From: seebs@... (Peter Seebach)
Date: 2007-04-30T13:30:30+09:00
Subject: Re: Why is 0x7FFFFFFF Bignum and not Fixnum ?

In message <b6597fbb7656f667941d80d0f00bfd63@ruby-forum.com>, Mr Magpie writes:
>> Because 0x40000000 is either the sign bit or the magic flag.  

>0x80000000 would be the sign bit. What is this "magic flag" that seems 
>to be occupying 0x40000000 ?

Remember how it said you get "32 bits, minus one"?

What that means is you have a total of 31 bits available for values.

So.  The largest 31-bit number is 0x7FFF,FFFF.  But wait!  What about
negative numbers?  How do we represent those?  If we are using a 31-bit
representation, then the topmost of *those 31 bits* must be the sign
bit, so the highest number is 0x3FFF,FFFF -- because 0x7FFF,FFFF is
negative.

Unless something else similar is being done; I never read the source.  I
just know that, if you have 31 bits available for positive and negative
numbers, you can't represent 0x7FFF,FFFF as a positive number in that
range, and Ruby seems to have no unsigned types.

-s