Subject:
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Re: FW: Something else is needed, I think...
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Newsgroups:
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lugnet.robotics
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Date:
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Wed, 5 May 1999 20:33:46 GMT
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Viewed:
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995 times
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Paul Speed <pspeed@augustschell.com> wrote:
> Mario Ferrari wrote:
> > ... I succeeded in writing a legOS program that performs a lot of trig
> > math with fixed point (four decimals). ...
>
> So far every discussion I've seen about fixed point math
> that's gone to any detail to explain the implementation has had it
> a little off. Usually, trying to impose the decimal system onto
> your fixed point routines makes them both harder to code and slower
> to run.
The one advantage to working with radix 10, on the RCX at least, is that
the display routines use this radix, so even if you prefer working with
some power of two radix, you might find yourself stuck converting in the
end. Not that this is bad; I prefer to work in radix 2 for the same
reasons you mentioned.
Also, as far as explaining fixed point goes, the analogy to base 10 is
certainly easier to understand, even if it is misleading when it comes to
implementation.
It is also not clear what Mario meant by four decimals. Maybe he meant
four fractional places?
Regarding what is needed in the byte code for fixed point: the processor
has everything you need; the byte code has bit operations but is missing
shifts; those would need to be added if you do not want to have to use
multiplies instead. Also, 16-bit fixed point requires 32-bit temporaries;
these would have to be added too.
-Kekoa
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Message has 2 Replies:
 | | Re: FW: Something else is needed, I think...
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| (...) That's true. My very first fixed point implementation was in radix 10. At the time I didn't even know what I was doing was called fix point. Later, in graphics work, it just seemed obvious to use radix 2 since there are several tricks that (...) (25 years ago, 5-May-99, to lugnet.robotics)
| | | Re: FW: Something else is needed, I think...
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| (...) I must admit I didn't think at radix 2 for fixed point math. It is obviously the best choice to implement. I used radix 10 fixed point math because it came more natural to me. (...) Sorry I was not clear (it happens when you write in an idiom (...) (25 years ago, 5-May-99, to lugnet.robotics)
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Message is in Reply To:
| | Re: FW: Something else is needed, I think...
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| (...) So far every discussion I've seen about fixed point math that's gone to any detail to explain the implementation has had it a little off. Usually, trying to impose the decimal system onto your fixed point routines makes them both harder to (...) (25 years ago, 5-May-99, to lugnet.robotics)
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