What does it mean to isolate a field in a word?
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 &amp; 7;
y = (w &gt;&gt; 3) &amp; 3;
z = w &gt;&gt; 5;