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From: Simon Wright <simon@pushface.org>
Newsgroups: comp.lang.ada
Subject: Re: How do I write directly to a memory address?
Date: Wed, 09 Feb 2011 10:42:40 +0000
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Date: 2011-02-09T10:42:40+00:00
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"Vinzent Hoefler" <0439279208b62c95f1880bf0f8776eeb@t-domaingrabbing.de>
writes:

> Adam Beneschan wrote:
>
>> On Feb 8, 12:01 pm, Hyman Rosen <hyro...@mail.com> wrote:
>>> On 2/8/2011 2:43 PM, Dmitry A. Kazakov wrote:
>>>
>>> > Three candidates in a row I have interviewed for a C/C++/C#
>>> > position could not answer the question how to test the 3rd bit of
>>> > a byte in C. All three had 3+ years of programming "experience."
>>>
>>>      (byte & (1 << (3 - 1)))
>>
>> That's not the correct answer.  The correct answer is to ask, "Third
>> bit from which end?"
>
> From the less significant end, unless you want the bit numbers to
> change when assigned to different sized variables.

Well, on big-endian hardware like standard PowerPC, they - in some sense
- do.

With a project on PowerPC involving lots of different groups/companies,
some writing in Ada and some in C, we ended up drawing maps: this is the
sort of thing I mean; it describes (the first word of) an NTP datagram,
which is sent in network byte order:

   --   0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1
   --  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+
   --  |LI | VN  |Mode |    Stratum    |     Poll      |   Precision   |
   --  +-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

(this one is my own work, for project purposes we put the byte numbers
in as well).

We found that most (UK-raised) programmers could intuit that bit 31 of
this 32-bit structure was the lsb of Precision, however unnatural they
thought it.

I don't think I've ever met anyone who naturally thought big-endian;
perhaps it's a first-machine effect.