Question 1

What is the difference between a bit flag and a multi-bit field?

Accepted Answer

A single-bit flag occupies exactly one bit position and represents a boolean (set or clear) — e.g. TCP SYN at bit 1. A multi-bit field spans a contiguous range of bits (e.g. bits 2-3 = MODE) and can hold an integer value 0..(2^width - 1). This tool handles both in a single layout: you give each field a name, a start bit (LSB0 or MSB0 numbering) and a width, and the tool validates that fields do not overlap and stay within the chosen word width (8/16/32/64-bit).

Question 2

How does encoding and decoding work?

Accepted Answer

Encoding: for each field you enter a value, the tool shifts it into the field's bit range and ORs the result into a packed integer (e.g. A=1, MODE=2 in a 4-bit layout → 0b1001). Decoding: you paste a packed integer in hex/binary/decimal, the tool extracts each field by masking and shifting, and shows the raw value, the enum label if any, and the boolean state for single-bit flags. All arithmetic uses BigInt so 64-bit layouts round-trip exactly with no Number precision loss.

Question 3

Which languages are supported for code generation?

Accepted Answer

Five languages: C/C++ (anonymous struct with bitfields plus #define masks and shifts), Rust (bitflags! macro for the single-bit flags and const helpers for multi-bit fields), Python (IntFlag class with @property accessors), Go (const block with masks/shifts and getter/setter funcs), and TypeScript (const enum + helper functions). The generated code round-trips the layout you designed, so decoding in code matches the in-browser decode exactly.

Question 4

What presets are included?

Accepted Answer

Five well-known real-world layouts: (1) POSIX file mode (12 bits — setuid/setgid/sticky + rwx × user/group/other), (2) TCP header flags (9 bits — CWR, ECE, URG, ACK, PSH, RST, SYN, FIN, NS), (3) FAT file attributes (8 bits — read-only, hidden, system, volume-label, directory, archive), (4) ARM CPSR (32-bit — mode bits, Thumb, FIQ/IRQ disable, etc.), and (5) x86 EFLAGS (selected status/control flags). Each preset is a complete layout you can edit and re-export as code.

Question 5

What extra features does this tool have versus other bitmask designers?

Accepted Answer

(1) Mixed single-bit flags and multi-bit fields in one layout. (2) 8/16/32/64-bit word widths via BigInt (no JS Number precision loss). (3) LSB0 / MSB0 bit-numbering toggle. (4) Overlap & out-of-range validation with field-level error messages. (5) Enum value labels per field (e.g. MODE 0=off, 1=auto, 2=on). (6) Reserved-bit marking for unused regions. (7) Live encode + decode round-trip. (8) Code generation for C/C++, Rust, Python, Go and TypeScript. (9) Five real-world presets (POSIX mode, TCP, FAT, ARM CPSR, x86 EFLAGS). (10) Set/clear/toggle/test helper generation. (11) Save/load layouts in localStorage. (12) History (last 20). (13) Shareable URL with full layout round-trip. (14) Color-coded visual bit strip. 100% client-side and offline — your register definitions never leave the device.

Bit Field / Bitmask / Flags Designer & Decoder

Fields (2)

Bit layout (16-bit, MSB-first display)

Encode result

Code generation

Color legend

How to use

Fields (2)

Bit layout (16-bit, MSB-first display)

Encode result

Code generation

Color legend