/* A factorial programs you have written up to this point have not
 * allowed an input of 1. Modify your factorial program to return 1
 * if the input is 0 and turn on the green LED wired to Port B bit 0
 * on the CSULB shield if the input is greater than 8. 
 */

.INCLUDE <m328pdef.inc> 

.DSEG
A:     .BYTE 1
B:     .BYTE 1
C:     .BYTE 2
D:     .BYTE 2

.CSEG
.ORG 0x0000

RST_VECT:
rjmp setup

.ORG 0x0100 ; bypass IVT
.INCLUDE "spi_shield.inc"

setup:
    rcall  initShield   ; initialize the CSULB shield

Fact0_8:
	lds  r26, A        ; calculate r26!
	clr  r25
	clr  r24
	inc  r24           ; r25:r24 = 0x0001
	cbi  PortB,0       ; turn OFF error LED

    movw r1:r0,r25:r24 ; guess input is 0
    tst  r26
	breq fact0_8b
	cpi   r26, 9       ; if input >= 9 (complement of test is input < 9)
	brlo fact0_8a
	; error
	sbi  PortB,0       ; then turn ON the error LED
	rjmp fact0_8b
fact0_8a:            ; else, calculate factorial
    rcall muls16x8_24  ; r4:r3:r2 = r25:r24 x r26
    movw  r25:r24, r3:r2
    dec  r26
	brne fact0_8a
fact0_8b:
	sts  C, r0    ; least significant byte (little end)
	sts  C+1, r1  ; most  significant byte (big end)
	rjmp Fact0_8

/* Multiply a 16-bit unsigned number in the r25:r24 register pair by 
 * an 8-bit number in r22. Return the answer in r4:r3: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,r22     ; multiply LSB
	movw r3:r2,r1:r0 ; copy result to result register
	mul  r25,r22     ; multiply MSB
	add  r3,r0
    adc  r4,r1

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