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• 4v-operations

4v-wand

Four-valued semantics for a wired and.

(4v-wand a b) returns:

• F when either input is F, or
• Z when both inputs are Z, or
• T when one input is T and the other is T/Z, or
• X otherwise.

We use this to model what happens when multiple signals are connected in a wired and arrangement.

The full truth table is shown below. It intentionally corresponds to the Verilog semantics for wired ors (Section 4.6.2 of the Verilog 2005 specification).

     |   F   T   X   Z  |
-----+------------------+
  F  |   F   F   F   F  |
  T  |   F   T   X   T  |
  X  |   F   X   X   X  |
  Z  |   F   T   X   Z  |
-----+------------------+

Definitions and Theorems

Function: 4v-wand

(defun 4v-wand (a b)
  (declare (xargs :guard t))
  (mbe :logic (4vcases a
                       (t (4vcases b (z (4vt)) (& (4v-fix b))))
                       (f (4vf))
                       (z (4v-fix b))
                       (& (4vcases b (f (4vf)) (& (4vx)))))
       :exec (cond ((eq a (4vf)) (4vf))
                   ((eq b (4vf)) (4vf))
                   ((eq a (4vt))
                    (if (or (eq b (4vt)) (eq b (4vz)))
                        (4vt)
                      (4vx)))
                   ((eq a (4vz))
                    (if (or (eq b (4vt)) (eq b (4vz)))
                        b
                      (4vx)))
                   (t (4vx)))))

Theorem: 4v-wand-truth-table

(defthm 4v-wand-truth-table
  (and (equal (4v-wand (4vf) (4vf)) (4vf))
       (equal (4v-wand (4vf) (4vt)) (4vf))
       (equal (4v-wand (4vf) (4vx)) (4vf))
       (equal (4v-wand (4vf) (4vz)) (4vf))
       (equal (4v-wand (4vt) (4vf)) (4vf))
       (equal (4v-wand (4vt) (4vt)) (4vt))
       (equal (4v-wand (4vt) (4vx)) (4vx))
       (equal (4v-wand (4vt) (4vz)) (4vt))
       (equal (4v-wand (4vx) (4vf)) (4vf))
       (equal (4v-wand (4vx) (4vt)) (4vx))
       (equal (4v-wand (4vx) (4vx)) (4vx))
       (equal (4v-wand (4vx) (4vz)) (4vx))
       (equal (4v-wand (4vz) (4vf)) (4vf))
       (equal (4v-wand (4vz) (4vt)) (4vt))
       (equal (4v-wand (4vz) (4vx)) (4vx))
       (equal (4v-wand (4vz) (4vz)) (4vz)))
  :rule-classes nil)

Theorem: 4v-equiv-implies-equal-4v-wand-2

(defthm 4v-equiv-implies-equal-4v-wand-2
  (implies (4v-equiv b b-equiv)
           (equal (4v-wand a b)
                  (4v-wand a b-equiv)))
  :rule-classes (:congruence))

Theorem: 4v-equiv-implies-equal-4v-wand-1

(defthm 4v-equiv-implies-equal-4v-wand-1
  (implies (4v-equiv a a-equiv)
           (equal (4v-wand a b)
                  (4v-wand a-equiv b)))
  :rule-classes (:congruence))