threshold-subtractor4bit

4-bit subtractor. Computes A - B (modulo 16) with borrow output.

Function

subtractor4bit(A, B) = (A - B) mod 16, borrow_out

Truth Table (selected rows)

A	B	Diff	Borrow
7	3	4	0
5	5	0	0
3	7	12	1
0	1	15	1

Architecture

Ripple-borrow subtractor using full subtractor units.

For each bit i:

• diff_i = a_i XOR b_i XOR borrow_{i-1}
• borrow_i = majority(NOT(a_i), b_i, borrow_{i-1})

The borrow signal indicates A < B (unsigned comparison).

Parameters

Inputs	8 (a3-a0, b3-b0)
Outputs	5 (d3-d0, bout)
Neurons	25
Layers	8
Parameters	86
Magnitude	88

Usage

from safetensors.torch import load_file
# See model.py for full implementation

# 7 - 3 = 4
# subtractor4(0,1,1,1, 0,0,1,1) = [0,1,0,0, 0]

# 3 - 7 = 12 (wraps), borrow=1
# subtractor4(0,0,1,1, 0,1,1,1) = [1,1,0,0, 1]

License

MIT