detect overflow of multiply/ div32 result

[ruijc]

Hi guys,

I hope i'm not hijacking this thread but...

It's not complicated...
C=A*/B is same as C=(A*B)/$FF
C=A**B same as C=(A*B)/$FFFF

Maybe my mind is set to work with decimal only and this might be a dumb question but i cant understand what we can get from these operations

If A=1234 and B=5678 then

A*B=7006652 ( which in bynary is 0110 1010 ... )

I know that this operation keeps the higher 2 bytes 0110 1010 so
A**B= 106

Having said that, i know how they work... so my question is, when and why are these operations used?

Thanks

[richard]

its mainly about speed of execution and code size.

eg miles to kilometre conversion k=m*1.609

pbp has no floats so the 1.609 needs to be scaled up and then the result scaled back

this will work :-
you could do 1.609*1000=1609

k=m*1609 /1000

(k=m*1609
div32 1000)


this is better :-

using 1.609 * 256 = 411
so k=m*411/256 (eq to k=m*/411)

and will give you smaller faster code
and has a larger input range before overflowing also

[ruijc]

Thanks for the explanation Richard,

It's crystal clear now

Regards
Rui