Introduction to Assembly language with Freescale Part II

/*************************************************************************
*
* Lab 4 section 2 execise 1
* Funtion string reverse to reverse the string passed
*
* @version 29.10

*************************************************************************/
.text
.align 2
.global stringReverse
.type stringReverse function

/*************************************************************************

* Function String Reverse
* Reverses the string of character passed on from main
* Inputs:
* r0 - address of first charater on string

**************************************************************************
**************************************************************************

* Clears the registers r1, r2, r3
*************************************************************************/

stringReverse:

	mov r1, #0 /* Immediate 0 to r1 */
	mov r2, #0 /* Immediate 0 to r2 */
	mov r3, #0 /* Immediate 0 to r3 */
	push {r1} /* Push r1 onto the memory stack */



/*************************************************************************
* Function pushloop
* Push the string onto the stack in order starting from MSB to LBS

pushloop:
	ldr r1, [r0,r2] /* Load charater of string into r1 */
	push {r1} /* Push the character onto memory stack */
	add r2,r2,#1 /* Increase the count by 1 */
	cmp r1, #0 /* Compare r1 with null to test for end (i.e 0) */
	bne pushloop /* Loop back to pushloop if r1 is not equal to 0 */
	pop {r1} /* Pop r1(null value 0)from stack */



/*************************************************************************
* Function poploop
* Pop's back the string onto the stack in order starting from LSB to MSB
* Reverse order
************************************************************************/

poploop:

	pop {r1} /* Pop the character into r1 in the reverse order*/
	str r1, [r0,r3] /* Store the charater in the registers r0 from start */
	add r3, r3, #1 /* Increase the count of r3 by 1*/
	cmp r2,r3 /*Compare r3 and r2 (number of characters pushed(r2) to number of characters
	popped (r3))*/
	bne poploop /* Branch back to poploop and loop if r2 is not eqaul to r3 */
	bx lr /* Branch back to location stored in lr of main */