B.COM(8) B.COM(8)
NAME
b.com - PC bootstrap program
SYNOPSIS
(Under MS-DOS)
[ drive :][ path ]b [ bootfile ]
DESCRIPTION
B.com is an MS-DOS program that loads and starts a program
in Plan 9 boot format (-H3 option to 8l (see 2l(1))). B.com
loads the bootfile at the entry address specified by the
header, usually 0x80100020. After loading, control is
passed to the entry location.
B.com can be loaded two ways: either by booting MS-DOS and
typing b in the appropriate directory, or by preparing a
Plan 9 boot floppy using format (see prep(8)); such floppies
include and run b.com automatically. With a boot floppy,
the bootfile must be named explicitly to b.com; from DOS it
may be named as an argument to the command.
The bootfile can be specified in one of 3 ways, in order of
precedence:
command-line argument
configuration file option
default, based on available devices
The format of the bootfile name is device!unit!file. If
!file is omitted, the default for the particular device is
used. Supported devices are
fd An MS-DOS floppy disk. The bootfile is the contents of
the MS-DOS file. The default file is 9dos.
e Ethernet. File is [host:]pathname. The default file is
determined by the /lib/ndb (see ndb(6)) entry for this
PC.
h Hard (IDE) disk partition. The bootfile is the contents
of the partition given by file. The default partition is
boot.
hd Hard (IDE) disk MS-DOS partition. As for fd.
s SCSI disk boot partition. As for h.
sd SCSI disk MS-DOS partition. As for fd.
Unit specifies which disk or Ethernet interface to use. The
default is to use the lowest unit number found.
When b.com starts, it relocates itself to address 0x80000 in
the standard PC memory and switches to 32-bit mode. It then
double maps the first 16Mb of physical memory to virtual
addresses 0 and 0x80000000. Physical memory between 0x80000
and 0xA0000 and from 0x200000 upwards is used as program and
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B.COM(8) B.COM(8)
data space. Next, in order to find configuration informa-
tion, b.com searches all units on devices fd, hd, and sd, in
that order, for an MS-DOS file system containing a file
called plan9\plan9.ini or plan9.ini (see plan9.ini(8)). If
one is found, searching stops and the file is read into mem-
ory at physical address 0x400 where it can be found later by
any loaded bootfile. Some options in plan9.ini are used by
b.com:
console
baud Specifies the console device and baud rate
if not a display.
etherX (where X is a number) Ethernet interfaces.
These can be used to load the bootfile over
a network. Probing for Ethernet interfaces
is too prone to error.
bootfile=device!unit!file
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, b.com proceeds to
determine which bootfile to load. If there was no command-
line argument or bootfile option, b.com chooses a default
from the following prioritized device list:
fd e h hd s sd
B.com 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, b.com
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.
B.com prints the list of available device!unit pairs 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 device!unit!file in response to
the `Boot from:' prompt.
A control-P character typed at any time on the console
causes b.com to perform a hardware reset.
FILES
[drive:] [ path ]b.com
MS-DOS filesystem:\plan9.ini
SOURCE
/sys/src/boot/pc
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B.COM(8) B.COM(8)
SEE ALSO
plan9.ini(8)
BUGS
When looking for a plan9.ini file, SCSI disk controllers are
assumed to be at I/O port 0x330. Only one SCSI controller
is recognized.
Much of the work done by b.com is duplicated by the loaded
kernel.
The BIOS data area at physical address 0x400 should not be
overwritten, and more use made of the information therein.
If b.com detects an installed MS-DOS Extended Memory Man-
ager, 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 b.com.
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