Most, but not all, processors use big- or little-endian bit ordering within a byte that is consistent with big- or little-endian ordering of bytes within a multibyte scalar. Let us consider the Motorola 68030, which uses big-endian byte ordering. The documentation of the 68030 concerning formats is confusing. The user's manual explains that the bit ordering of bit fields is the opposite of bit ordering of integers. Most bit field operations operate with one endian ordering, but a few bit field operations require the opposite ordering. The following description from the user's manual describes most of the bit field operations:
A bit operand is specified by a base address that selects one byte in memory (the base byte), and a bit number that selects the one bit in this byte. The most significant bit is bit seven. A bit field operand is specified by: (1) a base address that selects one byte in memory; (2) a bit field offset that indicates the leftmost (base) bit of the bit field in relation to the most significant bit of the base byte; and (3) a bit field width that determines how many bits to the right of the base byte are in the bit field. The most significant bit of the base byte is bit field offset 0, the least significant bit of the base byte is bit field offset 7.
Do these instructions use big-endian or little-endian bit ordering?