute `ldr sp, =_stack_start`, sp value is 0x0 (Of course, _stack_start is not 0x0 and I check it in the assembler code).

Therefore, after `push` some registers and `pop` those registers, the data abort excetpions occurred!

How can I change the sp value?