Bootstrapping

• machines start their active lives not knowing much of ANYTHING.
• 'bootstrapping': a series of steps in which the computer successively "becomes smarter" about its environment.
• Each step builds upon the previous one and allows the next one.

Initial state: knowing nothing

• has a binary I/O system (BIOS) in rom.
• has empty memory.
• BIOS knows how to access the 'boot block' of a disk.

Boot block

• 'first' block of a disk
• contains a 'bootstrap loader'
• loader is specific to operating system.
• present on selected hard disks AND floppy disks.

Overview of generic booting

• load loader
• loader loads kernel
• kernel starts everything

Bootstrap loader

• goal: "get smart enough" to load further blocks from disk.
• linux: LILO (LInux LOader)
• now we know how to locate root filesystem and the system kernel /vmlinux (or /vmlinuz)
• load kernel /vmlinux into memory

Kernel

• goal: start up everything else
• locate and mount filesystems (/, /usr)
• check filesystems for errors.
• check which operating mode we should be in.
• start running scripts in /etc/rc* to turn on services.

Building Linux: a bootstrap process

• Chicken and egg problem.
• You can't build linux if there's no linux on the system.
• But you can't get linux on the system until you build it.
• You can't run linux on the filesystems you're building.

Overview of building linux

• Base state: nothing
• run mini-linux from floppy
• mount fake ramdisks
• run install program from fake system
• build real disks
• reboot real disks
• perform post-installation configuration

Base state:

• no linux at all.
• Mr. Bill rules.
• BIOS supports floppy disk boot.

Step 1: run a mini-linux

• boot linux boot floppy.
• LILO loads.
• LILO loads "special" vmlinuz

Step 2: mini-linux dissociates itself from real local filesystems

• vmlinuz builds 'ramdisks': fake disks that live in memory.
• Pretends that these are real.
• Runs installation program from ramdisk.

Step 3: Installation program

• allows you to contact network where all linux files reside.
• partitions disks
• mounts real disks in temporary location (/mnt)
• copies software onto real disks from network.
• builds real bootblock on real disk!

Step 4: Post-installation

• reboot using REAL bootblock.

Step 5: LILO

• loads from real hard disk
• starts loading kernel

Step 6: First boot

• Kernel loads
• one-time scripts are run to configure system
• asks about network information
• asks about root password
• now up and running!

What if it doesn't work...

• or: what we did so you won't have to...
• First, hardware must be put together correctly.
• Everything has to agree in several ways

Booting woes

• BIOS: often the ONLY thing you can't change.
• non-volatile memory failure: backup battery needed.

Non-volatile memory

• Determines boot devices and trial order (try floppy)?
• Controls clock and can exhibit y2k problems.
• Setting: press F1 or F2 while booting.
• Possible problems: backup batteries are BAD.

BIOS limits:

• disk size (addressability of IDE disks)
• in PROM if machine > 5 years old.
• old limit: 512MB or even less.
• difficult to change if in PROM.
• limits whether one can load bootblock!
• Problem: 512MB limit, 900MB partition.
• Old Solution: don't use bootblock from disk!
  • Use floppy disk to boot!
  • Avoids bios limitations on hard disk.
• Another solution: 10 MB partition /boot at beginning of disk
  • Create 10mb partition /boot
  • Kernel is put there when building.
  • even really old BIOS's can read it!
  • MUCH BETTER than relying on floppies! Do this in A1!

BIOS extensions:

• some devices have their OWN ROM's that they add to lower memory.
• Example: SCSI cards.
• Must map card BIOS to unused memory using onboard 'jumpers'.
• If two devices use same memory, system becomes unbootable.

A typical computer

• BUS: a group of 'wires' shared by all component 'cards'
• ISA, PCI: two form factors for cards (spacing of contacts on connectors).
• Many computers have both.

Hardware Configuration

• physically plug in cards.
• Configure cards (using software and hardware)

Card Configuration options:

• IRQ: a number, between 1 and 15, that specifies an 'interrupt' channel.
• Memory range: a pair of hex addresses between 0x8000 and 0xffff where read-only memory is created by plugging in the card.
• IO Port address: a hex address between 0x100? and 0x400? describing where 'ports' for the card reside.

Card configuration rules:

• All cards (and the computer, usually on a motherboard) must have non-conflicting and unique choices for IRQ, Memory, and Ports.
• If not, chaos ensues.
• Two kinds of cards: hard-jumper and software-configurable.