reactos/boot/freeldr/notes.txt

   1 FreeLoader notes
   2
   3 Memory layout:
   4
   5 0000:0000 - 0000:0FFF: Interrupt vector table & BIOS data
   6 0000:1000 - 0000:6FFF: Real mode stack area
   7 0000:7000 - 0000:7FFF: Cmdline (multiboot)
   8 0000:8000 - xxxx:xxxx: FreeLoader program & data area
   9 xxxx:xxxx - 7000:7FFF: Random memory allocation heap
  10 7000:8000 - 7000:FFFF: Protected mode stack area
  11 8000:0000 - 8000:FFFF: File system read buffer
  12 9000:0000 - 9000:FFFF: Disk read buffer for BIOS Int 13h
  13 A000:0000 - FFFF:FFFF: reserved
  14
  15
  16 FreeLoader Boot Process
  17
  18 FAT 12/16/32 Boot Sector
  19
  20         The BIOS loads the boot sector at 0000:7C00. The FAT32 boot sector
  21 relocates itself higher in memory at 9000:0000 and loads it's extra sector
  22 at 9000:0200 and then looks for freeldr.sys on the file system. Once found
  23 it loads freeldr.sys to 0000:7E00 and then jumps to it's entry point at
  24 0000:8000. The FAT12/16 boot sector does no relocation, it just searches for
  25 the freeldr.sys and loads the first 512 bytes to 0000:7E00. This extra code
  26 enables it to fully navigate the file allocation table. Then it loads
  27 freeldr.sys to 0000:7E00 and jumps to it's entry point at 0000:8000. Before
  28 FreeLoader gets control the boot sector saves the screen contents to a buffer
  29 at 9000:8000 and the cursor x & y position to bytes at 9000:8FA0 & 9000:8FA1
  30 respectively.
  31
  32
  33 ISO-9660 (CD-ROM) Boot Sector
  34
  35         The BIOS loads the boot sector (2048 bytes) at 0000:7C00. First, the
  36 boot sector relocates itself to 0000:7000 (up to 0000:7800). Then it looks
  37 for the LOADER directory and makes it the current directory. Next it looks for
  38 FREELDR.SYS and loads it at 0000:8000. Finally it restores the boot drive
  39 number in the DL register and jumps to FreeLoader's entry point at 0000:8000.
  40
  41
  42 Multiboot
  43
  44         Freeldr contains a multiboot signature and can itself be loaded by a
  45 multiboot-compliant loader (like Grub). The multiboot header instructs the
  46 primary loader to load freeldr.sys at 0x200000 (needs to be above 1MB). Control
  47 is then transferred to the multiboot entry point. Since freeldr.sys expects to
  48 be loaded at a base address 0000:8000 it will start by relocating itself there
  49 and then jumping to the relocated copy.
  50
  51
  52
  53 FreeLoader Initialization
  54
  55         When FreeLoader gets control it saves the boot drive, passed to it in
  56 the DL register, and sets up the stack, enables protected mode, and calls
  57 BootMain().