Bit order - Revision history

Jsummers: Removed from deprecated "Binary Data" category

2017-08-23T19:30:28Z

Removed from deprecated "Binary Data" category

Jsummers at 20:02, 4 December 2015

2015-12-04T20:02:40Z

Jsummers: Changed primary category

2015-12-04T18:52:04Z

Changed primary category

Jsummers: Typo

2014-07-20T15:46:06Z

Typo

Jsummers: Created page with "{{FormatInfo |formattype=electronic |subcat=Binary Data }} '''Bit order''' is the question of whether the most significant bit, or the least significant bit, of a byte comes "..."

2014-05-30T23:24:03Z

Created page with "{{FormatInfo |formattype=electronic |subcat=Binary Data }} '''Bit order''' is the question of whether the most significant bit, or the least significant bit, of a byte comes "..."

New page

{{FormatInfo
|formattype=electronic
|subcat=Binary Data
}}
'''Bit order''' is the question of whether the most significant bit, or the least significant bit, of a byte comes "first". For many formats, this is not a meaningful question to ask, but for some it is very important.

== Disambiguation ==
There is an orthogonal concept called '''bit numbering''', which is the question of whether the most significant bit, or the least significant bit, is known as "bit 0". This can confuse the issue, since you still have to decide whether "bit 0" or "bit 7" comes first.

The term '''bit endianness''' (or '''bit-level endianness''') is sometimes seen, but it seems that not everyone agrees on its meaning, due to conflation of the ''bit ordering'' and ''bit numbering'' concepts.

== Discussion ==
There are two sensible bit orders, which we can call '''MSB-first''' (most significant bit first), and '''LSB-first''' (least significant bit first).

Bit order issues are similar to those of (byte) [[endianness]], but with a conceptual difference. With endianness, that storage device (or other data source) assigns a natural ''order'' to the bytes, and the format specification decides which bytes are more ''significant''. With bits, it's usually the other way round. The storage device assigns a natural ''significance'' to the bits (by packing them into bytes), and the format specification decides their logical ''order''.

Bit order is relevant when data is interpreted as a bitstream, and two or more entities (or portions of entities) are encoded by a single byte. For example, it is often relevant to:
* Uncompressed bitmap image formats, if multiple pixel are packed into each byte.
* Some compressed data formats, such as [[Huffman coding]] and most forms of [[LZW]].
* Transfer encodings such as [[Base64]].

Image formats usually use MSB-first order. That usually means the most significant bit(s) of a byte correspond to the leftmost pixel of those encoded by that byte.

Serial communications devices usually use LSB-first order, and this may be reflected in formats related to them, such as fax formats.

Note that if a format uses byte-oriented data compression, it is possible for there to be two distinct layers of bit ordering. For example, an image format could use one bit order for extracting compression codes from the compressed datastream, and another for extracting pixels from the uncompressed datastream.

== Links ==
* [http://www.fileformat.info/mirror/egff/ch06_04.htm Bit Order], from the [[Encyclopedia of Graphics File Formats]]
* [[Wikipedia:Endianness|Wikipedia: Endianness]] - Has a section on bit endianness
* [[Wikipedia:Bit numbering|Wikipedia: Bit numbering]]
* [http://eli.thegreenplace.net/2005/11/22/when-bit-endianness-matters/ When bit endianness matters]

@@ Line 22: / Line 22: @@
 Image formats usually use MSB-first order. That usually means the most significant bit(s) of a byte correspond to the leftmost pixel of those encoded by that byte.
-Serial communications devices usually use LSB-first order, and this may be reflected in formats related to them, such as fax formats.
+Serial communications devices usually use LSB-first order, and this may be reflected in formats related to them, such as [[fax formats]].
 Note that if a format uses byte-oriented data compression, it is possible for there to be two distinct layers of bit ordering. For example, an image format could use one bit order for extracting compression codes from the compressed datastream, and another for extracting pixels from the resulting uncompressed datastream.

@@ Line 28: / Line 28: @@
 == Links ==
 * [http://www.fileformat.info/mirror/egff/ch06_04.htm Bit Order], from the [[Encyclopedia of Graphics File Formats]]
-* [[Wikipedia:Endianness|Wikipedia: Endianness]] - Has a section on bit endianness
+* [[Wikipedia: Endianness]] - Has a section on bit endianness
-* [[Wikipedia:Bit numbering|Wikipedia: Bit numbering]]
+* [[Wikipedia: Bit numbering]]
 * [http://eli.thegreenplace.net/2005/11/22/when-bit-endianness-matters/ When bit endianness matters]

@@ Line 1: / Line 1: @@
 {{FormatInfo
 |formattype=electronic
-|subcat=Binary Data
+|subcat=Elements of File Formats
 }}
 '''Bit order''' is the question of whether the most significant bit, or the least significant bit, of a byte comes "first". For many formats, this is not a meaningful question to ask, but for some it is very important.
@@ Line 31: / Line 31: @@
 * [[Wikipedia:Bit numbering|Wikipedia: Bit numbering]]
 * [http://eli.thegreenplace.net/2005/11/22/when-bit-endianness-matters/ When bit endianness matters]

@@ Line 13: / Line 13: @@
 There are two sensible bit orders, which we can call '''MSB-first''' (most significant bit first), and '''LSB-first''' (least significant bit first).
-Bit order issues are similar to those of (byte) [[endianness]], but with a conceptual difference. With endianness, that storage device (or other data source) assigns a natural ''order'' to the bytes, and the format specification decides which bytes are more ''significant''. With bits, it's usually the other way round. The storage device assigns a natural ''significance'' to the bits (by packing them into bytes), and the format specification decides their logical ''order''.
+Bit order issues are similar to those of (byte) [[endianness]], but with a conceptual difference. With endianness, the storage device (or other data source) assigns a natural ''order'' to the bytes, and the format specification decides which bytes are more ''significant''. With bits, it's usually the other way round: The storage device assigns a natural ''significance'' to the bits (by packing them into bytes), and the format specification decides their logical ''order''.
 Bit order is relevant when data is interpreted as a bitstream, and two or more entities (or portions of entities) are encoded by a single byte. For example, it is often relevant to:
@@ Line 24: / Line 24: @@
 Serial communications devices usually use LSB-first order, and this may be reflected in formats related to them, such as fax formats.
-Note that if a format uses byte-oriented data compression, it is possible for there to be two distinct layers of bit ordering. For example, an image format could use one bit order for extracting compression codes from the compressed datastream, and another for extracting pixels from the uncompressed datastream.
+Note that if a format uses byte-oriented data compression, it is possible for there to be two distinct layers of bit ordering. For example, an image format could use one bit order for extracting compression codes from the compressed datastream, and another for extracting pixels from the resulting uncompressed datastream.
 == Links ==