On 24/11/2010 16:22, Olin Lathrop wrote:
> Kerry Wentworth wrote:
>
>> Please pardon my ignorance, I'm here to learn.  Is 10.6 arithmetic
>> something that everybody but me uses?  Is it an assembly language
>> thing?
> Fixed point in general is commonly used on small systems like PICs.  10.6
> format is just one example, which has 6 fraction bits in a 16 bit number.
> Fixed point is common when you need to represent fractional values but ha=
ve
> a integer arithmetic processor.  It's more tricky than floating point
> because you have to keep track of where the imagined binary point is.
>
> One way to think of floating point is that it stores the binary point
> information in the number, and updates everything accordingly at run time=
..
> That takes more hardware, which small processors like PICs don't have.
>
>> I use a C compiler almost exclusively.
> There's the problem.  Wake up and smell the bits.  It is difficult to
> understand what is really going on if you keep a compiler between you and
> what is really going on.
>
>
> *
Other common option are 8.8 if only 8 bit "final" values are needed, as=20
then the higher byte is the Integer.

The compiler is irrelevant.

You are just using 16 bit integers.
You only have to think about the dp location after a multiply. Since an=20
Integer multiplication results in twice the bits on answer, you have to=20
think about that anyway. (maybe in C cast the input values to 32bits=20
from 16bit sized type before you multiply or it will overflow).

Where the DP is placed is application dependent. All the existing=20
Integer libraries work. You just may rescale by packing or dropping a=20
byte, or Left or right shifts before calling the existing math routines=20
or after calling them.

Maths done with paper and pen of currency is example of fixed point=20
integer. Infact most long hand on paper fixed decimal length maths is=20
not Floating point but simply Integer. It's really nothing magical, but=20
pre-calculator Primary School maths.

--=20
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