NAME

9load, ld, 9pxeload – 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 a 386 or amd64 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 procedures described below; it cannot be run directly by hand. 9pxeload is a version of 9load that can be booted using the PXE download (BOOTP/DHCP followed by TFTP) found in any reasonable ethernet card's BIOS. 9load will not use BIOS device drivers. 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 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 /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 disk or floppy, the BIOS loads the first sector of the medium at location 0x7C00. In the case of a disk, it is the master boot record (MBR). In the case of a floppy, this is the PBS. 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 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.
Thus the default sequence when booting from disk 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. Its 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 compiled from the same source.
9load begins execution at virtual address 0x80010000 (64K) and loads the bootfile at the entry address specified by the header, usually virtual 0xF0100020 for 386 kernels. After loading, control is passed to the entry location.
In summary, Plan 9 can be booted on a PC three different ways: either by using a PXE–capable BIOS to boot 9pxeload directly over the ethernet, by booting directly from a Plan 9 disk partition or boot floppy prepared using format to install the appropriate files and bootstrap sectors (see prep(8)), or rarely by booting MS–DOS and using ld to start 9load in the appropriate directory.

Bootfile
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 (see Kernel loading below). If the plan9.ini file contains multiple bootfile= entries, 9load will present a numerical menu of the choices; type the corresponding 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
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 kernel. file is determined by the /lib/ndb (see ndb(6)) entry for this PC.
sdCn     Normal 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. By
convention, this partition is called 9fat. There is no default partition or pathname.
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 normal disks, a partition may be given, but only dos is recognized: fd0!dos!file.
bios0    (Not in 9pxeload.) 9load loads from a FAT file system on the first LBA device in the BIOS's list of devices to try to boot from, using the BIOS INT 13 calls also used by pbslba. It does not understand any form of partition table; see the EXAMPLES in prep(8) for how to format such a device. This has been
mostly useful for booting from USB devices so far.
sdB0     (Not in 9pxeload.) A special case of sdCn that uses bios0 to read from a FAT file system. Partitions are understood.

Kernel loading
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.

9pxeload differs slightly in operation from 9load. It is initially loaded by the PXE BIOS at physical address 0x7C00. Only devices which can be automatically configured, e.g. most PCI ethernet adapters, will be recognised. If the file /cfg/pxe/ether can be located via a DHCP server, where ether is the lower–case MAC address of a recognised ethernet adapter, the contents are obtained and used as a plan9.ini.
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 available 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 prioritized 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 omitted 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.

Other facilities and caveats
9load parses the master boot record and Plan 9 partition tables (see prep(8)), leaving partitioning information appended to the in–memory contents of plan9.ini for the bootfile. This is used by sd(3) to initialize partitions so that fossil(4) or 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.
9pxeload differs slightly in operation from 9load. It is initially loaded by the PXE BIOS at physical address 0x7C00. Only devices which can be automatically configured, e.g. most PCI ethernet adapters, will be recognised. If the file /cfg/pxe/XXXXXXXXXXXX can be located via a DHCP server, where XXXXXXXXXXXX is the MAC address of a recognised ethernet adapter, the contents are obtained and used as a plan9.ini.

FILES

[drive:][path]9load
[drive:][path]ld
FAT–filesystem:\plan9\plan9.ini
FAT–filesystem:\plan9.ini
/386       
most of these binaries reside here
/cfg/pxe   directory of plan9.ini files on your TFTP server

SOURCE

/sys/src/boot/pc

SEE ALSO

booting(8), dhcpd(8), 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.
The way 9pxeload obtains the information normally found in a disc plan9.ini file, and thereby the kernel to load and boot, is not ideal and may change in the future.