Also, to the specific "0b1100011 in the lowest bits" question, page 8 of the spec shows that, for an instruction to be 32 bits, the lowest two bits must be 11 (

Also, to the specific "0b1100011 in the lowest bits" question, page 8 of the spec shows that, for an instruction to be 32 bits, the lowest two bits must be 11 (non-11 is used for 16-bit) and the next three bits must not all be 111 (111 is used for 48-bit and larger instructions).