term% cat index.txt B.COM(8) System Manager's Manual 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. Af‐
ter 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 automati‐
cally. 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 omit‐
ted, 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 deter‐
mined 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 stan‐
dard 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 data space. Next, in order to find configura‐
tion information, 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 memory 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 inter‐
faces 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 available 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 boot‐
file option, b.com chooses a default from the following prioritized de‐
vice list:
fd e h hd s sd
B.com then attempts to load the bootfile unless the bootfile=manual op‐
tion 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 omitted 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 boot‐
file=manual option. In prompt mode, the user is required to type a de‐
vice!unit!file in response to the 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
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 Manager, it at‐
tempts 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.
B.COM(8)