User:Madanra/Tasks before leaving Real Mode

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Before switching to Protected Mode or Long Mode, there's a number of tasks you'll want your bootloader to perform that are hard or impossible afterwards.

  • Get a map of the physical address space
The only safe and reliable way of getting a memory map is BIOS function INT 0x15, EAX=0xE820. See Detecting Memory (x86) and Memory Map (x86).
  • Video tasks
If you don't have a video driver for every graphics card in existence, you're going to need to use the BIOS. You may want to:
  • Get a list of video modes
  • Get the monitor's EDID
  • Set the video mode
See Getting VBE Mode Info.
  • Get a list of disk drives present and their "virtual cylinders, heads, and sectors" information
The CHS information is useful for correctly setting the CHS values in partition tables without guesswork, and it's useful to know the mapping from "BIOS device number" to disk.
  • If you booted from disk, find out what disk you booted from
todo
  • If you booted from network, get network information
todo
  • BIOS only
  • Determinte PCI access mechanism
  • Enable A20 line
  • Tell the BIOS what mode you're going to use
  • UEFI only
  • Get addresses of ACPI, SMBIOS etc. tables
  • Load your kernel/initrd/whatever
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