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


loader - kernel 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

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.

Builtin Commands

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:

autoboot [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.

boot kernelname [...]
boot -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.

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

chain device
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.

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

heap Displays memory usage statistics. For debugging purposes only.

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

include 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).

load [-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.

ls [-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.

lsdev [-v]
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.

lsmod [-v]
Displays loaded modules. If -v is specified, more details are

lszfs filesystem
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.

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

read [-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.

reboot [-f]
Immediately reboots the system. On a UEFI 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 variable
set variable=value
Set loader's environment variables.

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

unload Remove all modules from memory.

unset variable
Removes variable from the environment.

? Lists available commands.

ZFS Features

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).

Builtin Environment Variables

The loader has actually two different kinds of `environment' variables.
There are ANS Forth's environmental queries, 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

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.

boot_ask 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.

bootfile 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.

console 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).

currdev 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

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

prompt 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.

Builtin Parser

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.

+o \b , \f , \r , \n and \t are processed as in C.

+o \s is converted to a space.

+o \v is converted to ASCII 11.

+o \z is just skipped. Useful for things like "\0xf\z\0xf".

+o \0xN and \0xNN are replaced by the hex N or NN.

+o \NNN is replaced by the octal NNN ASCII character.

+o \" , \' and \$ will escape these characters, preventing them from
receiving special treatment in Step 2, described below.

+o \\ will be replaced with a single \ .

+o 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

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 must 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.

FICL Extra Words







compare This is the STRING word set's compare.





sliteral This is the STRING word set's sliteral.








Loader Extra Words

$ (--) 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.

fclose (fd --)
Closes a file.

fkey (fd -- char)
Reads a single character from a file.

fload (fd --)
Processes a file fd.

fopen (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.

fread (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

heap? (-- cells)
Return the space remaining in the dictionary heap, in cells.
This is not related to the heap used by dynamic memory allocation

inb (port -- char)
Reads a byte from a port.

isvirtualized? (-- addr len flag | flag)
Returns true and string with virtualization engine name or false.

key (-- char)
Reads a single character from the console.

key? (-- flag)
Returns true if there is a character available to be read from
the console.

ms (u --)
Waits u microseconds.

outb (port char --)
Writes a byte to a port.

seconds (-- u)
Returns the number of seconds since midnight.

tib> (-- addr len)
Returns the remainder of the input buffer as a string on the

Loader Extra Framebuffer Words

fb-bezier (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.

fb-drawrect (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 fill is true then the rectangle will be filled in.

fb-line (x1 y1 x2 y2 width --)
Draws a line from (x1,y1) to (x2,y2) in the current foreground
color and with the specified width.

fb-putimage (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 x2 or y2 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 x1 is "0", then the image will be placed as far over to the
right of the screen as possible. Similarly, if y1 is "0", then
the image will be placed as far down the screen as possible.

Flags is a bitfield; the following bits are defined:

1 Draw a single pixel border around the image in the
current foreground color.

2 Do not scroll the image with the rest of the screen.

128 Output diagnostic information (for debugging).

fb-setpixel (x y --)
Colors the pixel at (x,y) with the current foreground color.

term-drawrect (x1 y1 x2 y2 --)
Draws a rectangle with rounded corners using terminal coordinates
and the current foreground color.

Loader Defined Environmental Queries

TRUE if the architecture is IA32.

loader version.


The following values are thrown by loader:

100 Any type of error in the processing of a builtin.

-1 Abort executed.

-2 Abort" executed.

-56 Quit executed.

-256 Out of interpreting text.

-257 Need more text to succeed -- will finish on next run.

-258 Bye executed.

-259 Unspecified error.


/boot/loader.conf.local loader configuration files, as described in
/boot/loader.help Loaded by help. Contains the help messages.
/boot/loader.rc loader bootstrapping script.
/boot/forth/loader.4th Extra builtin-like words.
/boot/forth/support.4th loader.conf processing words.
/boot/loader 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.

illumos March 30, 2023 illumos