GameBoy\Opcode::opcode21 PHP Méthode

    public static function opcode21(Core $core)
    {
        $core->registerD = $core->unsbtub($core->registerD - 1);
        $core->FZero = $core->registerD == 0;
        $core->FHalfCarry = ($core->registerD & 0xf) == 0xf;
        $core->FSubtract = true;
    }