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    SubjectRe: i386/RTC: old problem, new solution?
    On 20 May 99, at 17:10, Riley Williams wrote:

    > Hi Ulrich.
    > ===8<=== CUT ===>8===
    > >> That's not quite what I understood your original email to be
    > >> referring to. Let's go for gold here and look at the boot
    > >> procedure:
    > >> 1. Reset button is pressed, and the ROM BIOS gets control.
    > >> 2. Amongst the tasks taken by ALL of the ROM BIOS's that I've
    > >> come across is to set the system's idea of the time from
    > >> the RTC if an RTC is found. Therefore, the system clock is
    > >> set BEFORE the kernel even gets a look in.
    > > The "system clock" is a collection of variables in the kernel,
    > > maintained by the kernel. I hope you agree. As The BIOS starts
    > > before Linux is in RAM, the BIOS can't set the time for Linux. I
    > > hope you also agree.
    > My understanding was that Linux reads the ROM BIOS clock and
    > initialises its system clock from that, in which case the BIOS
    > effectively initialises Linux's system clock.

    No: The RTC is a piece of hardware that is addressed directly as the
    serial ports and disc controllers. No need for a BIOS.

    > >> Granted, the ROM BIOS has no idea of timezones, but there's
    > >> precious little we can do about that.
    > > <soapbox>
    > > Well, Microsoft does not know about timezones, so why should a
    > > BIOS care to allow setting one? WE can do little about that, but
    > > i wonder why you can read the serial number of your CPU via DMI,
    > > and you can read the temperature of your mainboard, but you
    > > can't read the timezone.
    > > Maybe the BIOS developers are captured in a darf cellar and they
    > > don't ever meet reality.
    > > </soapbox>
    > LOL!!! That wouldnae surprise me though...
    > >> 3. When the ROM BIOS has finished whatever else it has been
    > >> told to do, it looks at the various drives, and ends up
    > >> loading the kernel image into memory via whatever procedure
    > >> the user has selected. It then executes the image so loaded
    > >> into memory.
    > >> It is at THIS stage in the proceedings that we first get an
    > >> opportunity to do something, and we already noted that the
    > >> RTC has previously been read and assigned to the system time.
    > > In case you want to look at it: LINUX reads the time from the
    > > hardware (=RTC) during boot. That's why the time is set.
    > Are you sure about this? I understood that it used the relevant ROM
    > BIOS call to read the BIOS clock before switching to protected mode,
    > and just used the value returned from that.

    I only found this on a quick scan:

    ! Read the CMOS clock. Return the seconds in al
    push cx
    mov ah,#0x02
    int 0x1a
    mov al,dh ! dh contains the seconds
    and al,#0x0f
    mov ah,dh
    mov cl,#0x04
    shr ah,cl
    pop cx

    and This will be against your theory:

    __initfunc(void time_init(void))
    xtime.tv_sec = get_cmos_time();
    xtime.tv_usec = 0;


    > > DST makes things harder: How can you know (after powering the
    > > system up) whether the RTC is already time-corrected or not?
    > There are basically three cases here:
    > 1. The RTC chip has a register that says whether DST is in use,
    > and that register is correctly set and reliable.

    If the bit is unchanged by other OSes it could be used. I doubt it.

    > 2. The RTC chip has a register that says whether DST is in use,
    > but that register is not reliably set.
    > 3. The RTC chip does not have a register that says whether DST
    > is in use.
    > I believe Win9x assumes case (1), as it throws up a warning screen if
    > it has shifted the RTC in to or out of DST.

    Yes and one version repeatedly changed time back one hour. I agree
    that the RTC should be set to UTC, but I'd like to have the correct
    time in DOSish systems too.


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