+;;; A one-byte instruction with a #x66 prefix, used to indicate an
+;;; operand size of :word.
+(sb!disassem:define-instruction-format (x66-byte 16
+ :default-printer '(:name))
+ (x66 :field (byte 8 0) :value #x66)
+ (op :field (byte 8 8)))
+
+;;; A one-byte instruction with a REX prefix, used to indicate an
+;;; operand size of :qword. REX.W must be 1, the other three bits are
+;;; ignored.
+(sb!disassem:define-instruction-format (rex-byte 16
+ :default-printer '(:name))
+ (rex :field (byte 5 3) :value #b01001)
+ (op :field (byte 8 8)))
+