PRODUCT  :  Borland C++                            NUMBER  :  722
  VERSION  :  2.0
       OS  :  DOS
     DATE  :  October 19, 1993                         PAGE  :  1/2

    TITLE  :  Using Floating Points in ISR's




  QUESTION:
          How do I use floating point within an interrupt service
          routine?

  ANSWER:
          The floating point emulator in Turbo C is NOT re-entrant.
          8087 instructions can only be used in interrupt routines
          if the chip is present, and all programs with 8087
          interrupt handlers should be compiled without emulation.
          Within the interrupt handler, the FNSAVE instruction
          should be used to save the state of the 8087 chip into a
          94 byte state record.  After saving the state, an
          implicit FINIT is executed to bring the chip into its
          default state.  In this state, exceptions are masked, so
          divide-by-zeros, overflows, etc. will produce NANs and
          INFs, not run-time errors. It so happens that this is
          exactly what we want (a run-time error during an
          interrupt would completely crash the system), so no FLDCW
          is required to load a different control word. The FNSAVE
          must be executed before the STI that re-enables
          interrupts. Furthermore, an FWAIT must precede STI to
          make sure that the state has been completely saved. The
          FRSTOR must be followed by an FWAIT to ensure that the
          state has been completely reloaded before the state
          variable is removed from the stack.

  EXAMPLE:
  #include <stdio.h>
  #include <conio.h>
  #include <dos.h>

  double x = 0;

  void interrupt (*save)(void);

  void interrupt func(void)
  {
    unsigned char state87[94];

  __emit__(
    0xDD,0x76,state87,       /* FNSAVE  [BP+<state87] */
    0x9B,                    /* FWAIT                 */
    0xFB);                   /* STI                   */













  PRODUCT  :  Borland C++                            NUMBER  :  722
  VERSION  :  2.0
       OS  :  DOS
     DATE  :  October 19, 1993                         PAGE  :  2/2

    TITLE  :  Using Floating Points in ISR's




    x += 1;

  __emit__(
    0x9B,0xDD,0x66,state87,  /* FRSTOR  [BP+<state87] */
    0x9B);                   /* FWAIT                 */
  }

  main()
  {
    char ch = 0;

    save = getvect(5); /* the <Shift-PrtScr> interrupt. */
    setvect(5, func);

    while (ch != 'q')
    {
      cprintf("%f\r\n", x); /* Each time you hit <Shift-PrtScr> */
      if (kbhit())          /* x will get incremented.          */
           ch = getch();
    }

    setvect(5, save);  /* restore original vector for INT 5 */
    return(0);
  }

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  subject to the terms of the No-Nonsense License Statement that
  you received with the Borland product to which this information
  pertains.