Privilege heap address 0 as a magic unset sentinal value
author |
Steve Losh <steve@stevelosh.com> |
date |
Mon, 09 May 2016 22:00:49 +0000 |
parents |
4050f38d9715 |
children |
12b69e64ece1 |
(in-package #:bones.wam)
;;; The cells of the WAM are essentially N bit bytes, with different chunks of
;;; bits representing different things. All cells have type tag bits in the
;;; low-order bits and their value in the higher-order bits:
;;;
;;; value type
;;; vvvvvvvvvvvvvvTT
;;;
;;; The contents of the value depend on the type of cell.
;;;
;;; NULL cells always have a value of zero.
;;;
;;; STRUCTURE cell values are an index into the heap, describing where the
;;; structure starts.
;;;
;;; REFERENCE cell values are an index into the heap, pointing at whatever the
;;; value is bound to. Unbound variables contain their own heap index as
;;; a value.
;;;
;;; FUNCTOR cell values are again split into two chunks of bits:
;;;
;;; index arity
;;; iiiiiiiiiiAAAA
;;;
;;; The index is the index into the WAM's functor table where this functor's
;;; symbol lives. Arity is the arity of the functor.
(defun* cell-type ((cell cell))
(:returns cell-tag)
(logand cell +cell-tag-bitmask+))
(defun* cell-value ((cell cell))
(:returns cell-value)
(ash cell (- +cell-tag-width+)))
(defun* cell-type-name ((cell cell))
(:returns string)
(eswitch ((cell-type cell) :test #'=)
(+tag-null+ "NULL")
(+tag-structure+ "STRUCTURE")
(+tag-reference+ "REFERENCE")
(+tag-functor+ "FUNCTOR")))
(defun* cell-type-short-name ((cell cell))
(:returns string)
(eswitch ((cell-type cell) :test #'=)
(+tag-null+ "NUL")
(+tag-structure+ "STR")
(+tag-reference+ "REF")
(+tag-functor+ "FUN")))
(defun* cell-functor-index ((cell cell))
(:returns functor-index)
(cell-value cell))
(defun* cell-aesthetic ((cell cell))
"Return a compact, human-friendly string representation of the cell."
(format nil "[~A~A]"
(cell-type-short-name cell)
(eswitch ((cell-type cell))
(+tag-null+ "")
(+tag-structure+
(format nil " ~X" (cell-value cell)))
(+tag-functor+
(format nil " ~X"
(cell-functor-index cell)))
(+tag-reference+
(format nil " ~X" (cell-value cell))))))
(defun* cell-null-p ((cell cell))
(:returns boolean)
(= (cell-type cell) +tag-null+))
(defun* cell-reference-p ((cell cell))
(:returns boolean)
(= (cell-type cell) +tag-reference+))
(defun* cell-functor-p ((cell cell))
(:returns boolean)
(= (cell-type cell) +tag-functor+))
(defun* cell-structure-p ((cell cell))
(:returns boolean)
(= (cell-type cell) +tag-structure+))
(defun* make-cell ((tag cell-tag) (value cell-value))
(:returns cell)
(values
(logior (ash value +cell-tag-width+)
tag)))
(defun* make-cell-null ()
(:returns cell)
(make-cell +tag-null+ 0))
(defun* make-cell-structure ((value cell-value))
(:returns cell)
(make-cell +tag-structure+ value))
(defun* make-cell-reference ((value cell-value))
(:returns cell)
(make-cell +tag-reference+ value))
(defun* make-cell-functor ((functor-index functor-index))
(:returns cell)
(make-cell +tag-functor+ functor-index))