How BIOS Works

• From the cold state a special hardware circuit sends a signal to the RESET pin of the CPU

• Some registers (cs, eip) are set to fixed values, and the code found at physical address 0xfffffff0 is executed

  • The address is mapped by hardware to BIOS ROM chip

• Linux does not use BIOS routines, BIOS must be executed in real mode

  • Only real mode addresses (seg*16+off) are available in the cold state
  • No GDT, LDT, or paging are needed, must be initialized in real mode

• Major BIOS tasks

  • POST
  • Hardware initialization (e.g. PCI configuration)
  • Search for an OS to boot (configurable)
  • Copies the first sector of the boot device to RAM (at 0x00007c00), jumps to it, and executes

• Invokes the bootloader