TH 9LOAD 8
SH NAME
9load, ld \- PC bootstrap program
SH SYNOPSIS
I "(Under MS-DOS)
br
[
I drive
:][
I path
RB ] 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.
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 0x7C00.  In the
case of a floppy, this is the PBS.  In the case of a hard
disk it it 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.
It's 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 address
B 0x80010000
(64K) and
loads the
I bootfile
at the entry address specified by the header,
usually
BR 0x80100020 .
After loading, control is passed to the entry location.
PP
In summary,
Plan 9 can be booted on a PC two different ways:
either by booting MS-DOS and using
I ld
to start
I 9load
in the appropriate directory,
or 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)).
PP
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
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.
PD
PP
When
I 9load
starts running at physical address 0x10000,
it switches to 32-bit mode.
It then double maps the first 16Mb of physical memory to
virtual addresses 0 and 0x80000000.
Physical memory from 0x300000 upwards is used as data
space.
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 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.
PP
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 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 :]
[
I path
RB ] 9load
br
RI [ drive :]
[
I path
RB ] 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
PP
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 .

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