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• Combine-functions

Combine16s

(combine16s a1 a0) merges unsigned bytes, producing the 16-bit signed interpretation of (a1 << 8) | a0.

Definitions and Theorems

Function: combine16s$inline

(defun combine16s$inline (a1 a0)
 (declare (type (unsigned-byte 8) a1 a0))
 (mbe
     :logic (logior (ash (fast-logext 8 (nfix a1)) 8)
                    (nfix a0))
     :exec
     (the (signed-byte 16)
          (logior (the (signed-byte 16)
                       (ash (the (signed-byte 8) (fast-logext 8 a1))
                            8))
                  a0))))

Theorem: combine16s-signed-byte

(defthm combine16s-signed-byte
  (implies (and (force (unsigned-byte-p 8 a1))
                (force (unsigned-byte-p 8 a0)))
           (signed-byte-p 16 (combine16s a1 a0))))