Plan 9's 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 data space. Next, in order to find configuration 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 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 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 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 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 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

BUGS