OS_Ust, Update small timer
Register parameters
RST 20H, DEFB $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