FreeBSD/Linux Kernel Cross Reference
sys/boot/common/loader.8

     .\" Copyright (c) 1999 Daniel C. Sobral
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    .\" $FreeBSD: releng/8.2/sys/boot/common/loader.8 188666 2009-02-16 02:42:17Z thompsa $
    .\"
    .Dd February 15, 2009
    .Dt LOADER 8
    .Os
    .Sh NAME
    .Nm loader
    .Nd kernel bootstrapping final stage
    .Sh DESCRIPTION
    The program called
    .Nm
    is the final stage of
    .Fx Ns 's
    kernel bootstrapping process.
    On IA32 (i386) architectures, it is a
    .Pa BTX
    client.
    It is linked statically to
    .Xr libstand 3
    and usually located in the directory
    .Pa /boot .
    .Pp
    It 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
    .Tn ANS
    Forth compatible Forth interpreter based on FICL, by
    .An John Sadler .
    .Pp
    During initialization,
    .Nm
    will probe for a console and set the
    .Va console
    variable, or set it to serial console
    .Pq Dq Li comconsole
    if the previous boot stage used that.
    If multiple consoles are selected, they will be listed separated by spaces.
    Then, devices are probed,
    .Va currdev
    and
    .Va loaddev
    are set, and
    .Va LINES
    is set to 24.
    Next,
    .Tn FICL
    is initialized, the builtin words are added to its vocabulary, and
    .Pa /boot/boot.4th
    is processed if it exists.
    No disk switching is possible while that file is being read.
    The inner interpreter
    .Nm
    will use with
    .Tn FICL
    is then set to
    .Ic interpret ,
    which is
    .Tn FICL Ns 's
    default.
    After that,
    .Pa /boot/loader.rc
    is processed if available, and, failing that,
    .Pa /boot/boot.conf
    is read for historical reasons.
    These files are processed through the
    .Ic include
    command, which reads all of them into memory before processing them,
    making disk changes possible.
   .Pp
   At this point, if an
   .Ic autoboot
   has not been tried, and if
   .Va autoboot_delay
   is not set to
   .Dq Li NO
   (not case sensitive), then an
   .Ic autoboot
   will be tried.
   If the system gets past this point,
   .Va prompt
   will be set and
   .Nm
   will engage interactive mode.
   Please note that historically even when
   .Va autoboot_delay
   is set to
   .Dq Li 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
   .Va autoboot_delay
   to
   .Dq Li -1 ,
   in this case
   .Nm
   will engage interactive mode only if
   .Ic autoboot
   has failed.
   .Sh BUILTIN COMMANDS
   In
   .Nm ,
   builtin commands take parameters from the command line.
   Presently,
   the only way to call them from a script is by using
   .Pa evaluate
   on a string.
   If an error condition occurs, an exception will be generated,
   which can be intercepted using
   .Tn 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.
   .Pp
   The builtin commands available are:
   .Pp
   .Bl -tag -width Ds -compact
   .It Ic autoboot Op Ar seconds Op Ar 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.
   .Pp
   .It Ic bcachestat
   Displays statistics about disk cache usage.
   For debugging only.
   .Pp
   .It Ic boot
   .It Ic boot Ar kernelname Op Cm ...
   .It Ic boot Fl flag Cm ...
   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.
   .Pp
   .Em WARNING :
   The behavior of this builtin is changed if
   .Xr loader.4th 8
   is loaded.
   .Pp
   .It Ic echo Xo
   .Op Fl n
   .Op Aq message
   .Xc
   Displays text on the screen.
   A new line will be printed unless
   .Fl n
   is specified.
   .Pp
   .It Ic heap
   Displays memory usage statistics.
   For debugging purposes only.
   .Pp
   .It Ic help Op topic Op subtopic
   Shows help messages read from
   .Pa /boot/loader.help .
   The special topic
   .Em index
   will list the topics available.
   .Pp
   .It Ic include Ar file Op Ar
   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
   .Sx ERRORS ) .
   .Pp
   .It Ic load Xo
   .Op Fl t Ar type
   .Ar file Cm ...
   .Xc
   Loads a kernel, kernel loadable module (kld), or file of opaque
   contents tagged as being of the type
   .Ar 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.
   Currently, argument passing does not work for the kernel.
   .Pp
   .It Ic load_geli Xo
   .Op Fl n Ar keyno
   .Ar prov Ar file
   .Xc
   Loads a
   .Xr geli 8
   encryption keyfile for the given provider name.
   The key index can be specified via
   .Ar keyno 
   or will default to zero.
   .Pp
   .It Ic ls Xo
   .Op Fl l
   .Op Ar path
   .Xc
   Displays a listing of files in the directory
   .Ar path ,
   or the root directory if
   .Ar path
   is not specified.
   If
   .Fl l
   is specified, file sizes will be shown too.
   .Pp
   .It Ic lsdev Op Fl v
   Lists all of the devices from which it may be possible to load modules.
   If
   .Fl v
   is specified, more details are printed.
   .Pp
   .It Ic lsmod Op Fl v
   Displays loaded modules.
   If
   .Fl v
   is specified, more details are shown.
   .Pp
   .It Ic more Ar file Op Ar
   Display the files specified, with a pause at each
   .Va LINES
   displayed.
   .Pp
   .It Ic pnpscan Op Fl v
   Scans for Plug-and-Play devices.
   This is not functional at present.
   .Pp
   .It Ic read Xo
   .Op Fl t Ar seconds
   .Op Fl p Ar prompt
   .Op Va variable
   .Xc
   Reads a line of input from the terminal, storing it in
   .Va variable
   if specified.
   A timeout can be specified with
   .Fl t ,
   though it will be canceled at the first key pressed.
   A prompt may also be displayed through the
   .Fl p
   flag.
   .Pp
   .It Ic reboot
   Immediately reboots the system.
   .Pp
   .It Ic set Ar variable
   .It Ic set Ar variable Ns = Ns Ar value
   Set loader's environment variables.
   .Pp
   .It Ic show Op Va variable
   Displays the specified variable's value, or all variables and their
   values if
   .Va variable
   is not specified.
   .Pp
   .It Ic unload
   Remove all modules from memory.
   .Pp
   .It Ic unset Va variable
   Removes
   .Va variable
   from the environment.
   .Pp
   .It Ic \&?
   Lists available commands.
   .El
   .Ss BUILTIN ENVIRONMENT VARIABLES
   The
   .Nm
   has actually two different kinds of
   .Sq environment
   variables.
   There are ANS Forth's
   .Em 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.
   .Pp
   Environment variables can be set and unset through the
   .Ic set
   and
   .Ic unset
   builtins, and can have their values interactively examined through the
   use of the
   .Ic show
   builtin.
   Their values can also be accessed as described in
   .Sx BUILTIN PARSER .
   .Pp
   Notice that these environment variables are not inherited by any shell
   after the system has been booted.
   .Pp
   A few variables are set automatically by
   .Nm .
   Others can affect the behavior of either
   .Nm
   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.
   .Bl -tag -width bootfile
   .It Va acpi_load
   Unset this to disable automatic loading of the ACPI module.
   See also
   .Va hint.acpi.0.disabled
   in
   .Xr device.hints 5 .
   .It Va autoboot_delay
   Number of seconds
   .Ic autoboot
   will wait before booting.
   If this variable is not defined,
   .Ic autoboot
   will default to 10 seconds.
   .Pp
   If set to
   .Dq Li NO ,
   no
   .Ic autoboot
   will be automatically attempted after processing
   .Pa /boot/loader.rc ,
   though explicit
   .Ic autoboot Ns 's
   will be processed normally, defaulting to 10 seconds delay.
   .Pp
   If set to
   .Dq Li 0 ,
   no delay will be inserted, but user still will be able to interrupt
   .Ic autoboot
   process and escape into the interactive mode by pressing some key
   on the console while kernel and
   modules are being loaded.
   .Pp
   If set to
   .Dq Li -1 ,
   no delay will be inserted and
   .Nm
   will engage interactive mode only if
   .Ic autoboot
   has failed for some reason.
   .It Va boot_askname
   Instructs the kernel to prompt the user for the name of the root device
   when the kernel is booted.
   .It Va boot_cdrom
   Instructs the kernel to try to mount the root file system from CD-ROM.
   .It Va boot_ddb
   Instructs the kernel to start in the DDB debugger, rather than
   proceeding to initialize when booted.
   .It Va boot_dfltroot
   Instructs the kernel to mount the statically compiled-in root file system.
   .It Va boot_gdb
   Selects gdb-remote mode for the kernel debugger by default.
   .It Va boot_multicons
   Enables multiple console support in the kernel early on boot.
   In a running system, console configuration can be manipulated
   by the
   .Xr conscontrol 8
   utility.
   .It Va boot_mute
   All console output is suppressed when console is muted.
   In a running system, the state of console muting can be manipulated by the
   .Xr conscontrol 8
   utility.
   .It Va boot_pause
   During the device probe, pause after each line is printed.
   .It Va boot_serial
   Force the use of a serial console even when an internal console
   is present.
   .It Va boot_single
   Prevents the kernel from initiating a multi-user startup; instead,
   a single-user mode will be entered when the kernel has finished
   device probing.
   .It Va boot_verbose
   Setting this variable causes extra debugging information to be printed
   by the kernel during the boot phase.
   .It Va bootfile
   List of semicolon-separated search path for bootable kernels.
   The default is
   .Dq Li kernel .
   .It Va comconsole_speed
   Defines the speed of the serial console (i386 and amd64 only).
   If the previous boot stage indicated that a serial console is in use
   then this variable is initialized to the current speed of the console
   serial port.
   Otherwise it is set to 9600 unless this was overridden using the
   .Va BOOT_COMCONSOLE_SPEED
   variable when
   .Nm
   was compiled.
   Changes to the
   .Va comconsole_speed
   variable take effect immediately.
   .It Va 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
   userland output (e.g.\& from
   .Xr init 8 ) .
   .It Va currdev
   Selects the default device.
   Syntax for devices is odd.
   .It Va init_chroot
   If set to a valid directory in the root file system, it causes
   .Xr init 8
   to perform a
   .Xr chroot 2
   operation on that directory, making it the new root directory.
   That happens before entering single-user mode or multi-user
   mode (but after executing the
   .Va init_script
   if enabled).
   .It Va init_path
   Sets the list of binaries which the kernel will try to run as the initial
   process.
   The first matching binary is used.
   The default list is
   .Dq Li /sbin/init:/sbin/oinit:/sbin/init.bak:\:/rescue/init:/stand/sysinstall .
   .It Va init_script
   If set to a valid file name in the root file system,
   instructs
   .Xr init 8
   to run that script as the very first action,
   before doing anything else.
   Signal handling and exit code interpretation is similar to
   running the
   .Pa /etc/rc
   script.
   In particular, single-user operation is enforced
   if the script terminates with a non-zero exit code,
   or if a SIGTERM is delivered to the
   .Xr init 8
   process (PID 1).
   .It Va init_shell
   Defines the shell binary to be used for executing the various shell scripts.
   The default is
   .Dq Li /bin/sh .
   It is used for running the
   .Va init_script
   if set, as well as for the
   .Pa /etc/rc
   and
   .Pa /etc/rc.shutdown
   scripts.
   The value of the corresponding
   .Xr kenv 2
   variable is evaluated every time
   .Xr init 8
   calls a shell script, so it can be changed later on using the
   .Xr kenv 1
   utility.
   In particular, if a non-default shell is used for running an
   .Va init_script ,
   it might be desirable to have that script reset the value of
   .Va init_shell
   back to the default, so that the
   .Pa /etc/rc
   script is executed with the standard shell
   .Pa /bin/sh .
   .It Va interpret
   Has the value
   .Dq Li OK
   if the Forth's current state is interpreting.
   .It Va LINES
   Define the number of lines on the screen, to be used by the pager.
   .It Va module_path
   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
   .Dq Li /boot/kernel;/boot/modules .
   .It Va num_ide_disks
   Sets the number of IDE disks as a workaround for some problems in
   finding the root disk at boot.
   This has been deprecated in favor of
   .Va root_disk_unit .
   .It Va prompt
   Value of
   .Nm Ns 's
   prompt.
   Defaults to
   .Dq Li "${interpret}" .
   If variable
   .Va prompt
   is unset, the default prompt is
   .Ql > .
   .It Va root_disk_unit
   If the code which detects the disk unit number for the root disk is
   confused, e.g.\& by a mix of SCSI and IDE disks, or IDE disks with
   gaps in the sequence (e.g.\& no primary slave), the unit number can
   be forced by setting this variable.
   .It Va rootdev
   By default the value of
   .Va currdev
   is used to set the root file system
   when the kernel is booted.
   This can be overridden by setting
   .Va rootdev
   explicitly.
   .El
   .Pp
   Other variables are used to override kernel tunable parameters.
   The following tunables are available:
   .Bl -tag -width Va
   .It Va hw.physmem
   Limit the amount of physical memory the system will use.
   By default the size is in bytes, but the
   .Cm k , K , m , M , g
   and
   .Cm G
   suffixes
   are also accepted and indicate kilobytes, megabytes and gigabytes
   respectively.
   An invalid suffix will result in the variable being ignored by the
   kernel.
   .It Va hw.pci.host_start_mem , hw.acpi.host_start_mem
   When not otherwise constrained, this limits the memory start
   address.
   The default is 0x80000000 and should be set to at least size of the
   memory and not conflict with other resources.
   Typically, only systems without PCI bridges need to set this variable
   since PCI bridges typically constrain the memory starting address
   (and the variable is only used when bridges do not constrain this
   address).
   .It Va hw.pci.enable_io_modes
   Enable PCI resources which are left off by some BIOSes or are not
   enabled correctly by the device driver.
   Tunable value set to ON (1) by default, but this may cause problems
   with some peripherals.
   .It Va kern.maxusers
   Set the size of a number of statically allocated system tables; see
   .Xr tuning 7
   for a description of how to select an appropriate value for this
   tunable.
   When set, this tunable replaces the value declared in the kernel
   compile-time configuration file.
   .It Va kern.ipc.nmbclusters
   Set the number of mbuf clusters to be allocated.
   The value cannot be set below the default
   determined when the kernel was compiled.
   .It Va kern.ipc.nsfbufs
   Set the number of
   .Xr sendfile 2
   buffers to be allocated.
   Overrides
   .Dv NSFBUFS .
   Not all architectures use such buffers; see
   .Xr sendfile 2
   for details.
   .It Va kern.maxswzone
   Limits the amount of KVM to be used to hold swap
   meta information, which directly governs the
   maximum amount of swap the system can support.
   This value is specified in bytes of KVA space
   and defaults to 32MBytes on i386 and amd64.
   Care should be taken
   to not reduce this value such that the actual
   amount of configured swap exceeds 1/2 the
   kernel-supported swap.
   The default of 32MB allows
   the kernel to support a maximum of ~7GB of swap.
   Only change
   this parameter if you need to greatly extend the
   KVM reservation for other resources such as the
   buffer cache or
   .Va kern.ipc.nmbclusters .
   Modifies kernel option
   .Dv VM_SWZONE_SIZE_MAX .
   .It Va kern.maxbcache
   Limits the amount of KVM reserved for use by the
   buffer cache, specified in bytes.
   The default maximum is 200MB on i386,
   and 400MB on amd64, sparc64, and sun4v.
   This parameter is used to
   prevent the buffer cache from eating too much
   KVM in large-memory machine configurations.
   Only mess around with this parameter if you need to
   greatly extend the KVM reservation for other resources
   such as the swap zone or
   .Va kern.ipc.nmbclusters .
   Note that
   the NBUF parameter will override this limit.
   Modifies
   .Dv VM_BCACHE_SIZE_MAX .
   .It Va machdep.disable_mtrrs
   Disable the use of i686 MTRRs (x86 only).
   .It Va net.inet.tcp.tcbhashsize
   Overrides the compile-time set value of
   .Dv TCBHASHSIZE
   or the preset default of 512.
   Must be a power of 2.
   .It Va vm.kmem_size
   Sets the size of kernel memory (bytes).
   This overrides the value determined when the kernel was compiled.
   Modifies
   .Dv VM_KMEM_SIZE .
   .It Va vm.kmem_size_min
   .It Va vm.kmem_size_max
   Sets the minimum and maximum (respectively) amount of kernel memory
   that will be automatically allocated by the kernel.
   These override the values determined when the kernel was compiled.
   Modifies
   .Dv VM_KMEM_SIZE_MIN
   and
   .Dv VM_KMEM_SIZE_MAX .
   .El
   .Ss 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.
   .Pp
   This special parser applies the following rules to the parsed text:
   .Pp
   .Bl -enum
   .It
   All backslash characters are preprocessed.
   .Bl -bullet
   .It
   \eb , \ef , \er , \en and \et are processed as in C.
   .It
   \es is converted to a space.
   .It
   \ev is converted to
   .Tn ASCII
   11.
   .It
   \ez is just skipped.
   Useful for things like
   .Dq \e0xf\ez\e0xf .
   .It
   \e0xN and \e0xNN are replaced by the hex N or NN.
   .It
   \eNNN is replaced by the octal NNN
   .Tn ASCII
   character.
   .It
   \e" , \e' and \e$ will escape these characters, preventing them from
   receiving special treatment in Step 2, described below.
   .It
   \e\e will be replaced with a single \e .
   .It
   In any other occurrence, backslash will just be removed.
   .El
   .It
   Every string between non-escaped quotes or double-quotes will be treated
   as a single word for the purposes of the remaining steps.
   .It
   Replace any
   .Li $VARIABLE
   or
   .Li ${VARIABLE}
   with the value of the environment variable
   .Va VARIABLE .
   .It
   Space-delimited arguments are passed to the called builtin command.
   Spaces can also be escaped through the use of \e\e .
   .El
   .Pp
   An exception to this parsing rule exists, and is described in
   .Sx BUILTINS AND FORTH .
   .Ss 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.
   .Pp
   If compiled, the builtin words expect to find, at execution time, the
   following parameters on the stack:
   .D1 Ar addrN lenN ... addr2 len2 addr1 len1 N
   where
   .Ar 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.
   .Pp
   If no arguments are passed, a 0
   .Em must
   be passed, even if the builtin accepts no arguments.
   .Pp
   While this behavior has benefits, it has its trade-offs.
   If the execution token of a builtin is acquired (through
   .Ic '
   or
   .Ic ['] ) ,
   and then passed to
   .Ic catch
   or
   .Ic execute ,
   the builtin behavior will depend on the system state
   .Bf Em
   at the time
   .Ic catch
   or
   .Ic execute
   is processed!
   .Ef
   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:
   .Dl : (boot) boot ;
   .Sh FICL
   .Tn 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.
   .Pp
   In
   .Nm ,
   each line read interactively is then fed to
   .Tn FICL ,
   which may call
   .Nm
   back to execute the builtin words.
   The builtin
   .Ic include
   will also feed
   .Tn FICL ,
   one line at a time.
   .Pp
   The words available to
   .Tn FICL
   can be classified into four groups.
   The
   .Tn ANS
   Forth standard words, extra
   .Tn FICL
   words, extra
   .Fx
   words, and the builtin commands;
   the latter were already described.
   The
   .Tn ANS
   Forth standard words are listed in the
   .Sx STANDARDS
   section.
   The words falling in the two other groups are described in the
   following subsections.
   .Ss FICL EXTRA WORDS
   .Bl -tag -width wid-set-super
   .It Ic .env
   .It Ic .ver
   .It Ic -roll
   .It Ic 2constant
   .It Ic >name
   .It Ic body>
   .It Ic compare
   This is the STRING word set's
   .Ic compare .
   .It Ic compile-only
   .It Ic endif
   .It Ic forget-wid
   .It Ic parse-word
   .It Ic sliteral
   This is the STRING word set's
   .Ic sliteral .
   .It Ic wid-set-super
   .It Ic w@
   .It Ic w!
   .It Ic x.
   .It Ic empty
   .It Ic cell-
   .It Ic -rot
   .El
   .Ss FREEBSD EXTRA WORDS
   .Bl -tag -width XXXXXXXX
   .It Ic \&$ Pq --
   Evaluates the remainder of the input buffer, after having printed it first.
   .It Ic \&% Pq --
   Evaluates the remainder of the input buffer under a
   .Ic catch
   exception guard.
   .It Ic .#
   Works like
   .Ic .
   but without outputting a trailing space.
   .It Ic fclose Pq Ar fd --
   Closes a file.
   .It Ic fkey Pq Ar fd -- char
   Reads a single character from a file.
   .It Ic fload Pq Ar fd --
   Processes a file
   .Em fd .
   .It Ic fopen Pq Ar addr len mode Li -- Ar fd
   Opens a file.
   Returns a file descriptor, or \-1 in case of failure.
   The
   .Ar mode
   parameter selects whether the file is to be opened for read access, write
   access, or both.
   The constants
   .Dv O_RDONLY , O_WRONLY ,
   and
   .Dv O_RDWR
   are defined in
   .Pa /boot/support.4th ,
   indicating read only, write only, and read-write access, respectively.
   .It Xo
   .Ic fread
   .Pq Ar fd addr len -- len'
   .Xc
   Tries to read
   .Em len
   bytes from file
   .Em fd
   into buffer
   .Em addr .
   Returns the actual number of bytes read, or -1 in case of error or end of
   file.
   .It Ic heap? Pq -- Ar cells
   Return the space remaining in the dictionary heap, in cells.
   This is not related to the heap used by dynamic memory allocation words.
   .It Ic inb Pq Ar port -- char
   Reads a byte from a port.
   .It Ic key Pq -- Ar char
   Reads a single character from the console.
   .It Ic key? Pq -- Ar flag
   Returns
   .Ic true
   if there is a character available to be read from the console.
   .It Ic ms Pq Ar u --
   Waits
   .Em u
   microseconds.
   .It Ic outb Pq Ar port char --
   Writes a byte to a port.
   .It Ic seconds Pq -- Ar u
   Returns the number of seconds since midnight.
   .It Ic tib> Pq -- Ar addr len
   Returns the remainder of the input buffer as a string on the stack.
   .It Ic trace! Pq Ar flag --
   Activates or deactivates tracing.
   Does not work with
   .Ic catch .
   .El
   .Ss FREEBSD DEFINED ENVIRONMENTAL QUERIES
   .Bl -tag -width Ds
   .It arch-i386
   .Ic TRUE
   if the architecture is IA32.
   .It FreeBSD_version
   .Fx
   version at compile time.
   .It loader_version
   .Nm
   version.
   .El
   .Ss SYSTEM DOCUMENTATION
   .Sh FILES
   .Bl -tag -width /boot/defaults/loader.conf -compact
   .It Pa /boot/loader
   .Nm
   itself.
   .It Pa /boot/boot.4th
   Additional
   .Tn FICL
   initialization.
   .It Pa /boot/boot.conf
   .Nm
   bootstrapping script.
   Deprecated.
   .It Pa /boot/defaults/loader.conf
   .It Pa /boot/loader.conf
   .It Pa /boot/loader.conf.local
   .Nm
   configuration files, as described in
   .Xr loader.conf 5 .
   .It Pa /boot/loader.rc
   .Nm
   bootstrapping script.
   .It Pa /boot/loader.help
   Loaded by
   .Ic help .
   Contains the help messages.
   .El
   .Sh EXAMPLES
   Boot in single user mode:
   .Pp
   .Dl boot -s
   .Pp
   Load the kernel, a splash screen, and then autoboot in five seconds.
   Notice that a kernel must be loaded before any other
   .Ic load
   command is attempted.
   .Bd -literal -offset indent
   load kernel
   load splash_bmp
   load -t splash_image_data /boot/chuckrulez.bmp
   autoboot 5
   .Ed
   .Pp
   Set the disk unit of the root device to 2, and then boot.
   This would be needed in a system with two IDE disks,
   with the second IDE disk hardwired to wd2 instead of wd1.
   .Bd -literal -offset indent
   set root_disk_unit=2
   boot /kernel
   .Ed
   .Pp
   See also:
   .Bl -tag -width /usr/share/examples/bootforth/X
   .It Pa /boot/loader.4th
   Extra builtin-like words.
   .It Pa /boot/support.4th
   .Pa loader.conf
   processing words.
   .It Pa /usr/share/examples/bootforth/
   Assorted examples.
   .El
   .Sh ERRORS
   The following values are thrown by
   .Nm :
   .Bl -tag -width XXXXX -offset indent
   .It 100
   Any type of error in the processing of a builtin.
   .It -1
   .Ic Abort
   executed.
   .It -2
   .Ic Abort"
   executed.
   .It -56
   .Ic Quit
   executed.
   .It -256
   Out of interpreting text.
   .It -257
   Need more text to succeed -- will finish on next run.
   .It -258
   .Ic Bye
   executed.
   .It -259
   Unspecified error.
   .El
   .Sh SEE ALSO
   .Xr libstand 3 ,
   .Xr loader.conf 5 ,
   .Xr tuning 7 ,
   .Xr boot 8 ,
   .Xr btxld 8
   .Sh STANDARDS
   For the purposes of ANS Forth compliance, loader is an
   .Bf Em
   ANS Forth System with Environmental Restrictions, Providing
   .Ef
   .Bf Li
   .No .( ,
   .No :noname ,
   .No ?do ,
   parse, pick, roll, refill, to, value, \e, false, true,
   .No <> ,
   .No 0<> ,
   compile\&, , erase, nip, tuck
   .Ef
   .Em and
   .Li marker
   .Bf Em
   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
   .Ef
   .Bf Li
   \&.s,
   bye, forget, see, words,
   \&[if],
  \&[else]
  .Ef
  .Em and
  .Li [then]
  .Bf Em
  from the Programming-Tools extension word set, Providing the
  Search-Order extensions word set.
  .Ef
  .Sh HISTORY
  The
  .Nm
  first appeared in
  .Fx 3.1 .
  .Sh AUTHORS
  .An -nosplit
  The
  .Nm
  was written by
  .An Michael Smith Aq msmith@FreeBSD.org .
  .Pp
  .Tn FICL
  was written by
  .An John Sadler Aq john_sadler@alum.mit.edu .
  .Sh BUGS
  The
  .Ic expect
  and
  .Ic accept
  words will read from the input buffer instead of the console.
  The latter will be fixed, but the former will not.

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