EP80579 Datasheet, PDF (1091/1916 Pages) Intel Corporation – Intel® EP80579 Integrated Processor Product Line

Intel^® EP80579 Integrated Processor
27.9
27.9.1
Alternate (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 (legacy) 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 IICH implements an alternate access mode.
If the ALT access mode is entered and exited after reading the registers of IICH
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 IICH 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 must 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.
Write Only Registers with Read Paths in Alternate Access Mode
The registers described in Table 27-37 have read paths in alternate access mode. The
access number field in the table indicates which register will be returned per access to
that port.
August 2009
Order Number: 320066-003US
Intel^® EP80579 Integrated Processor Product Line Datasheet
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