Lecture 019

Requirement of Booting:

• initialize machine to a known state

• initialize different set of hardware each time it boot

• load and run operating system

BIOS (Basic Input Output System)

Code: read-only memory (flash / EEPROM / EPROM / PROM / ROM) on motherboard, contains firmware

Do:

• Execution begins in real mode (with only 1MB address space) at 0xFFFF0

• power on self test (POST): check if CPU, RAM, keyboard, etc. are working

  • Graphics card: look for driver code at 0xC0000 (640KB - 1MB), if found, load it and run it
  • Disk: look for driver code at 0xC8000 (768KB - 1MB), if found, load it and run it
  • Keyboard: "Keyboard missing, press F1 to continue"
  • (the code is on hardware imported to CPU cache, so it can be different for different hardware, but the interface is the same)

• configure RAM refresh rate, bus speed, password protection, boot order

• Load OS for floppy disk: only support 1 OS

  • look for AA55 signature in the first sector of each device in boot order, if found, proceed
  • boot sector (512 bytes) is loaded to 0x7C00 and executed

• Load OS for hard disk: can support multiple OS

  • Boot sector ("Master Boot Record", has maximum 512 bytes of code) is loaded to 0x7C00
  • Boot sector code read partition table, find active partition
  • move itself off 0x7C00 and load its boot sector to 0x7C00 and execute it

• Boot loader: load OS kernel from disk to memory, and jump to it

  • Why:
    • MBR is only 512 bytes, can't have complex logic
    • MBR assume kernel lives in raw sector location of disk with fixed offset (no filesystem (ext4, NTFS, FAT))
    • MBR has no OS selection menu, recovery modes, etc.
    • MBR provides no boot arguments (grub.cfg), hardware configuration hints to OS
  • GRUB: Grand Unified Bootloader, support multiple OS, can load from network, support different file system, etc .
    • can understand partition label (and extended partition labels)
    • can understand different file system (directories, inodes, symbolic links)
    • can decompress .gz files to load /boot/kernel.gz
    • multiboot header: file should contain magic number 0x1badb002 and some flags in first 8192 bytes
  • PEE (Preboot Execution Environment): load OS kernel from network (DHCP protocol extension) by download via 2nd-stage boot via TFTP with UDP
  • APM (Advanced Power Management): power-saving mode is difficult
    • Problem
      • different hardware have different power-saving features
      • some hardware should enter power saving mode before other hardware
      • user can close lid
    • Solution: power kernel
      • it's a small kernel runs in 16 bits real mode
      • it handles power-saving features of different hardware
      • it context switch to main kernel sometimes
  • ACPI (Advanced Configuration and Power Interface): to replace APM
    • instead of context switch, BIOS provide power management code for OS to interpret and execute.
    • Windows ACPI spec is more accurate, and Linux kernel sometimes has to fake it as Windows
    • very complicated
  • EFI (Extensible Firmware Interface): to replace ACPI
    • more device drivers
    • drivers can be signed, certified, protected
    • more, larger interfaces

• provide device driver to OS

Links:

• BIOS: https://sites.google.com/site/pinczakko

• LinuxBIOS: coreboot.org, openbios.info

• PXE: pxespec.pdf, ipxe.org

• ACPI: uefi.org/specifications

• UEFI: uefi.org/specifications

• EFI: intel.com/technology/efi (use archive.org to read old version, so that it is short and contain main idea)

• Quick Boot by Peter Dice: https://doi.org/10.1515/9781501506819