What does func means in R-Format instruction set?

Question

I am very new to Assembly language. I was reading about MIPS architecture and I am stuck with the last field of the Register Format (R-Format). Here is its visual representation, Can anyone please help me out with what does the sixth field means and how do we calculate it? Thanks in advance.

Answer 1

As the description mentions all R-type instructions (e.g. ADD, AND, SLL and others) have the 6 most significant bits (= op) set to 0, that means the only way to distinguish between them is to look at the 6 least significant bits (= funct). In other words they determine the instruction type. Perhaps an example will help:

ADD $1, $2, $3 

has:

op = 0 (as all R-type instructions)
rs = 2
rt = 3
rd = 1
shamt = 0
funct = 0x20 = 0b00000100000 = 32

The encoding will thus be:

0000 0000 0100 0011 0000 1000 0010 0000

For e.g. XOR (another R-type) instruction funct is 0b100110 = 0x26 = 38. So you "calculate" it by looking up what instruction you want to encode.

(taken from MIPS Instruction Reference).