(use srfi-18 posix)

(let* [(time (current-milliseconds))
       (slice-used? #f)
       (c 0)
       (timer-reason 255)
       (t (make-thread
	   (lambda ()
	     (print "thread started")
	     (let [(old-hook ##sys#interrupt-hook)
		   (ct ##sys#current-thread)]
	       (set! ##sys#interrupt-hook
		   (lambda (reason state)
		     (print "c:" c " " reason  " "(current-milliseconds))
		     (if (fx= reason timer-reason)
			 (if (< (- (current-milliseconds) time) 1000)
			     (begin
			       (set! c (add1 c))
			       (##core#inline "C_set_initial_timer_interrupt_period" (##sys#slot ct 9))
			       ;; what does 0 mean?
			       (old-hook 0 state))
			     (begin
			       (set! slice-used? #t)
			       ;;(set! ##sys#interrupt-hook old-hook)
			       (old-hook reason state)
			       ;;(##sys#schedule)
			       ))
			 (old-hook reason state)))))
	     (let lp [(i 0)]
	       (lp (add1 i)))
	     (print "thread done"))))]
  (thread-start! t)
  (thread-yield!)
  (print "back to pri slice-used?: " slice-used? " num-ints: " c))

...
c:4051 255 1017.0
c:4052 255 1017.0
c:4053 255 1017.0
back to pri slice-used?: #t num-ints: 4053