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| author | Kaz Kylheku <kaz@kylheku.com> | 2019-10-17 18:55:43 -0700 |
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| committer | Kaz Kylheku <kaz@kylheku.com> | 2019-10-17 18:55:43 -0700 |
| commit | 45fa8653f7a3ece2eb0c3e92057fdfa9bc5b3780 (patch) | |
| tree | 0e5b995fb749830dafeadd4da1c7fde7cc7ef860 | |
| parent | 2a0033ebad7883109bc2a2053986aeaed7be9525 (diff) | |
| download | txr-45fa8653f7a3ece2eb0c3e92057fdfa9bc5b3780.tar.gz txr-45fa8653f7a3ece2eb0c3e92057fdfa9bc5b3780.tar.bz2 txr-45fa8653f7a3ece2eb0c3e92057fdfa9bc5b3780.zip | |
doc: bitfield allocation rules: endiannness.
* txr.1: Add discussion about endinanness to Bitfield
Allocation Rules for completeness.
| -rw-r--r-- | txr.1 | 31 |
1 files changed, 27 insertions, 4 deletions
@@ -64883,10 +64883,33 @@ has a nonzero value, then the offset .I O is incremented by one to cover that byte. -Lastly, the size of the structure is then padded up to a size -which is a multiple of the alignment of the most strictly aligned -member. - +As the last allocation step, the size of the structure is then padded up to a +size which is a multiple of the alignment of the most strictly aligned member. + +The description isn't complete without a treatment of byte and bit order. +Bitfield allocation follows an imaginary "bit endianness" whose direction +follows the machine's byte order: most significant bits are allocated first on +big endian, least significant bits first on little-endian. + +If a one-bit-wide bitfield is allocated into a hitherto empty structure, it +will be placed into the first byte of that structure, regardless of the +machine's endianness, and regardless of the underlying storage unit size for +that bitfield. Within that first byte, it will be placed into the most +significant bit position on a big-endian machine (bit 7); and on a +little-endian machine, it will be placed into the least significant bit +position (bit 0). If another one-bit-wide is allocated, it is placed into +bit 6 on big-endian, and bit 1 on little-endian. + +More generally, whenever a bitfield is allocated for a big-endian machine, and +the storage unit is determined into which that bitfield shall be placed, that +field will be placed at that unit's most significant bit position that is still +available for allocation. On a little-endian machine, that field will be placed +at the least significant available bit position of the storage unit. On either +type of machine, that field will be placed at the lowest-addressed byte or +bytes that are available, and within that byte or those bytes, the field +will occupy the most significant bit positions that are available. If the +field partially occupies a byte, the bits that remain free are the least +significant ones. .SS* FFI Call Descriptors |