2.5 KiB
2.5 KiB
What is bucts?
Many Intel southbridges provide a mechanism called Back Up Control Top Swap (BUC.TS).
This functionality allows to have the southbridge fetch the reset vector or
the beginning of the bootblock at a 64K/128/256K offset from the usual top of flash.
This can be useful in different ways:
- Have a backup bootblock in case of bootblock hacking;
- Some vendor BIOS only write protect their bootblock so this tool makes it possible to circumvent this protection to allow flashing and booting coreboot.
The BUC.TS status is stored in a nvram bit. To clear it one has to remove the RTC battery.
Operation Description
First compile bucts by running make:
$ make
Then you can view the current system settings:
$ ./bucts --print
To flip the decode address of the bootblock, by setting BUC.TS to 1:
$ ./bucts --set
To set the behavior the regular mapping, by setting BUC.TS to 0
$ ./bucts --unset
Details
Example Bootblock size of 64KB (this is only configurable to be something else like 128 or 256K on PCH Intel targets).
+-------------+ -> 0x200000 +-------------+ -> 0xFFFFFFFF +-------------+ -> 0xFFFFFFFF
| | | | | |
| bootblock_0 | | bootblock_0 | | bootblock_1 |
| | | | | |
+-------------+ -> 0x1F0000 +-------------+ -> 0xFFFF0000 +-------------+ -> 0xFFFF0000
| | | | | |
| bootblock_1 | | bootblock_1 | | bootblock_0 |
| | | | | |
+-------------+ -> 0x1E0000 +-------------+ -> 0xFFFE0000 +-------------+ -> 0xFFFE0000
| | | | | |
| | | | | |
Z Z Z Z Z Z
Z Z Z Z Z Z
| | | | | |
| | | | | |
FLASH (2M) Memory Map BUC.TS=0 Memory Map BUC.TS=1