BCD

Binary-coded decimal is a way of representing decimal digits (0 through 9) using four-bit binary sequences. Instead of converting a number like 47 directly to binary (which would be 101111), BCD encodes each decimal digit separately: the 4 becomes 0100 and the 7 becomes 0111, giving 01000111. This hybrid approach lets digital systems work with human-readable decimal values without the conversion overhead.

The four-bit limitation of BCD means each nibble represents a single digit from 0 to 9, leaving six unused combinations (1010 through 1111) when decoded. This inefficiency is the trade-off: BCD wastes storage compared to pure binary, but it simplifies interface logic, display drivers, and systems where decimal readability matters more than memory economy. Early calculators, test equipment, and industrial controllers relied heavily on BCD for this reason.

BCD appears in applications that output numerical data to seven-segment LED or LCD displays, since each digit maps directly to a display segment driver without further translation. Similarly, weight scales, counters, and frequency meters often use BCD internally to drive their indicator panels. Some microcontrollers and PLCs still support packed BCD arithmetic (storing two digits per byte) for applications where decimal precision and readability are required, such as financial equipment or dosing systems.

The codec has real limits. BCD addition requires correction logic: when two BCD digits sum to more than 9, the processor must add 6 to the result to account for the skipped binary combinations. Multiplication and division become clunky, and floating-point BCD is expensive in silicon. Modern processors have largely abandoned BCD for general computing, though some instruction sets retain a BCD operation or two for backward compatibility.

Today BCD survives in embedded systems with direct display coupling, where the simplicity of digit-to-display mapping outweighs the cost of slower arithmetic. It also appears in legacy equipment, maintenance firmware, and in technical standards where decimal representation is mandated, such as certain alarm thresholds or calibration records that must never round or convert.