bitrange

Bitrange, or bit range, refers to a contiguous sequence of bits within a larger binary word. The range is defined by a high and a low index, and its width is high minus low plus one. Bitranges are used to reference, extract, or modify specific fields inside a word, such as a status flag, a counter, or a protocol field.

Notation and languages vary. In Verilog, a bit range is written as [MSB:LSB], for example reg [31:0] r; and r[15:8] selects eight bits. In VHDL, a range is expressed as (MSB downto LSB) or (LSB to MSB) depending on context, such as signal f : std_logic_vector(31 downto 0); and f(15 downto 8) for a field. Many software languages use similar concepts under the name bitfield or mask, with extraction and assignment accomplished via shifts and masks or by language-specific slice syntax.

Typical operations include extracting a field by masking and shifting, for instance field = (word >> low) & ((1 << width) - 1), where width = high - low + 1. Setting a field involves clearing the target bits and writing the new value into that slice, e.g., word = (word & ~(mask << low)) | ((value & mask) << low) with mask = (1 << width) - 1. Bitranges also determine the width of a field, which affects masking, zero/sign extension, and hardware register semantics.

Endianness affects byte order across multiple words but does not change the internal bit numbering within a single word. Bitranges are fundamental in designing and interpreting hardware registers, communication protocols, and low-level software interfaces.