I want to calculate:

    (ticks * <microsecondsPerTick>) / 1000

to return the number of milliseconds elapsed based on a count  
maintained by a timer ISR.  microsecondsPerTick is a nice constant  
number like 1024 or 2048, so the multiplication becomes a shift, and  
factoring out twos I get:

    (ticks * (microsecondsPerTick/8)) / 125

"ticks" is at least 40 bits, however, and the multiplication MUST NOT  
OVERFLOW (been there, trying to fix that!), so I'm looking at at at  
least a 48bit intermediate value.

It would be nice if the code were in C.  Avr gcc, in particular.   
There are 8, 16, 32, and even 64 bit math functions available, but of  
course they're *C*, so they don't let you actually do mixed-sized  
math, nor are they likely to optimized

The code is not particularly time critical, but it would be nice if it  
were as small and fast as possible (using 64bit long longs seems  
excessive.)

Thoughts?  Any pointers to mixed-size C math functions in general?  In  
principle, I think I understand how to do this, but ...  Is there a  
way to take advantage of the constant?  Of 125 in particular?  (lots  
of one bits there...)

Thanks
Bill W

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