(import chicken scheme extras)
(use miscmacros srfi-4)

(define v (make-u8vector 16000000 100))

(time
 (let ((pos 0)
       (mask-pos 0)
       (len (u8vector-length v)))
   (let loop ()
     (bitwise-xor 1 1)
     (modulo mask-pos 4)
     (set! pos (+ pos 1))
     (when (< pos len) (loop))))
 )