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Commit deeba1dc authored by Treehugger Robot's avatar Treehugger Robot Committed by Automerger Merge Worker
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Merge "PDL: Document the layout of fields according to the endianness" am: c5dfa41e 

Original change: https://android-review.googlesource.com/c/platform/packages/modules/Bluetooth/+/2148678



Change-Id: Ic7e6540ea356e6c7d6354fc4301a690b57a57137
Signed-off-by: default avatarAutomerger Merge Worker <android-build-automerger-merge-worker@system.gserviceaccount.com>
parents 432a1c47 c5dfa41e

tools/pdl/doc/reference.md

+47 −0
Original line number Diff line number Diff line
@@ -54,6 +54,53 @@ packet Brew { 
}

```



The endianess affects how fields of fractional bit-size (hence named bit-fields) are parsed or

serialized. Such fields are grouped together to the next byte boundary, least

significant bit first, and then byte-swapped to the required endianess before

being written to memory, or after being read from memory.



```

packet Coffee {

  a: 1,

  b: 15,

  c: 3,

  d: 5,

}



// Little endian layout

//     LSB                                   MSB

//     0    1    2    3    4    5    6    7

//     +---------------------------------------+

//  0  | a  |             b[6:0]               |

//     +---------------------------------------+

//  1  |               b[14:7]                 |

//     +---------------------------------------+

//  2  |       c      |             d          |

//     +---------------------------------------+



// Big endian layout

//     LSB                                   MSB

//     0    1    2    3    4    5    6    7

//     +---------------------------------------+

//  0  |               b[14:7]                 |

//     +---------------------------------------+

//  1  | a  |             b[6:0]               |

//     +---------------------------------------+

//  2  |       c      |             d          |

//     +---------------------------------------+

```



Fields which qualify as bit-fields are:

- [Scalar](#fields-scalar) fields

- [Size](#fields-size) fields

- [Count](#fields-count) fields

- [Fixed](#fields-fixed) fields

- [Reserved](#fields-reserved) fields

- [Typedef](#fields-typedef) fields, when the field type is an

  [Enum](#enum)



Fields that do not qualify as bit-fields _must_ start and end on a byte boundary.



## Identifiers



- Identifiers can denote a field; an enumeration tag; or a declared type.