readme.md
1# What is bucts?
2Many Intel southbridges provide a mechanism called Back Up Control Top Swap (`BUC.TS`).
3This functionality allows to have the southbridge fetch the reset vector or
4the beginning of the bootblock at a 64K/128/256K offset from the usual top of flash.
5
6This 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
11The BUC.TS status is stored in a nvram bit. To clear it one has to remove the RTC battery.
12
13# Operation Description
14First compile bucts by running make:
15
16 $ make
17
18Then you can view the current system settings:
19
20 $ ./bucts --print
21
22To flip the decode address of the bootblock, by setting `BUC.TS` to 1:
23
24 $ ./bucts --set
25
26To set the behavior the regular mapping, by setting `BUC.TS` to 0
27
28 $ ./bucts --unset
29
30
31# Details
32Example Bootblock size of 64KB (this is only configurable to be something else
33like 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>
53