;initialize the SP to point 
	;to the last location of RAM (Stack_Top)
	;Assume Stack_Top = 0xFF8
	LDR	R13,=Stack_Top	;load SP
	LDR  	R0,=0x125	 ;R0 = 0x125
	LDR  	R1,=0x144	 ;R1 = 0x144
	MOV  	R2,#0x56	 ;R2 = 0x56
	BL	MY_SUB	 ;call a subroutine
	ADD	R3,R0,R1	 
		;R3 = R0 + R1 = 0x125 + 0x144 = 0x269
	ADD	R3,R3,R2	 
		;R3 = R3 + R2 = 0x269 + 0x56 = 0x2BF
HERE	B	HERE		 ;stay here
	;---------------------------	
MY_SUB
	;-------------------
	;save R0, R1, and R2 on stack 
	;before they are used by a loop
	STR  	R0,[R13]	 ;save R0 on stack
	SUB  	R13,R13,#4	 
		;R13 = R13 - 4, to decrement the stack pointer
	STR  	R1,[R13]	 ;save R1 on stack
	SUB  	R13,R13,#4	 
		;R13 = R13 - 4, to decrement the stack pointer
	STR  	R2,[R13]	 ;save R2 on stack
	SUB  	R13,R13,#4	 
		;R13 = R13 - 4, to decrement the stack pointer
	;--------R0,R1, and R2 are changed
	MOV  	R0,#0	 	 ;R0 = 0
	MOV  	R1,#0	 	 ;R1 = 0
	MOV  	R2,#0	 	 ;R2 = 0
	;------------------------------------------
	;restore the original registers contents from stack
	ADD	R13,R13,#4	 
		;R13 = R13 + 4 to increment the stack pointer
	LDR	R2,[R13]	 ;restore R2 from stack
	ADD	R13,R13,#4	 
		;R13 = R13 + 4 to increment the stack pointer
	LDR	R1,[R13]	 ;restore R1 from stack
	ADD	R13,R13,#4	 
		;R13 = R13 + 4 to increment the stack pointer
	LDR	R0,[R13]	 ;restore R0 from stack

	BX	LR		 ;return to caller