CMOS写入失败的可能原因是什么?

下面是代码片段,我从注册表中读取的值(最后一行)总是与我刚写给注册表的值不同。有人能帮我理解吗?

    val = CMOS_READ(RTC_FREQ_SELECT);
    printk(KERN_INFO "reading val=%d from RTC_FREQ_SELECT.\n", val);
    val &= 0xf0;
    val |= 2;
    printk(KERN_INFO "writing val=%d to RTC_FREQ_SELECT.\n", val);
    CMOS_WRITE(val, RTC_FREQ_SELECT);
    printk(KERN_INFO "reading val=%d from RTC_FREQ_SELECT.\n", CMOS_READ(RTC_FREQ_SELECT));

我使用的是linux 2.6内核,我尝试的平台是PPC和x86。

更新:

   /* disable periodic interrupts */
    tmp_control = CMOS_READ(RTC_CONTROL);
    tmp_control &= ~RTC_PIE;
    CMOS_WRITE(tmp_control, RTC_CONTROL);

    CMOS_READ(RTC_INTR_FLAGS);

    val = CMOS_READ(RTC_FREQ_SELECT);
    printk(KERN_INFO "reading val=%d from RTC_FREQ_SELECT.\n", val);

    val &= 0xf0;
    /* ...... */
    val |= 2;
    printk(KERN_INFO "writing val=%d to RTC_FREQ_SELECT.\n", val);

    CMOS_WRITE(val, RTC_FREQ_SELECT);
    printk(KERN_INFO "reading val=%d from RTC_FREQ_SELECT.\n", CMOS_READ(RTC_FREQ_SELECT));


    /* Enable periodic interrupts */
    tmp_control = CMOS_READ(RTC_CONTROL);
    tmp_control |= RTC_PIE;

    CMOS_WRITE(tmp_control, RTC_CONTROL);

    /* read the flags register to start interrupts */
    CMOS_READ(RTC_INTR_FLAGS);