82801BA Datasheet, PDF (225/498 Pages) Intel Corporation – Intel 82801BA I/O Controller Hub 2 (ICH2) and Intel 82801BAM I/O Controller Hub 2 Mobile

Register and Memory Mapping
6.4.1 Boot-Block Update Scheme
The ICH2 supports a “Top-Block Swap” mode that has the ICH2 swap the top block in the FWH
(the boot block) with another location. This allows for safe update of the Boot Block (even if a
power failure occurs). When the “top-swap” enable bit is set, the ICH2 will invert A16 for cycles
targeting FWH BIOS space. When this bit is 0, the ICH2 will not invert A16. This bit is
automatically set to 0 by RTCRST#, but not by PCIRST#.
The scheme is based on the concept that the top block is reserved as the “boot” block, and the block
immediately below the top block is reserved for doing boot-block updates.
The algorithm is:
1. Software copies the top block to the block immediately below the top
2. Software checks that the copied block is correct. This could be done by performing a
checksum calculation.
3. Software sets the “Top-Block Swap” bit. This inverts A16 for cycles going to the FWH.
Processor access to FFFF_0000 through FFFF_FFFF are directed to FFFF_0000 through
FFFE_FFFF in the FWH. Processor accesses to FFFE_0000 through FFFE_FFFF are directed
to FFFF_0000 through FFFF_FFFF.
4. Software erases the top block
5. Software writes the new top block
6. Software checks the new top block
7. Software clears the top-block swap bit
If a power failure occurs at any point after step 3, the system will be able to boot from the copy of
the boot block that is stored in the block below the top. This is because the top-swap bit is backed
in the RTC well.
Note: The Top-Block Swap mode may be forced by an external strapping option (See Section 2.20.1).
When Top-Block Swap mode is forced in this manner, the Top-Swap bit cannot be cleared by
software. A re-boot with the strap removed will be required to exit a forced Top-Block Swap mode.
Note: top-Block Swap mode only affects accesses to the FWH BIOS space, not feature space.
Note: The Top Block Swap mode has no effect on accesses below FFFE_0000.
82801BA ICH2 and 82801BAM ICH2-M Datasheet
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