9boot, 9bootpbs, 9load, 9loadusb, pbs – PC bootstrap programs


9boot is a specialized Plan 9 kernel loaded by the PXE download (BOOTP/DHCP followed by TFTP) found in any reasonable Ethernet card's BIOS; it bootstraps Plan 9 by using PXE to load another 386 or amd64 kernel and start it.

9bootpbs, 9load and 9loadusb are less–commonly–used variants that reside in a FAT file system under the name 9load and bootstrap Plan 9. 9bootpbs is like 9boot but it can be started by a partition boot sector (PBS), as can 9load and 9loadusb. It is intended to PXE boot older machines without working PXE ROMs. 9load and 9loadusb read FAT file systems. 9loadusb will use only the BIOS's device drivers, and thus can load from FAT file systems on USB devices. In contrast, 9load will not use BIOS device drivers and cannot read USB devices.

This profusion of loaders is unfortunate, but at least they are compiled from the same source. The division into separate programs is dictated by the need to fit within the first 640K of memory and the need to avoid intermixing BIOS and non–BIOS disk access.

These programs are run automatically by the boot procedures described below; they cannot be run directly by hand. There are two bootstrap sequences:
–PXE BIOS, 9boot, kernel
–BIOS, MBR, disk partition PBS, 9load, kernel

In summary, Plan 9 is usually booted on a PC by using a PXE–capable BIOS to boot 9boot directly over the ethernet. File servers that must be able to boot when other machines are down boot directly from a Plan 9 disk partition prepared using format to install the appropriate files and bootstrap sectors (see prep(8)).

Details follow.

Kernel loading
9boot is a bootstrap program that loads and starts a program, typically the kernel, on a PC. It is run by the PXE boot ROM of a PC, which loads 9boot at physical address 0x7C00 (31K). When it starts running, it switches to 32–bit mode. It then double maps the first 16Mb of physical memory to virtual addresses 0 and 0x80000000. 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 by TFTP and used as a plan9.ini(8). 9boot then loads the bootfile named within via TFTP, trying each ethernet in sequence, at the entry address specified by the kernel executable's header, usually virtual 0xF0100020. After loading, 9boot creates a Gnu Multiboot header in low memory for the benefit of the loaded kernel and control is passed to the entry location in 32–bit protected mode, even for 64–bit kernels. So far, only amd64 kernels expect Multiboot headers.

Some options in plan9.ini are used by 9boot:
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.
bootfile=bootfile    Specifies the bootfile.
bootfile=manual   After determining which devices are available for loading from, enter prompt mode.

9load is a similar bootstrap program, loaded by the PC partition boot sector program (PBS), which usually resides in the first sector of the active disk partition. It is initially loaded at physical address 0x10000(64K); it begins execution at virtual address 0x80010000. In order to find configuration information, 9load searches all units on devices sd?[0–9]* (all sd devices), for a file called plan9.ini (see plan9.ini(8)) on a FAT 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.

When the search for plan9.ini is done, 9load proceeds to determine which bootfile to load. If there was no bootfile option, 9load searches sd?[0–9]* FAT partitions for a kernel (any file named 9pc*, 9k8* or 9k10*) and if it finds exactly one kernel in a given FAT partition, chooses it. 9load then attempts to load the bootfile.

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.

The bootfile can be specified to these programs as a bootfile= entry in plan9.ini, or if booting from the ethernet, by a BOOTP server (see Kernel loading below). If loading with 9load, the bootfile may be a Plan 9 boot image compressed with gzip(1). In all cases, the uncompressed kernel must be in Plan 9 boot image, ELF or ELF64 format. If the plan9.ini file contains multiple bootfile= entries, these programs 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, 9boot only. 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.
sdCn     Normal disk, 9load only. The device name format is described in sd(3). A partition must be given and must normally name a partition containing a FAT file system. 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, but if file is omitted, 9load will load a kernel directly from the named partition without any interpretation of a file system.
biosn    USB or other BIOS device, 9loadusb only. 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
sdBn     USB or other BIOS device's partition, 9loadusb only. A special case of sdCn that uses biosn to read from a FAT file system.

Boot Sectors
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, 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). 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 limitations. 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.

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 may be read for NVRAM contents or fossil(4) or kfs(4) file systems can be mounted as the root file system. On ISO 9660 CDs, 9load treats the contents of a file named /bootdisk.img as a 9fat partition, and it is assumed to contain the image of a FAT file system. A more extensive partitioning is typically done by fdisk and prep as part of termrc or cpurc (see cpurc(8)). 9boot cannot parse partition tables, as it lacks disk drivers, so add readparts= to the machine's /cfg/pxe file, per plan9.ini(8), if needed.

A control–P character typed at any time on the console causes 9boot to perform a hardware reset (Ctrl–Alt–Del can also be used on a PC keyboard).

9load must be contiguously allocated on the disk. See dossrv(4) for information on ensuring this.

/386       these programs reside here
/cfg/pxe   directory of configuration (plan9.ini) files on your TFTP server


/sys/src/boot/pc          first–stage disk boot sectors (MBR, PBS)
/sys/src/9/pcboot          PC–bootstrap–specific source
/sys/src/9/^(pc port ip)   common kernel source

8l(1), cons(3), booting(8), dhcpd(8), fshalt(8), mkusbboot(8), plan9.ini(8), prep(8)

Some of the work done by 9boot is duplicated by the loaded kernel, but usually by the same source code.

bios and sdB usually only work on the first LBA device in the BIOS's list of boot devices, if they work at all.

USB keyboards will only work with 9boot if the BIOS emulates a PS/2 keyboard (and that is enabled).

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