boot/freeldr/notes.txt

   1 FreeLoader notes (for x86 PC architecture only!)
   2 ================================================
   3
   4 Memory layout (WARNING: may be out-of-date)
   5 -~-~-~-~-~-~-
   6
   7 0000:0000 - 0000:0FFF: Interrupt vector table & BIOS data
   8 0000:1000 - 0000:6FFF: Real mode stack area
   9 0000:7000 - 0000:7FFF: Cmdline (multiboot)
  10 0000:8000 - xxxx:xxxx: FreeLoader program & data area
  11 xxxx:xxxx - 7000:7FFF: Random memory allocation heap
  12 7000:8000 - 7000:FFFF: Protected mode stack area
  13 8000:0000 - 8000:FFFF: File system read buffer
  14 9000:0000 - 9000:FFFF: Disk read buffer for BIOS Int 13h
  15 A000:0000 - FFFF:FFFF: reserved
  16
  17
  18 FreeLoader Boot Process
  19 -~-~-~-~-~-~-~-~-~-~-~-
  20
  21 FAT 12/16/32 Boot Sector
  22
  23     The BIOS loads the boot sector at 0000:7C00. The FAT12/16 boot sector just
  24 searches for FREELDR.SYS and loads its first 512 bytes to 0000:F800. This extra
  25 helper code enables it to fully navigate the file allocation table. The boot
  26 sector then jumps to FREELDR.SYS entry point at 0000:F803 and the helper code
  27 takes the relay. It finishes loading the FREELDR.SYS image and finally jumps to
  28 its final entry point at 0000:FA00.
  29     The FAT32 boot sector loads its extra sector at 0000:7E00 and looks for
  30 FREELDR.SYS on the file system. Once found it loads FREELDR.SYS to 0000:F800
  31 and jumps to its entry point at the same address. This allows it to jump over
  32 the FAT12/16 extra helper code situated at this address, and go to the final
  33 entry point at 0000:FA00.
  34
  35
  36 ISO-9660 (CD-ROM) Boot Sector
  37
  38     The BIOS loads the boot sector (2048 bytes) at 0000:7C00. First, the
  39 boot sector relocates itself to 0000:7000 (up to 0000:7800). Then it looks
  40 for the LOADER directory and makes it the current directory. Next it looks
  41 for FREELDR.SYS and loads it at 0000:F800. Finally it restores the boot drive
  42 number in the DL register and jumps to FreeLoader's entry point at 0000:F800.
  43
  44
  45 Multiboot
  46
  47     Freeldr contains a multiboot signature and can itself be loaded by a
  48 multiboot-compliant loader (like GRUB). The multiboot header instructs the
  49 primary loader to load FREELDR.SYS at 0x200000 (needs to be above 1MB). Control
  50 is then transferred to the multiboot entry point. Since FREELDR.SYS expects to
  51 be loaded at a base address 0000:F800 it will start by relocating itself there
  52 and then jumping to the relocated copy.
  53
  54
  55 FreeLoader Initialization
  56 -~-~-~-~-~-~-~-~-~-~-~-~-
  57
  58     When FreeLoader gets control it saves the boot drive and partition, passed
  59 to it respectively in the DL and DH registers, and sets up the stack, enables
  60 protected mode, and calls BootMain().