Consider an 8-bit word whose contents can be described as follows:
- Bits 0–2 encode some value $x$.
- Bits 3–4 encode some value $y$.
- Bits 5–7 encode some value $z$.
Each of these different values is known, in this context, as a field. You can think of it as a field in some form.
Isolating a field out of a word means extracting just the value of the field. For example, denoting the entire word $w$, here is how we can isolate each of the fields:
- $x$: AND $w$ with the mask $7$ (which is $111$ in binary).
- $y$: Right-shift $w$ by 3 positions, and AND the result with the mask $3$.
- $z$: Right-shift $w$ by 5 positions.
In C, you could implement these as follows:
x = w & 7;
y = (w >> 3) & 3;
z = w >> 5;