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Path: utzoo!hcrvax!hugh
From: hugh@hcrvax.UUCP (Hugh Redelmeier)
Newsgroups: net.micro.68k,net.arch,net.lang
Subject: Re: Inconsistent bit addressing in the 68020: big- AND little-endian
Message-ID: <1136@hcrvax.UUCP>
Date: Fri, 31-Aug-84 10:52:06 EDT
Article-I.D.: hcrvax.1136
Posted: Fri Aug 31 10:52:06 1984
Date-Received: Sat, 1-Sep-84 00:32:14 EDT
References: loral.444 <491@turtlevax.UUCP> <1185@rti-sel.UUCP>
Organization: Human Computing Resources, Toronto
Lines: 15

Randy Buckland says the VAX is entirely little-endian.  If you look at
the floating point format, it seems to be middle-endian or something:
the exponent is in the middle of the fraction, splitting the fraction
into two parts.  Furthermore, the fraction components contained in succeeding
16-bit words decreases in significance (step-function-endian??).

Perhaps the reason is that in changing the precision of integers, the low order
bits are important (in narrowing, you pray the high order bits are the same;
in widening you make them so), whereas in floating point, the high order
fraction bits are the important ones.  Thus, for various tricks, you want
to address an integer by the location of its low bits, and a float by
the location of its exponent & high order bits.

At first glance, the VAX f.p. format looks like a botch; at second glance,
it makes some sense.  Any other thoughts?