---
license: mit
tags:
- pytorch
- safetensors
- threshold-logic
- neuromorphic
---

# threshold-signbit8

Extract sign bit (MSB) of 8-bit number.

## Function

signbit8(a7..a0) = a7

In 2's complement representation:
- Output 0: non-negative (0 to 127)
- Output 1: negative (-128 to -1)

## Truth Table (selected)

| unsigned | signed | signbit |
|---------:|-------:|:-------:|
| 0 | 0 | 0 |
| 127 | +127 | 0 |
| 128 | -128 | 1 |
| 255 | -1 | 1 |

## Architecture

Single neuron extracting MSB:

- Weights: [1, 0, 0, 0, 0, 0, 0, 0]
- Bias: -1

Fires when: a7 - 1 >= 0, i.e., a7 >= 1

## Parameters

| | |
|---|---|
| Inputs | 8 |
| Outputs | 1 |
| Neurons | 1 |
| Layers | 1 |
| Parameters | 9 |
| Magnitude | 2 |

## Usage

```python
from safetensors.torch import load_file
import torch

w = load_file('model.safetensors')

def signbit8(a7, a6, a5, a4, a3, a2, a1, a0):
    inp = torch.tensor([float(a7), float(a6), float(a5), float(a4),
                        float(a3), float(a2), float(a1), float(a0)])
    return int((inp @ w['neuron.weight'].T + w['neuron.bias'] >= 0).item())

print(signbit8(0, 1, 1, 1, 1, 1, 1, 1))  # 0 (value = +127)
print(signbit8(1, 0, 0, 0, 0, 0, 0, 0))  # 1 (value = -128)
```

## License

MIT