Register parameters
RST 20H, DEFW $78
IN: BC = new timer value OUT: Fc = 0 Fz = 0, old time value non-zero Fz = 1, old time value zero BC = old time value
IN: BC = new timer value OUT: Fc = 0 Fz = 0, old time value non-zero Fz = 1, old time value zero BC = old time value