IN: BC = new timer value
Fc = 0
Fz = 0, old time value non-zero
Fz = 1, old time value zero
BC = old time value
Internal operating system usage.
The timer latches at $0000. An initial value of $FFFF will not be decremented. The timer decrements once per centiseconds. This code uses same timer as OS_Tin and OS_Dly.