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From: dewar@merv.cs.nyu.edu (Robert Dewar)
Subject: Re: Byte/Bit Twiddling in Ada
Date: 1997/02/15
Message-ID: <dewar.856015719@merv>#1/1
X-Deja-AN: 218979989
References: <5dvfnn$olj@neocad.xilinx.com> <33048844.7AEB@elca-matrix.ch>
Organization: New York University
Newsgroups: comp.lang.ada
Date: 1997-02-15T00:00:00+00:00
List-Id: <comp.lang.ada>


Mats said

<<A(0 .. 15) := A(16 .. 31);  -- copy low-order 16 bits
                            -- into high order 16 bits

A(0 .. 30) := A(1 .. 31);  -- right shift
A := A(31) & A(0 .. 30);   -- rotate left>>


You are making entirely unwarranted assumptions about the mapping of
bit positions to high or low order, there is no such guarantee. Even
if you know the endianness, you cannot make such assumptions.

If you want a right shift, the portable way to write this is to use
the predefined right shift operation!

Note that in GNAT, the shift operators can be defined for any integer
type, including user defined ones, by writing the appropriate 
pragma Import (Intrinsic) declarations. This however is defintiely
NOT required to work by the RM. The only thing the RM guarantees is
that the predefined shift operations in interfaces will work.