-;;; the main types. These types are represented by the low three bits of the
-;;; pointer or immeditate object.
-(defenum (:suffix -type)
- even-fixnum
- function-pointer
- other-immediate-0
- list-pointer
- odd-fixnum
- instance-pointer
- other-immediate-1
- other-pointer)
+;;; Tags for the main low-level types are stored in the low three
+;;; bits to identify the type of a machine word. Certain constraints
+;;; apply:
+;;; * EVEN-FIXNUM-LOWTAG and ODD-FIXNUM-LOWTAG must be 0 and 4: code
+;;; which shifts left two places to convert raw integers to tagged
+;;; fixnums is ubiquitous.
+;;; * LIST-POINTER-LOWTAG + 4 = OTHER-POINTER-LOWTAG: NIL is both a
+;;; cons and a symbol (at the same address) and depends on this.
+;;; See the definition of SYMBOL in objdef.lisp
+(eval-when (:compile-toplevel :load-toplevel :execute)
+ ;; The EVAL-WHEN is necessary (at least for Lispworks), because the
+ ;; second DEFENUM uses the value of OTHER-IMMEDIATE-0-LOWTAG, which is
+ ;; defined in the first DEFENUM. -- AL 20000216
+ (defenum (:suffix -lowtag)
+ even-fixnum
+ ;; Note: CMU CL, and SBCL < 0.pre7.39, had FUN-POINTER-LOWTAG
+ ;; here. We swapped FUN-POINTER-LOWTAG and
+ ;; INSTANCE-POINTER-LOWTAG in sbcl-0.pre7.39 in order to help with a
+ ;; low-level pun in the function call sequence on the PPC port.
+ ;; For more information, see the PPC port code. -- WHN 2001-10-03
+ instance-pointer
+ other-immediate-0
+ list-pointer
+ odd-fixnum
+ fun-pointer
+ other-immediate-1
+ other-pointer))