Re: Numerical calculations: Why not use fixed point types for everything?

[Dennis Lee Bieber <wlfraed@ix.netcom.com>]

On Fri, 18 Jan 2013 01:17:37 -0800 (PST), Ada novice <shai.lesh@gmx.com>
declaimed the following in comp.lang.ada:

> 
> I have read (John Mc Cormick: Building parallel...with Ada's book) that fixed point arithmetic is faster than floating-point arithmetic since integer instructions are faster so I would ask about the second point put by Adam that mathematical functions are slow with fixed-point numbers. Can you please elaborate on that?
>
	Integer operations may be faster than floating point... But fixed
point with a non-zero decimal place add in the overhead of having to
track where the point is and shifting results to match the declared data
type.

25 * 25 => 725

2.5 * 2.5 => 7.25 -- but if the data type is only one decimal place this
now has to be rounded and shifted to become 7.3.

	All these adjustments and tracking are being done at the high-level
instruction set, not in the processor hardware. In contrast, a floating
point processor does all the mantissa and exponent adjustments in the
hardware, which may be faster than having to do things like --

ld i1, 25
mult i1, 25
add i1, 5
div i1, 10

-- the sequence to treat integer registers as fixed 1-decimal place data
compared to (the add/div would be added by the compiler based upon the
number of decimal places to be kept in the result, based upon the number
of places that were in the source 

ld f1, 2.5E0
mult f1, 2.5E0

-- 
	Wulfraed                 Dennis Lee Bieber         AF6VN
        wlfraed@ix.netcom.com    HTTP://wlfraed.home.netcom.com/