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limine/CONFIG.md
mintsuki 0c96d0a973 misc: Clarify config docs on E9 and COM1
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Limine configuration file

Location of the config file

Limine scans for a config file on the boot drive. Every partition on the boot drive is scanned sequentially (first partition first, last partition last) for the presence of either a /limine.cfg or a /boot/limine.cfg file, in that order.

Once the file is located, Limine will use it as its config file. Other possible candidates in subsequent partitions or directories are ignored.

It is thus imperative that the intended config file is placed in a location that will not be shadowed by another potentially candidate config file.

Structure of the config file

The Limine configuration file is comprised of assignments and entries.

Entries and sub-entries

Entries describe boot entries which the user can select in the boot menu.

An entry is simply a line starting with : followed by a newline-terminated string. Any locally assignable key that comes after it, and before another entry, or the end of the file, will be tied to the entry.

An entry can be a directory, meaning it can hold sub-entries. In order for an entry to become a directory, it needs to have a sub-entry following right after it.

A sub-entry is an entry with a number of : greater than 1 prepended to it. Each : represents 1 level deeper down the tree hierarchy of directories and entries.

Directories can be expanded (meaning they will not show up as collapsed in the menu) by default if a + is put between the :s and the beginning of the entry's name.

Assignments

Assignments are simple KEY=VALUE style assignments. VALUE can have spaces and = symbols, without requiring quotations. New lines are delimiters.

Some assignments are part of an entry (local), some other assignments are global. Global assignments can appear anywhere in the file and are not part of an entry, although usually one would put them at the beginning of the config. Some local assignments are shared between entries using any protocol, while other local assignments are specific to a given protocol.

Some keys take URIs as values; these are described in the next section.

Globally assignable keys are:

  • TIMEOUT - Specifies the timeout in seconds before the first entry is automatically booted. If set to no, disable automatic boot. If set to 0, boots default entry instantly (see DEFAULT_ENTRY key).
  • DEFAULT_ENTRY - 1-based entry index of the entry which will be automatically selected at startup. If unspecified, it is 1.
  • GRAPHICS - If set to yes, do use graphical VESA framebuffer for the boot menu, else use text mode.
  • MENU_RESOLUTION - Specify screen resolution to be used by the Limine menu in the form <width>x<height>. This will only affect the menu, not any booted OS. If not specified, Limine will pick a resolution automatically. If the resolution is not available, Limine will pick another one automatically. Ignored if GRAPHICS is not yes.
  • MENU_BRANDING - A string that will be displayed on top of the Limine menu.
  • MENU_FONT - URI path to a font file to be used instead of the default one for the menu. The font file must be a code page 437 character set comprised of 256 consecutive 8x16 glyphs bitmaps (4096 byte font file). Each glyph's bitmap must be expressed left to right (1 byte per row), and top to bottom (16 bytes per whole glyph).
  • THEME_COLOURS - Specifies the colour palette used by the terminal (AARRGGBB). It is a ; separated array of 8 colours: black, red, green, brown, blue, magenta, cyan, and gray, respectively. Ignored if GRAPHICS is not yes.
  • THEME_COLORS - Alias of THEME_COLOURS.
  • THEME_MARGIN - Set the amount of margin around the terminal. Ignored if GRAPHICS is not yes.
  • THEME_MARGIN_GRADIENT - Set the thickness in pixel for the gradient around the terminal. Ignored if GRAPHICS is not yes.
  • BACKGROUND_PATH - URI where to find the background .BMP file. Ignored if GRAPHICS is not yes.
  • STAGE2_MAP - URI where to find the stage2 symbol map file for stacktraces and symbol name resolution at runtime.
  • E9_OUTPUT - If set to yes, output to port 0xe9 for debugging purposes.
  • COM1_OUTPUT - If set to yes, output to COM1 for debugging purposes.

Locally assignable (non protocol specific) keys are:

  • PROTOCOL - The boot protocol that will be used to boot the kernel. Valid protocols are: linux, stivale, stivale2, chainload.
  • CMDLINE - The command line string to be passed to the kernel. Can be omitted.
  • KERNEL_CMDLINE - Alias of CMDLINE.

Locally assignable (protocol specific) keys are:

  • Linux protocol:

    • KERNEL_PATH - The URI path of the kernel.
    • MODULE_PATH - The URI path to a module (such as initramfs).

    Note that one can define this last variable multiple times to specify multiple modules.

  • stivale and stivale2 protocols:

    • KERNEL_PATH - The URI path of the kernel.
    • MODULE_PATH - The URI path to a module.
    • MODULE_STRING - A string to be passed to a module.

    Note that one can define these 2 last variable multiple times to specify multiple modules. The entries will be matched in order. E.g.: the 1st module path entry will be matched to the 1st module string entry that appear, and so on.

    • KASLR - If set to yes, it enables Kernel Address Layout Randomisation for 64-bit relocatable kernels.
    • RESOLUTION - The resolution to be used should the kernel request a graphical framebuffer. This setting takes the form of <width>x<height>x<bpp> and overrides any resolution requested by the kernel, or automatic resolution requests. If the resolution is not available, Limine will pick another one automatically.

  • Chainload protocol:

    • DRIVE - The 1-based BIOS drive to chainload.
    • PARTITION - The 1-based BIOS partition to chainload, if omitted, chainload drive.

URIs

A URI is a path that Limine uses to locate resources in the whole system. It is comprised of a resource, a root, and a path. It takes the form of:

resource://root/path

The format for root changes depending on the resource used.

A resource can be one of the following:

  • boot - The root is the 1-based decimal value representing the partition on the boot drive (values of 5+ for MBR logical partitions). If omitted, the partition containing the configuration file on the boot drive is used. For example: boot://2/... will use partition 2 of the boot drive and boot:///... will use the partition containing the config file on the boot drive.
  • bios - The root takes the form of drive:partition; for example: bios://3:1/... would use BIOS drive 3, partition 1. Partitions and BIOS drives are both 1-based (partition values of 5+ for MBR logical partitions). Omitting the drive is possible; for example: bios://:2/.... Omitting the drive makes Limine use the boot drive.
  • guid - The root takes the form of a GUID/UUID, such as guid://736b5698-5ae1-4dff-be2c-ef8f44a61c52/.... The GUID is that of either a filesystem, when available, or a GPT partition GUID, when using GPT, in a unified namespace.
  • uuid - Alias of guid.
  • tftp - The root is the IP address of the tftp server to load the file from. If the root is left empty (tftp:///...) the file will be loaded from the server Limine booted from. This resource is only available when booting off PXE.

A URI can optionally be prefixed by a $ character to indicate that the file pointed to be the URI is a gzip-compressed payload to be uncompressed on the fly. E.g.: $boot:///somemodule.gz.