/* Calculate A^3 where A is an 8-bit unsigned variable.
 * The result is placed into 24-bit variable C. 
 * The 24-bit result is saved using little endian byte ordering.
 * C = A^3
 *
 * Solution: To find the cube of A, I begin by finding the square of A,
 *           load calling arguments needed by muls16x8_24 using the 
 *           move word instruction (movw), call muls16x8_24
 *           subroutine, and finally save the result into C.
 *            
 */

 .INCLUDE <m328pdef.inc> 

.DSEG
A:     .BYTE 1
C:     .BYTE 3

.CSEG

A_Cubed:
    lds  r26,A    ; load
	mul  r26,r26
    movw r25:r24, r1:r0 
    rcall muls16x8_24
	sts  C,r2     ; least significant byte (little end)
	sts  C+1,r3   ; most  significant byte (big end)
	sts  C+2, r4

	rjmp A_Cubed

/* Multiply a 16-bit unsigned number 
 * in the r25:r24 register pair by an 8-bit number in r26.
 * return the answer in r4:r3:r2
 *
 * Multiply
 *    r25:r24
 *    x   r26
 * ----------
 *    0:r3:r2  = r16*r18  first byte
 * + r1:r0     = r17*r18  second byte
 * --------
 *   r4:r3:r2  = (r1 + c):(r3 + r0):r2
 */

muls16x8_24:
    push r15         ; save contents of registers modified by this subroutine
	in   r15, SREG
    push r24
	push r1
	push r0

    clr  r4
    mul  r24,r26     ; multiply LSB
	movw r3:r2,r1:r0 ; copy result to result register
	mul  r25,r26     ; multiply MSB
	add  r3,r0
    adc  r4,r1

	pop  r0
	pop  r1
	pop  r24
	out  SREG, r15
	pop  r15
	ret