# SXMicrocontrollerMathMethod

## BCD (packed) subtraction

From: Scott Dattalo

Translated and optimized for the Scenix SX by Nikolai Golovchenko

```;******************************************
;bcd_sub
;
; Computes  x = x - y
; where x and y are all 8-bit packed BCD numbers
; Exits with C=0 if x<y, and with z=1 if x-y = 0
;
; Size: 14 instructions
; Execution time including return: 14/15 cycles

bcd_sub
mov	W, y		;x = x - y
sub	x, W

clr	W
sb	DC		;if lsn of x < lsn of y (dec)
or	W, #\$06	;then substract extra 6
snb	C		;Similarly for the msn
jmp	bcd_sub2

or	W, #\$60
sub	x, W		;correct result
clrb	C		;restore carry
ret
bcd_sub2
sub	x, W		;correct result
setb	C		;restore carry
ret
```

With result in a different register:

```;******************************************
;bcd_sub_to
;
; Computes  s = x - y
; where x and y are all 8-bit packed BCD numbers
; Exits with C=0 if x<y, and with z=1 if x-y = 0
;
; Size: 15 instructions
; Execution time including return: 15/16 cycles

bcd_sub_to
mov	W, y		;x = x - y
mov	W, x-W
mov	s, W

clr	W
sb	DC		;if lsn of x < lsn of y (dec)
or	W, #\$06	;then substract extra 6
snb	C		;Similarly for the msn
jmp	bcd_sub_to2

or	W, #\$60
sub	s, W		;correct result
clrb	C		;restore carry
ret
bcd_sub_to2
sub	s, W		;correct result
setb	C		;restore carry
ret
```

 file: /Techref/scenix/lib/math/sub/bcdp2_sx.htm, 1KB, , updated: 2004/6/10 14:40, local time: 2019/5/23 13:13, TOP NEW HELP FIND:  54.157.61.68:LOG IN

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