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Boot Process.md

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Boot Process.md

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Pure64 boot process

Pure64 uses two different first stage loaders depending on the firmware of the system. Older systems will be using BIOS while newer systems will be using UEFI.

BIOS boot (bios.asm):

  • BIOS loads the first 512-byte sector from the boot disk to 0x7C00
  • The boot code pulls some data about the system (memory map), sets the video mode, and loads the second stage loader to 0x8000.
  • The boot code transitions from 16-bit real mode to 32-bit protected mode and then jumps to the second stage.
  • The second stage immediately transitions from 32-bit protected mode to a minimal 64-bit long mode. It then follows the unified boot path.

UEFI boot (uefi.asm):

  • UEFI loads \EFI\BOOT\BOOTX64.EFI from the FAT32 partition of the boot disk.
  • The boot code sets the video mode, copies the second stage loader to 0x8000, gets the system memory map, and then jumps to the second stage loader.
  • The second stage then follows the unified boot process as the system is already in 64-bit mode.

Unified boot with the second stage (pure64.asm):

  • Once in 64-bit mode it configures the PIC, PIT, serial port, sets up the proper 64-bit environment.
  • Patches the second stage loader so that the AP's can start at 0x8000 as well.
  • The software processes the system memory map and creates a list of memory that is usable.
  • All available memory above 4MiB is mapped to 0xFFFF800000000000
  • Exception gates are built at 0x0
  • ACPI tables are parsed
  • BSP and HPET are configured
  • PIC is activated
  • SMP is activated
  • An information table is built at 0x5000
  • The (up to) 26 KiB of binary data directly after Pure64 is copied to 0x100000
  • Jumps to 0x100000 to start the kernel