Bitendianness

Bitendianness refers to the ordering of bits within a data unit or stream. It is distinct from byte endianness, which concerns the order of bytes in multi-byte values. Bitendianness describes whether bits are transmitted or stored in a most-significant-bit first (MSB-first) or least-significant-bit first (LSB-first) order within a byte or bit sequence.

Within a byte, MSB-first means the bit with the highest value is processed first, while LSB-first processes the lowest-valued bit first. Many network and communication protocols adopt MSB-first bit ordering for bit fields transmitted over a channel. Some serial interfaces and hardware devices, however, can be configured to use either MSB-first or LSB-first, depending on the device’s design or the chosen communication mode.

Bitendianness is largely separate from the byte-level endianness that governs how multi-byte integers are arranged in memory. In practice, software on different architectures may share the same byte order but differ in bit order within bytes. The C and C++ standards do not specify a portable bit layout for bitfields, and many compilers implement bit-field order in ways that vary by platform, which can affect portability.

When designing interoperable data formats or hardware interfaces, it is important to specify the bit order explicitly. Protocols, file formats, and device manuals often define whether bit streams use MSB-first or LSB-first ordering. Where possible, data should be processed with explicit shifting and masking to avoid assumptions about internal bit order, and any conversion between host and transmitted representations should account for bitendianness as well as byte endianness.