// including forms involving multiple registers such as R1:R2.
func (p *Parser) register(name string, prefix rune) (r1, r2 int16, scale int8, ok bool) {
	// R1 or R(1) R1:R2 R1,R2 R1+R2, or R1*scale.
	r1, ok = p.registerReference(name)
	if !ok {
		return
	}
	if prefix != 0 && prefix != '*' { // *AX is OK.
		p.errorf("prefix %c not allowed for register: %c%s", prefix, prefix, name)
	}
	c := p.peek()

	validKVS, ok := DefaultNotificationKVS[subSys]
	if !ok {
		return nil
	}
	for tname, kv := range tgt {
		subSysTarget := subSys
		if tname != config.Default {
			subSysTarget = subSys + config.SubSystemSeparator + tname
		}
		if v, ok := kv.Lookup(config.Enable); ok && v == config.EnableOn {