harvey/sys/man/8/9load

.TH 9LOAD 8
.SH NAME
9load, 9pxeload, ld \- PC bootstrap program
.SH SYNOPSIS
.I "(Under MS-DOS)
.br
.RI [ drive\f(CW:\fP ][ path ] ld
[
.I 9load
]
.SH DESCRIPTION
.I 9load
and
.I ld
are programs that reside in a FAT file system and bootstrap Plan 9.
.I 9load
loads the kernel, but it cannot be run from DOS; use
.I ld
to bootstrap (by starting
.IR 9load )
if DOS is running.
.I 9load
is run automatically by the boot procedures described below;
it cannot be run directly by hand.
.I 9pxeload
is a version of
.I 9load
that can be booted using the PXE download
found in some ethernet card BIOSes.
There are three bootstrap sequences:
.IP \-
BIOS, MBR, disk partition PBS,
.IR 9load ,
kernel
.IP \-
BIOS, floppy PBS,
.IR 9load ,
kernel
.IP \-
BIOS, MBR, DOS,
.IR ld ,
.IR 9load ,
kernel.
.PP
Details follow.
.PP
.I 9load
is a bootstrap program that loads and starts a program,
typically the kernel, on a PC.
It is run by the PC partition boot sector program (PBS),
which usually resides in the first
sector of the active partition.
A copy of the Plan 9 PBS is kept in
.BR /386/pbs ,
but due to the ``cylinder-head-sector'' (CHS) addressing mode of old BIOSes, it can only
operate up to 8.5GB into the disk.
Plan 9 partitions further into the disk
can only be booted using
.BR /386/pbslba ,
and then only if the machine's BIOS supports
linear block addressing (LBA) mode for disk transfers.
.PP
When booting from floppy or hard disk, the BIOS loads the
first sector of the medium at location
.BR 0x7C00 .
In the case of a floppy, this is the PBS.
In the case of a hard disk, it is the master boot record (MBR).
The MBR copies itself to address
.BR 0x600 ,
finds the active partition and loads its PBS at address
.BR 0x7C00 .
A copy of the Plan 9 MBR is kept in
.BR /386/mbr ;
some commercial MBRs cannot read sectors
past 2GB.
The Plan 9 MBR can read sectors up to 8.5GB into
the disk, and further if the BIOS supports LBA.
The single file
.B /386/mbr
detects whether the BIOS supports LBA and
acts appropriately, defaulting to CHS mode
when LBA is not present.
The PBSs cannot do this due to code size considerations.
The Plan 9 MBR is suitable for booting non-Plan 9
operating systems,
and (modulo the large disk constraints just described)
non-Plan 9 MBRs are suitable for booting Plan 9.
.PP
Thus the default sequence is: BIOS, MBR, PBS,
.IR 9load ,
kernel.
.PP
Because it contains many device drivers for different
disks and networks,
.I 9load
is larger than 64K and cannot be run as a DOS
.RB `` .com ''
executable.
A stripped-down version that knows about disks but not networks,
called
.I ld
(really
.BR ld.com ),
fits in 64K and can be used under DOS to load and start a program (default
.IR 9load )
from the FAT16 partition.
Its command line argument is of the same format as the
.I bootfile
specifiers described below.
This profusion of loaders is unfortunate, but at least
.I ld
and
.I 9load
are compiled from the same source.
.PP
.I 9load
begins execution at virtual address
.B 0x80010000
(64K) and
loads the
.I bootfile
at the entry address specified by the header,
usually virtual
.BR 0xF0100020 .
After loading, control is passed to the entry location.
.PP
In summary,
Plan 9 can be booted on a PC three different ways:
either by booting MS-DOS and using
.I ld
to start
.I 9load
in the appropriate directory,
by booting directly from a Plan 9 boot floppy or disk
partition
prepared using
.B format
to install the appropriate files and bootstrap sectors
(see
.IR prep (8)),
or by using a PXE capable BIOS to boot
.I 9pxeload
directly over the ethernet.
.SS Bootfile
The
.IR bootfile ,
which may be compressed with
.IR gzip (1),
can be specified to
.I 9load
as a
.B bootfile=
entry in
.IR plan9.ini ,
or if booting from the ethernet, by a BOOTP server.
If the
.B plan9.ini
file contains multiple
.B bootfile=
entries,
.I 9load
will present a numerical menu of the choices; type
the corresponding number to select an entry.
.PP
The format of the
.I bootfile
name is
.IB device ! file
or
.IB device ! partition ! file\f1.
If
.BI ! file
is omitted, the default for the particular
.I device
is used.
Supported
.I devices
are
.TF \fLethern
.PD
.TP
.BI fd n
An MS-DOS floppy disk.
.I N
specifies the floppy drive, either
0 or 1.
The
.I bootfile
is the contents of the MS-DOS
.IR file .
There is no default file.
For compatibility with hard disks, a
.I partition
may be given, but only
.B dos
is recognized:
.BI fd0!dos! file\f1.
.TP
.BI ether n
Ethernet.
.I N
specifies the Ethernet device number.
If a
.I partition
is specified, it is taken to be the name of a host machine
from which to load the kernel.
.I file
is determined by the
.B /lib/ndb
(see
.IR ndb (6))
entry for this PC.
.TP
.BI sd Cn
Non-floppy disk.
The device name format is described in
.IR sd (3).
A
.I partition
must be given and must
name a partition containing a FAT file system.
The name
.B dos
refers to the first DOS partition on a given device.
It is common for Plan 9 partitions to contain a small
FAT file system for configuration.
By convention, this partition is called
.BR 9fat .
There is no default partition or pathname.
.TP
.B bios0
(Not in
.IR 9pxeload .)
.I 9load
loads from a FAT file system on
the first LBA device
in the BIOS's list of devices to try to boot from,
using the BIOS INT 13 calls also used by
.IR pbslba .
It does not understand any form of partition table;
see the EXAMPLES in
.IR prep (8)
for how to format such a device.
This is mostly useful for booting from USB devices so far.
.TP
.B sdB0
(Not in
.IR 9pxeload .)
A special case of
.BI sd Cn
that uses
.B bios0
to read from a FAT file system.
Partitions are understood.
.SS Kernel loading
When
.I 9load
starts running at physical address
.BR 0x10000 ,
it switches to 32-bit mode.
It then double maps the first 16Mb of physical memory to
virtual addresses
.B 0
and
.BR 0x80000000 .
Physical memory from
.B 0x300000
upwards is used as data space.
.PP
.I 9pxeload
differs slightly in operation from
.IR 9load .
It is initially loaded by the PXE BIOS at physical address
.BR 0x7C00 .
Only devices which can be automatically configured,
e.g. most PCI ethernet adapters,
will be recognised.
If the file
.BI /cfg/pxe/ XXXXXXXXXXXX
can be located via a DHCP server,
where
.I XXXXXXXXXXXX
is the MAC address of a recognised ethernet adapter,
the contents are obtained and used as a
.IR plan9.ini .
.PP
Next, in order to find configuration information,
.I 9load
searches all units on devices
.BR fd
and
.BI sd Cn \fR,
in that order, for a file called
.B plan9\eplan9.ini
or
.B plan9.ini
(see
.IR plan9.ini (8))
on a partition named
.B dos
or
.BR 9fat .
If one is found, searching stops and the file is read into memory
at physical address
.B 0x1200
where it can be found later by any loaded
.IR bootfile .
Some options in
.B plan9.ini
are used by
.IR 9load :
.TF bootfile=manual
.TP
.B console
.TP
.B baud
Specifies the console device and baud rate if not a display.
.TP
.BI ether n
Ethernet interfaces. These can be used to load the
.I bootfile
over a network.
Probing for Ethernet interfaces is too prone to error.
.TP
.BI bootfile= bootfile
Specifies the
.IR bootfile .
This option is overridden by a command-line argument.
.TP
.B bootfile=auto
Default.
.TP
.B bootfile=local
Like
.IR auto ,
but do not attempt to load over the network.
.TP
.B bootfile=manual
After determining which devices are available for loading from,
enter prompt mode.
.PD
.PP
When the search for
.B plan9.ini
is done,
.I 9load
proceeds to determine which bootfile to load.
If there was no
.I bootfile
option,
.I 9load
chooses a default
from the following prioritized device list:
.EX
	fd sd ether
.EE
.I 9load
then attempts to load the
.I bootfile
unless
the
.B bootfile=manual
option was given, in which case prompt mode is entered immediately.
If the default device is
.BR fd ,
.I 9load
will prompt the user for input before proceeding with the
default bootfile load after 5 seconds;
this prompt is omitted if
a command-line argument or
.I bootfile
option
was given.
.PP
.I 9load
prints the list of available
.IR device s
and
enters prompt mode on encountering any error
or if directed to do so by a
.B bootfile=manual
option.
In prompt mode, the user is required to type
a
.IB bootfile
in response to the
.L "Boot from:
prompt.
.SS Other facilities and caveats
.I 9load
parses the master boot record and Plan 9 partition tables
(see
.IR prep (8)),
leaving partitioning information appended to the
in-memory contents of
.I plan9.ini
for the
.IR bootfile .
This is used by
.IR sd (3)
to initialize partitions so that
.IR fossil (4)
or
.IR kfs (4)
file systems can be mounted as the root file system.
A more extensive partitioning is typically done by
.I fdisk
and
.I prep
as part of
.I termrc
or
.I cpurc
(see
.IR cpurc (8)).
.PP
A
control-P
character typed at any time on the console causes
.B 9load
to perform a hardware reset
(Ctrl-Alt-Del can also be used on a PC keyboard).
.PP
When loaded from a PBS (rather than from
.IR ld.com ),
.I 9load
must be contiguously allocated on
the disk.
See
.IR dossrv (4)
for information on ensuring this.
.SH FILES
.RI [ drive\f(CW:\fP ][ path ]\c
.B 9load
.br
.RI [ drive\f(CW:\fP ][ path ]\c
.B ld
.br
.IB "FAT-filesystem" :\eplan9\eplan9.ini
.br
.IB "FAT-filesystem" :\eplan9.ini
.SH SOURCE
.B /sys/src/boot/pc
.SH "SEE ALSO"
.IR plan9.ini (8),
.IR prep (8)
.SH BUGS
Much of the work done by
.B 9load
is duplicated by the loaded kernel.
.PP
If
.I ld
detects an installed MS-DOS Extended Memory Manager,
it attempts to de-install it, but the technique
used may not always work.
It is safer not to install the Extended Memory Manager before running
.IR ld .
.PP
The way
.I 9pxeload
obtains the information normally found in a disc
.I plan9.ini
file,
and thereby the kernel to load and boot,
is not ideal and may change in the future.
.PP
BIOS bugs force some limitions on reading via the BIOS.
.B bios0
and
.B sdB0
only work on the first LBA device in the BIOS's list of boot devices.