util/bucts/readme.md

   1 # What is bucts?
   2 Many Intel southbridges provide a mechanism called Back Up Control Top Swap (`BUC.TS`).
   3 This functionality allows to have the southbridge fetch the reset vector or
   4 the beginning of the bootblock at a 64K/128/256K offset from the usual top of flash.
   5
   6 This can be useful in different ways:
   7 - Have a backup bootblock in case of bootblock hacking;
   8 - Some vendor BIOS only write protect their bootblock so this tool makes it
   9   possible to circumvent this protection to allow flashing and booting coreboot.
  10
  11 The BUC.TS status is stored in a nvram bit. To clear it one has to remove the RTC battery.
  12
  13 # Operation Description
  14 First compile bucts by running make:
  15
  16         $ make
  17
  18 Then you can view the current system settings:
  19
  20         $ ./bucts --print
  21
  22 To flip the decode address of the bootblock, by setting `BUC.TS` to 1:
  23
  24         $ ./bucts --set
  25
  26 To set the behavior the regular mapping, by setting `BUC.TS` to 0
  27
  28         $ ./bucts --unset
  29
  30
  31 # Details
  32 Example Bootblock size of 64KB (this is only configurable to be something else
  33 like 128 or 256K on PCH Intel targets).
  34
  35 <pre>
  36     +-------------+ -> 0x200000         +-------------+ -> 0xFFFFFFFF       +-------------+ -> 0xFFFFFFFF
  37     |             |                     |             |                     |             |
  38     | bootblock_0 |                     | bootblock_0 |                     | bootblock_1 |
  39     |             |                     |             |                     |             |
  40     +-------------+ -> 0x1F0000         +-------------+ -> 0xFFFF0000       +-------------+ -> 0xFFFF0000
  41     |             |                     |             |                     |             |
  42     | bootblock_1 |                     | bootblock_1 |                     | bootblock_0 |
  43     |             |                     |             |                     |             |
  44     +-------------+ -> 0x1E0000         +-------------+ -> 0xFFFE0000       +-------------+ -> 0xFFFE0000
  45     |             |                     |             |                     |             |
  46     |             |                     |             |                     |             |
  47     Z             Z                     Z             Z                     Z             Z
  48     Z             Z                     Z             Z                     Z             Z
  49     |             |                     |             |                     |             |
  50     |             |                     |             |                     |             |
  51     FLASH (2M)                          Memory Map BUC.TS=0               Memory Map BUC.TS=1
  52 </pre>