82801FB Datasheet, PDF (167/786 Pages) Intel Corporation – Intel I/O Controller Hub 6 (ICH6) Family

Functional Description
5.14.10
ALT Access Mode
Before entering a low power state, several registers from powered down parts may need to be
saved. In the majority of cases, this is not an issue, as registers have read and write paths. However,
several of the ISA compatible registers are either read only or write only. To get data out of
write-only registers, and to restore data into read-only registers, the ICH6 implements an ALT
access mode.
If the ALT access mode is entered and exited after reading the registers of the ICH6 timer (8254),
the timer starts counting faster (13.5 ms). The following steps listed below can cause problems:
1. BIOS enters ALT access mode for reading the ICH6 timer related registers.
2. BIOS exits ALT access mode.
3. BIOS continues through the execution of other needed steps and passes control to the
operating system.
After getting control in step #3, if the operating system does not reprogram the system timer again,
the timer ticks may be happening faster than expected. For example DOS and its associated
software assume that the system timer is running at 54.6 ms and as a result the time-outs in the
software may be happening faster than expected.
Operating systems (e.g., Microsoft Windows* 98, Windows* 2000, and Windows NT*) reprogram
the system timer and therefore do not encounter this problem.
For some other loss (e.g., Microsoft MS-DOS*) the BIOS should restore the timer back to 54.6 ms
before passing control to the operating system. If the BIOS is entering ALT access mode before
entering the suspend state it is not necessary to restore the timer contents after the exit from ALT
access mode.
Intel^® I/O Controller Hub 6 (ICH6) Family Datasheet
167