CMOS.layout guide

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The wiki is being retired!

Documentation is now handled by the same processes we use for code: Add something to the Documentation/ directory in the coreboot repo, and it will be rendered to Contributions welcome!

CMOS layout

This page explain the workings of the CMOS layout file. The CMOS layout file can be linked into a board specific environment. By creating the cmos.layout file in the board directory. and set the HAVE_OPTION_TABLE in the Kconfig file of the board.

The CMOS layout file must contain the following sections;

  • entries
  • enumerations
  • checksums

The hash (#) character is the comment prefix, all characters until the eof of line are treated as comment.

entries section

This section defined all the variables used within coreboot. Each line consists out of the following parameters;

 <start-bit> <bit-length> <config> <config-id> <parameter-name>

The parameter <start-bit> is the start position is bit position where the parameter get stored.

The parameter <bit-length> is the length in bits of the parameter.

The parameter <config> indicates the type of parameter, currently there are three known types;

    • r = register
    • e = enumeration
    • h = ?

The parameter <config-id> is used when the <config> is set to 'e', than there must be at least one item in the enumeration section.

The parameter <parameter-name> is the name the coreboot retrieval and storage functions will refer to.


 0          384       r       0        reserved_memory
 384          3       e       5        baud_rate
 487          1       e       1        power_on_after_fail

enumerations section

Each line in the the enumerations section is contructed as follows;

 <config-id>  <value>  <label>

The parameter <config-id> is the same value as the <config-id> from the entries section.

The parameter <value> is the value that get stored at the bit location defined in the entries section, where that config-id is being used.

The parameter <label> is the label for displaying in the the presentation layer.


 1     0     Disable
 1     1     Enable
 5     0     115200
 5     1     57600
 5     2     38400
 5     3     19200
 5     4     9600
 5     5     4800
 5     6     2400
 5     7     1200

checksums section

The checksums section has one variable with three parameters;

 checksum <startbit> <endbit> <start-of-checksumbit>

The <startbit> parameter is the bit position where the calculation of the checksum starts from.

The <endbit> parameter is the bit position there the calculation of the checksum starts on.

The <start-of-checksumbit> parameter is the bit position there the checksum will be stored, there must be space for 16 bits of checksum data.


 checksum 392 983 984

Example cmos.layout file