LOADER(7) Standards, Environments, and Macros LOADER(7)

loaderkernel bootstrapping final stage

The loader is the final stage of illumos's kernel bootstrapping process. The actual name for the stage depends on the platform. On IA32 (i386) architectures with BIOS firmware, it is a BTX client and named loader. It is linked statically to libstand and usually located in the directory /boot.

loader supports booting from ZFS, UFS, PCFS, HSFS and NFS file systems. Additionally, loader can load files from the TFTP file service. The NFS and TFTP based boot is enabled via pxeboot(7). The loader also does support uncompressing gzip files while reading. The uncompression will happen automatically if the compressed file is stored without .gz suffix or if the file is accessed by leaving out the .gz suffix from the name. If the file is referred by full name, including .gz suffix, then the file content is read as is and the uncompression is not performed.

loader provides a scripting language that can be used to automate tasks, do pre-configuration or assist in recovery procedures. This scripting language is roughly divided in two main components. The smaller one is a set of commands designed for direct use by the casual user, called "builtin commands" for historical reasons. The main drive behind these commands is user-friendliness. The bigger component is an ANS Forth compatible Forth interpreter based on FICL, by John Sadler.

During initialization, loader will probe for a console and set the console variable, or set it to serial console (“ttya” - “ttyd”) if the previous boot stage used that. If multiple consoles are selected, they will be listed separated by commas. Then, devices are probed, currdev and loaddev are set, and screen-#cols, screen-#rows, and ISADIR are set. Next, FICL is initialized, the builtin words are added to its vocabulary. The inner interpreter loader will use with FICL is then set to interpret, which is FICL's default. After that, /boot/loader.rc is processed if available. These files are processed through the include command, which reads all of them into memory before processing them, making disk changes possible.

At this point, if an autoboot has not been tried, and if autoboot_delay is not set to “NO” (not case sensitive), then an autoboot will be tried. If the system gets past this point, prompt will be set and loader will engage interactive mode. Please note that historically even when autoboot_delay is set to “0” user will be able to interrupt autoboot process by pressing some key on the console while kernel and modules are being loaded. In some cases such behaviour may be undesirable, to prevent it set autoboot_delay to “-1”, in this case loader will engage interactive mode only if autoboot has failed.

In loader, builtin commands take parameters from the command line. If an error condition occurs, an exception will be generated, which can be intercepted using ANS Forth exception handling words. If not intercepted, an error message will be displayed and the interpreter's state will be reset, emptying the stack and restoring interpreting mode.

The builtin commands available are:

[seconds [prompt]]
Proceeds to bootstrap the system after a number of seconds, if not interrupted by the user. Displays a countdown prompt warning the user the system is about to be booted, unless interrupted by a key press. The kernel will be loaded first if necessary. Defaults to 10 seconds.

Displays statistics about disk cache usage. For debugging only.

kernelname [...]
-flag ...
Immediately proceeds to bootstrap the system, loading the kernel if necessary. Any flags or arguments are passed to the kernel, but they must precede the kernel name, if a kernel name is provided.

: The behavior of this builtin is changed if loader.4th(7) is loaded.

Chain load another boot loader from the specified device. Device can be either disk name, partition or file name. In case of x86 BIOS boot, the file must be copy of MBR or PBR. For UEFI boot, the file is efi application.

[-n] [⟨message⟩]
Displays text on the screen. A new line will be printed unless -n is specified.

Displays memory usage statistics. For debugging purposes only.

[topic [subtopic]]
Shows help messages read from /boot/loader.help. The special topic will list the topics available.

file [file ...]
Process script files. Each file, in turn, is completely read into memory, and then each of its lines is passed to the command line interpreter. If any error is returned by the interpreter, the include command aborts immediately, without reading any other files, and returns an error itself (see ERRORS).

[-t type] file ...
Loads a kernel or file of opaque contents tagged as being of the type type. Kernel and modules can be either in a.out or ELF format. Any arguments passed after the name of the file to be loaded will be passed as arguments to that file.

[-l] [path]
Displays a listing of files in the directory path, or the root directory if path is not specified. If -l is specified, file sizes will be shown too.

Lists all of the devices from which it may be possible to load modules. In addition to disks and partitions, ZFS pools are also listed. If -v is specified, more details are printed. For ZFS pools the output resembles zpool status output.

Displays loaded modules. If -v is specified, more details are shown.

A ZFS extended command that can be used to explore the ZFS filesystem hierarchy in a pool. Lists the immediate children of the filesystem. The filesystem hierarchy is rooted at a filesystem with the same name as the pool.

file [file ...]
Display the files specified, with a pause at each screen-#rows displayed.

[-t seconds] [-p prompt] [variable]
Reads a line of input from the terminal, storing it in variable if specified. A timeout can be specified with -t, though it will be canceled at the first key pressed. A prompt may also be displayed through the -p flag.

Immediately reboots the system. On a system, if the -f flag is set, or the BOOT_TO_FW_UI environment variable exists, and the firmware supports this feature, the system will automatically enter the firmware setup after reboot.

Set loader's environment variables.

Displays the specified variable's value, or all variables and their values if variable is not specified.

Remove all modules from memory.

Removes variable from the environment.

Lists available commands.

loader supports the following format for specifying ZFS filesystems which can be used wherever loader refers to a device specification:


where pool/filesystem is a ZFS filesystem name as described in zfs(8).

The loader has actually two different kinds of ‘environment’ variables. There are ANS Forth's , and a separate space of environment variables used by builtins, which are not directly available to Forth words. It is the latter type that this section covers.

Environment variables can be set and unset through the set and unset builtins, and can have their values interactively examined through the use of the show builtin. Their values can also be accessed as described in BUILTIN PARSER.

Notice that these environment variables are not inherited by any shell after the system has been booted.

A few variables are set automatically by loader. Others can affect the behavior of either loader or the kernel at boot. Some options may require a value, while others define behavior just by being set. Both types of builtin variables are described below.

Number of seconds autoboot will wait before booting. If this variable is not defined, autoboot will default to 10 seconds.

If set to “NO”, no autoboot will be automatically attempted after processing /boot/loader.rc, though explicit autoboot's will be processed normally, defaulting to 10 seconds delay.

If set to “0”, no delay will be inserted, but user still will be able to interrupt autoboot process and escape into the interactive mode by pressing some key on the console while kernel and modules are being loaded.

If set to “-1”, no delay will be inserted and loader will engage interactive mode only if autoboot has failed for some reason.

Will set kernel(8) -a option.
Will set kernel(8) -d option.
Will set kernel(8) -k option.
Will set kernel(8) -r option.
Will set kernel(8) -s option.
Will set kernel(8) -v option.
Will set custom arguments for the kernel. If set in loader configuration, the loader startup will parse the boot-args value to set boot prefixed variables listed above, any unrecognized options are added to kernel command line verbatim.
The name of the kernel.
If set, contains the device or file name used with chain command and will cause chain command menu entry to appear on loader main menu. The alternate method to use chain loader is to add menu entries into menu.lst(5) file.
Defines the current console or consoles. Multiple consoles may be specified. In that case, the first listed console will become the default console for the kernel(8).
Selects the default device. Syntax for devices is odd.
Has the value “ok” if the Forth's current state is interpreting.
Define the number of lines on the screen, to be used by the pager.
Sets the list of directories which will be searched for modules named in a load command or implicitly required by a dependency. The default value for this variable is “/platform/i86pc/${ISADIR}
Value of loader's prompt. Defaults to “${interpret}”. If variable prompt is unset, the default prompt is ‘>’.
If set, the value is used to set kernel(8) console property.
Throttles the output of the ‘twiddle’ I/O progress indicator displayed while loading the kernel and modules. This is useful on slow serial consoles where the time spent waiting for these characters to be written can add up to many seconds. The spinner is updated only once every twiddle_divisor operations. The default value for twiddle_divisor is 16.

Other variables are used for loader or to set kernel properties or for informational purposes.

When a builtin command is executed, the rest of the line is taken by it as arguments, and it is processed by a special parser which is not used for regular Forth commands.

This special parser applies the following rules to the parsed text:

  1. All backslash characters are preprocessed.
    • \b , \f , \r , \n and \t are processed as in C.
    • \s is converted to a space.
    • \v is converted to ASCII 11.
    • \z is just skipped. Useful for things like “\0xf\z\0xf”.
    • \0xN and \0xNN are replaced by the hex N or NN.
    • \NNN is replaced by the octal NNN ASCII character.
    • \" , \' and \$ will escape these characters, preventing them from receiving special treatment in Step 2, described below.
    • \\ will be replaced with a single \ .
    • In any other occurrence, backslash will just be removed.
  2. Every string between non-escaped quotes or double-quotes will be treated as a single word for the purposes of the remaining steps.
  3. Replace any $VARIABLE or ${VARIABLE} with the value of the environment variable VARIABLE.
  4. Space-delimited arguments are passed to the called builtin command. Spaces can also be escaped through the use of \\ .

An exception to this parsing rule exists, and is described in Builtins And FORTH.

All builtin words are state-smart, immediate words. If interpreted, they behave exactly as described previously. If they are compiled, though, they extract their arguments from the stack instead of the command line.

If compiled, the builtin words expect to find, at execution time, the following parameters on the stack:

addrN lenN ... addr2 len2 addr1 len1 N
where addrX lenX are strings which will compose the command line that will be parsed into the builtin's arguments. Internally, these strings are concatenated in from 1 to N, with a space put between each one.

If no arguments are passed, a 0 be passed, even if the builtin accepts no arguments.

While this behavior has benefits, it has its trade-offs. If the execution token of a builtin is acquired (through ' or [']), and then passed to catch or execute, the builtin behavior will depend on the system state

at the time catch or execute is processed!
This is particularly annoying for programs that want or need to handle exceptions. In this case, the use of a proxy is recommended. For example:
: (boot) boot

FICL is a Forth interpreter written in C, in the form of a forth virtual machine library that can be called by C functions and vice versa.

In loader, each line read interactively is then fed to FICL, which may call loader back to execute the builtin words. The builtin include will also feed FICL, one line at a time.

The words available to FICL can be classified into four groups. The ANS Forth standard words, extra FICL words, extra FreeBSD words, and the builtin commands; the latter were already described. The ANS Forth standard words are listed in the STANDARDS section. The words falling in the two other groups are described in the following subsections.

This is the STRING word set's compare.
This is the STRING word set's sliteral.

Evaluates the remainder of the input buffer, after having printed it first.
Evaluates the remainder of the input buffer under a catch exception guard.
Works like . but without outputting a trailing space.
(fd --)
Closes a file.
(fd -- char)
Reads a single character from a file.
(fd --)
Processes a file fd.
(addr len mode -- fd)
Opens a file. Returns a file descriptor, or -1 in case of failure. The mode parameter selects whether the file is to be opened for read access, write access, or both. The constants O_RDONLY, O_WRONLY, and O_RDWR are defined in /boot/forth/support.4th, indicating read only, write only, and read-write access, respectively.
(fd addr len -- len')
Tries to read len bytes from file fd into buffer addr. Returns the actual number of bytes read, or -1 in case of error or end of file.
(-- cells)
Return the space remaining in the dictionary heap, in cells. This is not related to the heap used by dynamic memory allocation words.
(port -- char)
Reads a byte from a port.
(-- addr len flag | flag)
Returns true and string with virtualization engine name or false.
(-- char)
Reads a single character from the console.
(-- flag)
Returns true if there is a character available to be read from the console.
(u --)
Waits microseconds.
(port char --)
Writes a byte to a port.
(-- u)
Returns the number of seconds since midnight.
(-- addr len)
Returns the remainder of the input buffer as a string on the stack.

(x1 y1 x2 y2 x3 y3 width --)
Draws a quadratic Bezier curve in the current foreground color using the three provided points and specified line with.
(x1 y1 x2 y2 fill --)
Draws a rectangle to the screen with the top left at (x1,y1) and the bottom right at (x2,y2) , using the current foreground color. If is true then the rectangle will be filled in.
(x1 y1 x2 y2 width --)
Draws a line from (x1,y1) to (x2,y2) in the current foreground color and with the specified width.
(flags x1 y1 x2 y2 addr len -- flag)
Outputs an image to the screen. Returns true if the image was able to be written and false otherwise. Only truecolor PNG images are supported and the path to the file must be provided through the addr and len arguments on the stack. The image will be displayed in the rectangular screen region with the top left at (x1,y1) and the bottom right at (x2,y2)

Either or can be set to "0" which causes it to be calculated to maintain the aspect ratio of the image. If both are "0" then the native resolution of the image will be used.

If is "0", then the image will be placed as far over to the right of the screen as possible. Similarly, if is "0", then the image will be placed as far down the screen as possible.

Flags is a bitfield; the following bits are defined:

Draw a single pixel border around the image in the current foreground color.
Do not scroll the image with the rest of the screen.
Output diagnostic information (for debugging).
(x y --)
Colors the pixel at with the current foreground color.
(x1 y1 x2 y2 --)
Draws a rectangle with rounded corners using terminal coordinates and the current foreground color.

if the architecture is IA32.
loader version.

The following values are thrown by loader:

Any type of error in the processing of a builtin.
Out of interpreting text.
Need more text to succeed -- will finish on next run.
Unspecified error.

loader configuration files, as described in loader.conf(5).
Loaded by help. Contains the help messages.
loader bootstrapping script.
Extra builtin-like words.
loader.conf processing words.
loader itself.

Boot in single user mode:

boot -s

Load the kernel, a boot_archive, and then autoboot in five seconds. Notice that a kernel must be loaded before any other load command is attempted.

load /platform/i86pc/kernel/amd64/unix
load -t rootfs /platform/i86pc/amd64/boot_archive
autoboot 5

Set the default device used for loading a kernel from a ZFS filesystem:

set currdev=zfs:rpool/ROOT/knowngood:

Although setting the currdev as shown in the example above is supported, it is advisable to use loader beadm command or boot environment menu instead. The reason is, the beadm or menu selection will also instruct loader to clean up the currently set configuration and load configuration from the new boot environment.

btxld(1onbld), loader.conf(5), menu.lst(5), boot(8)

For the purposes of ANS Forth compliance, loader is an

ANS Forth System with Environmental Restrictions, Providing
.(, :noname, ?do, parse, pick, roll, refill, to, value, \, false, true, <>, 0<>, compile, , erase, nip, tuck
and marker
from the Core Extensions word set, Providing the Exception Extensions word set, Providing the Locals Extensions word set, Providing the Memory-Allocation Extensions word set, Providing
.s, bye, forget, see, words, [if], [else]
and [then]
from the Programming-Tools extension word set, Providing the Search-Order extensions word set.
March 30, 2023 OmniOS