Subnormal number: definition · Chip Field Guide

Beginner

An extra-small number a format can still store by giving up some of its detail, instead of jumping straight to zero.

Novice

A floating-point value smaller than the smallest normal number. Its exponent field is all zeros and the hidden leading 1 becomes a 0, so it has fewer significant bits. Subnormals fill the gap between the smallest normal value and zero.

Expert

Encodings with $E = 0$ , valued $(-1)^S \times 2^{1 - \text{bias}} \times 0.M$ . They give gradual underflow ( $x - y = 0$ only if $x = y$ ) at the cost of precision; hardware sometimes flushes them to zero for speed. In FP16 they reach down to $2^{-24}$ .