BCD adder & subtractor

It's a modular BCD adder/subtractor, operating entirely via signal strength.
The architecture is fully modular and infinitely expandable; runtime scales linearly with the number of digits.

At its core lies a comparator system that outputs |a − b|.
Inputs a, b, and the output can be inverted, resulting in 8 operating modes.
Each mode is valid within a specific number range. Once this range is exceeded, a mode switch is required.
Switching follows these two rules:

Addition: if a + b > 9

Subtraction: if a > b

Negative numbers are handled using 2’s complement representation.

Next to the number pad, there are four control buttons for operation modes. Currently, only addition (+) and subtraction (−) are implemented.
The remaining two buttons are used to reset the registers and to shift the result into upper memory.

The number pad follows the standard layout found on most keyboards.
Note: If a button is pressed before the processing indicator lamp turns off, the input may be written incorrectly.