(defun shl (x width bits)
  "Compute bitwise left shift of x by 'bits' bits, represented on 'width' bits"
  (logand (ash x bits)
          (1- (ash 1 width))))

(defun shr (x width bits)
  "Compute bitwise right shift of x by 'bits' bits, represented on 'width' bits"
  (logand (ash x (- bits))
          (1- (ash 1 width))))