Z80 Routines:Math:Signed Math

Here come useful routines that handle signed math.

Change Sign

;---> negate a
negA:
	neg     ;or: cpl / inc a
 

;absolute value of A
AbsA:
 bit 7,a
 ret z
 neg
 ret
 

;Input: e - 8-bit signed number
;Output: a - 8-bit signed number
;Sample: abs(-2) = 2; abs(2) = 2 
AbsAp:
	ld a,e
	rlca
	sbc a,a  ;($FF if signed, $00 if not)
	ld b,a
	xor e
	sub b
;OR
	xor a
	sub e
	jp p,$+4
	ld a,e
 

;---> negate hl
;input: hl
;ouput: hl negated
;destroys a
negHL:
	xor a
	sub l
	ld l,a
	sbc a,a
	sub h
	ld h,a
	ret
 

Extend Sign

;a to bc (extend sign)
;inputs:  a  - 8-bit signed number
;outputs: bc - same 16-bit signed number
AtoBCextendendsign:
	ld c,a
	rlca		; or rla
	sbc a,a
	ld b,a
	ret
 

; sign-extends e into de
EtoDEextendsign:
	ld a, e
	rlca		; move sign bit into carry flag
	sbc a, a	; a is now 0 or 11111111, depending on the carry
	ld d, a
	ret
 

; sign-extends de into hlde
DEtoHLDEextendsign:
	ld h, d
	add hl, hl	; move sign bit into carry flag
	sbc hl, hl	; hl is now 0 or 11111111 11111111, depending
	ret