Question regarding assembly language.

Question

I am reading the book The Art of Assembly Language. I came across this paragraph.

 To determine a particular instruction’s opcode, you need only select the
 appropriate bits for the iii, rr, and mmm fields. For example, to encode the 
mov ax, bx instruction you would select iii=110 (mov reg, reg), rr=00 (ax), 
and mmm=001 (bx). This produces the one-byte instruction 11000001 or 0C0h.

Here I can't understand what is iii, rr and mmm? Can anybody please explain? Thanks in advance.

Answer 1

They're bits in the opcode. The sequence iiirrmmm makes up the opcode byte.

Answer 2

"iii" represents a three-bit field (because there are three i's), "rr", a two-bit field and "mmm" another three-bit field. The reason letters are used like this is so that when you see "iiirrmm", you know which bits correspond to which fields in the opcode.