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diff --git a/util/bucts/readme.md b/util/bucts/readme.md new file mode 100644 index 0000000000..1bf9beebc8 --- /dev/null +++ b/util/bucts/readme.md @@ -0,0 +1,52 @@ +# 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). + +<pre> + +-------------+ -> 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 +</pre> |