MS Fortran bug

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3 total messages Started by Duncan Murdoch Sun, 19 Nov 1989 15:31

#260

Author: Duncan Murdoch
Date: Sun, 19 Nov 1989 15:31

30 lines
1245 bytes

The following little program crashes an XT clone with an 8086/8087.  The crash
seems to occur in the start-up code where the run-time library is trying to 
determine the coprocessor type, and goes away if I single-step or if I
link in more library routines by referencing something like the NAG random
number generator.  Can anyone suggest any fix for this?


      external Eval, Cube
      real*8 Cube,x
      x = 1.0
      x = Cube(x)
      stop
      end
c
      subroutine Eval(Fn,N,X,Value)
      real*8   Fn
      integer  N
      real*8   X(N),Value
      Value = Fn(N,X)
      return
      end
c
      real*8 function Cube(X)
      real*8 X
      Cube = X**3
      return
      end
     
Duncan Murdoch

Re: MS Fortran bug

#261

Author: Duncan Murdoch
Date: Mon, 20 Nov 1989 02:26

36 lines
1745 bytes

In article <863@maytag.waterloo.edu> I wrote:
>
>The following little program crashes an XT clone with an 8086/8087.  

It turned out to be a bug in the run-time library routine __init80x87.
It had the sequence
  FSTCW  [BP-4]
  FNINIT
  MOV    CX,01Eh
  LOOP   in place
What happens on an 8087 is that the FNINIT aborts the FSTCW instruction, and 
the control word never got stored.  Later on a garbage control word was loaded
and the system crashed.  A fix that seems to work is to put the delay loop 
before the FNINIT.  I'm not sure what the original programmer would have 
been thinking of writing it this way; the next instruction is an arithmetic
one that is preceded by a WAIT anyways.

To patch the library LLIBFOR7.LIB, search for the original bytes

  9B D9 7E FC     
  DB E3
  B9 1E 00
  E2 FE

and replace them with

  9B D9 7E FC     ; no change
  B9 1E 00        ; line 3 above
  E2 FE           ; line 4 above
  DB E3           ; line 2 above

In my copy of the library the first 6 bytes of the original gave a unique search
string.  I'd guess it's safe to do the same patch on any other library; the 
original code would never make sense.

Duncan Murdoch

Re: MS Fortran bug

#262

Author: Duncan Murdoch
Date: Mon, 20 Nov 1989 12:18

5 lines
685 bytes

Whoops!  As one correspondent pointed out, I forgot to say which version of
Fortran had the bug in either the original report or my suggested patch.  It's
MS Fortran 5.0 that you have to watch out for.

Duncan Murdoch

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