DEMultBC

; DEMultBC {
; Output in HL
; Destroys A
; Preserves everything else (except DE, if result is negative)
DEMultBC:
    call AbsDE
    ld a,$B7
    jp m,+_ ; Jump if BC was POSITIVE
    xor a,%10000000
_:  call AbsBC
    jp m,+_ ; Jump if DE was POSITIVE
    xor a,%10000000
_:  ld (MultSMC),a
    or a,a \ sbc hl,hl
    dec sp \ push bc \ inc sp \ pop af
                rla \ jr nc,$+3 \ add hl,de
    add hl,hl \ rla \ jr nc,$+3 \ add hl,de
    add hl,hl \ rla \ jr nc,$+3 \ add hl,de
    add hl,hl \ rla \ jr nc,$+3 \ add hl,de
    add hl,hl \ rla \ jr nc,$+3 \ add hl,de
    add hl,hl \ rla \ jr nc,$+3 \ add hl,de
    add hl,hl \ rla \ jr nc,$+3 \ add hl,de
    add hl,hl \ rla \ jr nc,$+3 \ add hl,de
    ld a,b
    add hl,hl \ rla \ jr nc,$+3 \ add hl,de
    add hl,hl \ rla \ jr nc,$+3 \ add hl,de
    add hl,hl \ rla \ jr nc,$+3 \ add hl,de
    add hl,hl \ rla \ jr nc,$+3 \ add hl,de
    add hl,hl \ rla \ jr nc,$+3 \ add hl,de
    add hl,hl \ rla \ jr nc,$+3 \ add hl,de
    add hl,hl \ rla \ jr nc,$+3 \ add hl,de
    add hl,hl \ rla \ jr nc,$+3 \ add hl,de
    ld a,c
    add hl,hl \ rla \ jr nc,$+3 \ add hl,de
    add hl,hl \ rla \ jr nc,$+3 \ add hl,de
    add hl,hl \ rla \ jr nc,$+3 \ add hl,de
    add hl,hl \ rla \ jr nc,$+3 \ add hl,de
    add hl,hl \ rla \ jr nc,$+3 \ add hl,de
    add hl,hl \ rla \ jr nc,$+3 \ add hl,de
    add hl,hl \ rla \ jr nc,$+3 \ add hl,de
    add hl,hl \ rla \ jr nc,$+3 \ add hl,de
    res 7,a ; make sure the sign does not lie
    push af \ inc sp \ push hl \ inc sp \ pop hl \ inc sp ; A -> HLu -> H -> L
MultSMC:    or a,a
    ret nc
    ex de,hl
    or a,a \ sbc hl,hl
    sbc hl,de
    ret
; }

; Result in DE
; S flag is set to opposite of input sign
AbsDE:
    or a,a
    sbc hl,hl
    sbc hl,de
    ret m ; If ( 0 - DE ) is negative, then DE is positive (so return)
    ex de,hl
    ret

; Result in BC
; S flag is set to opposite of input sign
AbsBC:
    or a,a
    sbc hl,hl
    sbc hl,bc
    ret m ; If ( 0 - BC ) is negative, then BC is positive (so return)
    push hl \ pop bc
    ret