BD82QM67-SLJ4M Datasheet, PDF (183/934 Pages) Intel Corporation – Intel® 6 Series Chipset and Intel® 6 Series Chipset and

Functional Description
5.13.9
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 PCH implements an ALT access mode.
If the ALT access mode is entered and exited after reading the registers of the PCH
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 PCH 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
Microsoft MS-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 (such as Microsoft Windows* 98 and Windows* 2000) reprogram
the system timer and therefore do not encounter this problem.
For other operating systems (such as 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.
Datasheet
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