/* Using the indirect addressing mode instructions ld and st
 * write a subroutine to save registers  r0 to r15 to a 
 * stack buffer located at SRAM 0x04FF
 * Hint: SRAM address 0x0000 to 0x0001F map to registers r0 to r31 
 */

.INCLUDE <m328pdef.inc>

Setup:
    ldi  YH,0x05   ; YH = r29, initialize register stack pointer
	ldi  YL,0x00   ; YL = r28, pre-decrement, first saved location is 04FF

    rcall PreReg0_20
	rcall SaveReg0_15

done:
	rjmp  done


SaveReg0_15:
    clr  XH      ; r27
	clr  XL      ; r26
	clr  r25     ; starting with r0
    ldi  r24,16  ; save 16 registers
save_reg:  
    ld   r25, X+
	st   -Y, r25
	dec  r24
	brne save_reg
    ret

/* 
 * Preload registers  r0 to r20 with values 21 to 1
 */
PreReg0_20:
    clr  XH      ; r27
	clr  XL      ; r26
    ldi  r24,21  ; clear registers r0 to r20
reg_loop:  
    st   X+,r24
	dec  r24
	brne reg_loop
	ret