f80 to decimal conversion

f80.rkt

#lang racket

(define one #x3fff8000000000000000)
(define two #x40008000000000000000)
(define three #x4000c000000000000000)

(define (fits-in-80bit? x)
  (zero? (arithmetic-shift x -80)))

(define (sign x)
  (bitwise-and (arithmetic-shift x -79) #x1))

(define (exponent x)
  (bitwise-and (arithmetic-shift x -64) #x7fff))

(define (significand x)
  (bitwise-and x #xffffffffffffffff))

(define (f80->dec x)
  (define sgn (sign x))
  (define exp (exponent x))
  (define sig (significand x))
  (define unbiased-exp (- exp 16383))

  (define f80
    (* (expt -1 sgn)
       (expt 2 unbiased-exp)
       (for/sum ([i (range 64)]
                 #:when (bitwise-bit-set? sig (- 63 i)))
         (printf "Bit ~a contributes ~a~n" (- 63 i) (expt 2 (- i))) 
         (expt 2 (- i)))))

  (printf "Sign: 0x~x~n" sgn)
  (printf "Unbiased Exponent: 0x~x~n" unbiased-exp)
  (printf "Significand: 0x~x~n" sig)
  (printf "decimal: ~a~n" f80))