not Pseudoinstruction.

The not pseudoinstruction changes each bit in a register. Each 1 is changed to a 0 and each 0 is changed to a 1. (This is sometimes called "flipping" each bit or "reflecting" each bit.)

not   d,s  # load register d with the 
           #  bitwise not of register s
           # (pseudoinstruction)

This is a pseudoinstruction. The assembler translates it into a single basic assembly instruction, a nor instruction. The nor operation is given in the table. The nor of two bits is equivalent to applying not to the or of the bits. The nor instruction uses two registers as operands and writes the result to a third register:

nor  d,s,t  # $d <— bitwise NOR $s with $t.
            # (basic instruction)

Look at the first two columns of the table where the first operand is zero. The nor of the two operands is the not of the second operand.