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From: Paul Graham <pgraham@cadence.com>
Subject: Re: Disallowing Pre-Defined Operations
Date: 2000/03/17
Message-ID: <38D2A073.A30FF041@cadence.com>#1/1
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Charles Hixson wrote:
> 
> Re: Units:
> mks? cgs? or English System? (And is a gallon an Imperial Gallon or a US
> gallon?)  Are angles measures in degrees, radians, or mils? (etc.)

VHDL has a syntax for defining units for a given physical type.  For
instance:

    type distance is range integer'left to integer'right 
	units angstrom;
	nm = 10 angstrom;
	mm = 1000_000 nm;
	inch = 25 mm;
    end units;

Internally a value of a physical type is represented in terms of its
base unit, in this case angstroms.  This representation is obtained from
the 'pos attribute.  To convert from one unit to another (e.g., for
display purposes) you can do:

    variable d : distance;	-- vhdl declaration syntax
    variable d_inches : integer;
    variable d_mm : integer;

    d_inches := distance'pos(d) / distance'pos(1 inch);
    d_mm := distance'pos(d) / distance'pos(1 mm);

It's also possible to combine different units of the same type:

    d := 3 mm + 4 inch + 2 angstrom;

Paul