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Text File | 1993-10-08 | 6KB | 199 lines

  PRODUCT  :  Borland C++                           NUMBER  :  1703
  VERSION  :  All
       OS  :  DOS
     DATE  :  October 8, 1993                          PAGE  :  1/3

    TITLE  :  Possible conflicts with DOS time and date functions




  Every IBM compatible PC is capable of keeping track of the
  current date and time.  Your PC can keep the current time
  even when it is turned off, because of a clock that is built
  in to the CMOS.  However, this CMOS clock is only used once,
  when your computer is turned on.  From this point on the
  date/time is maintained by both BIOS and DOS working together.
  Because of some quirks in the way BIOS and DOS work, some
  problems can occur if you are not careful in your program.

  To understand these problems and how to avoid them, we first
  need to see how BIOS and DOS stores the time.  The BIOS keeps
  two variables in memory at the following locations:

    0x0040:0x006C - # of ticks since midnight (double word)
    0x0040:0x0070 - midnight flag (byte)

  These values are updated 18.2 times a second, by the system
  timer.  This timer does nothing but increment the tick count,
  and check to see if midnight has passed.  If midnight passes,
  the tick count is reset to 0, and the midnight flag is set
  to 1.  It is important to note the midnight flag is a Boolean,
  not a count.  That is, it doesn't count the number of times
  midnight has passed, it only indicates that midnight has passed
  "at least once".

  It is also important to note that the BIOS doesn't update
  the actual clock time or date.  It only updates the timer tick
  count and midnight flag.  DOS maintains the time and date, but
  only when you ask it to retrieve these values.

  There are several ways to find the current date and time.  Listed
  below are BIOS/DOS interrupts, and their corresponding functions
  from the Borland C++ run-time library:

   BIOS int 0x1A,0x00 - read system time (biostime)
   BIOS int 0x1A,0x01 - DOS system time  (biostime)
   DOS  int 0x21,0x2A - get DOS date     (getdate, _dos_getdate)
   DOS  int 0x21,0x2B - set DOS date     (setdate, _dos_setdate)
   DOS  int 0x21,0x2C - get DOS time     (gettime, _dos_gettime)
   DOS  int 0x21,0x2D - set DOS time     (settime, _dos_settime)

  The DOS functions operate by first calling the BIOS function
  to get the timer tick count and midnight flag, and then













  PRODUCT  :  Borland C++                           NUMBER  :  1703
  VERSION  :  All
       OS  :  DOS
     DATE  :  October 8, 1993                          PAGE  :  2/3

    TITLE  :  Possible conflicts with DOS time and date functions




  calculating the appropriate values.  The date is only updated
  when you call one of the DOS date functions.

  The first problem that can occur is due to the fact that the
  midnight flag is Boolean.  DOS only updates the date when you
  call a DOS date function, and the midnight flag is reset at
  this time.  Consider the following scenario:

    actual                                           DOS date
    date/time   | F |  event                      |  after event
    ------------+---+-----------------------------+---------------
    12/01 12:00 |   |  turn on computer           |  12/01
    12/02 00:00 | X |  midnight flag is set       |  12/01
    12/02 15:45 |   |  call DOS getdate function  |  12/02
    12/03 00:01 | X |  midnight flag set          |  12/02
    12/04 00:00 | X |  midnight flag set (again)  |  12/02
    12/04 11:30 |   |  call DOS getdate function  |  12/03

  If an entire day passes with no call to a date function, the
  internal DOS date never gets updated, causing a day to be missed.

  The second problem comes in because of how BIOS int 0x1A
  operates.  Whenever you call this function to retrieve the
  system time (the current timer tick value) it also returns the
  current midnight flag and RESETS THE FLAG.  But since the BIOS
  function doesn't update the DOS date, the next time you ask for
  the date, it will not be updated correctly.  DOS is aware of
  the behavior, so when you call any DOS function, the midnight
  flag is maintained correctly.  If you call BIOS int 0x1A
  yourself, you MUST check the midnight flag value and turn it
  back on if it was set.

  The following function is a safe replacement for the biostime()
  function.  It properly restores the midnight flag, avoiding the
  problem described above:

  long safebiostime(int cmd, long newtime) {
    long temptime;
    asm {
      mov     ax,cmd
      mov     dx,word ptr newtime
      mov     cx,word ptr newtime+2
      int     0x1a













  PRODUCT  :  Borland C++                           NUMBER  :  1703
  VERSION  :  All
       OS  :  DOS
     DATE  :  October 8, 1993                          PAGE  :  3/3

    TITLE  :  Possible conflicts with DOS time and date functions




      mov     word ptr temptime,dx
      mov     word ptr temptime+2,cx
      or      al,al
      jz      not_midnight
      mov     ax,0x40
      mov     es,ax
      mov     bx,0x70
      mov     byte ptr es:[bx],1
    }
  not_midnight:

    return temptime;
  }

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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.