CMOS: Difference between revisions

CMOS (and the Real-Time Clock) can only be accessed through IO Ports 0x70 and 0x71. The function of the CMOS

memory is to store 50 (or 114) bytes of "Setup" information for the BIOS while the computer is turned off --

as follows:

 

 

Once a register is selected, you either read the value of that register on Port 0x71 (with inb or an equivalent

function), or you write a new value to that register -- also on Port 0x71 (with outb, for example):

 

 

Note1: Reading or writing Port 0x71 seems to default the "selected register" back to 0xD. So you need to

holes", it ignores memory mapped hardware, and it ignores memory reserved for important ACPI system tables.

 

But this number is untrustworthy when the system has more than 4G, it ignores "memory holes", it ignores

memory mapped hardware, and it ignores memory reserved for important ACPI system tables.

Better information can always be obtained via BIOS function INT13h AH=8, or by sending an ATA 'Identify'

command to the disk in [[ATA PIO Mode]].

 

== The Real-Time Clock ==

 

<pre>

0x0A Status Register A

0x0B Status Register B

 

If there is no century register then software can guess. For example, a piece of software written in 1990 can use the (2 digit) year register to determine the most likely century - if the RTC year register is higher than or equal to 90 then the year is probably be "19YY" and if the RTC year register is less than 90 than the year must be "20YY". In this way, software can correctly determine the century for up to 99 years after the software is written.

 

==== Century Register As Time and Date Sanity Check ====

Essentially; the method (described above) for guessing the century when there is no century register is much more reliable than the CMOS/RTC century register (if it exists). This means that the century register (if/when present) can be used in reverse, as a way to check if the CMOS/RTC time and date are sane (or if the CMOS/RTC has a flat battery or something).

 

 

=== Weekday Register ===

 

 

The correct way to determine the current day of the week is to calculate it from the date (see [http://en.wikipedia.org/wiki/Weekday_determination the article on Wikipedia] for details of this calculation).

 

=== RTC Update In Progress ===

 

The second alternative is to be prepared for dodgy/inconsistent values and cope with them if they occur. To do this, make sure the "Update in progress" flag is clear (e.g. "''while(update_in_progress_flag != clear)''") then read all the time and date registers; then make sure the "Update in progress" flag is clear again (e.g. "''while(update_in_progress_flag != clear)''") and read all the time and date registers again. If the values that were read the first time are the same as the value that were read the second time then the values must be correct. If any of the values are different you need to do it again, and keep doing it again until the newest values are the same as the previous values.

 

=== Format of Bytes ===

 

=== Reading from the CMOS ===

ReadFromCMOS (unsigned char array [])

{

}

}

 

=== Writing to the CMOS ===

WriteTOCMOS(unsigned char array[])

{

}

}

 

=== Reading All RTC Time and Date Registers ===

 

int century_register = 0x00; // Set by ACPI table parsing code if possible

cmos_data = 0x71

};

 

int get_update_in_progress_flag() {

return (in_byte(cmos_data) & 0x80);

}

 

unsigned char get_RTC_register(int reg) {

return in_byte(cmos_data);

}

 

void read_rtc() {

}

}

 

== See Also ==

[[Category:X86]]

[[Category:Time]]