Linux 2.4.20 RTC Timer bug

From: Richard B. Johnson (
Date: Wed Jul 16 2003 - 15:31:15 EST

#if 0

In Linux 2.4.20, some rogue is incrementing the value
in register 0x71 at about 1 second intervals! This
port is the index register for the CMOS timer chip
and it must be left alone when the chip is not being
accessed and it must be left at an unused offset,
typically 0xff, after access. This is to prevent
destruction of CMOS data during the power-down

This code clearly shows that somebody is mucking with
this chip. Here, I have reviewed the only drivers
installed, SCSI and network, and have not found anybody
messing with this chip so I think it must be something
in the kernel proper. The numbers increase at 1 second
intervals from 0 to 89 and then restart. This shows that
it is not residual from the system clock code that will
read only the timer registers.

These are the only modules installed...

Module Size Used by
ipchains 41400 7
3c59x 28224 1 (autoclean)
nls_cp437 4376 4 (autoclean)
BusLogic 35768 7
sd_mod 10184 14
scsi_mod 54572 2 [BusLogic sd_mod]

This running of the CMOS timer index register is the
reason why the CMOS checksum and parameters are being
lost on several systems that run 2.4.20. If any of
these system are turned off when the index register
points to checksummed data, the byte at that location
will get smashed to whatever is on the bus when the
power fails. To non-believers, note that the chip-select
goes low to enable ... and that's what a power failure
does ... goes low, while internally, the chip still has
power from its battery.


#include <stdio.h>

__inline__ int inb()
        register int eax = 0;
        __asm__ volatile ("inb $0x71, %%al" : "=eax" (eax));
        return eax;

extern int iopl(int);
extern int usleep(int);

int main()

        printf("%d\n", inb());

Dick Johnson
Penguin : Linux version 2.4.20 on an i686 machine (797.90 BogoMips).
            Note 96.3% of all statistics are fiction.

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