Byte and Bit Order Dissection

Discussing the differences between big and little endianness, bit and byte order and what it all means.
Summary

The topic of byte and bit endianness can go even further than what we discussed here. Hopefully this article has covered the main aspects of it. See you next time in the maze.

Kevin Kaichuan He is a senior system software engineer at Solustek Corp. He currently is working on board bring-up, embedded Linux and networking stacks projects. His previous work experience includes being a software engineer at Cisco Systems and a research assistant in Computer Science at Purdue University. In his spare time, he enjoys digital photography, PS2 games and movies.

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Memory bit order of int

Anonymous's picture

On a system with little endian and int holding 32 bits. If one want to bit twiddle and set bits by memory order, would this be correct?

* To set bit 1,2,8,29 of int my_int in memory, in c, one would set bit:

bit 1: my_int |= 1 << 7;
10000000 00000000 00000000 00000000 (0x80000000)
bit 2: my_int |= 1 << 6;
11000000 00000000 00000000 00000000 (0xc0000000)
bit 8: my_int |= 1 << 15;
11000000 10000000 00000000 00000000 (0xc0800000)
bit 29: my_int |= 1 << 26;
11000000 10000000 00000000 00000100 (0xc0800004)

If so, is there any trick to calculate the left-shift from a given value, i.e. "Set bit 29" - give 26.

As the left-shift order would be:

set  left
bit  shift
 0    7
 1    6
 2    5
 3    4
 4    3
 5    2
 6    1
 7    0
 8   15
 9   14
10   13
11   12
12   11
..   ..

have tried with various ((set_bit%BITS_PER_INT / CHAR_BIT) * CHAR_BIT) + ...; variants but always become a big huge pile of mods and so on.

Any good trick for this?

(I'm perhaps thinking wrong here, - starting to get to late)

Bit order?

Anonymous's picture

What is bit order? In which machines do bits have addresses?

Ethernet Address Endianess

Nick Collins's picture

I think your description of Ethernet Addressing is mistaken. In your example where the MAC address 12:34:56:78:9a:bc, you say that "12" will appear on the line first. This is not correct. The "bc" will appear first. Refer to section 3.2 of the 802.3 spec. It explicitly states the byte ordering of the Length field and the CRC are high-order byte first. So, I'm led to believe that the SA and DA are low-order byte first.

This would make sense because we know that the first bit on the wire determines multicast or unicast and that this is the LSB of the entire field...which is the last byte (not of the 1st byte).

Void

Nick Collins's picture

Nevermind my previous posting. It was a late night.

Errata: dot2ip() function

kingchurch's picture

It's incomplete in this on-line version which should be:

/* dot2ip - convert a dotted decimal string into an
* IP address
*/
uint32_t dot2ip(char *pdot)
{
uint32_t i,my_ip;

my_ip=0;
for (i=0; i
my_ip = my_ip*256+atoi(pdot);
if ((pdot = (char *) index(pdot, '.')) == NULL)
break;
++pdot;
}
return my_ip;
}

Re: Errata: dot2ip() function

kingchurch's picture

Already fixed by LJ, thanks!

Errata for ASCII Graphs

kingchurch's picture

Most of the ASCII graphs inlined in this on-line version of
the article are not formatted properly.

I'm contacting LJ to correct the format. In the meanwhile
you can reference my original article here if you get
confused of the ASCII graphs:

http://www.employees.org/~hek2000/articles/endianess-v0.7.html

Re: Errata for ASCII Graphs

kingchurch's picture

Already fixed. by LJ. Thanks!

Re: Byte and Bit Order Dissection

feiyunw's picture

1. A great article;
2. I suggest you create a HOWTO in the Linux Documentation Project (www.tldp.org) so that more people can benefit from your article;
3. As I know, bit0 is the MSB in Motorola PowerPC Manual; maybe you should clarify your bit numbering explicitly;

Re: Byte and Bit Order Dissection

kingchurch's picture

Thank you!
I'll consider the HOWTO suggestion.
About the 3rd comment, have you seen the "Typo"
discussion thread other readers brought up ?
Hopefully my correction to the typo can address your
doubt too.

- kevin

Typo?!

Anonymous's picture

"That is, in a big endian system the most significant bit is stored at the lowest bit address; in a little endian system, the least significant bit is stored at the lowest bit address."

Re: Typo?! -- -Yes, it's an error

kingchurch's picture

In fact, it is an error. In the original article I submitted
to LJ , I wrote:

"That is, in a big endian system, the most significant
bit is stored at the lowest bit address and in a
little endian system, the least significant bit is
stored at the lowest bit address." ---- Correct
^^^^^^
But somehow it was changed to the following
in the on-line version without notifying me.

"That is, in a big endian system, the most significant
bit is stored at the lowest bit address and in a
little endian system, the least significant bit is
stored at the highest bit address." --- Wrong
^^^^^^^^
I'm contacting LJ to correct this error now,
in the meanwhile please reference my original
sentence.

Thanks,

Kevin

Big and Little Endians

Anonymous's picture

Thank you for the pow wow concerning big endians and little endians. One thing is clear, although there are several kinds of endians, there are neither good endians nor bad endians. It would be nice to have but one type of endian, but uniting all endian tribes of thought under one teepee is not likely for the forseeable future. Nevertheless, it would be nice to hold a big council, so let me know when and where, and I'll make a reservation to attend.

Re: Typo?!

kingchurch's picture

The sentence you quoted follows "Bit order usually follows the same endianness as the byte order for a given computer system. ".

So I'm illustrating what the bit order will look like if it follows
the byte order on the same architecture. In another
word, in some systems where bit order doesn't follow
byte order, the quoted sentence is not applicable.

Thanks,

Kevin

Re: Typo?!

Anonymous's picture

No not really. What is meant is that in big-endian, bit 0 is the most significant bit and in little-endian, bit 7 is the most significant bit (for a single byte).

Example:
In most RISC architectures a 64-bit bus would be represented as 64bus

In an Intel system a 64-bit bus would be represented as 64bus

Re: Byte and Bit Order Dissection

Anonymous's picture

You left out everyone's favorite forgotten case: Middle endian! And a mention of the origin of "endian" (we have the Lilliputians to thank for this).

Seriously, though -- good article.

Re: Byte and Bit Order Dissection

kingchurch's picture

Thank you for the input. It must be more complete to include
"Middle Endian" in the discussion.
On the other hand, I have a word count limitation for the article
which forces me include only the most typical cases :p

Kevin

Re: Byte and Bit Order Dissection

Anonymous's picture

No, I was kidding about Middle Endian. It's an obsolete format (or rather, _they're_ obsolete formats). But no byte order discussion is complete without a mention of "Gulliver's Travels". Right after "First introduced by Danny Cohen in 1980, it describes the method a computer system uses to represent multi-byte integers." should be something like, "This was a reference to the disagreement about which side of an egg was the proper side to crack first."

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