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Topics - eric

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QB64 Discussion / _SHR bug with sign bit of _UNSIGNED _INTEGER64
« on: December 03, 2019, 06:20:30 pm »
Hello,

I use qb64 to test some algorithms on integers when i got a strange garbage.

So, i try :

Code: QB64: [Select]
  2.  
  3. n = &H0100000000000000
  4.  
  5. PRINT , HEX$(n)
  6. n = _SHL(n, 7)     'shift 1 to sign bit left most
  7. PRINT , HEX$(n)
  8. n = _SHR(n, 7)     'revert to first position
  9. PRINT , HEX$(n)
  10.  
  11. m~&& = &H0100000000000000
  12. PRINT , HEX$(m~&&)
  13. m~&& = _SHL(m~&&, 7)
  14. PRINT , HEX$(m~&&)
  15. m~&& = _SHR(m~&&, 7)
  16. PRINT , HEX$(m~&&)
  17.  

both display result as :

     100000000000000
     8000000000000000
     FF00000000000000


I think _SHR is not shifting but expanding the "sign" bit to the right.

The last print is the negative of the first.

Is it a solution ?

Thanks.


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