What does it mean to isolate a field in a word?

Question

I have been reading in the 'computer organization and design' book and I encountered this section:

what operations isolate a field in a word

I know that the answer is logic operations and shift instructions from the book but. I want to know what does it even mean to isolate a field

Answer 1

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;