threshold-ripplecarry2bit

Adds two 2-bit numbers with carry-in. The classic ripple-carry architecture using cascaded full adders.

Circuit

       a0  b0  cin          a1  b1
        │   │   │            │   │
        └───┼───┘            └───┼───┘
            ▼                    ▼
       ┌─────────┐          ┌─────────┐
       │   FA0   │──────────│   FA1   │
       └─────────┘   c0     └─────────┘
            │                    │    │
            ▼                    ▼    ▼
           s0                   s1   cout


    Input:  (a1 a0) + (b1 b0) + cin
    Output: (cout s1 s0)

The carry ripples from FA0 to FA1, hence "ripple-carry."

Example

  11 (a1=1, a0=1)
+ 11 (b1=1, b0=1)
+ 0  (cin=0)
────
 110 (cout=1, s1=1, s0=0)

3 + 3 + 0 = 6 = 0b110

Architecture

Component	Neurons
FA0	9
FA1	9

Total: 18 neurons, 42 parameters, 8 layers

Inputs/Outputs

Type	Bits	Meaning
a	a1, a0	First 2-bit number
b	b1, b0	Second 2-bit number
cin	1 bit	Carry in
s	s1, s0	2-bit sum
cout	1 bit	Carry out

Usage

from safetensors.torch import load_file

w = load_file('model.safetensors')

def ripple_carry_2bit(a0, a1, b0, b1, cin):
    # FA0: a0 + b0 + cin -> s0, c0
    # FA1: a1 + b1 + c0 -> s1, cout
    # See model.py for full implementation
    pass

Scaling

This 2-bit adder demonstrates the pattern. For n bits:

• n full adders in cascade
• Depth grows linearly (slow for large n)
• Carry-lookahead or prefix adders would be faster but more complex

Files

threshold-ripplecarry2bit/
├── model.safetensors
├── model.py
├── config.json
└── README.md

License

MIT