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