REX byte
note: The REX prefix must immediately precede the opcode byte(s).
| bit | name | description |
| 7..4 | n/a | 0100b (yields 40..4Fh range) |
| 3 | W |
0 = default operand size 1 = 64bit operand size |
| 2 | R | mod R/M byte reg field extension |
| 1 | X | SIB byte index field extension |
| 0 | B |
mod R/M byte r/m field extension SIB byte base field extension opcode byte reg field extension |
