+;;;; prefixes
+
+(define-instruction x66 (segment)
+ (:printer x66 () nil :print-name nil)
+ (:emitter
+ (bug "#X66 prefix used as a standalone instruction")))
+
+(defun emit-prefix (segment name)
+ (ecase name
+ ((nil))
+ (:lock
+ #!+sb-thread
+ (emit-byte segment #xf0))
+ (:fs
+ (emit-byte segment #x64))
+ (:gs
+ (emit-byte segment #x65))))
+
+(define-instruction fs (segment)
+ (:printer seg ((fsgs #b0)) nil :print-name nil)
+ (:emitter
+ (bug "FS prefix used as a standalone instruction")))
+
+(define-instruction gs (segment)
+ (:printer seg ((fsgs #b1)) nil :print-name nil)
+ (:emitter
+ (bug "GS prefix used as a standalone instruction")))
+
+(define-instruction lock (segment)
+ (:printer byte ((op #b11110000)) nil)
+ (:emitter
+ (bug "LOCK prefix used as a standalone instruction")))
+
+(define-instruction rep (segment)
+ (:emitter
+ (emit-byte segment #b11110011)))
+
+(define-instruction repe (segment)
+ (:printer byte ((op #b11110011)) nil)
+ (:emitter
+ (emit-byte segment #b11110011)))
+
+(define-instruction repne (segment)
+ (:printer byte ((op #b11110010)) nil)
+ (:emitter
+ (emit-byte segment #b11110010)))
+