Velocity Reviews - Computer Hardware Reviews

Velocity Reviews > Newsgroups > Programming > C Programming > bitfield diagram - interpretation welcomed!

Reply
Thread Tools

bitfield diagram - interpretation welcomed!

 
 
Doug
Guest
Posts: n/a
 
      05-18-2007
Hi all,

A workmate was recently bitching to me about an RFC.

(Apologies - the RFC number eludes me at present (it's related to the
DIAMETER protocol, that's all I can remember) but I will try to find
out and update the thread.)

He was moaning how the RFC laid out the description of an 8-bit
field. According to him, the diagram was (fixed-width font required,
but it won't really matter, and I've omitted the schematic-like use of
-,| and + that the RFCs usually use):

0 1 2 3 4 5 6 7
F F X X X Y Y Y P

The odd thing for him (and me) was the ordering of the bit numbers in
the diagram - increasing from left to right. According to him, there
was absolutely no description of what this diagram meant. In other
words, is the 0-bit in the diagram the Most Significant Bit, or the
Least Significant Bit?

We work with network protocol specfications, so suffice to say this
lack of precision was surprising to him (and on his relaying the
story, is surprising to me). He has already found products during
interop testing with the two different interpretations.

This is apparently an early draft of the RFC, and he will be
requesting clarification, but I was wondering what your people's take
on it was? How would you intepret this?

Thanks in advance,
Doug

 
Reply With Quote
 
 
 
 
Doug
Guest
Posts: n/a
 
      05-18-2007
On 18 May, 21:08, Doug <(E-Mail Removed)> wrote:

<snip>

> 0 1 2 3 4 5 6 7
> F F X X X Y Y Y P


Ahem, please ignore the 'P'. 'P'retend it isn't there.

Thanks,
Doug

 
Reply With Quote
 
 
 
 
Terminal Crazy
Guest
Posts: n/a
 
      05-18-2007
In article <(E-Mail Removed) .com>,
Doug <(E-Mail Removed)> wrote:
> Hi all,


> A workmate was recently bitching to me about an RFC.


> (Apologies - the RFC number eludes me at present (it's related to the
> DIAMETER protocol, that's all I can remember) but I will try to find
> out and update the thread.)


> He was moaning how the RFC laid out the description of an 8-bit
> field. According to him, the diagram was (fixed-width font required,
> but it won't really matter, and I've omitted the schematic-like use of
> -,| and + that the RFCs usually use):


> 0 1 2 3 4 5 6 7
> F F X X X Y Y Y P


> The odd thing for him (and me) was the ordering of the bit numbers in
> the diagram - increasing from left to right. According to him, there
> was absolutely no description of what this diagram meant. In other
> words, is the 0-bit in the diagram the Most Significant Bit, or the
> Least Significant Bit?



It doesn't matter, bit 0 is always the lowest.

HTH

--
Mitch

http://www.velocityreviews.com/forums/(E-Mail Removed)
www.sand-hill.freeserve.co.uk/terminal_crazy

 
Reply With Quote
 
Doug
Guest
Posts: n/a
 
      05-18-2007
Ahem,

Please ignore me completely. The bitfield diagram appears within a
PDU diagram, the bit numbering is completely normal for an RFC.

My apologies for wasting your time and bandwidth.

Doug

 
Reply With Quote
 
Ben Pfaff
Guest
Posts: n/a
 
      05-18-2007
Terminal Crazy <(E-Mail Removed)> writes:

> In article <(E-Mail Removed) .com>,
>> The odd thing for him (and me) was the ordering of the bit numbers in
>> the diagram - increasing from left to right. According to him, there
>> was absolutely no description of what this diagram meant. In other
>> words, is the 0-bit in the diagram the Most Significant Bit, or the
>> Least Significant Bit?

>
> It doesn't matter, bit 0 is always the lowest.


Most of the world agrees, but IBM mainframers number their bits
"backward", so that bit 0 is the MSB.
--
Comp-sci PhD expected before end of 2007
Seeking industrial or academic position *outside California* in 2008
 
Reply With Quote
 
Doug
Guest
Posts: n/a
 
      05-18-2007
On 18 May, 21:43, Terminal Crazy <Terminal_Cr...@sand-
hill.freeserve.co.uk> wrote:
> In article <(E-Mail Removed) .com>,
> Doug <(E-Mail Removed)> wrote:
>
>
>
>
>
> > Hi all,
> > A workmate was recently bitching to me about an RFC.
> > (Apologies - the RFC number eludes me at present (it's related to the
> > DIAMETER protocol, that's all I can remember) but I will try to find
> > out and update the thread.)
> > He was moaning how the RFC laid out the description of an 8-bit
> > field. According to him, the diagram was (fixed-width font required,
> > but it won't really matter, and I've omitted the schematic-like use of
> > -,| and + that the RFCs usually use):
> > 0 1 2 3 4 5 6 7
> > F F X X X Y Y Y P
> > The odd thing for him (and me) was the ordering of the bit numbers in
> > the diagram - increasing from left to right. According to him, there
> > was absolutely no description of what this diagram meant. In other
> > words, is the 0-bit in the diagram the Most Significant Bit, or the
> > Least Significant Bit?

>
> It doesn't matter, bit 0 is always the lowest.
>
> HTH
>
> --
> Mitch


Hi Mitch,

Thanks for your reply.

Actually, in this case - and I thought the same as you when my
workmate described this to me - bit 0 in that diagram is actually the
MSB. The context I was missing (but have since found in a code
comment) is that it appears within an RFC digram of a PDU. These
diagrams assume 8 bit bytes, and are written left to right 0, 1,
2, ... usually to 32/64, so the diagram lines up nicely over multiple
rows. The numbering doesn't reflect the MSB/LSB ordering - the right-
most bit in each octet in the diagram is still the LSB.

Sorry for any confusion!

Thanks,
Doug

 
Reply With Quote
 
Flash Gordon
Guest
Posts: n/a
 
      05-18-2007
Ben Pfaff wrote, On 18/05/07 22:18:
> Terminal Crazy <(E-Mail Removed)> writes:
>
>> In article <(E-Mail Removed) .com>,
>>> The odd thing for him (and me) was the ordering of the bit numbers in
>>> the diagram - increasing from left to right. According to him, there
>>> was absolutely no description of what this diagram meant. In other
>>> words, is the 0-bit in the diagram the Most Significant Bit, or the
>>> Least Significant Bit?

>> It doesn't matter, bit 0 is always the lowest.

>
> Most of the world agrees, but IBM mainframers number their bits
> "backward", so that bit 0 is the MSB.


It's not only IBM that use 0 as the MSB. To confuse things further I've
seen documents numbering bits from 1.

Moral of the story, always read the full specification and never assume
you know what it will say in advance.
--
Flash Gordon
 
Reply With Quote
 
Barry
Guest
Posts: n/a
 
      05-19-2007

"CBFalconer" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> Ben Pfaff wrote:
>> Terminal Crazy <(E-Mail Removed)> writes:
>>> In article <(E-Mail Removed) .com>,
>>>
>>>> The odd thing for him (and me) was the ordering of the bit
>>>> numbers in the diagram - increasing from left to right.
>>>> According to him, there was absolutely no description of what
>>>> this diagram meant. In other words, is the 0-bit in the diagram
>>>> the Most Significant Bit, or the Least Significant Bit?
>>>
>>> It doesn't matter, bit 0 is always the lowest.

>>
>> Most of the world agrees, but IBM mainframers number their bits
>> "backward", so that bit 0 is the MSB.

>
> Also the HP3000.
>


Of course C was not the language of choice on the HP3000.


 
Reply With Quote
 
Barry
Guest
Posts: n/a
 
      05-19-2007

"Ben Pfaff" <(E-Mail Removed)> wrote in message
news:(E-Mail Removed)...
> Terminal Crazy <(E-Mail Removed)> writes:
>
>> In article <(E-Mail Removed) .com>,
>>> The odd thing for him (and me) was the ordering of the bit numbers in
>>> the diagram - increasing from left to right. According to him, there
>>> was absolutely no description of what this diagram meant. In other
>>> words, is the 0-bit in the diagram the Most Significant Bit, or the
>>> Least Significant Bit?

>>
>> It doesn't matter, bit 0 is always the lowest.

>
> Most of the world agrees, but IBM mainframers number their bits
> "backward", so that bit 0 is the MSB.


Which is precisely why the protocols are not defined based upon
the architecture of the transceivers. CBFalconer has stated below
that the HP3000 was "backward", but the network devices
connected to its system bus were not necessarily .


 
Reply With Quote
 
Kenneth Brody
Guest
Posts: n/a
 
      05-21-2007
Barry wrote:
>
> "CBFalconer" <(E-Mail Removed)> wrote in message
> news:(E-Mail Removed)...
> > Ben Pfaff wrote:

[...]
> >>> It doesn't matter, bit 0 is always the lowest.
> >>
> >> Most of the world agrees, but IBM mainframers number their bits
> >> "backward", so that bit 0 is the MSB.

> >
> > Also the HP3000.

>
> Of course C was not the language of choice on the HP3000.


My first exposure to computers was, I believe, an HP-3000. I forget
the exact model number. I never actually saw the machine, either,
as it was in some other city at the other end of a telephone line.
The only language we had access to was some primitive form of BASIC.

On the other hand, I still recall the login:

HELLO-B669,<password>

(I still use that password on some systems.)

--
+-------------------------+--------------------+-----------------------+
| Kenneth J. Brody | www.hvcomputer.com | #include |
| kenbrody/at\spamcop.net | www.fptech.com | <std_disclaimer.h> |
+-------------------------+--------------------+-----------------------+
Don't e-mail me at: <(E-Mail Removed)>


 
Reply With Quote
 
 
 
Reply

Thread Tools

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

BB code is On
Smilies are On
[IMG] code is On
HTML code is Off
Trackbacks are On
Pingbacks are On
Refbacks are Off


Similar Threads
Thread Thread Starter Forum Replies Last Post
transforming bitfield to readable data hortitude.eyeball@gmail.com XML 0 05-25-2006 07:59 PM
bitfield & union strange ?! Claudio C++ 2 08-02-2004 09:23 AM
bitfield optimizations zb32 C++ 1 07-13-2004 05:13 AM
typecasting a bitfield to unsigned long long Sushil C Programming 1 11-28-2003 06:00 AM
Bitfield structs that are not padded to the size of an int? Davide Bruzzone C Programming 9 08-27-2003 09:04 AM



Advertisments