9LOAD(8) 9LOAD(8)
NAME
9load, ld - PC bootstrap program
SYNOPSIS
(Under MS-DOS)
[ drive :][ path ]ld [ 9load ]
DESCRIPTION
9load and ld are programs that reside in a FAT file system
and bootstrap Plan 9. 9load loads the kernel, but it cannot
be run from DOS; use ld to bootstrap (by starting 9load) if
DOS is running. 9load is run automatically by the boot pro-
cedures described below; it cannot be run directly by hand.
There are three bootstrap sequences:
- BIOS, MBR, disk partition PBS, 9load, kernel
- BIOS, floppy PBS, 9load, kernel
- BIOS, MBR, DOS, ld, 9load, kernel.
Details follow.
9load is a bootstrap program that loads and starts a pro-
gram, 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 /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 /386/pbslba,
and then only if the machine's BIOS supports linear block
addressing (LBA) mode for disk transfers.
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 0x600, finds the active partition and loads its PBS
at address 0x7C00. A copy of the Plan 9 MBR is kept in
/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
/386/mbr detects whether the BIOS supports LBA and acts
appropriately, defaulting to CHS mode when LBA is not pre-
sent. The PBSs cannot do this due to code size considera-
tions. 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.
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9LOAD(8) 9LOAD(8)
Thus the default sequence is: BIOS, MBR, PBS, 9load, kernel.
Because it contains many device drivers for different disks
and networks, 9load is larger than 64K and cannot be run as
a DOS ``.com'' executable. A stripped-down version that
knows about disks but not networks, called ld (really
ld.com), fits in 64K and can be used under DOS to load and
start a program (default 9load) from the FAT16 partition.
It's command line argument is of the same format as the
bootfile specifiers described below. This profusion of
loaders is unfortunate, but at least ld and 9load are com-
piled from the same source.
9load begins execution at address 0x80010000 (64K) and loads
the bootfile at the entry address specified by the header,
usually 0x80100020. After loading, control is passed to the
entry location.
In summary, Plan 9 can be booted on a PC two different ways:
either by booting MS-DOS and using ld to start 9load in the
appropriate directory, or by booting directly from a Plan 9
boot floppy or disk partition prepared using format to
install the appropriate files and bootstrap sectors (see
prep(8)).
The bootfile, which may be compressed with gzip(1), can be
specified to 9load as a bootfile= entry in plan9.ini, or if
booting from the ethernet, by a BOOTP server. If the
plan9.ini file contains multiple bootfile= entries, 9load
will present a numerical menu of the choices; type the cor-
responding number to select an entry.
The format of the bootfile name is device!file or
device!partition!file. If !file is omitted, the default for
the particular device is used. Supported devices are
fdn An MS-DOS floppy disk. N specifies the floppy
drive, either 0 or 1. The bootfile is the contents
of the MS-DOS file. There is no default file. For
compatibility with hard disks, a partition may be
given, but only dos is recognized: fd0!dos!file.
ethern Ethernet. N specifies the Ethernet device number.
If a partition is specified, it is taken to be the
name of a host machine from which to load the ker-
nel. file is determined by the /lib/ndb (see
ndb(6)) entry for this PC.
sdCn Non-floppy disk. The device name format is
described in sd(3). A partition must be given and
must name a partition containing a FAT file system.
The name 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.
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9LOAD(8) 9LOAD(8)
By convention, this partition is called 9fat. There
is no default partition or pathname.
When 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, 9load
searches all units on devices fd and sdCn, in that order,
for a file called plan9\plan9.ini or plan9.ini (see
plan9.ini(8)) on a partition named dos or 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 bootfile. Some options in plan9.ini are used by
9load:
console
baud Specifies the console device and baud rate
if not a display.
ethern Ethernet interfaces. These can be used to
load the bootfile over a network. Probing
for Ethernet interfaces is too prone to
error.
bootfile=bootfile
Specifies the bootfile. This option is
overridden by a command-line argument.
bootfile=auto Default.
bootfile=local Like auto, but do not attempt to load over
the network.
bootfile=manual After determining which devices are avail-
able for loading from, enter prompt mode.
When the search for plan9.ini is done, 9load proceeds to
determine which bootfile to load. If there was no bootfile
option, 9load chooses a default from the following priori-
tized device list:
fd sd ether
9load then attempts to load the bootfile unless the
bootfile=manual option was given, in which case prompt mode
is entered immediately. If the default device is fd, 9load
will prompt the user for input before proceeding with the
default bootfile load after 5 seconds; this prompt is omit-
ted if a command-line argument or bootfile option was given.
9load prints the list of available devices and enters prompt
mode on encountering any error or if directed to do so by a
bootfile=manual option. In prompt mode, the user is
required to type a bootfile in response to the `Boot from:'
prompt.
9load parses the master boot record and Plan 9 partition
tables (see prep(8)), leaving partitioning information
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9LOAD(8) 9LOAD(8)
appended to the in-memory contents of plan9.ini for the
bootfile. This is used by sd(3) to initialize partitions so
that kfs(4) file systems can be mounted as the root file
system. A more extensive partitioning is typically done by
fdisk and prep as part of termrc or cpurc (see cpurc(8)).
A control-P character typed at any time on the console
causes 9load to perform a hardware reset (Ctrl-Alt-Del can
also be used on a PC keyboard).
When loaded from a PBS (rather than from ld.com), 9load must
be contiguously allocated on the disk. See dossrv(4) for
information on ensuring this.
FILES
[drive:] [ path ]9load
[drive:] [ path ]ld
FAT filesystem:\plan9\plan9.ini
FAT filesystem:\plan9.ini
SOURCE
/sys/src/boot/pc
SEE ALSO
plan9.ini(8), prep(8)
BUGS
Much of the work done by 9load is duplicated by the loaded
kernel.
If 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 ld.
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